Flow extremes and benthic organic matter shape themetabolism of a headwater Mediterranean stream

VICENC ACUNA,* ADONIS GIORGI , † ISABEL MUNOZ,* URS UEHLINGER ‡

AND SERGI SABATER §

*Department of Ecology, Faculty of Biology, University of Barcelona, Barcelona, Spain†CONICET, Departamento Ciencias Basicas, Universidad Nacional de Lujan, Lujan, Argentina‡Department of Limnology, EAWAG, Switzerland§Institute of Aquatic Ecology and Department of Environmental Sciences, University of Girona, Campus Montilivi, 17071 Girona,

Spain

SUMMARY

1. Single-station diel oxygen curves were used to monitor the oxygen metabolism of an

intermittent, forested third-order stream (Fuirosos) in the Mediterranean area, over a

period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP)

were estimated and related to organic matter inputs and photosynthetically active

radiation (PAR) in order to understand the effect of the riparian forest on stream

metabolism.

2. Annual ER was 1690 g O2 m)2 year)1 and annual GPP was 275 g O2 m)2 year)1.

Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16.

3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O2 m)2 day)1. The maximum

values of GPP occurred during a few weeks in spring, and ended when the riparian

canopy was fully closed. The phenology of the riparian vegetation was an important

determinant of light availability, and consequently, of GPP.

4. On a daily scale, light and temperature were the most important factors governing the

shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in

the P–I relationships. Hysteresis-type curves were characteristic of late autumn and winter.

Light saturation responses (that occurred at irradiances higher than 90 lE m)2 s)1) were

characteristic of early spring. Linear responses occurred during late spring, summer and

early autumn when there was no evidence of light saturation.

5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g

O2 m)2 day)1. ER was highest in autumn 2001, when organic matter accumulations on the

streambed were extremely high. By contrast, the higher discharge in autumn 2002

prevented these accumulations and caused lower ER. The Mediterranean climate, and in

its effect the hydrological regime, were mainly responsible for the temporal variation in

benthic organic matter, and consequently of ER.

Keywords: ecosystem metabolism, flow regime, Mediterranean streams, organic matter dynamics,riparian forest

Introduction

Aquatic ecosystems and their adjacent terrestrial

environments are closely connected through a variety

of material inputs and energy flows (Hill, Mulholland

& Marzolf, 2001). These connections are particularly

Correspondence: Vicenc Acuna, Department of Ecology, Faculty

of Biology, University of Barcelona, Avgda. Diagonal 645,

E-08028 Barcelona, Spain.

E-mail: vicencacuna@ub.edu

Freshwater Biology (2004) 49, 960–971

960 � 2004 Blackwell Publishing Ltd

tight in forested headwater streams, where riparian

forest provides large amounts of particulate organic

matter to benthic food webs enhancing respiration

processes (Cummins et al., 1989), and the shade cast

by riparian plants is an important constraint on lotic

primary production (Rosenfeld & Roff, 1991; Hill,

Ryon & Schilling, 1995). Therefore, metabolism of

headwater streams has been generally considered to

be heterotrophic (Vannote et al., 1980; Cummins et al.,

1989; Rosenfeld & Roff, 1991).

Against this general background, variability of

metabolism in forested streams has been attributed

to light availability and to the dynamics of allochtho-

nous organic matter (e.g. Minshall, 1978; Hill et al.,

1995). During the annual cycle changes in the amount

of light reaching stream beds in temperate deciduous

forests can be large and rapid, because of the annual

cycles of leaf emergence and abscission (Hill et al.,

2001). Organic matter of riparian origin may influence

stream metabolism both in terms of the contribution

of allochtonous matter as well as the influence on the

accumulation processes, which depend respectively

on riparian forest phenology and the retentive capa-

city of the stream (e.g. Smock, Metzler & Gladden,

1989; Gregory et al., 1991).

Indeed, it is true that water flow is the driving force

for organic matter transport and accumulation in

streams. Flow may have a strong impact on the

metabolism of medium sized rivers (Young & Huryn,

1996; Uehlinger, 2000), while in low order streams the

relevance of flow for the dynamics of benthic organic

matter (BOM) accumulation may be affected by local

climate, topography and geomorphology. This may be

particularly true in Mediterranean streams, where

flow extremes are common and often unpredictable

(Gasith & Resh, 1999). However our understanding of

how these temporal patterns may affect metabolism of

streams is still limited.

In this paper, we aim to determine how the seasonal

variations of water flow, light and temperature could

influence the metabolism of an undisturbed, forested,

oligotrophic Mediterranean stream. The large varia-

bility of climate conditions characteristic of Mediter-

ranean streams may influence the hydrological regime

and the input and retention of organic matter (Sabater

et al., 2001). Thus, climate variability may result in

extremely large accumulations of organic matter

during low flow or, conversely, in losses during

floods. It is our hypothesis that these differences

would be reflected in stream metabolism, and that

water flow could be a driving force for metabolism,

mainly through its effect on organic matter accumu-

lation or transport. In this study, stream metabolism

was estimated using open system measurements with

diel O2 variations being recorded using a single-

station method, over a period of 18 months.

Methods

Study site

Fuirosos is an intermittent third order stream drain-

ing a catchment area of 16.2 km2 in the Montnegre-

Corredor Natural Park, a forested range close to the

Mediterranean sea (50 km north of Barcelona, north-

east Spain). The climate is typically Mediterranean,

with mild winters and warm springs and summers.

Monthly air temperatures range from 4 �C in

December to 28 �C in July and August; during

winter, air temperatures below 0 �C are infrequent.

Precipitation mostly occurs in autumn and spring

with occasional storms in summer (Bernal, Butturini

& Sabater, 2002). Mean annual discharge was about

32 L s)1 in 2001 and 87 L s)1 in 2002, with monthly

averages ranging from 0.07 L s)1 in September in

2001 to 626 L s)1 in May 2002. Permanent flow

usually ceases from July/August to September/

October (Sabater et al., 2001). At a discharge of

30 L s)1, the stream was 3–4 m wide with depths

ranging from 0.1 to 0.2 m in riffles and 0.4 m in

pools. A 50 m long reach was selected in a riffle

section of uniform channel morphology and channel

slope (0.9%). The stream has a well developed

riparian area 10–20 m wide, which forms a closed

canopy from May to October dominated by alder

(Alnus glutinosa, L.), hazelnut (Corylus avellana, L.)

and plane (Platanus acerifolia, Aiton-Willd.). The

study reach was unreplicated, so that only tentative

conclusions about riparian influences on ecosystem

metabolism may be drawn. However, these conclu-

sions are based on data from two highly different

hydrological periods.

Physico-chemical characteristics

The water level was continuously monitored from

December 2000 to December 2002 using a pressure

transducer PDCR 1830 (Druck, Leicester) connected to

Riparian influences on stream metabolism 961

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

an automatic sampler (Sigma 900 Max). Discharge

was measured on several occasions using the slug-

injection method with sodium chloride as tracer

(Gordon, McMahon & Finlayson, 1992). These mea-

surements and the corresponding stage provided an

empirical stage-discharge relationship that was used

to calculated discharge based on the water level

record (Bernal et al., 2002).

Global radiation (GLR) was continuously recorded

from June 1998 to November 2002, using a SP-1110

Pyranometer Sensor (Campbell Scientific, U.K.) con-

nected to a data logger (Campbell Scientific CR-10,

U.K.) placed outside the riparian forest, about 50 m

from the study reach. GLR was converted to PAR

according to McCree (1971). Light interception by the

canopy was measured with a LAI-2000 Plant Canopy

Analyzer (LI-COR Inc., Lincoln, NA, U.S.A.). Derived

results were converted to PAR reaching the streambed

using the empirical coefficient of refraction by the

water surface of 0.49 m)1 (averaged value of 15 sets of

measurements throughout the studied period). To

validate these calculations we measured PAR reaching

the streambed by means of an underwater quantum

sensor (sensor LI-192SA fitted to LI-250 Quantum

Meter, both LI-COR Inc., Lincoln, NA, U.S.A.). The

correlation between measured and calculated values

was highly significant (n ¼ 116, r2 ¼ 0.85, P < 0.01).

Temperature and dissolved oxygen were continu-

ously recorded (see below). The pH was measured

in situ (WTW MultiLine P4, Welheim, Germany).

Stream water for nutrient analyses was filtered in situ

through glass fiber filters (Whatman GF/F filters) and

stored at 4 �C until analysis. N-NO3 and N-NH4 were

analysed using a Skalar (Breda, The Netherlands) auto

analyser, while P-PO4 was analysed with a Perkin-

Elmer spectrophotometer (APHA, 1992).

Algal and organic matter

Samples from sand and rocks were collected every

20 days for the determination of benthic chlorophyll a

(chl a). Sandy substrata were sampled using a corer

(diameter 2 cm), collecting five cores at random in the

reach. Chlorophyll on rocks was estimated from

artificial substrata (unglazed tiles 1 · 1 cm, 40 days

colonisation), which were used as surrogates for

natural communities. The tiles were glued with

silicone to natural cobbles, and were distributed in

the study reach. On each sampling date, nine tiles were

collected at random. Both cores and tiles were trans-

ported to the laboratory and chlorophyll was extracted

with 90% acetone. Routine measurements of chloro-

phyll followed Jeffrey & Humphrey (1975). Results

were expressed as chl a m)2 of streambed by using the

percentage cover of rock and sand (see below).

Direct input of particulate organic matter was

monitored using traps suspended 0.6 m above the

stream surface. The traps consisted of a square wooden

frame (1 · 1 m) and a nylon net (mesh size ¼ 1 mm)

that formed the bottom of the trap. Lateral transport to

the channel was determined by means of 10 traps

equally distributed near the channel margins. Lateral

traps consisted of a wooden frame holding a nylon net

(mesh size ¼ 1 mm). Total input was considered to be

the sum of direct and lateral input. BOM was collected

monthly with triplicate core samples with a diameter

of 20 cm diameter (mesh size 1 mm). Organic matter

samples were dried at 105 �C to a constant weight and

combusted at 450 �C for about 4 h to estimate ash free

dry mass (AFDM). Water samples for dissolved

organic carbon (DOC, <0.7 lm) analysis were analysed

using a high-temperature catalytic oxidation Shim-

adzu TOC 5000 analyzer (Shimadzu, Rydalmere,

Australia). Carbon content was calculated by applying

the empirical factor of 2.4 to the ratio gram

AFDM : gram C (Margalef, 1983).

The macroscopic distribution of substrata within

the study reach was mapped along eleven equidistant

transects using an underwater viewer 0.4 · 0.4 m.

Within this area the following were estimated:

(i) percentage streambed covered by the different

inorganic substrata (rock, cobble, sand, gravel),

(ii) percentage streambed covered by algal patches

and (iii) percentage streambed covered by the differ-

ent types of BOM (leaves, branches or fine detritus).

Finally, the average value of all transects was also

calculated for every sampling date. These averages

were used to scale up chl a and BOM to a reach scale.

Ecosystem metabolism

Assessment of metabolism rates was based on the

single-station diel O2 method (Odum, 1956). Dis-

solved oxygen concentration was continuously recor-

ded 40 times, for about 36 h. Oxygen was measured

with an oxygen meter (WTW Oxi 340-A, Weilheim,

Germany) and temperature with a 107 Temperature

Probe (Campbell Scientific, UK). The oxygen meter

962 V. Acuna et al.

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

and temperature probe were connected with a data-

logger (Campbell-Scientific CR 10-X, Shepshed,

U.K.), and data recorded at 0.5 and 15 min intervals,

respectively. The oxygen probe was air-calibrated

before each measuring interval. Both probes were

placed in the thalweg area of the stream, about 5 cm

below the water surface. The saturation concentra-

tion of O2 was calculated using temperature and

elevation above sea level, according to Buhrer (1975).

Net Production rate of O2 [b(t)] in g O2 m)2 h)1 was

calculated using:

bðtÞ ¼ DO2

Dt� KO2

ðO2sat � O2Þz ð1Þ

where DO2/Dt is the change of O2 concentration

between two subsequent measurements, z is the mean

depth (m), O2sat is the saturation concentration of O2

(mg O2 L)1) and Ks is the reaeration coefficient of O2.

From the 40 oxygen series we excluded nine because

of supersaturation during the night, probe malfunc-

tioning, or sudden flow increase.

The reaeration coefficient Ks was determined with

propane using methods reported in Genereux &

Hemond (1992). Propane and a solution of sodium

chloride (conservative tracer) were continuously

injected about 10 m upstream of the upper end of

the study reach. Conductivity was continuously

measured at 10 and 40 m downstream of the injection

point in order to determine when the steady state in

conductivity was reached along the entire reach

(Stream Solute Workshop, 1990). These conductivity

measurements were later used to estimate the lateral

inflow through the reach. Samples for volatile tracer

concentrations were collected at 0, 5, 10 and 20 and

40 m downstream of the injection point after a steady

state was achieved. This was carried out in order to

assure that the propane diffusion into the water body

was correctly completed. The propane exchange

coefficient, Kpropane was calculated as:

Kpropane ¼1

sln

G1C2

G2C1

� �ð2Þ

where s is travel time of water (min)1) between an

upstream station, at 10 m from the injection point (1)

and a downstream station, at 40 m from the injection

point (2), Gi is the steady state concentration of

propane at the respective sites, and Ci is the steady

state conservative tracer concentration at the respect-

ive sites, corrected for the background concentration.

The reaeration rate of propane was converted to

oxygen using a factor of 1.39 (Rathbun et al., 1978).

The DO changes integrate the response of a stream

ecosystem over a reach length £3v/Ks, where v is the

average velocity and Ks is the reaeration coefficient

(Uehlinger, Konig & Reichert, 2000). In Fuirosos, the

average stream length considered by the metabolism

estimates was therefore 346 m, and ranged from 50 to

1000 m. The derivative (DO2/Dt) at the time ti (day)

needed for the calculation of metabolism rates (eqn 1)

was the analytical derivative of a 2nd second order

polynomial fitted to the data. For this fit the data were

weighted with a normal distribution centred at ti and

with a standard deviation of 0.05 day. The calculation

of the derivative with the polynomial fitting technique

strongly reduces the effect of errors in O2 measure-

ments, which is greatly amplified by the differenti-

ation process (Shoup, 1983). Such errors are ‘averaged

out’ when daily rates are calculated but they strongly

affect the determination of hourly of half-hourly rates

needed to evaluate photosynthesis–irradiance (P–I)

curves. Three metabolic parameters were based on net

production rates of O2 (eqn 1): ecosystem respiration

(ER), gross primary production (GPP), and net

ecosystem production. ER was calculated as the sum

of net O2 production rate [b(t)] during the dark period

and respiration values during the light period, these

being calculated as the linear interpolation between

the net O2 production rate values of sunrise and

sunset of the nights before and after the day of

interest. GPP was the sum of net O2 production rate

during the light period and respiration rates during

the light period, as explained above. Annual rates of

GPP and ER for 2002 were obtained by numerical

integration that was based on 26 measurements.

Stepwise regression analysis was used to explore the

potential influence of season and riparian forest GPP

and ER. Factors used were discharge (Q), water

temperature (T), daily accumulated PAR (I), per cent

of cobbles cover (%), chl a (C) and BOM (OM). These

factors were not or only moderately inter-correlated.

The regression model was forced to return non-

negative output because negative values for ER and

GPP are not reasonable. The full regression model was:

X¼maxð0;aþb �Qþc�Tþd�Iþe �%þf �Cþg�OMÞð3Þ

where the dependent variable X is ER or GPP and a, b,

c, d, e, f and g are model parameters. Sub-models can

Riparian influences on stream metabolism 963

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

be derived from the full model by ignoring the

influence of some factors by setting the corresponding

model parameters to zero. A backward analysis was

performed starting with the full model (3) and

iteratively eliminating the least important factor at

each step. For each dependent variable 2p¢)1 models

were tested (p¢ is the number of parameters of the full

model, light was not considered as a factor influen-

cing ER). This procedure ranked the factors for the

modelled variables GPP and ER according to their

influence. The decision, which factors should be

considered to be significant, based on the Schwarz

Bayesian Criterion (Schwarz, 1978). A detailed des-

cription of the applied regression procedure is given

by Uehlinger et al. (2000).

Photosynthesis–irradiance relationships

To evaluate the relationship between GPP and PAR

reaching the streambed, we identified the parameters

of a hyperbolic tangent function (eqn 4; Jassby & Platt,

1976) by non-linear regression (STATISTICA, version

5.5; StatSoft Inc., Tulsa, OK, U.S.A.).

GPP ¼ Pmax tanhaI

Pmaxð4Þ

where Pmax is light saturated photosynthesis, a is the

initial slope of the P–I curve (or Pmax/Ik), and I is PAR

reaching the streambed. The half-saturation light

intensity (Ik) was calculated as Pmax/a (Henley, 1993).

Results

Physicochemical characteristics

Annual rainfall was only 512 mm in 2001 (Fig. 1a). In

spite of this low input, a major flood event in January

2001 completely altered the substrata distribution in

the streambed. Later on, discharge declined almost

continuously to the end of June, when the stream

dried out (except for a few pools scattered in the

stream reach that persisted for another 2 weeks).

Flow resumed in mid October 2001. The year 2002

was wetter (annual rainfall of 850 mm). Floods

occurred between January and May, and between

September and December, and flow persisted

throughout the year. The conservative tracer injec-

tions did not indicate the accrual of water through the

study reach.

The reaeration coefficient ranged from 3.8 day)1 in

July 2002 to 187 day)1 in May 2002, and averaged

57 day)1 (Table 1). Reaeration coefficients were

strongly correlated with the product of average water

velocity and slope (r2 ¼ 0.79, P < 0.05), consistent

with the empirical relationship proposed by Tsivog-

lou & Neal (1976) to predict the reaeration coefficient.

The seasonal patterns of PAR reaching the

streambed (Fig. 1b) reflected the development of the

riparian canopy and interception by the hill slopes.

PAR increased from March to the end of April but

declined rapidly with the emergence of leaves. How-

ever, autumnal leave abscission only had a minor

effect on PAR because the hill slope caused effective

shading because of low sun angle during autumn.

Thus, except for a short period in spring, PAR was

usually <20 lE m)2s)1. Water temperatures varied

between 4 and 36 �C (Fig. 1b). Soluble reactive phos-

phorus in the stream water varied from <1 to 17 lg

P-PO4 L)1, ammonia from <1 to 45 lg N-NH4 L)1 and

nitrate from 4 to 2714 lg N-NO3 L)1. Concentrations

peaked in autumn (Table 1).

01 04 07 10 01 04 07 10 01

PA

R (

µE m

–2s–1

)

0

10

20

30

40

50

60

70

80

Wat

er t

emp

erat

ure

(°C

)

0

5

10

15

20

25

30

35

40

PARWater temperature

Dis

char

ge

(L s

–1)

0

200

400

600

800

10006000

9000

12 000(a)

(b)

Dry

per

iod

20022001

Fig. 1 (a) Discharge in Fuirosos Stream from January 2001 to

January 2003. (b) Average daily (from sunrise to sunset) pho-

tosynthetically active radiation (PAR) reaching the streambed

(continuous line) and water temperature (dotted line) in Fuiro-

sos from January 2001 to January 2003.

964 V. Acuna et al.

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

Algal biomass and organic matter

Algal biomass (measured as chl a) was higher in

spring and summer and lower in autumn and winter

(ANOVAANOVA, P < 0.05). In spring 2001, chl a reached

71 mg m)2, declined after leaf emergence, and peaked

again but only in isolated pools (51 mg m)2) just

before the entire reach dried out. In the following

year, the vernal chl a peak was only 32 mg m)2,

presumably because of the preceding floods, and the

response to canopy closure was less distinct. Chloro-

phyll concentration on sand was significantly lower

than on tiles (ANOVAANOVA, P < 0.05), the values ranging

between 0.2 and 14 mg m)2.

The BOM ranged from 0.2 to 30 g C m)2, with the

lowest values occurring after the flood in January

2001 (Fig. 2). Total input (TI) peaked in autumn, at

around 2 g C m)2 day)1. Two different patterns in the

relationship between TI and BOM could be distin-

guished. In 2001, because of low water availability

during spring and summer, the litter fall started in

July and finished in November. In 2002, however,

most of the litter fall input occurred in November.

The lack of flow during summer as well as the low

discharge in autumn 2001 caused leaf accumulation

on the streambed to reach 30 g C m)2. Small debris

dams were formed, which increased water depth and

reduced current velocity. In 2002, permanent flow

and a few floods in late summer and autumn kept

BOM at low levels (maximum BOM of 10 g C m)2 in

November).

Daily and annual metabolism rates

There was significant seasonal variation in ER

(ANOVAANOVA, P < 0.05; Fig. 3). After the dry period of

summer 2001, high rates of ER (32 g O2 m)2 day)1)

coincided with large leaf litter accumulations in the

wetted channel. On the contrary, relatively low ER

rates (3.3–5.6 g O2 m)2 day)1) in autumn 2002 corres-

ponded with moderate BOM concentrations. GPP

peaked at 1.9 g O2 m)2 day)1 in spring 2002, but

subsequently declined to 0.34 g O2 m)2 day)1 when

leaves started to emerge. Seasonal variation in GPP

was less distinct, but significant (ANOVAANOVA, P < 0.05).

During winter, GPP varied between 0.1 to 1.1 g

Table 1 Physical, chemical and biological characteristics of Fuirosos, averaged on a seasonal basis. Data were obtained from January

2001 to December 2002; data from summer are those of 2002

Season Autumn Winter Spring Summer

Wetted width (cm) 215 ± 29 255 ± 2.6 241 ± 2.4 210 ± 44

Depth (cm) 11.7 ± 5.8 18 ± 3.2 23.1 ± 14.8 10 ± 11

Water velocity (m s)1) 0.13 ± 0.15 0.18 ± 0.14 0.13 ± 0.05 0.05 ± 0.01

Chlorophyll (mg chl a m)2) 7.7 ± 7.4 3.2 ± 2 15.7 ± 5.6 1.4 ± 0.8

Benthic organic matter (g C m)2) 54 ± 15 29 ± 43 23.2 ± 0.9 6.9 ± 2

Total input of organic matter (mg C m)2 day)1) 1117 ± 581 305 ± 83 506 ± 47 612 ± 102

O2 reaeration rate at 20 �C (day)1) 56 ± 26 33 ± 26 188 ± 149 5 ± 0.6

pH 7.2 ± 0.18 7.9 ± 0.13 7.6 ± 0.08 7.2 ± 0.11

Conductivity (mS cm)1) 316 ± 29.3 229 ± 24 173 ± 11.9 285 ± 51

Dissolved organic carbon (mg DOC L)1) 4.4 ± 1.7 6.8 ± 2 3.1 ± 0.54 9.8 ± 6.2

SRP concentration (mg P-PO4 L)1) 9.3 ± 10.9 3.3 ± 3.8 0.02 ± 0.007 5.7 ± 8.5

Nitrate concentration (mg N-NO3 L)1) 590 ± 161 511 ± 85 29.2 ± 0.62 3.9 ± 0.2

Ammonia concentration (mg N-NH4 L)1) 29.4 ± 4.8 15 ± 17 0.63 ± 0.07 3.4 ± 5.2

05 07 09 11 01 03 05 07 09 11 01

ER

(g

O2

m–1

day

–1)

0

5

10

15

20

25

30

35

BO

M (

g C

m–2

)

0

5

10

15

20

25

30

35ERBOM

2001 2002

Dry

per

iod

Fig. 2 Ecosystem respiration (ER, in g O2 m)2 day)1) in relation

to benthic organic matter (BOM, in g C m)2) in Fuirosos. The

non-flow period is represented by dotted vertical lines.

Riparian influences on stream metabolism 965

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

O2 m)2 day)1. GPP and ER were not significantly

correlated (r2 ¼ 0.08, P > 0.05).

The annual rates of ER and GPP in 2002 were

respectively 1690 and 275 g O2 m)2 year)1, and

annual net ecosystem production equalled )1415 g

O2 m)2 year)1. P : R ratios ranged from 0.01 to 4.2

(this latter being recorded immediately before the leaf

emergence), and averaged 0.16 in 2002.

Ecosystem respiration was significantly correlated

with BOM (r2 ¼ 0.48, P < 0.05), while GPP was

better predicted by discharge (r2 ¼ 0.28, P < 0.01).

The selected model of the stepwise regression

analysis for ER, explained 60% of the variance

and included (in order of importance) BOM,

discharge and chl a as the most influential para-

meters. A three parameter model explaining 57% of

the variation was selected for GPP, with (in order of

importance) discharge, per cent of cobbles coverage

and chl a. We found no significant relationship

between daily metabolism rates and nutrient con-

centrations.

Photosynthesis-Irradiance relationships

The relationship between the instantaneous GPP rate

and PAR varied with season (Fig. 4). The relationship

was linear during summer, early autumn and late

spring (Fig. 4a). Light saturation of GPP occurred in

early spring, when PAR values were higher than

90 lE m)2s)1 (Fig. 4b). In late autumn and winter the

P–I relationship followed a temperature hysteresis

curve (Fig. 4c). The two parameters Ik and Pmax were

maximal in spring (Table 2) but differences between

seasons were not significant (ANOVAANOVA, P > 0.05),

except for Pmax which was significantly smaller

during summer (ANOVAANOVA, P ¼ 0.05). The model used

to identify these parameters (eqn 4) only provides

information on an average P–I curve because hyster-

esis is not considered.

05 07 09 11 01 03 05 07 09 11 01

g O

2 m–2

day

–1

–35

–30

–25

–20

–15

–10

–5

0

5

GPPER

2001 2002

Dry

per

iod

Fig. 3 Gross Primary production (GPP) and ecosystem respir-

ation (ER) in Fuirosos stream from June 2001 to December 2002.

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

PAR (µE m–2 s–1)

0

0 15 30 45 60 75 90 105

2 4 6 8 10 12 14 16 180.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Morning valuesAfternoon values

Gro

ss p

rim

ary

pro

du

ctio

n (

g O

2 m–2

h–1

)

(a)

(b)

(c)

0 5 10 15 20 25 30 35 40 45 500.00

0.04

0.08

0.12

0.16

0.20

0.24

Fig. 4 Gross Primary production versus PAR in three typical

situations (see text): (a) 02/07/02 (summer) (b) 28/04/02

(spring) and (c) 13/02/02 (winter). Measurements in the morn-

ing are indicated as black circles and measurements in the

afternoon as white circles.

966 V. Acuna et al.

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

Discussion

The results obtained in Fuirosos support the view

that in oligotrophic and heavily shaded streams the

organic matter input from riparian forests is a major

determinant of ER (Webster, Wallace & Benfield,

1995). However, it is obvious in this type of system

that the seasonal effects of the changing canopy on

GPP can be extraordinary (Guasch & Sabater, 1995;

Hill & Dimick, 2002). Moreover, the hydrological

regime may significantly modify the riparian influ-

ence on ER through its effect on the amount of

allochthonous organic matter stored in the channel

(Gregory et al., 1991). This effect may be a major

driver in Mediterranean streams (Gasith & Resh,

1999). However, to reduce the limitation of an

unreplicated study similar investigations in other

Mediterranean systems are needed.

Gross primary production

In comparison with other systems, primary production

of Furiosos was relatively small (Table 3). The riparian

forest had several effects on the primary producers

within the Fuirosos. On one hand, riparian shade

conjointly with shade by hill slope at low sun angle

(from November to January), produced a clear annual

pattern of light availability (Fig. 1b). This pattern in

light availability caused relatively low GPP values for

most of the year (Table 3), except in early spring (Fig. 3)

when shade was minimal. On the other hand, detritus

covered the algal community for extended periods,

further impeding light reaching them. High discharges

had a ‘cleaning’ function by removing BOM and

allowing light to reach the primary producers, until

then partially covered by BOM. A large proportion of

the algal community in Fuirosos is made up of

encrusting red algae and cyanobacteria, which may

receive more light after the litter is removed. It is

therefore not surprising that discharge was the best

single predictor of GPP, as well as the first factor in

order of importance in the stepwise regression analysis.

Discharge has been related to GPP in other studies,

although the reasons proposed have been diverse.

Lamberti & Steinman (1997) attributed the positive

Table 2 Pmax (light saturated photosynthesis) and Ik (half sat-

uration light intensity) estimated in Fuirosos

Autumn Winter Spring Summer

Pmax (g O2 m)2

day)1)

2.8 ± 2.9 2.8 ± 2.9 3.3 ± 1.1 0.7 ± 0.3

Ik(lE m)2 s)1) 13.5 ± 9.3 13.5 ± 8.3 23.1 ± 13.1 15.3 ± 10.6

Table 3 Range or average and standard deviation of GPP, CR and P : R from low to medium size stream systems reported in the

literature

Author Method Stream

GPP

(g O2 m)2 day)1)

ER

(g O2 m)2 day)1) P : R

This study Single-station Fuirosos Stream 0.05–1.9 0.4–32.1 0.16

Bott et al., 1985 Chambers Inter-biome 0.2–3.4 0.4–2.9

Chessman, 1985 Single-station La Trobe River 0.15–1.90 3.0–4.6

Edwards & Meyer, 1987 Single-station Ogeechee River 0.5–14.0 3.7–11.5 0.25

Edwards & Owens, 1962 Single-station Ivel River 9.6 8.5 1.13

Fellows, Valett & Dahm, 2001 Two-station Rio Calaveras 0.5 0.19

Fellows et al., 2001 Two-station Gallina Creek 1.7 0.1

Fisher & Carpenter, 1976 Single-station Fort River 1.8 3.7

Hall, 1972 Single-station New Hope Creek 0.8 1.3 0.61

Hoskin, 1959 Single-station Neuse River 0.3–9.8 1.7–21.5

Kaenel, Buehrer & Uehlinger, 2000 Single-station Muhlibach 12.5 ± 4.5 8.9 ± 6.84 1.07 ± 0.08

Marzolf, Mulholland

& Steinman, 1998

Two-station Walker Branch 1.4 ± 1.8 6.5 ± 1.9

Molla et al., 1996 Single-station Montesina Stream 2.4 ± 2.1 2.2 ± 1.6 0.59 ± 0.69

Mulholland et al., 2001 Two-station Inter-biome <0.1–15 2.4–11

Uehlinger & Naegeli, 1998 Single-station River Necker 2.5 3.5 0.73

Uehlinger, Naegeli & Fisher, 2002 Single-station Hassayampa River 0.3 ± 0.1 1.65 ± 0.13 0.17 ± 0.05

Young & Huryn, 1996 Single-station Taieri River <0.3–9.6 0.7–9.8

Young & Huryn, 1999 Two-station Sutton 0.8 ± 0.3 4.6 ± 1.1 0.2 ± 0.1

Young & Huryn, 1999 Two-station Three O’Clock 3.7 ± 0.9 2.7 ± 0.9 1.5 ± 0.2

Riparian influences on stream metabolism 967

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

relationship between GPP and discharge to high

nutrient loading associated with high discharges.

However, Uehlinger & Naegeli (1998) and Uehlinger

(2000) reported that high discharges could shift

ecosystem metabolism towards heterotrophy because

benthic primary producers are more susceptible to the

abrasion by moving bed sediments or suspended solids

than hyporheic biofilms largely contributing to ER.

Chlorophyll a concentration was also a significant

predictor (r2 ¼ 0.24, P < 0.01) of GPP in Fuirosos.

Chlorophyll responded to the high PAR values in

spring by showing a peak, and these changes had their

effect on stream metabolism. PAR explained 64% of

the variation in GPP and 54% of the variation in chl a

during spring. At the end of April, the canopy closure

caused a 78% reduction in PAR in Fuirosos, which was

accompanied by a remarkable decrease in GPP.

Therefore, it can be concluded that the phenology of

riparian vegetation was an important determinant of

GPP, as stressed by Mulholland et al. (2001). Rosenfeld

& Roff (1991) also reported that light availability, as

mediated by the development of riparian vegetation,

was the most important factor regulating GPP.

Furthermore, only during some weeks in spring were

PAR values higher than Ik, while during most of the

year they were lower, indicating that photosynthesis

was generally light limited (Hill & Dimick, 2002) in

Fuirosos. PAR reaching the streambed after canopy

closure was usually much lower than Ik values, a result

consistent with experimental evidence of light limita-

tion under full canopy (Steinman, 1992; Hill et al.,

1995). These changes in stream metabolism because of

light availability did not have the same effect on ER, in

spite of the increase in chl a concentration. Further-

more, the correlation between chl a and ER was

negative (r ¼ )0.36; P < 0.05), suggesting that break-

down of allochthonous material is driving ER rather

than autotrophic respiration.

Seasonal variations in light and temperature were

not only relevant at an annual scale, but also affected

the daily response of benthic communities. Three

patterns could be differentiated in the P–I curves

obtained in Fuirosos (Fig. 4). GPP was a linear or

quasi-linear function of PAR in late spring, summer

and early autumn. This linear relationship (Fig. 4a)

disappeared in spring with increasing PAR. However,

light saturation of GPP rates was only observed in

April. During this period, Fuirosos GPP became light

saturated when PAR exceeded 90 lE m)2 s)1 (Fig. 4b).

These values are relatively low when compared with

other systems. Hill et al. (1995) gathered data from

multiple studies and showed that photosaturation

typically occurred between 200 and 400 lE m)2 s)1. In

addition, Young & Huryn (1996) provided the first

ecosystem level evidence for light saturation of GPP,

which usually occurred at light intensities over

250 lE m)2 s)1. However, saturation irradiance values

around 100 lE m)2 s)1 are not uncommon in shaded

Mediterranean streams (Guasch & Sabater, 1995). The

hysteresis of the GPP rates (Fig. 4c) observed in late

autumn and winter may be explained by the water

temperature, which, in contrast to PAR, distinctly

differed between morning and afternoon (e.g. by 7 �C).

Ecosystem respiration

Fuirosos is a heterotrophic system because of the

predominance of respiration processes for most of the

year. The P : R ratio averaged 0.16 and annual net

ecosystem production was )1415 g O2 m)2 year)1.

Most of the ER values were reasonably similar to

analogous systems (Table 3). However, ER in autumn

was higher than values recorded previously for

similar forested streams (Table 3). However, there

have been reported by far higher ER values in Prairie

streams, which raise 177 g O2 m)2 year)1 (Wiley,

Osborne & Larimore, 1990). Duffer & Dorris (1966)

reported similar values from North American grass-

land river systems.

The main reason for this extremely high ER in

autumn was the BOM stored in the stream during

summer, which resulted in a high respiration at some

weeks after the flow onset (Fig. 2). Young & Huryn

(1999) also concluded that organic matter supply,

together with light availability, had the most dramatic

influence on community metabolism. Conversely,

Mulholland et al. (2001) determined, in a comparison

of eight streams from several biomes in U.S.A., that

benthic detritus standing crop accounted for only 17%

of ER variation. Also, Webster et al. (1995) reported

that evidence for a positive effect of BOM storage on

ER in eastern U.S.A. streams was weak. The large

BOM accumulation in Fuirosos during low flows

emphasises that its influence on ER is variable not

only on a seasonal basis, but also on an inter-annual

basis. In Fuirosos, one year was drier and allowed

greater storage of BOM, but the following year flow

was sufficient to move BOM downstream.

968 V. Acuna et al.

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 960–971

The second best single predictor of ER in Fuirosos

was water temperature but the relationship between

ER and temperature observed was negative (r2 ¼ 0.37,

P < 0.01). In the Fuirosos, maximum temperatures

were recorded in summer, but this period had

minimum ER values. From a different perspective,

Edwards & Meyer (1987) reported that microbial

respiration during cooler months was stimulated by

large inputs of dissolved and particulate organic

substrates, helping to explain high winter respiration

rates. Therefore, it appears that temperature does not

universally regulate respiration on an annual scale in

stream ecosystems, but other factors (such as BOM

supply and quality, Sinsabaugh, 1997) may also

account for the observed negative relationship

between temperature and ER. In the case of Mediter-

ranean streams, overall climatic variability has to be

considered. Variability in rainfall, which is the prin-

cipal attribute of the Mediterranean-type climate

(Gasith & Resh, 1999), can alter the patterns of the

leaf fall because of drought stress during spring and

summer and, of course, determine the discharge

regime. Therefore, variability in rainfall may have a

major impact on stream metabolism.

In the drier period of this study, the amount of

BOM in Fuirosos was relatively high and steady for

about 3 months. This pattern may be explained by the

enhanced organic matter retention in the stream

channel during low flow (Bilby & Likens, 1980;

Speaker, Moore & Gregory, 1984). However, in

autumn 2002, leaf fall was concentrated in November

and, because of the continuous discharge during this

period, there was not the same BOM accumulation as

recorded the year before. As a consequence, the

values of ER were much higher in autumn 2001 than

in autumn 2002 (Fig. 2). Discharge could also be

controlling a second peak of ER in the middle of

March 2002, related to the remarkable amount of

BOM accumulated during the low winter flow. This

stable discharge period abruptly finished at the end of

March, the BOM decreased to minimum values and

ER also reached minimum values. It is worth empha-

sising that the ER peaks had a certain time lag with

respect to the peaks in BOM (Fig. 2), reflecting the

particular dynamics of leaf detritus processing

(Webster et al., 1995; Schade & Fisher, 1997).

The stream metabolism in Fuirosos seems to

depend on the hydrology and riparian forest inputs,

as they affected both the functioning of primary

producers as well as BOM accumulation and its

respiration. The hydrological regime was mainly

responsible for temporal variation in BOM, and

consequently ER. The rate and timing of entry of

organic matter to stream are major determinants of

stream metabolism.

The lowest stream metabolism activities were

reported in summer, and further research will be

needed to elucidate the importance of pool isolation

during this period. Data from Fuirosos show that there

can be high interannual variability in ER, and that this

is mainly related to hydrological variations between

years. Although these patterns may not be unique to

Mediterranean-type systems (e.g. Uehlinger, 2000), it

is obvious in these systems that the influence of flow

extremes, such as spates or extended periods of low

flow, may be able to modify the relative contributions

of both GPP and ER to lotic metabolism.

Acknowledgments

Andrea Butturini, Nuria Morral, Anna Romanı, Joan

Artigas, Elena Guerra, Meritxell Aznar and Ainhoa

Gaudes assisted in the stream and in the laboratory.

Maurice Lock (University of Wales) and two anony-

mous reviewers provided useful comments on the

manuscript. This research was funded by the CICYT

projects AMB99-0499 and REN2002-04442-C02-02/

GLO of the Spanish Science Ministry.

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(Manuscript accepted 23 April 2004)

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