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Continental Shelf Research 27 (2007) 1096–1103

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Transport, sloughing and settling rates of estuarinemacrophytes: Mechanisms and ecological implications

M.R. Flindta,�, C.B. Pedersenb, C.L. Amosc, A. Levyd,A. Bergamascoe, P.L. Friendc

aInstitute of Biology, Centre of Environmental Technology, University of Southern Denmark, Campusvej 55, 5230 Odense M, DenmarkbCounty of Funen, Denmark

cSchool of Ocean and Earth Science, National Oceanography Centre Southampton, Southampton SO14 3ZH, UKdAcadia University, Nova Scotia, Canada

eCNR, Messina, Italy

Available online 11 January 2007

Abstract

The study of plant-bound nutrient transport has been largely neglected in estuaries. Lately however, it has been shown

that nutrients bound to macroalgae and seagrasses can constitute a major part of the nutrient transport in shallow tidal

estuaries. Organic detritus in estuaries comes from various sources. This paper looks into the source of detritus from

sloughing, and the transport behaviour of plant detritus under unidirectional flows. In order to determine the extent of the

sloughing of macrophytes, the threshold current velocities for the traction and resuspension of the most common

submersed macrophytes in Venice Lagoon (Ulva lactuca, Enteromorpha sp., Ceramium rubrum, Cladophora sp., and

Chaetomorpha linum) were studied in a laboratory flume. It was found that all macrophytes subjected to flows of

1.5–3.0 cm s�1 move initially as bed load. The threshold for suspension of the macrophyte tissue was at current speeds

43 cm s�1. The exception was the filamentous macroalgae, C. linum, which moved as bed load at all current speeds. This

implies that the advection of plant-bound nutrients in Venice Lagoon is widespread and takes place over virtually all stages

of the tide.

Initial experiments were carried out on unattached macroalgae. A second study focused on the sloughing of attached

macroalgae by steady currents. The threshold current speeds at which sloughing commenced varied between different types

of algae, and sloughing rate was related largely to current speed. The resuspension rates were uniform between the different

macrophyte groups.

Our results help explain why plant matter has been trapped in nets close to the bed of Venice Lagoon on ebbing tides.

It shows that a major component of the bedload is organic in origin. The results verify that a large proportion of the net

nutrient export from estuaries is bound in macrophyte tissue. These findings need to be included in future ecological

models that describe the resuspension, sloughing and settling of macrophytes.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Macrophyte; Seagrass; Traction; Resuspension; Sloughing; Settling; Transport

e front matter r 2007 Elsevier Ltd. All rights reserved.

.2006.08.011

ng author. Tel.: +45 65502606; fax: +45 65930457.

ss: mrf@biology.sdu.dk (M.R. Flindt).

ARTICLE IN PRESSM.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–1103 1097

1. Introduction

The external nutrient loading of many estuarieshas recently been decreasing due to better wastewater treatment, with some estuaries only nowbeginning to adjust to this change (Borum, 1996).As a result, water quality has improved whenmeasured as summer and winter average values,and phytoplankton growth-limiting periods arelonger, resulting in a better benthic light climatewith the potential for reinvasion by benthic macro-phytes. This phase of recovery often shows non-linear behaviour and can be longer than expected(Borum, 1997). National monitoring programmes,focusing on nutrient loading, water quality and theexport of nutrients, have been developed to acquiremore information about the recovery phase. Inparticular, estimates are made of nutrient massbalances at the outer boundary of lagoons andestuaries. However, measurements have been lim-ited to dissolved inorganic nutrients (ammonia,nitrite, nitrate, phosphate), and the traditionalparticulate detritus fraction. Possible transport ofnutrients bound in macro-detritus and macrophytesis often not taken into account, although somestudies have shed light on this aspect of estuarineplant dynamics (Flindt et al., 1997, 1999; Salomonsenet al., 1997, 1999).

A major fraction of submerged estuarine macro-phytes are found at the bed, either unattached (Ulva

sp. and Chaetomorpha sp.) or loosely attached(Enteromorpha sp., Ceramium sp. and Polysiphonia

sp.). These macrophytes are subject to the action ofbottom currents, which transport them close to thebed. It has recently been shown that macrophyte-and macro-detritus-bound nutrient transport can bethe dominating form of nutrient transport inestuaries (Flindt et al., 1997, 1999; Salomonsenet al., 1997, 1999). This occurs particularly in thegrowth season, when opportunistic macroalgae andseagrasses in shallow eutrophic systems may reachvery high biomasses (Sfriso et al., 1992; Sfriso, 1995;Viaroli et al., 1991, 1996). Macrophytes have beencollected as drifting macro-detritus in VeniceLagoon in such high quantities, that plant-boundnutrient export may constitute more than 90% ofthe total nutrient export (Flindt et al., 1999). Thisplant-bound nutrient transport far exceeds tradi-tionally measured nutrient transport.

There is a correlation between current velocityand advection of macrophyte material in VeniceLagoon (Flindt et al., 1997). Studies show that some

macrophytes are transported as bed load (Chaeto-

morpha sp.), whilst others travel dispersed through-out the water column (Ulva sp.). The majority ofseagrasses float at the surface. It is important toaccount for this plant-bound export and/or disper-sion of macroalgae to determine accurate nutrientbudgets. In order to simulate such export thethreshold velocity for mobilisation of macrophytes,the mode of transport as a function of currentvelocity, the current threshold for sloughing ofattached macrophytes, and the settling rates of thedifferent dominating macroalgae must be known.The aim of this study is therefore: (1) to examine themodes of transport of different forms of macroalgaesubject to unidirectional flows; (2) to determine thecurrent threshold for macroalgal transport; (3) todefine the relationship between plant velocity andmean current speed; (4) to define the currentthreshold for sloughing of macrophyte thalli; and(5) to examine the settling rates of macroalgae instill water. A final objective is to explain thedifferences in export and import of macrophytetissue, measured in Venice Lagoon by Flindt et al.(1997).

2. Materials and methods

Three sets of experiments were undertaken onUlva lactuca, Ceramium rubrum, Cladophora sp.,and Chaetomorpha linum: resuspension, sloughingand settling. The macroalgae and seagrass leaveswere treated similarly in all three experiments. Thatis, they were washed gently in estuarine water toremove surface air bubbles. Epiphytes, fauna andinorganic particulate matter were removed in orderto eliminate errors due to changes in mass.

2.1. Resuspension experiments

Measurements of transport modes and resuspen-sion of macrophytes were made in Lab Carousel(Amos et al., 2000; Thompson and Amos, 2002).The flume is a 2m diameter acrylic annulus, with achannel width of 0.15m. A current is generated by arotating lid fitted with eight equidistantly-spacedpaddles. Lid speed is controlled by an E-tracks ACinverter motor controller. Current speed is mea-sured using a Marsh-McBirney EMCM. A 4 cmthick bed was formed (by deposition from suspen-sion) in the flume using 50 kg of cohesive sedimentcollected from a local intertidal mudflat. Thesediment was sieved through a 1mm mesh to

ARTICLE IN PRESSM.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–11031098

remove shells and gravel. The bed was allowed tostabilise for 24 h before experiments. The bed thenhad a clearly defined sediment–water interface, andwas capable of supporting the introduced macro-algae. The flume was filled with pre-filtered seawater(temperature 18 1C) to a water depth of 0.36m.

Four types of macroalgae were collected from thesite of the sediment sample: U. lactuca, C. rubrum,Cladophora sp., and C. linum. The macroalgae wereplaced gently on the sediment surface in still water,and were thus unattached. In most of the experi-ments, the algae frontal surface area constituted lessthan 5% (and a few times between 5% and 10%) ofthe hydraulic cross-section of the flume, ensuring anunrestricted free flow. Mean current speed wasincreased in steps of 0.5 cm s�1 in 10min increments.The free-stream current speed at which the macro-phytes started to move as bedload was consideredthe threshold velocity for transport. This free-stream velocity has been studied in detail byThompson et al. (2004). In all experiments, macro-phytes started to move as bed load, and then movedinto suspension at higher current velocities. Thecurrent speed at which it was observed that themacrophytes lifted into suspension was defined asthe threshold for suspension. Algae speed (70.5 s)was calculated using the known flume circumferenceand a stop watch to record the time taken forindividual species to travel 5 times around theannulus.

2.2. Sloughing of macroalgal tissue

Sloughing was evaluated using the same labora-tory set-up as for the resuspension experiments.Five different species of macrophyte were collected,of which Enteromorpha sp., Ulva sp. and Porphyra

sp. were attached to a mixture of bivalves, gastro-pod shells, and small stones, and Cladophora sp. andFucus sp. were attached mainly to stones. Onlyhealthy specimens were used. Ten attached speci-mens of each species were placed evenly in theflume. Experiments were carried out one species at atime. The current speed was increased from stillwater in steps of 0.5 cm s�1 until the macroalgaestarted to move. Any sloughed macroalgal materialwas collected manually within each increment incurrent speed. The biomass of the sloughed macro-algae, and the thalli still attached at the end of eachexperiment, were quantified. Intercepts from a least-squares regression between sloughed biomass andcurrent speed were used to estimate sloughing

threshold velocities. The linear regression was alsoused to estimate when all algae tissue (y ¼ 1) wouldbe lost by the sloughing process.

2.3. Macrophyte settling rate

All settling rates were measured on living, washedmacrophyte material with the exception of deadseagrass leaves (Cymodocea sp.). This, becauseseagrass leaves can settle only when the aerenchyme(air-storing tissue) is filled with water, whichhappens only with senescence. The settling ratewas measured first against different sizes of algae.Discs of U. lactuca were cut into different sizesfollowing the technique described by Geertz-Hansenet al. (1993). Squares of algae were also made inorder to evaluate the effect of shape on settling.

The settling experiment was carried out using asettling column 2.25m high and 30 cm in diameter,filled with pre-filtered estuarine water at a tempera-ture of 24 1C. The algae were stored at the sametemperature prior to the experiment. The settlingrate was measured over a distance of 2.0m. Resultsof algae that were observed to touch the walls of thecylinder were excluded. The settling mode ofcircular and square sheets of algae (either transverseor normal to settling) was recorded in all cases.Experiments were repeated using different macro-algae and seagrass leaves, in order to determine thevariation in settling rates. In most of the settlingexperiments the algae area was less than 10% of thearea of the settling column insuring an unaffectedsettling pattern.

3. Results

3.1. Macroalgal resuspension thresholds

Eight pieces of Ulva sp. and five pieces ofC. linum, Ceramium sp. and Cladophora sp., eachdiffering in weight by a factor of 10, were monitoredat increasing current speeds (up to 10 cm s�1). Thethreshold velocity of the smallest piece was 3 cm s�1,and the suspension threshold was 7 cm s�1. Thelargest piece of Ulva sp. had a threshold velocity of2 cm s�1 and a suspension threshold of 9 cm s�1. Thevelocity increase of both larger and smaller pieceswas proportional to stepwise increases in currentspeed (Fig. 1(A)). There was no significant differ-ence between threshold velocities or transport ratesfor the different sizes. The data below the suspen-sion threshold fitted a straight line (r2 ¼ 0.97). The

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Ulva lactuca

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 0.02 0.04 0.06 0.08 0.1 0.12

Current speed (m s-1

)

Alg

ae s

peed

(m

s-1

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Alg

ae s

peed

(m

s-1

)

Alg

ae s

peed

(m

s-1

)A

lgae s

peed

(m

s-1

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1:1

Chaetomorpha sp.

0 0.02 0.04 0.06 0.08 0.1 0.12

Current speed (m s)-1

1:1

Ceramium sp.

1:1

Cladophora sp.

1:1

0 0.02 0.04 0.06 0.08 0.1 0.12

Current speed (m s-1

)

0 0.02 0.04 0.06 0.08 0.1 0.12

Current speed (m s-1

)

A B

C D

Fig. 1. Algae speed as a function of free stream current speed for four species of macroalgae. Linear correlations below the suspension

threshold are shown for: (A) Ulva lactuca. (r2 ¼ 0.97, n ¼ 8); (B) Chaetomorpha sp. (r2X0.96, n ¼ 5); (C) Cladophora sp. (r2X0.93, n ¼ 5);

(D) Ceramium sp. (r2X0.93, n ¼ 5).

M.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–1103 1099

threshold velocity for C. linum (Fig. 1(B)) variedbetween 2 and 3 cm s�1, and was similar to that ofUlva sp. None of the five C. linum pieces passed intosuspension but continued to tumble along the bed atall current speeds. All data below the suspensionthreshold fitted a straight line (r2 ¼ 0.96). Thefilamentous red algae Ceramium sp. (Fig. 1(C))showed a transport pattern similar to the greenalgae Cladophora sp., with a threshold velocityvarying from 2 to 3 cm s�1. Four pieces of algae hada suspension threshold of 9 cm s�1; the fifth piecewas not resuspended. Five similar sized pieces ofCladophora sp. (Fig. 1(D)) were used. These algaestarted to move along the bottom at a current speedof between 2 and 3 cm s�1. The first piece ofCladophora sp. was suspended at 5 cm s�1, theremaining two at 9 cm s�1. The data set below thesuspension threshold for both Ceramium sp. andCladophora sp. fitted a straight line (r2 ¼ 0.92).Algal speed and current speed should be equal whenthe algae are resuspended, and this convergence wasindeed apparent at higher flows.

Average current speeds at which the macrophytesmoved as bedload and suspended load are illu-

strated in Fig. 2. All species of macroalgae began tomove at very low current speeds (2–4 cm s�1). Oncein motion, the algae moved slowly in traction alongthe bed. With increasing current speed, the algaebegan to tumble along the bed. At current speeds ofbetween 6 and 10 cm s�1, three of the algae specieswere resuspended. Only Chaetomorpha sp. was notresuspended, but persisted in moving as bedload.

Macrophytes release oxygen from their tissue,which can be trapped as attached bubbles therebychanging their apparent density causing them tobecome buoyant. Thus, floatation and buoyancy areresponsible for part of the resuspension of themacroalgae. These experiments were undertakenwithout oxygen bubbles; thus, results are conserva-tive. All the macroalgae were selected so that onlyhealthy algae without weaker sporulation areaswere used in the studies, which ensured conservativeand reproducible results.

3.2. Current-induced sloughing

The first attached macroalgal species to losebiomass were Porphyra sp. and Ulva sp., with

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3–4% of biomass lost at a current speed of12 cm s�1, increasing to 4–7% lost at 15 cm s�1

(Fig. 3). At a current speed of 40 cm s�1, 40–50%was lost, and the algae and attachment moved alongthe bed. The sloughing of Enteromorpha sp. wasslightly less sensitive to current speed: at 20 cm s�1,only 1% of the biomass was sloughed, increasing to20% at 35 cm s�1. Above this current speed, shellsand attached Enteromorpha sp. started to move. Themost resistant species to sloughing was Fucus sp.,which started to lose small pieces of tissue at a speedof 30 cm s�1. The loss was only 1.5% of the biomassat a speed of 35 cm s�1, and 4% of the biomass at50 cm s�1. Fucus sp. was attached to larger stonesthat did not move at the highest current speeds inthe experiment.

Macroalgae sloughing

0.0

0.2

0.4

0.6

0 0.2 0.4 0.6

Current speed (m s-1)

Slo

ug

hed

bio

mass

tota

l b

iom

ass

Fucus

Ulva

Porphyra

Entero

Fig. 3. Relationship between sloughed macroalgal biomass as a

proportion of total biomass, and current speed.

0.00

0.05

0.10

0.15

Ulva Chaet. Cer. Clad.

Cu

rren

t sp

eed

(m

s-1

)

bed load suspension

Fig. 2. Transport mode (bed load, suspended load) for four

species of macroalgae: Ulva lactuca, Chaetomorpha sp., Ceramium

sp. and Cladophora sp. Results from all of the Lab Carousel

experiments. Error lines indicate standard deviation of mean.

Linear regressions between current speed andsloughed biomass for ten replicates of each speciesare presented in Table 1. The statistics show thatmacroalgae sloughing has a high dependency oncurrent speed, and that the differences in sloughingthresholds are much greater than the measuredvariation in resuspension thresholds for unattachedmacroalgae (Fig. 2). A rough estimate of the currentspeed at which all macroalgal tissue is lost can befound by extrapolation of the sloughed biomass/total biomass (see Fig. 2) to a proportion of 1.0(Table 2).

3.3. Settling rates

This study involved macroalgae and seagrasscollected in Venice Lagoon during summer 1998.The results of the Ulva sp. settling measurements arepresented in Fig. 4. Based on correlation analysis, itwas clear that the settling rates were independent ofmacrophyte size even over a range of one order ofmagnitude (Ulva sp., square, r2 ¼ 0.03; Ulva sp.,circular, r2 ¼ 0.02). The settling rates also seemed tobe independent of the shape of the algae. Roundand square forms had the same range of settlingrates: square pieces of Ulva sp. settled at rates of17–70mh�1, and circular pieces at rates of17–75mh�1. In the Ulva sp. settling experiment,the algae began settling by random motion and

Table 1

Estimated sloughing thresholds calculated from linear regression

between sloughed biomass and current speed for four species of

macroalgae

Species Threshold

(cm s�1)

Regression r2

Porphyra sp. 9 y ¼ 0.0127x�0.12 0.87

Ulva sp. 12 y ¼ 0.0143x�0.17 0.91

Enteromorpha sp. 21 y ¼ 0.0094x�0.2 0.86

Fucus sp. 217 y ¼ 0.0023x�0.5 0.99

Table 2

Estimated current speeds at which total sloughing is reached for

four species of macroalgae

Species Total sloughing

(cm s�1)

Porphyra sp. 88

Ulva sp. 82

Enteromorpha sp. 128

Fucus sp. 652

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0

20

40

60

80

0 2 4 6 8 10

Ulva area (cm2)

Se

ttli

ng

ra

te (

m h

-1)

Fig. 4. Settling rates for different sizes and forms of Ulva lactuca.

The size is given by the area (cm2). Circles and squares indicate

circular and square discs of the algae, respectively. There were no

significant differences between forms or size classes.

0

50

100

150

200

0.0 2.5 5.0 7.5 10.0

Plant mass (g wwt)

Sett

lin

g r

ate

(m

h-1

)

Condrus Cymodocea Gracilaria Polysiphonia

Fig. 5. Settling rates for four different species of macrophyte:

Chondrus sp.E; Cymodocea sp. J; Gracilaria sp. n; Polysiphonia

sp. ’.

0

20

40

60

80

100

Sett

lin

g r

ate

(m

h-1

)

Ulva Light h Ulva Light v Ulva dark h Ulva dark v

Condrus Gracilaria Polysifonia Cymodocea

Fig. 6. Average settling rates for all the studied macrophytes.

The Ulva class was subdivided into light and dark individuals,

which settled either with the thallus placed horizontal (h) or

vertical (v).

M.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–1103 1101

settled either flow transverse (35mh�1) or flowparallel (60mh�1). Fig. 5 shows settling rates ofdifferent sized pieces of branched or filamentousmacroalgae, and indicates that settling rate withineach species is independent of shape or size of theindividual pieces.

The study of macroalgal and seagrass settlingrates showed variations among the different species,although all macrophytes settled relatively quickly(Fig. 6). The red algae, Chondrus sp. and Gracilaria

sp. had the highest settling rates (80–85mh�1). Thered algae, Polysiphonia sp., settled at a slower rate(30mh�1), whilst the seagrass, Cymodocea sp.,settled at an intermediate value of 55mh�1.Correlation analysis between plant mass andsettling rate gave r2 ¼ 0.06 for Condrus sp.,

r2 ¼ 0.02 for Grasilaria sp., r2 ¼ 0.01 for Cymodo-

cea sp., and r2 ¼ 0.04 for Polysifonia sp., indicatingthat the settling rate within each species is indepen-dent of the size of the individual species.

4. Discussion and conclusions

Unbalanced energy budgets for salt marshecosystems led Odum (1968) to propose the ‘‘Out-welling Hypothesis’’. This hypothesis has inspiredmany investigations and reviews (e.g., Nixon, 1980;Dame and Stilwell, 1984; Dame, 1989, 1994; Dameand Allen, 1996). Most of these studies focused onSpartina sp. dominated salt marsh dynamics, and toour knowledge not much attention has been given toshallow estuaries that are dominated by seagrassesand macroalgae.

A few studies on the transport of macrophytesand macro-detritus have been carried out in shallowestuaries and lagoons. One study in Venice Lagoon(Flindt et al., 1997) observed macroalgae thallidrifting at the surface with a velocity of more than0.5m s�1. In that study, a significant correlation wasfound between current velocity and the entrapmentof Ulva rigida and Chaetomorpha aerea. By extra-polating the regression lines to zero entrapment, itwas possible to evaluate the surface current velocityin Venice Lagoon at which resuspension started.The in situ thresholds (0.05–0.15m s�1) were higherthan the measured threshold velocities of thislaboratory study (Fig. 2). This is accounted for inthe height of measurement in the laboratory(0.15m) compared to the water depth in VeniceLagoon (2–3m). It was possible to find a massbalance of macrophytes only in Roskilde Fjord,

ARTICLE IN PRESSM.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–11031102

Denmark, when very low resuspension thresholds of1–2 cm s�1 for the dominating Ulva sp. were used(Salomonsen et al., 1999). This is within the range ofvalues presented here.

Within Venice Lagoon, 90% of the lagoonal areahas current velocities higher than the measuredthreshold velocities (Umgiesser et al., 2004) found inthe present study; it is, therefore, potentially subjectto macrophyte mobilisation. This is in line with thefindings of spring and summer campaigns in VeniceLagoon (Flindt et al., 1997). During these surveys,losses in biomass through resuspension were higherthan macrophyte growth rates. Macrophytes dis-appeared in some areas of the Lagoon when currentspeeds exceeded threshold conditions. At the sametime, growth was far below maximum rates.

A direct extrapolation of the results to VeniceLagoon is complicated by uncertainties of bedroughness, which may enhance algal sloughingand change the transition between algal bedloadand suspended load transport. Nevertheless, byapplying the sloughing rates measured herein, itmay be possible to give a first-order prediction ofsloughing in Venice Lagoon. The loss by sloughingrequires a much higher current speed than theresuspension threshold for unattached macroalgae,making the attached macroalgae less sensitive totransport losses. The opportunistic macroalgaestarted to lose biomass at current speeds of10–15 cm s�1, and there was considerable loss at30–50 cm s�1; at current speeds of 79–127 cm s�1 allbiomass was lost (Fig. 3 and Table 2). As theprevailing currents in many shallow estuaries are ofthis magnitude, this mechanism may be widespread.For Fucus sp., as well as for comparable macroalgaewith more firm structural thalli and a strongerholdfast, the sloughing rate will never be large.These results are based on conservative assumptionsbecause all measurements were carried out onhealthy macrophytes, which may not be representa-tive for an average macroalgal community.

The experimental studies on sloughing wereverified by qualitative field studies at different tidalcurrents. We observed that Ulva, Enteromorpha andPorphyra were transported in the estuaries (Ros-kilde Fjord, Southampton Waters and VeniceLagoon) attached to their shells and small stones,whilst we never saw Fucus sp. being transported(Flindt et al., 1997). Enteromorpha sp. had thehighest transportation frequency among attachedliving tissue. This may be explained by oxygenbubbles trapped in the tube-formed thalli. Thus,

much lower current speeds were needed to bring themacrophytes into motion. Fucus sp., attached tostones, may only be transported as bedload at veryhigh current speeds; however, this is difficult toobserve in turbid estuaries. The sloughing resultssuggest that the presence of Enteromorpha sp. inintertidal areas can be controlled partly by currentssloughing the tissue. This appears to be the case inthe Mondego Estuary (Martins et al., 2001); here,Enteromorpha is absent in the channel where thehighest current velocities are above 1m s�1 (seeTable 2) (Martins et al., 2001; Duarte et al., 2002).The algae are able to grow only in the shallow moreprotected intertidal areas.

The variation in settling rates may be explainedby natural variability. Some Cymodocea plants maystill have had small amounts of air trapped in theirleaves, thereby changing their specific mass, whilstindividuals of Condrus sp. were adapted to differ-ences in their environment, resulting in slightlydifferent morphology. It is also well known thatslower growing organisms have a relatively lowerwater content in their tissue, which can result indifferent specific masses.

Despite variability in the measured settling ratesof macrophytes, the experiment showed that resus-pended macrophyte material settles quickly. Thismay explain the general situation regarding theexport/import balance at the open boundary ofVenice Lagoon, where about 90% of the exportedmacrophyte biomass does not return on thesubsequent flooding tide (Flindt et al., 1999).Presumably, and depending upon prevailing condi-tions in the Adriatic Sea, the macrophyte materialeither disperses rapidly or settles within 15–40minon the ebb tidal deltas of the three inlets, at depthsof around 20m.

There are many explanations for the presence orabsence of macroalgae and seagrasses in differentareas of estuaries during the growth season. Thetransport, sloughing and settling patterns of macro-phytes described in this study are capable of havinga large influence on the macrophyte dynamics; thus,on the nutrient dynamics and fluxes in shallowestuaries, especially those influenced by tidal cur-rents and with a high production of unattached orloosely attached macrophytes. If the drifting ofmacrophytes in such regions is not taken intoaccount, calculations of plant production can begrossly under-estimated. This can have a dramaticeffect upon nutrient balance estimates, as a largeexport and/or import of nutrients bound in plant

ARTICLE IN PRESSM.R. Flindt et al. / Continental Shelf Research 27 (2007) 1096–1103 1103

material would be omitted. It is important that theresults of the present study are included in futureecological models that describe the resuspension,sloughing and settling of macrophytes.

Acknowledgements

These studies were financed by the EuropeanMAST III Program through activities linked to theF-ECTS project (MAS3-CT97-CT97-0145), theEuropean TMR Program through activities linkedto the WET project (FMRX-CT96-0051), and bythe Carlsberg Foundation. Funding for C.L. Amoswas through CORILA Linea 3.15, and for P.L.Friend through the EU BIOFLOW Project (EVR1-CT-2001-20008).

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