Macrophytic, epipelic and epilithic primary productionin a semiarid Mediterranean stream

J . VELASCO*, A. MILLAN*, M. R. VIDAL-ABARCA*, M. L. SUAREZ*, C. GUERRERO*

AND M. ORTEGA †

*Department of Ecology and Hydrology, University of Murcia, Murcia, Spain

†Department of Biology and Ecology, University of Almeria, Almeria, Spain

SUMMARY

1. Primary production by Chara vulgaris and by epipelic and epilithic algal assemblages

was measured in a semiarid, Mediterranean stream (Chicamo stream, Murcia, Spain)

during one annual cycle.

2. The rates of gross primary production (GPP) and community respiration (CR) were

determined for each algal assemblage using oxygen change in chambers. The net daily

metabolism (NDM) and the GPPd)1 : CR24 ratio were estimated by patch-weighting the

assemblage-level metabolism values.

3. Gross primary production and CR showed significant differences between assemblages

and dates. The highest rates were measured in summer and spring, while December was

the only month when there were no significant differences in either parameters between

assemblages. GPP was strongly correlated with respiration, but not with algal biomass.

4. Chara vulgaris showed the highest mean annual metabolic rates (GPP ¼ 2.80 ± 0.83 gC

m)2 h)1, CR ¼ 0.76 ± 0.29 gC m)2 h)1), followed by the epilithic assemblage

(GPP ¼ 1.97 ± 0.73 gC m)2 h)1, CR ¼ 0.41 ± 0.12 gC m)2 h)1) and epipelic algae

(GPP ¼ 1.36 ± 0.22 gC m)2 h)1, CR ¼ 0.39 ± 0.06 gC m)2 h)1).

5. The epipelic assemblage dominated in terms of biomass (82%) and areal cover (88%),

compared with the other primary producers. Epipelic algae contributed 84% of gross

primary production and 86% of community respiration in the stream.

6. Mean monthly air temperature was the best single predictor of macrophyte respiration

and of epipelic GPP and CR. However, ammonium concentration was the best single

predictor of C. vulgaris GPP, and suspended solid concentration of epilithon GPP and CR.

7. Around 70% of the variation in both mean GPP and mean CR was explained by the

mean monthly air temperature alone. A multiple regression model that included

conductivity, PAR and nitrates in addition to mean monthly air temperature, explained

99.99% of the variation in mean CR.

8. Throughout the year, NDM was positive (mean value 7.03 gC m)2 day)1), while the

GPP : CR24 ratio was higher than 1, confirming the net autotrophy of the system.

Keywords: chamber method, Chara vulgaris, gross primary production, Mediterranean stream, netdaily metabolism

Introduction

Important ecological questions concerning the energy

flow in stream ecosystems require estimates of algal

primary production (Morin, Lamoureux & Busnarda,

1999). Such estimates of primary production have

been made in lotic systems across biomes and

Correspondence: J. Velasco, Department of Ecology and

Hydrology, University of Murcia, Campus of Espinardo,

30100 Murcia, Spain. E-mail: jvelasco@um.es

Freshwater Biology (2003) 48, 1408–1420

1408 � 2003 Blackwell Publishing Ltd

latitudes in North America (Bott et al., 1985; Mulhol-

land et al., 2001) and in most regions of the world

(Lamberti & Steinman, 1997). However, primary

production studies in Mediterranean-climate regions

(areas surrounding the Mediterranean Sea, parts of

western North America, parts of west and south

Australia, southwestern South Africa and parts of

central Chile) are largely lacking. Rainfall seasonality

and variability are the principal attributes of the

Mediterranean-type climate, most precipitation result-

ing from a few major storm events that may produce

flooding (Gasith & Resh, 1999). Annual precipitation

is highly variable among Mediterranean areas, usually

ranging between 275 and 900 mm, while certain

Mediterranean-climate regions fall into the category

of semiarid regions (between 200 and 500 mm;

UNESCO, 1979).

The importance of primary production in Mediter-

ranean streams is generally expected to increase with

increasing gradients of aridity (Gasith & Resh, 1999),

following the pattern found in an analysis of 30

streams over a large geographic range (Lamberti &

Steinman, 19971 ), where GPP increased significantly

with mean temperature and declined with total

precipitation. To our knowledge, there are few data

on primary production in Mediterranean streams.

Four streams in the Spanish Mediterranean area have

been studied: La Solana and Riera Major, in the NE of

the Iberian Peninsula with 800–1000 mm mean annual

precipitation (Guasch & Sabater, 1994, 1998); Monte-

sina stream, in the south (Molla, Malchik & Casado,

1994) with 527 mm of mean annual precipitation; and

Chicamo stream (Suarez & Vidal-Abarca, 20002 ),

located in southeastern Spain, with a semiarid climate

(250 mm mean annual precipitation). Early estimates

of primary production and respiration in Chicamo

stream (Suarez & Vidal-Abarca, 2000) were higher

than those reported in the other Mediterranean

streams of wetter areas, but were similar to those

obtained in the warm desert stream, Sycamore Creek

(Busch & Fisher, 1981; Grimm & Fisher (1984).

Although autotrophy may be important in arid and

semiarid streams, the patchiness of aquatic vegeta-

tion, its seasonal growth pattern and the contribution

of periphyton and macrophytes to autochthonous

sources during the year may differ. The objectives of

this paper were to determine the contribution of

different algal assemblages to annual primary pro-

duction in a semiarid Mediterranean stream (Chicamo

stream), to confirm its autotrophic metabolism, to

identify climatic and environmental variables that

might govern primary production in this stream, and

to compare metabolic rates with those of other

Mediterranean and desert streams.

Study area

Chicamo stream is a saline and intermittent tributary

of the Segura River located in southeast Spain (38�Nlatitude), in the most arid area of the province of

Murcia. The climate is characterised by a mean annual

precipitation below 300 mm and a mean annual

temperature of 18�C. A long warm and dry summer

season is interrupted by spring and autumn rains, the

latter followed by a short and temperate winter.

Annual and interannual variations in rainfall can lead

to a high discharge variability, with extreme condi-

tions of flooding and drying. The highest flows

normally occur in early autumn, followed by a second

peak in spring.

Chicamo stream is a 4th-order stream, draining a

sedimentary watershed of 502 km2. Surface water

flow is intermittent in 10 km of its 59.4 km total

length. The natural cover on the watershed is open

Mediterranean scrub, although much is dedicated to

citrus and horticultural crops.

Studies were conducted in a permanent, braided,

shallow and unshaded reach, 100 m length, typical of

the middle section. The substrate consisted of con-

solidated and impermeable marls, with deposits of

gravels and sand in the erosional zones of runs, and

silts in pools and depositional zones. Chicamo

stream water is hyposaline and hard (7.6 g L)1 mean

salinity, 219.5 mg L)1 mean alkalinity), well oxygen-

ated and rich in nutrients, especially nitrates and

ammonium (Vidal-Abarca et al., 2000). Riparian

vegetation is sparse because of frequent floods.

There are no trees, but isolated shrubs such as

Phragmites australis, Tamarix canariensis and Juncus

maritimus are present.

Aquatic primary producers in the study site include

the macrophyte Chara vulgaris (with epiphytic algae)

in pools, an extensive diatom assemblage on fine

sediments, including Nitzschia, Amphora, Navicula,

Gyrosigma and Pleurosigma among the most abundant

genera (D. Ros, per. com.3 ), and epilithic periphyton

dominated by the cyanobacteria Calothrix and the

chlorophyte Kentrosphaera facciolae, although in spring

Primary production in a semiarid stream 1409

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

the filamentous green alga Cladophora glomerata pre-

dominates.

Fine benthic particulate organic matter (FPOM) is

the principal benthic fraction (about 70% of the total)

while coarse particulate organic matter (CPOM) rep-

resents only 6% of the total because of the scarcity of

riparian vegetation and the consequent low input of

allochthonous materials (Martınez et al., 19984 ). Dis-

solved organic carbon (DOC) is the principal organic

carbon source flowing in the Chicamo stream, partic-

ularly when flash-floods occur (Vidal-Abarca et al.,

2001).

Methods

Metabolism experiments were conducted six times

between July 1998 and May 1999 to record the annual

variability. Macrophyte, epipelic and epilithic primary

production and community respiration were meas-

ured using oxygen change in clear plastic chambers

(25.5 · 13.8 · 8 cm) placed in the stream. Because of

the low discharge and current in the stream, we did

not use a pump for water recirculation. Chamber

temperature never exceeded stream temperature by

more than 3 �C. Four replicate estimates of production

and respiration were obtained from each assemblage.

The macrophyte, C. vulgaris, and epipelic samples

were collected 15 days before the experiment, tran-

ferred to plastic trays (21.5 · 12 · 2 cm) and placed

on the streambed contiguous with the sediment

surface for colonisation to continue. At the start of

the experiment, the trays were removed from the

stream and transferred to the chambers with minimal

disturbance to the community during removal. For

the epilithon, the stones used for each incubation were

collected from the stream immediately before the

experiment started.

All the chambers were filled with stream water,

closed and placed in situ. Each measurement of net

photosynthetic rate was made over a 3-h period (2 h

dark and 1 h light) during the morning. Chambers for

dark incubations were covered with aluminium foil.

Changes in O2 concentration were detected using

standard Winkler titration scaled to 10 mL sample

sizes taken with a needle and syringe. Winkler deter-

minations of dissolved oxygen (DO) were made before

and after the dark incubations, and after the light

incubations. Water temperature in the stream and in

each chamber was recorded at each sampling time.

Gross primary production (GPP) was calculated by

adding the change in DO measured during the dark

incubation to the change measured during the light

incubation. Because respiration rates include the

metabolism of heterotrophs such as microbes and

insects as well as autotrophs, respiration is termed

community respiration (CR). Oxygen generation was

converted to carbon fixation assuming a photosyn-

thetic quotient of 1.2 (Bott, 1997a).

After the incubation experiments, C. vulgaris was

removed from the chambers and stored on ice until it

was washed with tap water in the laboratory and

weighed. Three samples of 3 g of fresh macrophyte

with its epiphitic algae were taken from each chamber

to determine chlorophyll a (Chla) concentration by

spectrophotometry, following extraction in boiling

90% ethanol as described by Biggs (1995). Concentra-

tions were converted to algal carbon biomass using a

factor of 30, which is suitable for communities in

nutrient-rich, unshaded environments (Vollenweider,

1974; Bott et al., 1997). The rest of the macrophyte was

dried at 60 �C to constant dry weight (DW) and then

ashed at 450 �C for 4 h to estimate the content of ash

free dry weight (AFDW) in each chamber. Triplicate

sediment cores of 2.7 cm diameter were also collected

from each epipelic chamber for Chla and AFDW

analyses. The rocks were removed and returned to the

laboratory for determination of surface area and Chla

and AFDW content. Rock surface area was deter-

mined by tracing the outline of the top of the rock on

aluminium paper, cutting out the shape, and compar-

ing its weight with the weight of a known area of

paper. Periphyton on rocks was scraped, brushed and

washed from the rock surface. The extracts were

filtered onto Whatman A filters for Chla concentration

and AFDW determinations. Autochthonous detritus

standing crop was determined as the difference

between AFDW measurements (assuming a 50%

carbon content) and the periphyton biomass obtained

from Chla concentrations.

On each date, the area covered by each primary

producer assemblage in the stream was measured in

the study reach (100 m length) to extrapolate the

metabolism rates estimated for each assemblage to the

reach. Measurements of discharge, photosynthetically

active radiation (PAR), air and water temperatures,

conductivity, salinity and dissolved oxygen were

made in the stream during the experiments. Two

water samples were taken to determine suspended

1410 J. Velasco et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

solids, ammonium, nitrate and soluble reactive phos-

phorus (SRP) concentration on each date. The samples

were kept cool until their arrival at the laboratory,

where they were filtered onto preashed and pre-

weighed GF/F glass-fiber filters5 (Whatman) and

oven-dried at 60 �C to constant dry weight. Nutrients

were determined according to standard methods

(American Public Health Association (APHA), 1992):

nitrate by the cadmium reduction method, nitrites by

sulfanylic acid colorimetry, ammonium by phenol

nitroprussiate colorimetry and phosphate by ascorbic

acid colorimetry.

The days on which the experiments were performed

were sunny with mean PAR values during the experi-

ments of more than 1000 lE m)2 s)1, except in August

(914.78 lE m)2 s)1) and December (595.94 lE m)2 s)1).

At the stream reach scale, we estimated the net

daily metabolism (NDM) and the GPPd)1/CR24 ratio

by patch-weighting the assemblage-level metabolism

values. Chamber measurements of community res-

piration were extrapolated to a 24-h period, assu-

ming the respiratory rate during the night to be half

daylight respiration because of the decrease in

temperature:

GPP d�1 ¼ ðNPP h�1 � light hoursÞþ ðCR h�1 � light hoursÞ;

and

CR24 ¼ ðCR h�1 � light hoursÞþ ðCR h�1=2Þ � dark hours� �

Differences in gross primary production and res-

piration between primary producer assemblages

were analysed using analysis of variance (one way

ANOVAANOVA) and Tukey’s honest significant difference

test (HSD). All data were log(x + 1) transformed to

normalise distributions and equalise variance. Rela-

tionships of climatic and environmental variables to

metabolic rates were examined using bivariate cor-

relation and multiple linear regression approaches.

Pearson correlation analysis was used to indentify

relationships between single factors and metabolic

rates. Finally, a stepwise multiple linear regression

used all significant (P £ 0.05) correlated factors and the

response variables (GPP and CR). Statistical analyses

were performed using Systat (Wilkinson, 1996).

Results

Climatic and environmental conditions

Table 1 shows the mean values of physical and

chemical parameters measured for the six experiment

dates in Chicamo stream, together with monthly air

temperature and precipitation recorded in a nearby

metereological station. Total precipitation during the

study period (from July 1998 to May 1999) was

143.5 mm, mainly concentrated in autumn and winter

peaks. Discharge variation during the study period

was closely related to daily precipitation. Discharge

was very low (£ 3.5 L s)1) except following rainfall in

December and March. The mean annual temperature

Table 1 Mean (±1SE) values of physical and chemical parameters measured on the experiment dates in Chicamo stream, together

with monthly air temperature and precipitation recorded at a nearby meterological station

17/07/1998 19/08/1998 15/10/1998 28/12/1998 20/03/1999 28/05/1999

Monthly mean air temperature (�C) 26.4 26.5 18.9 9.7 14 20.7

Monthly precipitation (mm) 0 0 0 51.5 22.5 0

PAR during the experiment (lE m)2 s)1) 1788 915 1437 596 1512 1760

Air temperature during the experiment (�C) 33 27 25.5 7.5 23.5 30.5

Water temperature during the experiment (�C) 32.5 30 20 10 19.5 27.5

Discharge (L s)1) 3.5 0.17 0 73 48.6 3.3

Conductivity (mS cm)1) 14 13.5 13.5 11 12.7 13

Salinity 10.9 10 11 9 9.3 11

Total suspended solids (mgDW L)1) 69.6 ± 6.1 10.85 ± 5.7 33.82 ± 3.3 6.20 ± 1.45 18.13 ± 8.13 302 ± 98

Alkalinity (meq L)1) 4.88 ± 0 5.24 ± 0.04 4.28 ± 0.52 5.32 ± 0.28 5.4 ± 0.04 6.44 ± 0.12

Dissolved oxygen (mg L)1) 6.47 ± 0.03 5.35 ± 0 10.38 ± 0.45 9.78 ± 0.03 7.28 ± 0.06 6.08 ± 0.18

NH4-N (lg L)1) 173 ± 12 70 ± 1.2 55.5 ± 6.7 19.12 ± 1.64 0 46 ± 3.9

NO3-N (lg L)1) 8555 ± 529 549 ± 14 59.15 ± 4.9 2611 ± 129 3481 ± 145 1239 ± 124

SRP (lg L)1) 8.24 ± 0.4 255 ± 11 6.38 ± 0.74 2.17 ± 0.12 4.78 ± 0.5 7.71 ± 2.72

Primary production in a semiarid stream 1411

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

was 18.2 �C with August being the hottest month,

when the maximum temperature reached over 32 �C,

while December was the coldest month, with the

minimum recorded temperature being 4 �C. The

mean water temperature was 20 �C (max ¼ 30 �C in

August; min ¼ 7.5 �C in December).

Areal coverage and biomass of algal assemblages

Epipelic algae were the most widespread assemblage

throughout the study period, representing more than

70% of the total wetted surface (Table 2). The cover-

age by C. vulgaris was peaked in October, though

subsequent high winter flows buried it with sedi-

ments and reduced it to small patches. In March, the

macrophyte was absent, but it started to grow again in

May. Maximum epilithon cover was attained in

December and minimum in October. C. vulgaris cover

was negatively correlated with discharge and

monthly precipitation (r ¼ )0.92, P £ 0.005 and

r ¼ )0.84, P £ 0.05) while it was positively correlated

with ammonium (r ¼ 0.85, P £ 0.02). Epilithic algal

cover showed a positive correlation with discharge

and precipitation (r ¼ 0.95, P £ 0.001; r ¼ 0.89,

P £ 0.005, respectively) and a negative corrrelation

with conductivity (r ¼ )0.76, P £ 0.05). The epipelic

algal cover showed a negative correlation with the

dissolved oxygen (r ¼ )0.78, P £ 0.05).

Mean annual biomass values for macrophyte,

epipelic and epilithic assemblages were 25.26, 5.05

and 4.62 gC m)2, respectively (Table 2). C. vulgaris

registered the highest biomass values on all the dates

except in March when it was absent. Its biomass

peak occurred in December, and the minimum in

summer. It was positively correlated with the stand-

ing crop of detritus and ammonium concentration

Table 2 Summary of area covered in the reach, and of biomass and metabolism measurements for algal assemblages (Chara vulgaris,

epipelic and epilithic algae) in the chambers and extrapolated to reach scale. Gross primary production (GPP), community respiration

(CR) and net daily metabolism (NDM)

Date Assemblage

Coverage

(%)

Biomass

(gC m)2)

Reach

biomass

(%)

GPP

(gC m)2 h)1)

Reach

GPP

(%)

CR

(gC m)2 h)1)

Reach

CR

(%)

NDM

(gC m)2 d)1) GPP : CR24

17/07/1998 C. vulgaris 5.15 19.6 ± 5.5 22.38 4.74 ± 0.39 9.97 0.93 ± 0.12 8.36

Epipelic 90.79 3.53 ± 0.34 70.95 2.18 ± 0.26 80.89 0.53 ± 0.05 84.67

Epilithon 4.06 7.42 ± 2.06 6.67 5.46 ± 1.19 9.05 0.98 ± 0.17 6.96

16.61 2.69

19/08/1998 C. vulgaris 6.26 14.5 ± 2.2 16.85 3.88 ± 0.49 16.58 1.49 ± 0.20 20.65

Epipelic 91.30 4.69 ± 0.5 79.32 1.86 ± 0.39 81.43 0.55 ± 0.12 78.08

Epilithon 2.44 8.47 ± 1.46 3.83 1.71 ± 0.42 1.99 0.34 ± 0.04 1.27

9.59 1.94

15/10/1998 C. vulgaris 20.34 38 ± 11 75.85 2.06 ± 0.44 22.83 0.31 ± 0.36 17.63

Epipelic 78.71 3.15 ± 0.64 24.08 0.98 ± 0.51 76.79 0.37 ± 0.13 81.82

Epilithon 0.96 0.79 ± 0.16 0.07 0.72 ± 0.11 0.38 0.21 ± 0.05 0.57

2.85 1.51

28/12/1998 C. vulgaris 0.60 48 ± 11 10.98 1.15 ± 0.50 0.74 0.14 ± 0.09 0.47

Epipelic 70.63 2.88 ± 0.14 77.12 0.94 ± 0.40 71.23 0.15 ± 0.05 58.42

Epilithon 28.77 1.09 ± 0.29 11.90 0.91 ± 0.41 28.03 0.26 ± 0.05 41.10

2.47 1.92

20/03/1999 C. vulgaris 0 0 0 0 0 0 0

Epipelic 98.68 5.87 ± 0.77 97.82 0.84 ± 0.2 96.83 0.31 ± 0.15 98.46

Epilithon 1.32 9.76 ± 1.96 2.18 2.06 ± 0.9 3.17 0.36 ± 0.08 1.54

2.23 1.44

28/05/1999 C. vulgaris 1.50 31 ± 2.5 4.78 5.01 ± 0.64 5.45 1.68 ± 0.23 5.72

Epipelic 95.93 10.2 ± 2 95.17 1.40 ± 0.36 93.08 0.45 ± 0.11 93.31

Epilithon 2.50 0.21 ± 0.03 0.05 0.96 ± 0.11 1.47 0.29 ± 0.04 0.97

7.15 1.89

Mean C. vulgaris 5.65 25 ± 7 21.80 2.80 ± 0.83 8.96 0.76 ± 0.29 8.05

Epipelic 87.67 5.05 ± 1.12 74.08 1.37 ± 0.22 84 0.39 ± 0.06 85.91

Epilithon 6.68 4.62 ± 1.78 4.12 1.96 ± 0.73 7.05 0.41 ± 0.12 6.03

7.03 1.9

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(r ¼ 0.96, P £ 0.001 and r ¼ 0.80, P £ 0.05, respect-

ively).

The epilithic assemblage registered its highest bio-

mass value in March, when C. glomerata was the

dominant alga. A second peak of epilithon biomass

occurred in summer, when the dominant alga was the

cyanobacterium Calothrix. Epilithon biomass also

showed a positive correlation with the standing crop

of detritus (r ¼ 0.88, P £ 0.01). The peak of epipelon

biomass was reached in spring. Epipelon biomass was

positively correlated with alkalinity (r ¼ 0.83, P £ 0.05).

At the reach level, epipelic biomass accounted for

the highest portion of biomass on all dates, except in

October when C. vulgaris represented 76% of the total

biomass of the reach. Differences in algal biomass

between assemblages were significant on all dates

(Table 3).

Gross primary production and respiration rates

The mean gross primary production rate was

2.05 ± 0.42 gC m)2 h)1 and the mean respiration rate

0.52 ± 0.12 gC m)2 h)1, resulting in a mean net pri-

mary production rate of 1.53 ± 0.31 gC m)2 h)1. Res-

piration represented 25.36% of mean gross primary

production.

At the assemblage level, C. vulgaris showed the

highest mean metabolic rates (Table 2), followed by

the epilithic and the epipelic assemblages. Summer

and spring were the most productive seasons

(Fig. 1a). The maximum values measured correspon-

ded to epilithon in July (5.46 gC m)2 h)1) and C.

vulgaris in May (5.01 gC m)2 h)1).

Gross primary production rates showed highly

significant differences between assemblages, except

in December (Table 3). Macrophyte (Chara) production

exceeded epilithic and epipelic production for most of

the year (Fig. 1a), significantly so in August, October

and May (Table 3). Epipelic and epilithic production

rates were similar except in July, when epilithic

production reached its highest value (Fig. 1a).

Respiration and gross primary production were

highly correlated for all the assemblages (r ¼ 0.9,

P £ 0.005 for C. vulgaris; r ¼ 0.83, P £ 0.02 for epipelic

algae, and r ¼ 0.97, P £ 0.001, for epilithic algae).

However, no significant correlations were found for

any algal assemblage between metabolic rates and

biomass.

Community respiration rates followed a similar

pattern to the gross primary production rates

(Fig. 1b). The highest respiration rates were also

found in spring and summer, and the lowest in

Table 3 Summary of the analysis of variance (one-way A N O V AA N O V A) for the variables, biomass, gross primary production (GPP) and

community respiration (CR) between algal assemblages by date. P-levels of significance of the Tukey honest significant difference test

for all possible pairs of means are shown (n.s., not significant)

Variable

Factor

(date) d.f. MS F P

Epilithic versus

C. vulgaris

C. vulgaris

versus epipelic

Epilithic

versus epipelic

Biomass (gChla m)2) July 2 0.359 7.981 0.0101 n.s. 0.0008 n.s.

August 2 0.183 13.618 0.0019 n.s. 0.0016 n.s.

October 2 1.756 46.351 0.0000 0.0002 0.0004 n.s.

December 2 2.028 105.913 0.0000 0.0002 0.0002 0.0434

March 2 1.158 98.036 0.0000 0.0002 0.0002 n.s.

May 2 2.056 114.310 0.0000 0.0002 0.0007 0.0019

GPP (gC m)2 h)1) July 2 0.101 12.015 0.0029 n.s. 0.0086 0.0040

August 2 0.089 7.143 0.0139 0.0166 0.0379 n.s.

October 2 0.011 6.271 0.0197 0.0238 0.0489 n.s.

December 2 0.011 0.711 n.s. n.s. n.s. n.s.

March 2 0.016 12.228 0.0027 0.0023 0.0306 n.s.

May 2 0.007 39.054 0.0000 0.0002 0.0003 n.s.

CR (gC m)2 h)1) July 2 0.079 5.522 0.0273 n.s. n.s. 0.0381

August 2 0.433 24.903 0.0002 0.0004 0.0035 n.s.

October 2 0.016 0.087 n.s. n.s. n.s. n.s.

December 2 0.145 1.602 n.s. n.s. n.s. n.s.

March 2 11.535 153.897 0.0000 0.0002 0.0002 n.s.

May 2 0.735 11.688 0.0031 0.0054 0.0070 n.s.

Primary production in a semiarid stream 1413

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December. Significant differences were found be-

tween assemblages except in October and December

(Table 3). C. vulgaris respiration was significantly

greater than that of the periphyton assemblages in

August and May. Differences between epipelic and

epilithic respiration were not significant, except in

July.

Examination of the GPP/Chla ratio throughout

the year (Fig. 1c) showed the most photosyntethic

efficient assemblage to be the epilithon (mean

37.44 gC g)1 Chla h)1) which reached its highest

value in May (137.59 gC g)1 Chla h)1). Significant

negative correlation was found between GPP/Chla

and epilithon biomass. The epipelic algae and C.

vulgaris were less efficient, although they showed

greater efficiency in summer than during the rest of

the year.

The mean annual GPP and CR of the reach

studied were 285.19 ± 72.24 and 76.51 ± 20.12 gC h)1,

respectively, with the mean annual NPP being

208.68 ± 52.12 gC h)1. At the reach level, the epipelic

assemblage was the principal contributor to auto-

chthonous production in the stream, representing

84% of total production and 86% of total respiration

Fig. 1 Variation in gross primary production (a), community respiration (b) and gross primary production per unit of Chlorophyll a

(c) of the different algal assemblages during the study period (±1 SE).

1414 J. Velasco et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

(Table 2). Its contribution to GPP varied during the

year from 71.23% in December to 96.83% in March.

C. vulgaris and epilithon production represented

only 8.96 and 7.05% of the total, although the

contribution of the macrophyte and its epiphitic

algae was more important in summer and autumn,

while the epilithon showed a maximum in Decem-

ber. Patch respiration followed a similar pattern to

patch GGP.

Relationships between metabolic rates and climatic

and environmental variables

The primary production of C. vulgaris showed a

positive correlation with ammonium (r ¼ 0.92,

P £ 0.005), standing crop of detritus (0.82, P £ 0.01),

and mean monthly air temperature (r ¼ 0.76,

P £ 0.05), and was negatively correlated with monthly

precipitation (r ¼ )0.82, P £ 0.05). Ammonium con-

centration was the best single predictor of GPP in the

simple regression analysis, explaining 81% of its

variation, although the best multiple regression model

included detritus standing crop and monthly air

temperature, which explained 92% of the variation

(Table 4).

The respiration rate of C. vulgaris was positively

correlated with mean monthly air temperature

(r ¼ 0.78, P £ 0.05), and negatively with dissolved

oxygen concentration (r ¼ )0.76, P £ 0.05) and

monthly precipitation (r ¼ )0.77, P £ 0.05). Mean

monthly air temperature was the only significant

predictor of macrophyte community respiration,

explaining 51% of its variation (Table 4).

Epipelic production and respiration were also

positively correlated with mean monthly air tempera-

ture (r ¼ 0.84, P £ 0.05, and r ¼ 0.99, P £ 0.001,

respectively) and water temperature (r ¼ 0.78,

P £ 0.05, and r ¼ 0.98, P £ 0.001, respectively). Res-

piration rate was also positively correlated with

conductivity (r ¼ 0.92, P £ 0.005) and negatively with

monthly precipitation (r ¼ )0.89, P £ 0.005), dis-

charge (r ¼ )0.77, P £ 0.05) and dissolved oxygen

(r ¼ )0.766, P £ 0.05). Mean monthly air temperature

was the best single predictor of the epipelic produc-

tion and respiration, explaining 63 and 98%, respect-

ively, of its variation. Multiple regression analyses

provided a predictive model of GPP that included,

besides monthly air temperature, nitrate, ammonium

concentration and PAR as predictor variables,

accounting for 99.99% of the variation (Table 4). The

best multiple regression model for epipelic respiration

(r2adj ¼ 99.99%) included monthly air temperature,

dissolved oxygen and conductivity as independent

variables (Table 4).

Table 4 Results of stepwise multiple

regression analysis for rates of gross pri-

mary production (GPP) and community

respiration (CR) for each algal assemblage

and for mean assemblage rates (n ¼ 6 for

each regression)

Community

Dependent

variable Independent variable r2adj P

C. vulgaris GPP Detritus standing crop 0.582 0.0146

Mean monthly air temperature 0.342 0.0223

Full model 0.924 0.0097

CR Mean monthly air temperature 0.507 0.0683

Epipelic GPP Mean monthly air temperature 0.629 0.0095

Nitrate 0.0016

Ammonium 0.0034

PAR 0.0041

Full model 0.999 0.0018

CR Mean monthly air temperature 0.980 0.0299

Dissolved oxygen 0.0279

Conductivity 0.0429

Full model 0.999 0.0066

Epilithon GPP Suspended solids 0.690 0.0253

CR Suspended solids 0.753 0.0157

Mean assemblages GPP Mean monthly air temperature 0.638 0.0352

CR Mean monthly air temperature 0.712 0.0018

Conductivity 0.0032

PAR 0.0053

Nitrates 0.0219

Full model 0.999 0.0029

Primary production in a semiarid stream 1415

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

However, epilithon GPP and CR only showed a

positive correlation with the suspended solid concen-

tration (r ¼ 0.87, P £ 0.05, and r ¼ 0.78, P £ 0.05,

respectively). This independient variable explained

69% of epilithon production and 75% of variation in

its respiration rate, and it was the only predictor

variable in the multiple regression analysis (Table 4).

In general, the mean rates of gross primary pro-

duction and respiration of algal assemblages were

positively correlated (P £ 0.05) with mean monthly air

temperature (r ¼ 0.88 and r ¼ 0. 87, respectively), and

respiration was also negatively correlated with

monthly precipitation (r ¼ )0.79) and dissolved oxy-

gen (r ¼ )0.83). The results obtained by simple

regression analysis using the mean values for the

assemblages as a whole, indicated that 64% of the

variation in mean GPP and 71 % of the variation of

mean CR could be explained by the mean monthly air

temperature alone. For mean CR, a significant mul-

tiple regression model including, besides mean

monthly air temperature, conductivity, PAR and

nitrate concentration, explained 99.99% of its vari-

ation (Table 4).

Net ecosystem metabolism and GPP : CR24 ratio

Daily net metabolism in the reach was positive on all

the dates studied, ranging from 2.23 gC m)2 day)1 in

March to 16.61 gC m)2 day)1 in July, with a mean

value of 7.03 gC m)2 day)1 (Table 2).

The maximum rate of daily GPP was

26.41 gC m)2 day)1 in July, and the minimum

5.15 gC m)2 day)1 in December. The rates of daily

benthic respiration were much lower than those of

GPP, with values ranging from 2.67 to

10.22 gC m)2 day)1. Mean daily GPP and CR24 were

13.70 and 6.88 gC m)2 day)1, respectively. The

GPP : CR24 ratios on all dates were higher than 1,

with a mean value of 1.9, a maximum value of 2.7

being reached in July.

Discussion

The exceptionally high rates of primary production

found in Chicamo stream are only possible as a result

of high temperatures, high light availability and the

intensive internal recycling of nutrients. Mean

monthly air temperature was the best single predictor

of mean GPP and mean CR in Chicamo stream,

explaining more than 63 and 71% of their variation,

respectively. Morin et al. (1999), using multiple regres-

sion models fitted to predict the primary production

of stream periphyton, found that the production of

algae increased with both Chla standing crop and

water temperature, although Chla was most strongly

correlated with primary production, accounting for

65% of the variation, while temperature accounted for

a much smaller portion (4%) of the variability. Other

comparative studies of streams from different biomes

found that temperature explained 33 (Bott et al., 1985)

and 38% (Sinsabaugh, 1997) of the variation in R,

although Mulholland et al. (2001) found no evidence

for any effect of water temperature on R or GPP. On a

local-scale, Uehlinger, Konig & Reichert (2000)6 during

one annual cycle of a Swiss river, found that R was

significantly related to water temperature, although

temperature explained only 22% of its variation.

Temperature interacts with other environmental fac-

tors, such as nutrients, gases, metabolites, develop-

ment stage, trophic interactions and, especially, light,

although the individual effect of each factor is not

clear (DeNicola, 1996). In Chicamo stream, high

temperatures were associated with high irradiance,

low or null precipitation, low discharge, high con-

ductivity, low dissolved oxygen levels and high

ammonium concentrations.

Although photoinhibition usually ocurrs at irradi-

ances greater than 600 lmol m)2 s)1 (Hill, 1996), in

our study, exposure of the experimental chambers to

higher natural light levels did not appear to cause any

photoinhibition. Regarding photosynthesis-irradiance

responses, benthic algae inhabiting ‘‘high-light’’ envi-

ronments (such as open sites in clear streams) are

probably under considerable selective pressure to

develop mechanisms that reduce the potentially

damaging effects of high irradiances, whether in the

form of accessory pigments (carotenids) or sheath

pigments such as scytonemin (Garcia-Pichel &

Castenholz, 1991). On the other hand, high irradiance

at the surface of benthic algal mats may inhibit

photosynthesis by surface cells, while shaded subsur-

face cells receive only saturating or subsaturating

levels of irradiance. Photosynthesis by underlayers at

high surface irradiance may compensate for inhibition

in surface layers, so there is no evidence of photoin-

hibition at community level (Hill, 1996).

In our study, PAR was a secondary predictor

variable of both epipelic GPP and mean CR in the

1416 J. Velasco et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

Ta

ble

5S

um

mar

yo

fd

aily

gro

ssp

rim

ary

pro

du

ctio

n(G

PP

ing

Cm

)2

day

)1)

and

dai

lyco

mm

un

ity

resp

irat

ion

(CR

ing

Cm

)2

day

)1)

fro

mst

ream

so

fd

iffe

ren

td

eser

tan

d

Med

iter

ran

ean

area

s

Stu

dy

area

GP

PC

RC

om

mu

nit

yS

tud

yp

erio

dM

eth

od

Ref

eren

ce

Sy

cam

ore

Cre

ek(S

C),

Ho

td

eser

tst

ream

,A

Z,

U.S

.A.

4.68

2.50

Wh

ole

com

mu

nit

y27

May

1997

Tw

o-s

tati

on

diu

rnal

ox

yg

ench

ang

e

Mu

lho

llan

det

al.,

2001

Dee

pC

reek

(DC

),

Co

ol

des

ert

stre

am,

ID,

U.S

.A.

1.18

1.15

Mac

rop

hy

tes

and

per

iph

yto

nA

nn

ual

aver

age

(mo

nth

lym

easu

res)

Tw

o-s

tati

on

diu

rnal

ox

yg

ench

ang

e

Min

shal

l,19

78

Rat

tles

nak

eC

reek

(RC

),

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ol

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ert

stre

am,

WA

,U

.S.A

.

9.3

8.1

Wh

ole

com

mu

nit

y

(per

iph

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nan

dw

ater

cres

s)

An

nu

alav

erag

e

(mo

nth

lym

easu

res)

Tw

o-s

tati

on

die

l

curv

e-p

H-C

O2

Cu

shin

g&

Wo

lf,

1984

Tec

op

aB

ore

(TB

),

Th

erm

ald

eser

tsp

rin

g,

CA

,U

.S.A

.

3.25

Alg

alm

ats

An

nu

alav

erag

e

(mo

nth

lym

easu

res)

14C

up

tak

eN

aim

an,

1976

Rie

raM

ajo

r(R

M),

Med

iter

ran

ean

stre

am,

Sp

ain

0.11

0.08

Ep

hil

itic

alg

aeA

nn

ual

aver

age

(mo

nth

lym

easu

res)

Ph

oto

syn

thes

is-i

rrad

ian

cecu

rve

Gu

asch

&S

abat

er,

1998

La

So

lan

a(L

S),

Med

iter

ran

ean

stre

am,

Sp

ain

0.27

0.23

Ep

hil

itic

alg

aeA

nn

ual

aver

age

(mo

nth

lym

easu

res)

Ph

oto

syn

thes

is-i

rrad

ian

cecu

rve

Gu

asch

&S

abat

er,

1998

Mo

nte

sin

a(M

),

Med

iter

ran

ean

stre

am,

Sp

ain

0.71

0.69

Mac

rop

hy

tes

and

per

iph

yto

nF

ou

rd

ates

aver

age

(J,

M,

M,

J)

Ox

yg

ench

ang

ein

cham

ber

sM

oll

aet

al.,

1994

16

Ch

icam

o(C

H),

Sem

iari

d

Med

iter

ran

ean

stre

am,

Sp

ain

1.27

1.18

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ole

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mu

nit

yF

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rd

ates

aver

age

(J,

N,

F,

J)

On

e-st

atio

nd

iurn

alo

xy

gen

chan

ge

Su

arez

&V

idal

-Ab

arca

,20

0017

13.7

6.88

Cha

ravu

lgar

isS

ixd

ates

aver

age

(J,

A,

O,

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M)

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ench

ang

ein

cham

ber

sT

his

stu

dy

Ep

hil

itic

alg

ae

Ep

ipel

ical

gae

Primary production in a semiarid stream 1417

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

multiple regression models. Of the nutrients exam-

ined, only the ammonium concentration was signifi-

cantly related to C. vulgaris GPP and the nitrate and

ammonium concentration to epipelic GPP.

Measurements of GPP and CR showed within-year

differences between assemblages that reflected the

phenological characteristics of the primary producers

studied in relation to seasonal variations of tempera-

ture and precipitation. In addition, proximate factors

such as resource availability, ecophysiology, life his-

tory characteristics, stress, allelophathy, competition

and predation, are causes of differential species

performance (Stevenson, 1996). In unshaded streams,

the flood disturbance regime is perhaps the funda-

mental factor determining habitat suitability and the

spatial and temporal patterns of benthic algae, and is

a major biomass loss mechanism (Biggs, 1996). Floods

in the Chicamo stream increase the habitat suitability

for epilithon and nutrient availability in the water

column, but have a negative effect on the production

of C. vulgaris which is buried by transported sedi-

ment. Although epipelic algae biomass was largely

removed by flooding, recolonisation was rapid and

biomass and production values returned to predis-

turbance levels in less than one month. The availab-

ility of algal propagules and rapid growth rates make

epipelic algae resilient in the face of flooding.

In Chicamo stream, the greater extent of epipelic

assemblages, the high abundance of gathering collec-

tors and the low diversity of algal grazers (Martınez

et al. 19987 ) suggest that neither the macrophyte

C. vulgaris nor epilithic algae are an important food

source, although epipelic algae with detritus derived

from sloughing periphyton are. High rates of primary

production in Chicamo stream support correspond-

ingly high rates of secondary production of aquatic

macroinvertebrates. The annual production estimates

of the gathering-collector Caenis luctuosa presented the

highest value ever reported for Caenis species

(6.35 gDW m)2 year)1; Peran, Velasco & Millan,

1999).

A comparison of the primary production values

obtained in this study with those reported for other

desert and Mediterranean streams (Table 5) shows

that our rates of GPP were higher than those reported

earlier from other streams, including desert streams

which had higher GPP than a variety of streams from

different biomes and locations throughout the world

(Naiman, 19768 ; Lamberti & Steinman, 1997; Mulhol-

land et al., 2001). The expected pattern of increasing

GPP with increasing mean annual temperature and

decreasing total precipitation was observed (Fig. 2), as

found by Lamberti & Steinman (1997) in a multi-

stream comparison study.

Net metabolism measurements in the Mediterra-

nean streams also point to differences in the degree of

autotrophy as aridity increases. Autotrophic metabo-

lism prevailed throughout the year in Chicamo

stream, the most arid stream, but only during some

periods of the year (principally spring) in the rest of

the streams (Guasch & Sabater, 1994, 1998; Molla et al.,

1994). The mean net metabolism in Chicamo was

7.03 gC m)2 day)1, resulting in the stream being a net

producer of organic carbon.

Early estimates of daily GPP and CR in Chicamo

stream made by the one station-open system

method (Suarez & Vidal-Abarca, 2000) were lower

than our estimates made using the chamber method,

because the former only reflects a particular local

community. The chamber method employed in our

study provides a better estimate of stream metabo-

lism because the contribution of the different algal

Fig. 2 Gross primary production (gC m)2 year)1) versus mean

annual temperature (a) and mean annual precipitation (b), for

the desert and Mediterranean streams used in the comparative

analysis. The thermal desert spring Tecopa Bore was not inclu-

ded in the regression analysis because of its extremely low

precipitation (41 mm) and high temperature (>25 �C).

1418 J. Velasco et al.

� 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 1408–1420

assemblages present is recorded and can be extra-

polated to the total reach. Bott et al. (1978), in a

comparison of methods for measuring primary

productivity and community respiration in streams,

found that open methods provided lower NPP

estimates than the chamber methods. Therefore,

inter and intra stream differences should be inter-

preted with caution because comparative analyses of

primary production are difficult because of the large

differences in analytical methods, conversion factors,

communities and sampling frequencies employed

(Wetzel & Ward, 1992).

In conclusion, Chicamo stream is a net producer

of organic carbon, with autotrophic metabolism

throughout the year, epipelic algae being the most

important primary producer in the stream. Tem-

perature dominates the control of mean GPP and

mean CR in the stream, perhaps because it integ-

rates many features relevant to primary production.

In addition, the frequency and intensity of flood

disturbance has a considerable influence on the

biomass and production of different algal assem-

blages. Nutrient concentrations, especially of ammo-

nium and nitrates, appeared to be a secondary

determinant of GPP.

Thus, Chicamo stream resembles desert and prairie

streams as regards the dominant role of autochtho-

nous organic matter as the base of the food web.

Acknowledgments

We thank J.L. Moreno, B. Martınez, A. Peran and A.

Mellado for sampling assistance and D. Ros for algal

identification. Comments made on a early version of

the manuscript by Professor R.I. Jones and two

anonymous reviewers have enabled us to improved

the paper. This research was supported by Projectt

PB96-1113 (National Program CICYT).

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