Pelagic-benthic coupling, food webs, and organic matter degradationin the Chukchi Sea: Insights from sedimentary pigments and stablecarbon isotopes

Nathan D. McTigue,* Philip Bucolo, Zhanfei Liu, Kenneth H. DuntonMarine Science Institute, The University of Texas at Austin, Port Aransas, Texas

Abstract

An understanding of the origin and fate of organic matter deposited on the seabed in the Chukchi Sea is

critical to our understanding of how regional changes in climate will affect this highly productive ecosystem.

We measured the concentrations of chlorophyll a, pheopigments, and accessory pigments in sediments at 39

stations using high-performance liquid chromatography (HPLC) to investigate the degradation pathways of

chlorophyll-containing organic matter. Chlorophyll a concentrations were among the highest ever reported

in marine sediments (up to 665 mg m-2). Measurements of pheopigments, including pheophytin a,

pheophorbide a, and pyropheophorbide a, revealed substantial degradative processing of organic matter that

began � 2 weeks after ice breakup. However, the ratio of chlorophyll a to total pheopigments (Chl a : pheo)

was > 1 in most stations, indicating that chlorophyll a was preserved despite grazing and senescent degrada-

tion pathways that produced accumulated pheopigments. Increased d13C values of bulk sediment organic

matter was also positively correlated to concentrations of pheophytin a (p < 0.01, r50.46), and total pheopig-

ment concentration was correlated to the d13C values of the epibenthic scavenging snow crab Chionoecetes

opilio (p < 0.01, r 5 0.92) and the surface deposit feeding bivalves Macoma spp. (p < 0.01, r 5 0.69). Based on

these observations, we hypothesize that organic matter degraded by microbial and herbivorous grazing

pathways produces an enriched 13C signal that is transferred to macrofaunal consumers, providing a critical

source of carbon to a rich and diverse benthic food web.

In the Arctic, weak grazing pressure from low zooplank-

ton abundance during spring allows a large proportion of

pelagic primary producers to reach the shallow benthos, pro-

viding a major carbon subsidy to the benthic food web

(Coyle and Pinchuk 2002). The occurrence is exemplified by

the high diversity and biomass of benthic fauna that corre-

sponds to high water column chlorophyll in a variety polar

shelves (Grebmeier et al. 1988; Dunton et al. 2005). The con-

cept of pelagic-benthic coupling has received particular

attention in shallow polar shelf ecosystems where a short

photosynthetic growing season mediated by ice cover sus-

tains diverse and abundant benthic faunal communities

(Dunton et al. 2005; Grebmeier et al. 2006). Vertical trans-

port of epontic and pelagic microalgae during ice retreat is

responsible for a predominant proportion of carbon flux that

sustains the benthic community (Legendre et al. 1992;

Arrigo et al. 2012).

The notion that polar shelf sediments act as repositories

for the various pelagic microalgae that sink to the benthos

led Mincks et al. (2005) and Pirtle-Levy et al. (2009) to con-

clude that such “food banks” provide a considerable amount

of reduced organic matter that becomes a rich substrate for

the microbial community, and ultimately, the benthic graz-

ers. Measurement of sedimentary pigments to assess the

standing stocks of food resources, especially in regions where

rapid polar ice pack retreat in spring fosters ice edge microal-

gal blooms, provides the information necessary to assess the

importance of the ice-sediment linkage (Sakshaug 2004).

Sedimentary pigments provide an estimate of the amount

of pelagic production exported to the benthos and can indi-

cate how this organic matter degrades on the seafloor (Day-

ton et al. 1986; Morata and Renaud 2008). Although a

variety of different chlorophyll a (Chl a) derivatives can exist

in marine sediments, the formation of these compounds

depends on specific diagenetic events. Herbivorous metazoan

grazing forms primarily pheophorbide a by cleaving the phy-

tol chain and removing the Mg21 ion from the porphyrin

ring in the Chl a molecule; pheophytin a, formed after Mg

hydrolysis from the porphyrin ring, is predominantly a prod-

uct of microbial degradation but can also be produced by

metazoan and protozoan grazing (Bianchi et al. 1988; Sun

et al. 1993). The differentiation of pheopigments can provide*Correspondence: mctigue@utexas.edu

429

LIMNOLOGYand

OCEANOGRAPHY Limnol. Oceanogr. 60, 2015, 429–445VC 2015 Association for the Sciences of Limnology and Oceanography

doi: 10.1002/lno.10038

insights into metazoan grazing vs. microbial degradation of

organic matter in sediments. Few studies have described con-

centrations of accessory pigments or pheopigments to differ-

entiate the degradation pathways in arctic shelf sediments

(Morata and Renaud 2008, and references within; Morata

et al. 2011), and currently, only spatially limited sediment

Chl a concentrations have been measured for the northeast

Chukchi Sea using fluorometry (Cooper et al. 2009; Pirtle-

Levy et al. 2009).

Recent studies in the Chukchi Sea have indicated that

benthic fauna assimilate a carbon source that is 13C-enriched

relative to phytoplankton, indicating that the benthic food

web is more complicated than previously conceived (Lov-

vorn et al. 2005; McTigue and Dunton 2014). Microbial deg-

radative activities are shown to cause 13C-enrichment of

organic matter, but the extent that the arctic microbial com-

munity processes organic matter prior to macrofaunal assim-

ilation is poorly understood despite its previously postulated

role in benthic food web dynamics (Lovvorn et al. 2005). We

hypothesize that major degradation processes of chlorophyll-

containing organic matter that cause 13C enrichment occur

before macrofaunal food web assimilation. To address this

hypothesis, we compared the concentrations of pheophytin

a to pheophorbide a in sediments throughout the Chukchi

Sea shelf and identified relationships between the d13C val-

ues of benthic invertebrates and sediments in relation to pig-

ment composition and concentration.

Methods

Study area and sample collection

The study area was located in the northeastern Chukchi

Sea with sampling stations between the Alaska coastline and

the U.S.-Russia maritime border at 169�W, and from approxi-

mately 68.9�N to 72.4�N (Fig. 1). Samples were collected

between 25 July and 16 August 2010 aboard the R/V Moana

Wave. Stations were chosen using a hexagonal tessellation

technique to ensure random selection and even distribution

of stations. Station depth was 42 6 7 m deep (x 6 standard

deviation; range 5 22–53 m). The region is dominated by

Bering Shelf-Anadyr Water (BSAW), with Alaska Coastal

Water (ACW) predominant on the inner shelf (Walsh et al.

1989). Souza et al. (2014a) addressed the mixing of multiple

water masses in the Chukchi Sea during this same expedition

based on temperature and salinity measurements collected at

23 stations. They concluded that at the time of our sam-

pling, the ACW was not prevalent in the study area (<10%),

although it contributed 50–65% to the bottom water at

Stas. 1 and 11, providing evidence for penetration of ACW

onto the shelf.

Suspended particulate organic matter (SPOM) was concen-

trated by vacuum filtration onto precombusted 25 mm GF/F

filters (Whatman) from water collected at near-surface

(within 2–8 m of the surface) and near-bottom (approxi-

mately 3 m above the seafloor) depths. Sediment samples for

pigments, stable isotopes, total organic carbon (TOC), and

total nitrogen (TN) were collected using a 20 cm3 syringe

barrel (1.8-cm diameter, 2-cm depth) at all stations from

undisturbed surface sediments retrieved by a van Veen grab

(0.1 m2). Sediment aliquots were placed in pre-labeled Falcon

tubes (Beckton Dickinson) and immediately frozen in dark-

ness to 220�C. Sediment was transported to the University

of Texas Marine Science Institute (UTMSI) and preserved at

280�C until prepared for pigment analysis.

Benthic fauna were collected using either a 3.05 m

plumb-staff beam trawl with a 7 mm mesh and a 4 mm cod-

end liner or a 0.1 m2 van Veen grab. Of the organisms

obtained, the snow crab Chionoecetes opilio and the bivalves

Macoma calcarea and M. moesta were collected in both a wide

spatial extent and in sufficient replication at stations where

pigments samples were also retrieved for analysis in this

study. The bivalves are hereafter aggregated as Macoma spp.

to enhance spatial coverage. Both bivalve species exhibit the

same trophic habits (Macdonald et al. 2010), which implies

that their stable isotope values are comparable. Fauna were

washed with ambient surface water to remove extraneous

organic matter and sediment. Muscle tissue was extracted

from the organism for stable isotope analyses. Tissue samples

were dried on board ship in aluminum dishes at 60�C.

Pigment analyses

Pigments targeted for analysis in the sediments were Chl

a, the pheopigments pheophytin a, pheophorbide a, pyro-

pheophorbide a, and the accessory pigments Chl b, Chl c

(defined as the sum of c1 and c2), fucoxanthin, peridinin,

Fig. 1. Location of sampling stations in the northeast Chukchi Sea withthe timing of sea ice retreat in 2010. The blue to white gradient repre-sents the earliest to most recent retreat. Station numbers appear on the

map while colors represent values.

McTigue et al. Chukchi Sea sedimentary pigments

430

prasinoxanthin, and 19-hex-fucoxanthin. Chl a was targeted

as a proxy for fresh algal organic matter in sediments while

the pheopigments were used to assess the standing stock of

degraded chlorophyll products from different pathways (see

Introduction). Specific microalgal groups were targeted using

the following pigment biomarkers: Chl b was targeted to

identify the presence of chlorophytes; Chl c is found in

some diatoms and dinoflagellates; fucoxanthin is an acces-

sory pigment mainly found in diatoms; peridinin is predomi-

nantly associated with dinoflagellates; prasinoxanthin is

found in prasinophytes; and 19-hex-fucoxanthin is a hapto-

phyte biomarker (Wright et al. 1996; Morata and Renaud

2008; Szymczak-Zyla et al. 2008). Sediment pigments were

extracted using 10 mL of 100% acetone as residual porewater

in the sample diluted the acetone concentration (Sun et al.

1991). Volume of porewater was determined by evaporation

and accounted for in the extract volume. Samples were soni-

cated in chilled water for 15 min in darkness during acetone

extraction. After centrifuging samples for five minutes at

4000 rotations per minute, supernatant was decanted and fil-

tered through 0.2 lm nylon filters. To ensure the complete

extraction of pigments from sediments, each sample was

extracted twice and the extracts were combined. If the com-

bined extract was cloudy, the entire 20 mL was refiltered

through a 0.2 lm nylon filter.

The high-performance liquid chromatography (HPLC) pig-

ment analysis followed the protocol of the Danish Hydraulic

Institute (DHI). Briefly, a binary gradient of 28 mmol L21

tetrabutylammonium acetate (TBA) in methanol (30% : 30%,

v : v; eluent A) and methanol (eluent B) was used. Eluant B

was ramped from 5% to 95% in 22 min, and held for seven

minutes before falling back to 5% within two minutes. A C8

HPLC column (Agilent Eclipse XDB, 3.5 lm, 4.6-mm

diameter 3 150-mm length) was used, and the eluted pig-

ment was detected by ultraviolet-visible absorbance (wave-

length 5 450 nm). Concentrations were determined by

comparing pigment peaks of equal retention time to those

of certified commercial standards (DHI, VWR, and Sigma-

Aldrich).

TOC, TN, and stable C and N isotopic analyses

To remove carbonates that would interfere with stable

carbon isotope analysis of organic carbon, a subsample of

sediments were soaked in 1 mol L21 HCl until bubbling

stopped, then rinsed in deionized water and dried at 60�C to

a constant weight. Sediment subsamples prepared for stable

nitrogen isotope analysis were not subjected to acidification.

Muscle tissue excised from shell or exoskeleton was not

acidified.

Dried tissue samples were manually homogenized with a

mortar and pestle, and tissue, sediment, and filter samples

were weighed in tin capsules to the nearest 1026 g. Samples

were analyzed on an automated system for coupled d13C and

d15N measurements using a Finnigan MAT Delta Plus mass

spectrometer attached to an elemental analyzer (CE Instru-

ments, NC 2500). Samples were combusted at 1020�C and

injected into the mass spectrometer with continuous flow.

Isotopic ratios are denoted in standard d notation relative to

carbon and nitrogen standards of Vienna PeeDee Belemnite

and atmospheric N2, respectively where

dX5½ðRsample=RstandardÞ21�31000 (1)

and X is either 13C or 15N of the sample and R corresponds

to the 13C : 12C or 15N : 14N ratio. Instrumental analytical

error was 60.20& based on internal standards (casein and

glutamic acid) checked against certified standards from the

U.S. National Institute of Science and Technology and the

International Atomic Energy Agency. We reported TN

instead of total organic nitrogenas inorganic nitrogen, espe-

cially ammonium, can bind to clay minerals common to

marine sediments and inflate the amount of measured

nitrogen.

Ice cover data

Shapefiles of daily ice cover (resolutions to 50 m2) pro-

vided courtesy of the National Ice Center (NIC), were pro-

jected onto a basemap containing all sampling stations in

ArcMap 10.0 (ESRI). The NIC defines the demarcation

between ice coverage and the marginal ice zone (MIZ) as

80% ice cover. This line was tracked daily to determine the

day that each station was initially covered by < 80% sea ice.

Photosynthetically active radiance

Quantum scalar irradiance was measured using a LI-COR

LI-193 underwater spherical quantum sensor attached to a

LI-COR LI-1000 datalogger. Light attenuation was calculated

using the Beer–Lambert equation

Io5Ize2kz (2)

where Io is the surface irradiance, Iz is the irradiance at depth

z, and k is the extinction coefficient. Calculating light

attenuation vs. measuring only light at depth compensates

for variables such as cloud cover or diurnal variation of vari-

able photosynthetically active radiance (PAR). The concen-

tration of reaching sediments was calculated using surface

irradiance and the extinction coefficients that were measured

only at Stations 14, 27, and 38 due to logistical time

restraints involved in deploying equipment.

Statistical analyses

All statistics were computed using R 2.14.0 (www.r-pro-

ject.org). Coefficient of determination (R2) is reported for

correlations except in cases that the sign of the relationship

is important, for which we instead report r values. Signifi-

cance for all tests was set at a 5 0.05. Data interpolation

were performed in ArcMap 10.0 (ESRI) in Lambert Azimuthal

Equal Area projection to prevent the distortion of rasters at

high latitudes. Inverse distance weighting was used to

McTigue et al. Chukchi Sea sedimentary pigments

431

interpolate all pigment values to depict overall trends. Krig-

ing was used to interpolate sediment stable carbon isotope

values. Values created in between actual data points are esti-

mates and are meant to convey patterns; no computations

were made on these estimated values. Data for IP25 concen-

trations were provided by Taylor and Harvey (pers. comm.).

Results

Chl a, pheopigments, and accessory pigments

As the sea ice edge fosters an intense microalgal bloom,

we investigated the number of days that had elapsed

between the ice edge retreat from each station and when the

station was sampled (Fig 1). We used the MIZ demarcation

defined as 80% ice cover by the NIC because this is an indi-

cator of when phytoplankton blooms can occur at the ice

edge (Reigstad et al. 2002). Furthermore, extensive under-ice

(100% ice cover) microalgae blooms have been observed

(Arrigo et al. 2012), so 0% ice cover is a poor timestamp for

the initiation of ice edge primary production. No significant

linear trends were found between days ice free prior to sam-

pling and Chl a concentration, but a second-order (para-

bolic) regression explained the pattern more clearly (Fig. 2).

There is a noticeable lack of data between days 20 and 40

due to the nature of the cruise track. The parabolic pattern

we observed is largely a function of very low Chl a concen-

trations at both the low and high ice-out extremes of the

dataset. Although data within the 20 to 40 day gap would be

insightful, we assume that values within that window would

not drastically change our results.

We found that sediment Chl a concentrations were lowest

at stations that had experienced ice retreat between 82 and

101 days prior to our occupation. Stations that transitioned

from ice cover to open water conditions within just a week

of our occupation contained sediments with relatively low

concentrations of Chl a. However, areas within the study

site that had been in open water conditions for approxi-

mately 35 to 77 days contained the highest concentrations

of Chl a.

Sediment Chl a concentrations ranged from 10.2 mg m22

to 665 mg m22 (0.33 lg g21 to 81.7 lg g21 dry weight;

Table 1; Fig. 3a). The northwestern area of our study site

(Stas. 35, 36, 37, 1030) contained the highest concentrations

of pigments (43.6–81.7 lg g21; 430–665 mg m22) where ice

had retreated 45 days to 56 days prior. Stas. 108, 46, and 109

near Hanna Shoal, the near-shore Stas. 4 and 14, and the

southernmost Stas. 103 and 105 contained relatively low

concentrations of Chl a (0.33–16.0 lg g21; 12.5–44.8 mg

m22). When pigment concentrations expressed by mg m22

and lg g21 were regressed, the two values for Chl a moder-

ately agreed (R2 5 0.72), which suggests that the porosity

was different among different sampling stations.

Pheophytin a was in low concentrations at stations

recently transitioned to open water phase (< 3 days) and

where open water had dominated for over 70 days, which

coincided with stations where Chl a concentrations were

lowest (Figs. 1, 3b). Pheophytin a was in relatively high con-

centrations in two regions: the southern Stas. 103 and 105,

which had experienced ice retreat 16 and 13 days prior to

sampling, respectively, and in the northwest region of the

study site where Chl a was also highest (Figs. 1, 3b). Pheo-

phorbide a, a marker for metazoan grazing, followed a

similar trend to the pheophytin distribution (Fig. 3c). Pyro-

pheophorbide a, the secondary degradation product of pheo-

phorbide a, had not yet formed at the southern stations

where pheophorbide was high or at stations where Chl a was

in low concentrations (Fig. 3d). However, pyropheophorbide

a concentrations were high in the northwest sector of the

study site and in the northeast sector near the mouth of Bar-

row Canyon. In some cases (e.g., Stas. 46, 108, 1015, 27,

1010), the Chl a : pyropheophorbide a ratios were < 1 (Table

1). Total pheopigment concentrations (sum of pheophytin a,

pheophorbide a, and pyropheophorbide a) spanned to rela-

tively high values (> 190 mg m22; 20 lg g21; Fig. 4) and pos-

itively correlated with Chl a concentrations (r 5 0.715, p <

0.001; Table 2).

Despite high pheopigment concentrations at some sta-

tions, the ratio of chlorophyll to total pheopigments (Chl a :

pheo) was > 1 at 32 of 39 stations (Fig. 4). There was no

apparent temporal trend between Chl a : pheo ratios and ice

retreat as stations that had been sub-80% ice cover for both

1 day and 100 days had Chl a : pheo ratios > 1. Some of the

stations that had experienced ice retreat less than two weeks

before sampling exhibited low values (< 1) indicating that

either substantial Chl a had not yet reached the seafloor and

the initial onset of fresh Chl a was rapidly degraded, or the

Chl a was degraded in the water column before it reached

the sediments.

Fig. 2. Second-order regression depicting the trend between time (indays) since ice retreat and sedimentary Chl a concentration. Ice retreatdefined as <80% sea ice (NIC).

McTigue et al. Chukchi Sea sedimentary pigments

432

Molar concentrations of pheophytin a were higher than

pheophorbide a at 25 of 39 stations (Fig. 5). At 10 of these

stations, pheophorbide a concentrations were below the

detection limit. Sta. 46 was the only station where pheophor-

bide a and pheophytin a were both undetectable. There were

no clear spatial or temporal trends aligning stations that had

higher pheophorbide a concentrations than pheophytin a.

Both pheophytin a and pheophorbide a were significantly

and positively correlated to Chl a (p < 0.001, r 5 0.78 and

0.54, respectively; Table 2). Pheophytin a was highly corre-

lated to fucoxanthin (p < 0.001, r 5 0.82), whereas pheophor-

bide a was less so (p < 0.01, r 5 0.50; Table 2).

Table 1. Concentrations of sedimentary pigments (both on a dry weight and areal basis), bottom water temperature, and bottomwater salinity in the northeast Chukchi Sea.

Chl

a

Pheophytin

a

Pheophorbide

a

Pyropheophor-

bide a Peridinin Fucoxanthin Salinity

Temp-

erature

Station lg g21 mg m22 lg g21 mg m22 lg g21 mg m22 lg g21 mg m22 lg g21 mg m22 lg g21 mg m22 (�C)

1 7.34 117 1.45 23.3 3.98 63.9 2.41 38.7 0.00 0.00 3.92 62.90 29.95 6.03

4 0.47 25.6 0.52 29.3 0.00 0.00 0.00 0.00 0.16 9.20 0.45 25.4 32.15 0.86

5 1.43 21.1 0.45 6.65 0.00 0.00 0.00 0.00 0.17 2.53 0.73 10.7 32.06 0.20

6 8.44 103 1.34 16.4 1.53 18.7 3.25 39.7 1.10 13.5 2.35 28.8 32.21 20.22

9 30.6 438 6.39 91.5 2.35 33.7 4.75 68.0 0.00 0.00 9.85 141 32.46 21.47

10 2.74 30.9 1.83 20.7 1.55 17.5 0.00 0.00 0.30 3.34 1.92 21.7 32.53 20.42

11 13.5 196 1.52 22.0 1.45 21.0 0.00 0.00 1.01 14.7 4.22 61.2 32.04 0.53

14 0.49 12.5 0.11 2.75 0.00 0.00 0.00 0.00 0.12 3.13 0.38 9.66 31.83 1.37

15 75.0 191 9.39 23.9 0.00 0.00 0.00 0.00 0.63 1.61 31.4 79.9 32.60 21.53

19 2.96 26.1 1.56 13.8 0.53 4.71 0.63 5.56 0.00 0.00 0.41 3.61 32.24 0.26

20 28.5 296 4.87 50.8 2.32 24.2 0.00 0.00 0.26 2.68 6.51 67.8 32.58 21.68

21 9.19 109 3.14 37.5 0.60 7.15 0.00 0.00 0.50 6.01 2.05 24.4 32.73 21.72

22 2.40 29.8 0.67 8.30 1.00 12.5 0.00 0.00 0.39 4.85 0.90 11.2 32.28 20.80

24 23.8 125 1.79 9.43 1.94 10.3 7.99 42.2 0.00 0.00 6.94 36.6 30.57 21.41

27 6.27 107 0.94 16.0 0.14 2.43 3.87 66.2 0.00 0.00 1.79 30.7 32.23 20.78

29 36.3 114 3.62 11.4 1.53 4.83 1.85 5.85 0.00 0.00 7.70 24.3 32.08 21.14

30 5.07 55.6 1.54 16.9 0.64 6.97 0.00 0.00 1.13 12.4 1.04 11.4 32.92 21.70

35 81.7 603 7.80 57.6 4.88 36.0 0.00 0.00 0.37 2.74 22.0 162 32.64 21.60

36 46.5 616 5.14 68.1 2.03 26.9 1.51 20.0 0.00 0.00 11.8 156 32.81 21.76

37 43.6 430 3.89 38.5 3.98 39.3 2.33 23.0 0.00 0.00 10.8 106 32.82 21.68

38 12.0 118 1.24 12.2 0.46 4.52 0.65 6.39 0.00 0.00 2.14 21.0 32.42 21.01

39 10.9 148 2.90 39.5 0.77 10.6 2.38 32.4 0.00 0.00 1.98 26.9 32.76 21.57

40 4.97 74.0 1.07 16.0 0.00 0.00 0.74 11.0 0.00 0.00 1.16 17.3 32.98 21.69

41 4.76 68.9 1.92 27.7 0.75 10.8 0.80 11.5 0.00 0.00 1.02 14.7 33.02 21.62

46 0.33 10.2 0.00 0.00 0.00 0.00 0.51 15.7 0.00 0.00 0.11 3.50 33.21 21.60

47 3.09 25.0 1.42 11.5 0.00 0.00 0.00 0.00 0.00 0.00 1.95 15.8 33.20 21.66

48 3.50 35.6 0.91 9.27 0.00 0.00 0.00 0.00 0.53 5.42 2.28 23.2 33.64 21.52

49 10.2 87.1 5.48 46.6 1.87 16.0 0.84 7.12 0.00 0.00 2.09 17.8 33.16 21.65

103 7.75 35.2 8.11 36.8 6.88 31.2 0.00 0.00 0.63 2.87 4.58 20.8 32.50 3.08

105 16.0 44.8 6.04 16.9 5.65 15.8 0.00 0.00 0.41 1.14 9.01 25.2 31.64 2.32

106 4.80 73.5 2.11 32.3 0.00 0.00 0.00 0.00 2.01 30.9 0.85 13.0 32.03 20.02

108 2.08 43.3 0.40 8.29 0.22 4.62 2.58 53.5 0.00 0.00 0.60 12.5 32.97 21.66

109 0.66 15.3 0.27 6.39 0.00 0.00 0.00 0.00 0.24 5.61 0.38 8.87 33.06 21.64

1010 51.1 284 5.53 30.9 4.68 26.1 9.77 54.5 0.00 0.00 16.3 91.1 33.30 21.41

1013 2.69 36.1 0.93 12.5 0.75 10.1 0.00 0.00 0.09 1.25 1.31 17.6 33.01 21.55

1014 3.51 43.4 0.60 7.39 0.41 5.06 0.00 0.00 0.31 3.86 1.53 18.9 32.05 0.19

1015 1.68 33.3 0.12 2.32 0.23 4.60 2.68 53.2 0.00 0.00 0.45 8.9 30.83 21.67

1016 0.71 14.8 0.08 1.71 0.01 0.23 0.00 0.00 0.01 0.14 0.40 8.39 32.18 0.22

1030 79.2 665 5.84 49.1 5.55 46.7 1.94 16.3 0.00 0.00 17.6 147 32.63 21.39

McTigue et al. Chukchi Sea sedimentary pigments

433

The pigments Chl b, Chl c, prasinoxanthin, and 19-hex-

fucoxanthin were found in very low concentrations or not at

all and displayed no significant trends within the study area;

therefore, these pigments are not reported here. The absence

of these pigments is likely attributed to the absence of the

microalgal groups that contain these accessory pigments.

Fucoxanthin, a biomarker for diatoms, was the predominant

accessory pigment in sediments (Table 1). Its high correla-

tion with sedimentary Chl a (r 5 0.948, p < 0.001; Table 2)

suggested that diatoms contributed a large proportion of the

Chl a standing stock in sediments. In addition to sedimen-

tary pheopigments, TOC and TN were also correlated to

fucoxanthin (Table 2). Contrarily, the dinoflagellate marker

peridinin was not significantly correlated to Chl a or any

other benthic parameter measured by this study. At 19 of 47

stations, no peridinin was detected (Table 1).

Stable carbon isotopes

SPOM stable carbon isotope values ranged from 226.2&

to 221.0&. Replicate values varied little around the mean

for all samples except for the near-seafloor SPOM at Sta. 37,

which had standard error spanning 1.4&. At all stations, the

mean near-bottom SPOM d13C value was more enriched

than the near-surface value (Fig. 6).

The sedimentary organic matter d13C values in our study

area ranged from 224.9& to 222.3&. The most 13C-depleted

Fig. 3. Interploation of sedimentary pigments (a) Chl a, (b) pheophytin a, (c) pheophorbide a, and (d) pyropheophorbide. The color scheme repre-sents pigment concentration (lg g21).

McTigue et al. Chukchi Sea sedimentary pigments

434

values occurred in coastal stations (1, 4, 14, 27). A distinct

gradient formed as stations further from the Alaskan coast

contained sediments with more 13C-enriched organic matter

(Fig. 7). A significant relationship existed where sediments

more 13C-enriched also contained higher pheophytin and

pheophorbide concentrations (Table 2).

C. opilio tissue ranged from 219.9& to 215.9& for stable

carbon isotope values while the d13C range for Macoma spp.

tissue was 221.6& to 216.8& (Table 3). The concentration

of total pheopigments was significantly and positively corre-

lated with the d13C values of the omnivorous scavenging

snow crab C. opilio (Fig. 8a) and the deposit feeding bivalves

Macoma spp. (Fig. 8b).

Photosynthetically active radiance

Surface irradiance measured with clear skies at 09:30 h

was 700 lmol photons m22 s21, while surface irradiance

measured at 14:00 h and clear skies was 1300 lmol photons

m22 s21. Attenuation coefficients (k) for Sta. 38 (0.07), Sta.

14 (0.13), and Sta. 27 (0.11) allowed 20 lmol photons m22

s21, 4.5 lmol photons m22 s21, and 4.6 lmol photons m22

s21 to reach the seafloor at 47 m, 40 m, and 50 m,

respectively.

Discussion

Organic matter inputs and retention in the benthos

The sedimentary Chl a concentrations quantified in this

study were among the highest ever recorded globally (Table

4). The Chukchi Sea and other arctic seas were historically

regarded as biological deserts as the photosynthetic growing

season is approximately 120 days (Sakshaug 2004). However,

studies in past decades showed that the arctic seas, and par-

ticularly the Chukchi Sea, undergo intense blooms of pri-

mary production that support rich and abundant benthic

fauna (Grebmeier et al. 2006).

The Chukchi Sea is host to three factors that, in concert,

allow the seafloor to become a repository for high concentra-

tions of photopigments. First, the entire 120,000 km2 study

area averaged 42-m deep, about half the average depth of

Fig. 4. Concentration of Chl a and total pheopigments (sum of pheophytin a, pheophorbide a, and pyropheophorbide a) at each station. Stas. 1, 4,10, 46, 103, 108, and 1015 contain less Chl a than total pheopigments.

Table 2. Pearson correlation matrix (r values) relating benthic pigment concentrations, biological characteristics, and environmentalfactors. Tpheo 5 total pheopigments; IP25 5 concentration of Ice Proxy 25 (ng g21); TOC 5 total organic matter; TN 5 total nitrogen.IP25 concentrations from K. Taylor and R. Harvey (pers. comm.). Only significant statistics are shown. Significance denoted as:*p<0.05; **p<0.01; ***p<0.001.

Explanatory Variable Fucoxanthin Pheophytin a Pheophorbide a Tpheo IP25 TOC TN

Fucoxanthin 0.82*** 0.50** 0.71*** 0.52*** 0.51**

Chl a 0.95*** 0.77*** 0.54*** 0.72*** 0.55*** 0.54***

Sediment d13C 0.46** 0.34* 0.56*** 0.56*** 0.64***

Days ice retreat 20.45**

Macoma spp. d13C 0.69**

C. opilio d13C 0.92*

McTigue et al. Chukchi Sea sedimentary pigments

435

the entire Chukchi Sea (Jakobsson 2002). Second, the Chuk-

chi Sea receives nitrate-rich water (<18 lmol L21) advected

northward through the Bering Strait (Codispoti et al. 2009),

facilitating relatively high primary productivity of >400 g C

m22 yr21, of which >160 g C m22 yr21 is “new” production

[see review by Sakshaug (2004)]. Third, this water column

primary production is largely ungrazed by zooplankton due

to an “extreme mismatch” in timing of zooplankton migra-

tion to the shallow seas and the rapid sinking of phytoplank-

ton (Sakshaug 2004).

Rates of zooplankton growth and reproduction during

spring are inhibited by low temperatures, which allow a major

fraction of primary production to sink directly to the seafloor

ungrazed, as shown in the Bering Sea (Coyle and Pinchuk

2002). Similarly in the Chukchi Sea, particle flux from the

water column is extremely high. Walsh et al. (2005) calculated

that in the Chukchi Sea 45% of net primary production settles

to the benthos in summer (July–August) at a rate of 320 mg C

m22 d21. At 67�N in the Chukchi Sea, the southern boundary

of our study area, the upper estimate of particle flux was meas-

ured at 38 mmol C m22 d21 (456 mg C m22 d21) using235Th/238U particle scavenging during the summer open water

season (Moran et al. 1997). Near the northernmost stations in

our study at 71 �N, particle export was quantified at 39 mmol

C m22 d21 (468 mg C m22 d21) during summer months

(Moran et al. 2005). Furthermore, a significant fraction of set-

tled organic matter is retained in Chukchi Sea sediments for

preservation up to decadal timescales based on 210Pb measure-

ments (Lepore et al. 2009).

The coupled three-dimensional circulation and ecological

model of the Chukchi Sea (Walsh et al. 2005) showed that

diatoms, which dominate the phytoplankton community,

are exported to 30 m in late July to represent a relatively

deep Chl a maximum. Diatoms can sink between 0.1 m

day21 to 30 m day21 depending on hydrographic and physi-

ological conditions (Smayda 1971). As at least 1% of surface

irradiance reaches 25% of the arctic continental shelves in

summer (Gattuso et al. 2006), the former value corroborated

by our own measurements, microalgae may not experience a

light-induced buoyancy trigger. Furthermore, dissolved inor-

ganic nitrogen (nitrate, nitrite, and ammonium) and phos-

phate gradients increased with depth during our summer

Fig. 5. Comparison of pheophytin a to pheophorbide a ratios within sediments in the Chukchi Sea. An asterisk denotes ratios of pheophytin:pheo-phorbide >1. A plus sign denotes pheophorbide a in concentrations below detection limit.

Fig. 6. d13C values of near-surface SPOM (closed circles) and near-bottom SPOM (open circles). Values are mean 6 standard error.

McTigue et al. Chukchi Sea sedimentary pigments

436

sampling from nutrient-poor surface waters to sediments

effluxing nutrients (Souza et al. 2014b), which can induce

negative buoyancy in microalgae (Richardson and Cullen

1995). Based on the above ancillary evidence, we assume

that a large component of organic matter produced in the

euphotic zone was rapidly transported out of the shallow

water column to the sediments.

Previous studies have shown trends that the western

boundary of our study site, influenced by the BSAW, con-

tains sediments with lower C : N values and higher TOC and

TN concentrations than the eastern Chukchi Sea sediments

underlying the ACW (Naidu et al. 2004). TOC and TN were

significantly (p < 0.001) and positively (r 5 0.55 and 0.54,

respectively) correlated with sedimentary Chl a standing

stock concentrations (Table 2), which indicated the consist-

ent contribution of microalgae to the sediments in this area.

The extent of the northwestward pigment gradient is incom-

plete since the U.S.-Russian maritime border bisecting the

Chukchi Sea prevented more westward sampling. Although

the photopigments measured here represent only a snapshot

of primary production that has reached the seafloor, the cor-

relation with time-integrated measures like C : N of sedi-

ments indicates the photopigment standing stock is

representative of primary production throughout the

summer season.

Chl a, pheopigments, and accessory pigments

Stations that experienced ice retreat 16 days and 13 days

prior to cruise occupation (103 and 105, respectively)

showed relatively low sedimentary Chl a values (7.75 lg g21

Fig. 7. Interpolated d13C values of sedimentary organic matter withinthe study site. Numbers on map are stations. Color scheme represents

gradient of d13C values (&).

Table 3. D13C for individual organisms, calculated as the difference between d13C values of sedimentary organic matter and con-sumers, in relation to total pheopigment concentrations in sediments. D13C for Macoma spp. is expected to be 0–1& if sedimentorganic matter is major source of assimilated carbon, and D13C for C. opilio is expected to be 1–2&.

Station

Total

Pheopigment

(lg g21)

Sediment

d13C (‰)

C. opilio

d13C (‰)

C. opilio

D13C (‰)

Macoma spp.

d13C (‰)

Macoma spp.

D13C (‰)

1 7.8 224.7 218.9 5.8

1 7.8 224.7 219.0 5.9

21 3.7 222.7 218.5 4.2 218.4 4.3

30 2.2 223.2 219.1 4.1

30 2.2 223.2 219.2 4.0

37 10.2 223.1 220.9 2.2

37 10.2 223.1 219.0 4.1

46 0.51 225.1 221.6 3.5

47 1.4 222.5 219.9 2.6 221.0 1.5

49 8.2 222.9 219.9 3.0

103 15.0 222.3 215.9 6.4 216.8 5.5

103 15.0 222.3 216.0 6.3 216.8 5.5

103 15.0 222.3 216.1 6.2 216.9 5.6

106 2.1 223.3 219.0 4.3

108 3.2 222.9 219.0 3.9

108 3.2 222.9 219.0 3.9

108 3.2 222.9 219.6 3.3

1013 1.7 222.8 219.1 3.7

1013 1.7 222.8 219.0 3.8

McTigue et al. Chukchi Sea sedimentary pigments

437

and 16.0 lg g21), but these values were higher than Stas. 109

(0.66 lg g21), 108 (2.08 lg g21), and 46 (0.33 lg g21) that

had experienced only 1–2 days of sub-80% ice cover prior to

sampling. Counterintuitive to the south-to-north sea ice

retreat paradigm for the Arctic, northwestern stations experi-

enced ice retreat earlier than the southern stations due to

the formation of the Chukchi Polynya in 2010 (Stringer and

Groves 1991; Weingartner et al. 2013). The highest values of

Chl a were measured at the northwest stations, which expe-

rienced sub-80% ice cover about six to eight weeks prior to

expedition occupation. This represents the approximate win-

dow of time for the initial ice edge bloom to occur and reach

the benthos. Stations with longer durations of sub-80% ice

conditions had lower Chl a concentrations, indicating

microalgae had already begun to degrade. Recent observa-

tions of microalgae blooms under thin, first-year ice (100%

cover) signify potential changes in future relationships

between the ice edge bloom and deposition to sediments

(Arrigo et al. 2012; Boetius et al. 2013).

The current understanding of water mass characteristics

affecting primary production in the Chukchi Sea would sug-

gest that our low sediment Chl a concentrations should

occur within the nutrient poor ACW, whereas higher con-

centrations should underlay the BSAW (Grebmeier et al.

2006). During our late summer sampling when mixing of

water masses on the Chukchi Sea shelf can occur (Winsor

and Chapman 2004), the ACW minimally contributed to the

bottom water within our study area (Souza et al. 2014a). We

found no trend between sediment Chl a concentration and

water mass. This is not unexpected as the pigment data are

“snapshot” measurements of standing stock, and the water

mass identity during our sampling is not necessarily the

same water mass that was present when the ice edge micro-

algae bloom formed. Based on these observations and the

date of ice retreat, it is apparent that neither the time since

ice out nor water mass over each station were sole, strong

predictors for Chl a concentration in sediment. However, in

tandem they can elucidate some of the sediment Chl a vari-

ability observed across the northeastern Chukchi Sea.

Previous studies have shown a positive correlation

between water column Chl a and sediment Chl a (Grebmeier

et al. 1988; Morata and Renaud 2008), a trend that was

reflected in our mid-summer Chukchi Sea data. When Sta.

103, a clear outlier, was omitted from the linear regression,

integrated water column Chl a (Grebmeier and Cooper

2012) and sedimentary Chl a correlated with each other (R2

5 0.74). The positive correlation corroborates the tight

nature of pelagic-benthic coupling on shallow arctic shelves

(Dunton et al. 2005; Morata and Renaud 2008). The correla-

tion between the concentration of Chl a in the water col-

umn and benthos implies either that a supply of fresh

microalgae must continually be replenished in the water col-

umn as it is delivered to the benthos, or microalgae simulta-

neously undergo intense production in the water column

and sediments. Engelsen et al. (2002) demonstrated with

remote sensing data that smaller blooms occur repeatedly

after an initial large bloom, and the second largest bloom

occurs up to 60 days post-ice retreat. The water column

bloom at Sta. 103, which occurred 16 days after ice retreat,

aligns with the timeline described by Engelsen et al. (2002)

where the initial ice edge bloom materializes two weeks

post-ice retreat. The secondary large bloom that can occur

�60 days post-ice retreat explains the positive correlation

between water column and sediment Chl a. Stations in the

42 to 64 day post-ice retreat window (e.g., Stas. 21, 35, 36)

had high concentrations of Chl a in the water column and

benthos simultaneously.

Although it is clear that pheophytin a and pheophorbide

a are formed during Chl a diagenesis, there are multiple deg-

radative pathways that lead to the formation of these

Fig. 8. Relationship between sedimentary pheopigments and d13C values of tissue from (a) C. opilio (p<0.01, R2 5 0.84, n 5 10) and (b) Macomaspp. (p<0.01, R2 5 0.47, n 5 14). Numbers next to data points are the stations where each individual was collected.

McTigue et al. Chukchi Sea sedimentary pigments

438

Tab

le4

.Sele

cted

prim

ary

litera

ture

valu

es

for

sed

imen

tary

Ch

la

an

dp

heop

igm

en

tco

nce

ntr

ati

on

s.V

alu

es

are

gro

up

ed

by

reg

ion

.A

nast

erisk

den

ote

sth

eva

lue

isest

imate

dfr

om

afig

ure

.

Stu

dy

Lo

cati

on

Meth

od

Seaso

nD

ep

th(m

)V

alu

es

Anta

rctic

stud

ies

Min

cks

et

al.

2005

Anta

rctica

,W

est

ern

An

tarc

tic

Pen

nin

sula

HPLC

an

dfluoro

metr

yA

ust

ralsu

mm

er–

win

ter

550–680

0.1

–3.0

7lg

g2

1C

hl

a

Dayto

net

al.

1986

Anta

rctic,

McM

urd

oSoun

dSp

ect

rop

hoto

metr

yA

ust

ralsu

mm

er

6–43

47.3

–913

mg

m2

2C

hl

a;

0.4

3–1

0.5

Ch

l

a:

pheo

Arc

tic

stud

ies

Math

eke

an

d

Horn

er

1974

Barr

ow

,A

lask

a(C

hukc

hiSea)

Sp

ect

rop

hoto

metr

yFe

bru

ary–A

ug

ust

�5

55–3

20

mg

m2

2C

hl

a

Glu

det

al.

2002

Youn

gSoun

d,

Gre

enla

nd

Sp

ect

rop

hoto

metr

yJu

ly–A

ug

ust

5–30

66–1

86

mg

m2

2C

hl

a

Coop

er

et

al.

2009

Ch

ukc

hiSea

(sh

elf

bre

ak)

Fluoro

metr

yM

ay–A

ug

ust

49–3760

0–20

mg

m2

2C

hl

a

Woelfelet

al.

2010

Kon

gsf

jord

en

,Sva

lbard

Sp

ect

rop

hoto

metr

yJu

ne–A

ug

ust

5–30

13–3

17

mg

m2

2C

hl

a

Mora

taan

dRen

aud

2008

West

ern

Bare

nts

Sea,

Sva

lbard

HPLC

May–A

ug

ust

195–503

0.1

63–2.8

01

lgg

21

Ch

la;

0.0

10–

1.8

10

lgg

21

pheop

horb

ide;

0.1

34–

3.4

91

lgg

21

pheop

hytin

;0.0

06–

0.4

54

lgg

21

pyro

ph

eop

horb

ide;

0.0

73–0.5

95

Ch

la

:p

heo

Mora

taet

al.

2011

Fran

klin

Bay,

Beaufo

rtSea

Fluoro

metr

yJa

nuary

–Ju

ne

250

1.5

–2.2

mg

m2

2C

hla

*0.0

6–0

.09

chl:p

heo*

Th

isst

ud

yC

hukc

hiSea

HPLC

July

–A

ug

ust

22–60

0.3

3–8

1.7

lg

g2

1C

hla;

10.2

–

665

mg

m2

2C

hla;

0.5

2–8

.0C

hl

a:p

heo

McTigue et al. Chukchi Sea sedimentary pigments

439

pigments. Pheophorbide a and pyropheophorbide a are

major products of macrozooplanktonic grazing formed via

the chlorophyllide pathway, not by enzymatic cleavage of

the phytol chain from pheophytin a (Lee et al. 2000). Pheo-

phytin a is likely a final end-product of microbial degrada-

tion not further converted into pheophorbide a. Sun et al.

(1993) showed that Chl a in sediments free of macrofauna

formed pheophytin a but did not further degrade to pheo-

phorbide a. Bianchi et al. (1988) demonstrated predominant

pathways of pheophytin a formation from microbial degra-

dation of Chl a in sediments while pheophorbide a was

formed by macrofaunal degradation of Chl a. However, there

are examples where both degradation products are produced

by grazing activities, such as from the cladoceran Daphnia

pulex (Daley 1973). Copepods and salps have also been

shown to produce both pheophytin a and pheophorbide a

after grazing on fresh microalgal material in the water col-

umn (Hallegraeff 1981). Pheophytin a and, to a lesser extent,

pheophorbide a were formed by ciliate and heterotrophic

dinoflagellate grazing (Strom 1993). Finally, protozoan graz-

ers are suspected of coprophagy that would accelerate the

conversion of pigments to colorless compounds and prevent

any pigments from accumulating in the sediments (Strom

1993).

In this study, pheophytin a and pheophorbide a were

found in relatively high concentrations at Stas. 103 and 105

where the ice edge bloom occurred approximately two weeks

prior. The rapid accumulation of pheopigments in sediments

demonstrated the efficiency of grazing pathways that

degrade fresh organic matter and shunt it through the food

web, whether microbial or metazoan (Boetius et al. 2013).

The northwestern region of the study area that contained

the highest Ch a concentrations also possessed some of the

highest pheopigment concentrations. Despite a lower

benthic faunal biomass in this region (Schonberg et al.

2014), pheophorbide a still accumulated in the sediments.

Pheophytin a concentrations were appreciably high through-

out the study area except for the coastal stations that experi-

enced the earliest ice edge conditions. In the coastal stations

where ice retreated earliest in 2010, benthic grazing path-

ways had the most time to rework the freshly deposited car-

bon in the sediments. These factors, in conjunction, are

likely responsible for the low chlorophyll and pheopigment

concentrations found here. The stations in the northeast of

the study site approach the 50 m isobath marking the

mouth of Barrow Canyon, a region of the Chukchi Sea

renown for its high benthic biomass (Grebmeier et al. 2006).

Here, at Sta. 29 and 1010, both pheophorbide a and pheo-

phytin a are relatively concentrated.

The molar ratios of pheophytin a to pheophorbide a in

the sediments (range 5 0.25 to 5.00; x6 SD 5 1.48 6 1.16)

at more than half of our stations exhibited a ratio > 1 (Fig.

5). 1 : 1 molar conversions of chlorophyll to pheopigments

after heterotrophic digestion are hypothetically possible

(Shuman and Lorenzen 1975), but pheopigments are eventu-

ally converted to colorless compounds, which reduces the

amount of pheopigments that can accumulate in sediments.

Hawkins et al. (1986) reported 45.9 6 26.7% of Chl a was

converted into pheopigments by the bivalve Mytilus edulus

while the remaining fraction was lost to colorless com-

pounds and heterotrophic metabolism. Helling and Baars

(1985) reported a 67% recovery of pheopigments from chlo-

rophyll after grazing by calanoid copepods. On the contrary,

Kiørboe and Tiselius (1987) measured only an 8% loss to col-

orless compounds by the copepod Acartia tonsa. The large

range of pheophytin : pheophorbide ratios and high concen-

trations of both pigments that we measured in the Chukchi

Sea sediments implies dynamic heterotrophic processes of

microbes, protozoans, and metazoans. For example, at Stas.

4, 5, 14, 15, 40, 48, and 106, pheophorbide a was not

detected, whereas pheophytin a was measurable. At those

stations, the community of heterotrophs and subsequent

degradation processes either did not produce pheophorbide

a but did produce pheophytin a, or pheophorbide a was pro-

duced but degraded faster than pheophytin a to colorless

compounds prior to our sampling. Some evidence suggests

the degradation rate of pheophytin a may be intrinsically

slower than the rate for Chl a or pheophorbide a, which

allows its accumulation in sediments (Sun et al. 1993).

Pyropheophorbide a, the degradation product of pheo-

phorbide a, followed a lag pattern of its parent molecule. In

the southern stations where high pheophorbide a had

recently accumulated in high concentrations from benthic

grazing of the recent microalgae deposition, pyropheophor-

bide a had not yet formed. Pyropheophorbide a was found

in high concentrations where both pheophorbide a was high

and sea ice had retreated more than three weeks prior to

sampling, with the exception of the northern Sta. 108 in the

Hanna Shoal region, which had experienced ice retreat less

than one week prior to sampling. Here, pyropheophorbide a

concentrations exceeded that of all other pigments at that

station. The high concentration of this pigment can be

explained as either the small amount of fresh organic matter

that had already sunk to the shallow benthos was rapidly

and thoroughly degraded, or pyropheophorbide a was a relic

pigment not associated with recently deposited organic mat-

ter. As pyropheophorbide a was not found throughout much

of the study area, the former hypothesis is more likely the

case.

Potential viability of the microphytobenthos

The prevalence of the diatom biomarker fucoxanthin over

the dinoflagellate biomarker peridinin in the sediments dem-

onstrates the importance of diatom carbon to the benthic

food web (Table 1). This concept is corroborated by the high

positive correlation between Chl a, TOC, and TN to fucoxan-

thin in the sediments (Table 2). Diatoms typically dominate

the initial onset of an ice edge bloom, whereas other

McTigue et al. Chukchi Sea sedimentary pigments

440

microalgae like dinoflagellates occur later in summer in low

nutrient-low silicate conditions (Carmack and Wassmann

2006). Siliceous tests make diatoms denser than seawater

(1.050–1.300 g cm23); thus, in the Arctic, diatoms of the

pennate morphology common to ice algae and benthic

microalgae accumulate more rapidly than other microalgae

on the seafloor (von Quillfeldt et al. 2003). Dinoflagellates

might predominate secondary or tertiary late-season blooms

if silicate is limiting, but dinoflagellate biomarkers have been

reported in low concentrations in the Chukchi Sea water col-

umn during summer (Hill et al. 2005).

The proportion of diatoms vs. dinoflagellates in sediments

(% diatoms) was estimated using pigment concentrations in

the equation

% diatoms5ðSfuco:Chl a=Wfuco:Chl aÞ=½ðSfuco:Chla=Wfuco:Chl aÞþ ðSperi:Chl a=Wperi:Chl aÞ� (3)

where S represents a sample’s ratio of fucoxanthin (fuco) or

peridinin (peri) to Chl a. To account for peridinin being

�50% of total dinoflagellate pigments and fucoxanthin

being �38% of diatom total pigments, we standardized S to

respective pigment ratios (W) determined by Wright et al.

(1996). In this way, Wfuco:chl 5 0.60 and Wperi:chl 5 1.06.

This allowed us to compare relative amounts of fucoxanthin

directly to relative amounts of peridinin despite one group

containing higher absolute concentration of pigments than

the other. This provided a rough approximation of the abun-

dance of diatoms and dinoflagellates as the fuco : chl and

peri : chl ratios can vary for various taxa and based on envi-

ronmental conditions. However, the approximation allowed

us to generally compare the concentrations of the two acces-

sory pigments to each other. When the pigments were com-

pared, diatoms accounted for at least 80% of the sum of

diatoms and dinoflagellates throughout Chukchi Sea study

area, except for Stas. 109 (74%), 30 (62%), and 106 (42%).

Although we did not measure the photosynthetic capacity

of sediments, the Chukchi Sea benthos could support a via-

ble microphytobenthos given sufficient PAR at seafloor

depths to support photosynthesis. K€uhl et al. (2001) report

viable benthic microalgae off the coast of Greenland at 5–35

m depths receiving 2–30 lmol photons m22 s21, which

encompasses the range of values observed from our in situ

measurements. Using Equation 2, we can coarsely assess the

potential for downwelling irradiance to reach the seafloor in

the Chukchi Sea using the study area mean depth of 42 m

(z) and applying the mean measured extinction coefficient

(k) of 0.10. Following the assumption of Gradinger (2009) of

an average surface irradiance (Io) of 1000 lmol photons m22

s21 for a sunny day, 13 lmol photons m22 s21 can reach the

seafloor (Iz) during open-water summer conditions, poten-

tially providing dark-adapted benthic microalgae with suffi-

cient PAR for photosynthesis in the Chukchi Sea.

Furthermore, the ratio of Chl a to total pheopigments (Chl a

: pheo) provides a metric to quantify the degree of degrada-

tion in microalgae. Most Chl a : pheo ratios throughout our

study site were > 1, which is indicative of viable cells. In

areas of low Chl a where sub-80% ice concentrations had

existed for >100 days, Chl a : pheo ratios were still >1 sug-

gesting that viable cells continued to produce and maintain

their photosynthetic pigments, counteracting the degrada-

tive grazing and senescent pathways.

The origin of benthic microalgae is likely tied to seasonal

water column and/or sea ice. A significant positive correla-

tion (p < 0.001, r 5 0.563; Table 2) between Ice Proxy 25

(IP25), a lipid synthesized by psychrophilic diatoms (Brown

et al. 2011), and sediment d13C suggests that the more 13C-

enriched sediments in the study area received a higher input

of ice algae, which can range in d13C value from 225.1& to

214.2& (Gradinger 2009). Also, interestingly, a negative cor-

relation (p < 0.01, r 5 20.452; Table 2) between IP25 and

ice-out date suggests that the stations with higher IP25 con-

centrations in their sediments experienced ice retreat more

recently, indicating a connection between sea ice algae and

sediments. However, even the more 13C-enriched sediments

in the study area (222.3&; Table 3) were much more 13C-

depleted than some reported ice algae values.

Sedimentary pigments as proxies for 13C-enrichment for

food webs

A hypothesis posited by previous studies (McConnaughey

and McRoy 1979; Lovvorn et al. 2005) implicates microbial

processing, which degrades organic matter and respires 13C-

depleted CO2, as a mechanism to provide 13C-enriched

organic matter to Bering Sea benthic fauna. The hypothesis,

to our knowledge, has not been directly tested in the Chuk-

chi Sea benthos, but our pigment and stable isotope data

provided some insight to the relationship between the degra-

dative processes in the sediments and the macrofaunal food

web.

Depth profiles of SPOM in our study area revealed organic

matter was more 13C-enriched near the seafloor than in sur-

face waters (Fig. 6). A possible explanation of this trend was

that as SPOM sinks toward the seafloor, it degraded and frac-

tionated in its stable isotope ratio as it lost 12CO2. Con-

versely, the slightly more 13C-enriched sedimentary organic

matter might be suspended by bottom currents to become a

constituent of near-bottom SPOM. Regardless, the d13C val-

ues of near-bottom SPOM did not account for the 13C-

enriched stable isotope values of the majority of benthic

fauna (McTigue and Dunton 2014). Therefore, if organic

matter degrades while it is suspended in the water column,

its d13C value will continue to increase as respiration pro-

gresses. This process will continue once it reaches the

seafloor.

There were significant trends between sediment d13C val-

ues and pheopigment concentrations (Table 2), implicating

degradation of organic matter is related to 13C enrichment.

McTigue et al. Chukchi Sea sedimentary pigments

441

The correlational trends of both sediment and organism

d13C values to pheopigment concentration may be con-

founded by sources of organic matter (marine vs. terrestrial)

to the system and the subsequent spatial patterns of stable

isotope values (i.e., isoscapes). We found the very distinct

pattern of sedimentary organic matter d13C values in the

Chukchi Sea also reported in the synthesis by Naidu et al.

(2004) during our summer sampling (Fig. 7). The previous

findings concluded that the coastal area (Stas. 1, 4, and 14 in

this study) received more terrestrially derived material that

was relatively 13C-depleted, whereas sediment from the areas

at the western and northern extent of our study area (e.g.,

Stas. 20, 21, 35, 36, 37, 1030, 38, 47, 49) contained a more13C-enriched signal of marine phytoplankton.

The gradient of 13C-depleted nearshore sediments to 13C-

enriched offshore sediments showed a striking resemblance

to the pattern of sedimentary pheophytin and pheophorbide

(Fig. 3), from low concentrations in the nearshore sediments

to high concentrations in offshore sediments. It is, then, dif-

ficult to separate the positive trends between organism d13C

values and pheopigments from potentially confounding sedi-

mentary d13C values. One useful metric to discount the pos-

sibility of this confounding trend was to assess the difference

between consumer and sediment stable isotope value

(d13Corganism 2 d13Corganic matter, or D13C).

If consumers were assimilating carbon from the sediment

organic matter, the difference in d13C values (D13C) for sec-

ond trophic level deposit feeding organisms like Macoma

spp. would range between 0& and 1&. Likewise, the D13C

for third trophic level organisms, like C. opilio, would range

from 1& to 2&. However, C. opilio individuals exhibited

D13C between 2.6& and 6.4&, and Macoma spp. ranged

between 1.5& and 5.6& for D13C (Table 3). The larger than

expected D13C suggests that bulk sedimentary organic mat-

ter, which is also isotopically similar to the overlying water

column organic matter, is not the primary source of carbon

to consumers. Benthic microalgae or degraded organic mat-

ter enriched in 13C within sediments may be diluted by

more plentiful, relatively 13C-depleted sources of organic

matter in our measurements of bulk sedimentary organic

matter. The correlation between the relatively high D13C for

these two organisms and the pigment concentrations in sedi-

ments indicate a potential relationship between enriched

d13C values in organisms and degraded organic matter (Fig.

8a,b).

Pigments are useful biomarkers for investigating the dia-

genetic processes of chlorophyll-containing organic matter.

However, the production of major pheopigments from mul-

tiple heterotrophic pathways confounds our ability to specif-

ically implicate the microbial food web as a major step in

organic matter processing vs. other protozoan and metazoan

activities. Future studies should experimentally investigate

predominant pigments formed by the diverse array of con-

sumers in the marine benthic system as most feeding-

pigment studies use zooplankton. An essential question still

remains for the carbon and nitrogen cycles in the Chukchi

Sea: what are the steps leading to the assimilation of organic

matter by benthic consumers? The pigment and stable iso-

tope data presented here suggest diagenetic intricacies, but

direct experimentation and measurement of these hypotheti-

cal processes are still needed.

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Acknowledgments

This work was funded by the U.S. Department of the Interior, Bureauof Ocean Energy Management (BOEM), Alaska Outer Continental Shelf

Region, Anchorage, Alaska under Contract Number M08PC20056 aspart of the Chukchi Sea Offshore Monitoring in Drilling Area-Chemicaland Benthos (COMIDA-CAB) Project and the BOEM Alaska Environmen-

tal Studies Program, and the University of Texas Marine Science Institute

(UTMSI) Awards for Students and Postdocs for Innovation and ResearchExcellence (ASPIRE) grant. We owe special thanks to the Contracting

Officer’s Technical Representative D. Prentki for his support during theCOMIDA-CAB project. The navigational expertise and shipboard assis-

tance of Capt. J. Seville and the crew of the R/V Moana Wave was essen-tial in the success of this project. We are appreciative for chief scientist J.Grebmeier for her leadership at sea. We extend our gratitude to A.

Souza, T. Connelly, K. Jackson, P. Garlough, and S. Liu for their contri-butions to this work, and for the quality digital products provided by

the National Ice Center (NIC) that facilitated our data analysis. We thanktwo anonymous reviewers for helping us improve this manuscript.

Submitted 23 April 2014

Revised 28 November 2014

Accepted 28 October 2014

Associate editor: Robert R. Bidigare

McTigue et al. Chukchi Sea sedimentary pigments

445