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Relationships between structure and function in streamscontrasting in temperature

NIKOLAI FRIBERG*, †, JOHN B. DYBKJÆR †, JON S. OLAFSSON ‡, GISLI MAR GISLASON§ ,

SØREN E. LARSEN † AND TORBEN L. LAURIDSEN †

*Macaulay Land Use Research Institute, Catchment Management Group, Craigiebuckler, Aberdeen, U.K.†Department of Freshwater Ecology, National Environmental Research Institute, University of Aarhus, Silkeborg, Denmark‡Institute of Freshwater Fisheries, Keldnaholt, Reykjavik, Iceland§Institute of Biology, University of Iceland, Sturlugata, Reykjavik, Iceland

SUMMARY

1. We studied 10 first-order Icelandic streams differing in geothermal influence in separate

catchments. Summer temperature (August–September) ranged between 6 and 23 �C.

2. Macroinvertebrate evenness and species overlap decreased significantly with temper-

ature whereas taxon richness showed no response. In total, 35 macroinvertebrate species

were found with Chironomidae the dominant taxonomic group. Macroinvertebrate

density increased significantly with temperature. Dominant species in the warm streams

were Lymnaea peregra and Simulium vittatum. Algal biomass, macrophyte cover and

richness were unrelated to temperature. Densities of trout (Salmo trutta), the only fish

species present, reflected habitat conditions and to a lesser degree temperature.

3. Density of filter-feeders increased significantly with temperature whereas scraper

density, the other dominant functional feeding group, was unrelated to temperature.

Stable isotope analysis revealed a positive relationship between d15N and temperature

across several trophic levels. No pattern was found with regard to d13C and temperature.

4. Leaf litter decomposition in both fine and coarse mesh leaf bags were significantly

correlated to temperature. In coarse mesh leaf packs breakdown rates were almost doubled

compared with fine mesh, ranging between 0.5 and 1.3 g DW 28 days)1. Nutrient diffusion

substrates showed that the streams were primarily nitrogen limited across the temperature

gradient while a significant additional effect of phosphorous was found with increasing

temperature.

5. Structural and functional attributes gave complementary information which all

indicated a change with temperature similar to what is found in moderately polluted

streams. Our results therefore suggest that lotic ecosystems could be degraded by global

warming.

Keywords: climate change, geothermal, Iceland, leaf litter, nutrients, stable isotopes

Introduction

Most bioassessments of streams have been based on

structural attributes and mostly using macroinverte-

brates (e.g. Hering et al., 2006; Mazor et al., 2006) while

surprisingly few studies cover multiple taxa (but see

Hering et al., 2006). The use of functional indicators in

detecting ecosystem stress has received considerable

recent attention (e.g. Gucker, Brauns & Pusch, 2006;

Bergfur et al., 2007) especially using leaf litter break-

down as a proxy for ecosystem functioning as advo-

cated by Gessner & Chauvet (2002). Other functional

proxies such as secondary production or whole-stream

Correspondence: Nikolai Friberg, Department of Freshwater

Ecology, National Environmental Research Institute, University

of Aarhus, Vejlsøvej 25, DK-8600 Silkeborg, Denmark.

E-mail: [email protected]

Freshwater Biology (2009) 54, 2051–2068 doi:10.1111/j.1365-2427.2009.02234.x

� 2009 Blackwell Publishing Ltd 2051

metabolism have been less frequently used (e.g.

Wallace, Grubaugh & Whiles, 1996; Gucker et al., 2006).

However, structural and functional attributes do not

necessarily respond in a similar manner to stress. Thus,

Mckie, Petrin & Malmqvist (2006) found negative

effects of liming on both macroinvertebrate assem-

blages and litter decomposition whereas Bergfur et al.

(2007) found that macroinvertebrate metrics per-

formed much better than leaf litter breakdown rates

along an enrichment gradient. Interpretation of dif-

ferences in response between structural and func-

tional attributes may not always be possible as

multiple stressors interact to affect structure and

function of ecosystems in different ways (Doroszuk

et al., 2007). Furthermore, studies on single trophic

level systems dominate research on ecosystem func-

tioning but all ecosystems contain multiple trophic

levels. Interactions between these trophic levels have

important effects on ecosystem structure and func-

tioning (Petchey et al., 2004).

Temperature is of key importance in the distribu-

tion and activity of organisms in ecosystems. Recent

studies have documented movements to higher lati-

tudes and altitudes by terrestrial species coinciding

with climatic warming (e.g. Parmesan et al., 1999;

Hickling et al., 2006). Analyses of long-term data sets

from freshwater ecosystems have suggested powerful

effects of temperature on species distributions and

local communities in freshwaters (e.g. Mouthon &

Daufresne, 2006). At present, however, most predic-

tions of community change are based on inferential

survey data that span large temperature gradients

across differences in latitude and altitude (e.g. Jacobsen,

Schultz & Encalada, 1997), confounding interpretation

of the temperature effect alone. Further, there are

virtually no empirical data to assess how increased

temperatures might alter ecosystem functioning,

despite theoretical predictions of changes in the

magnitude of energy fluxes, network topology and

altered body-size distributions within the food web

(Woodward & Warren, 2007). Studies on temperature

impacts on ecosystems have reached the top of the

global agenda due to recent and forecasted changes in

climate, and running waters are likely to be particu-

larly vulnerable to changing thermal regimes and

associated stressors (Giller et al., 2004a).

The overall aim of the present study was to

investigate how structural and functional attributes

of an entire stream ecosystem, encompassing all

trophic levels, responded to a strong environmental

gradient which was not confounded by a high degree

of natural variability or anthropogenic pressures. We

specifically wanted to study temperature effects as

this could provide valuable insights into how global

warming might change structural and functional

attributes of stream ecosystems and how best to

assess these changes. Our hypothesis was that tem-

perature, within the gradient studied, would have a

clearly detectable influence on both structure and

function. We used a unique natural ecosystem on

Iceland for this study which, due to biogeography,

consists of simple, species-poor stream ecosystems

with a geothermally induced range of temperatures

on a very small spatial scale.

Methods

Study site

The study was undertaken in geothermal active

Hengill area 30 km east of Reykjavik, Iceland

(64�03¢N: 021�18¢W, 350–420 m.a.s.l.). The area con-

tains a large number of mainly groundwater-fed

streams varying in geothermal influence and there-

fore also in temperature. Heating of the water is

indirect, either through heat transfer between

upwelling cold groundwater and surrounding geo-

thermally heated soils or through the stream bed.

The study area can be divided into two types of

landscapes: a flat plain dominated by different

species of grass and moss adjacent to a steeper,

more barren area characterised by hyaloclastite rock

of volcanic origin protruding through the sparse

grass vegetation. There are minimal allochthonous

inputs to the streams as there is no wooded vegeta-

tion present in the Hengill area. Except for landscape

changes, which date back more than 1000 years

(Saemundsson, 1967; Sæmundsson, 1995) there are

no anthropogenic pressures on the streams investi-

gated except for low density sheep grazing and

occasional hikers.

Selection of structural and functional attributes

We used a number of measures of community

structure including attributes relating to species rich-

ness, evenness, species overlap, abundance, cover or

biomass depending on the taxon. Functional attributes

2052 N. Friberg et al.

� 2009 Blackwell Publishing Ltd, Freshwater Biology, 54, 2051–2068

comprised two indirect and two direct measures of

ecosystem processes. We used analysis of functional

feeding groups (Cummins, 1973) as an indirect mea-

sure of function relying on deduction from structural

variables i.e. the species list (Gessner & Chauvet,

2002). Functional feeding groups represent an inter-

mediate level and a link between purely structural

and functional attributes. Likewise, we used stable

isotope ratios (d13C and d15N) as indirect functional

measures. They have previously been used to quan-

tify ecosystem change and can potentially indicate

which processes or components are most sensitive to

perturbation (Peterson & Fry, 1987; Bunn, Davies &

Mosisch, 1999; Udy et al., 2006). Direct measures of

ecosystem function used in this study included

organic matter transformation and algal biomass

accrual (Gessner & Chauvet, 2002; Giller et al., 2004b).

Sampling strategy

Ten first-order streams draining separate sub-catch-

ments were selected for this study. A pre-screening of

temperatures showed that they covered a gradient of

summer temperatures ranging from c. 7 to 24 �C. All

streams were situated within close proximity

(5–2000 m) to one other. In each stream, a 50 m

sampling reach was established and sampled during

the first week of August 2004 for physical–chemical

variables, algal biomass, macrophyte coverage, macr-

oinvertebrate community composition and fish. In

addition, samples were taken from all trophic levels

for stable isotope analysis. During a subsequent 28-

day period, from 9 August to 6 September 2004,

temperature was continuously measured and exper-

iments were undertaken using leaf bags and nutrient

diffusion substrates. Water chemistry was re-sampled

in September.

Physical and chemical variables

Temperature was measured at 30-min intervals in

each stream using Onset 32K StowAway TidBit

dataloggers in the period from 9 August to 6

September 2004. Data were retrieved using BoxCar

Pro 4 (version 4.0.7). At all reaches average width,

depth and coverage of dominant substrate types, e.g.

cobble ⁄ large pebble (>30 mm), small pebble ⁄gravel

(3–30 mm), sand (0.5–3 mm) and particulate organic

matter (POM), were recorded. The reach was divided

into 10 equally dispersed transects. Width (1 cm

accuracy) was measured for each transect. Depth

(1 cm accuracy) and substrate coverage were assessed

at four points across each transect corresponding to

0%, 25%, 50% and 75% of the stream width, totalling

40 points per reach. Slope was recorded in the

following manner. A measuring pole was placed at

the ends of a station and the difference in elevation

between the starting point and the end point of the

station measured with an optical range finder (Wild

NK01 Dumpy). Oxygen saturation, pH and conduc-

tivity were measured using a multi-probe sonde (YSI

600XLM). Water samples for chemical analysis were

obtained using a 1 L polyethylene bottle (not acid

washed) that was kept cool and dark until returning

to laboratory. Upon returning to laboratory, water

samples were immediately filtered and frozen

()18 �C) in new polyethylene bottles before subse-

quently being shipped to the analytical laboratory in

Denmark (NERI, Silkeborg). Samples were analysed

for nutrients (total-P, PO43), total-N, NH4

+, NO3)), the

macro-ions (SO42), Mg2+

, K+, Cl), Na+, Ca2+) and

total-Fe. Analyses were undertaken using Quick

Chem Fia+ 8000 and Perkin Elmer 4100 apparatus in

accordance with the following Danish ⁄European

standards (DS – Danish Standard 221 (1975): Total

N; DS – Danish Standard 238 (1985): Ca2+ and Mg2+;

DS – Danish Standard 263 (1982): Total-Fe, Na+ and

K+; DS – Danish Standard 292 (1985): Total P; DS ⁄EN

ISO 10304-1 (1996): Cl), PO43), NH4

+, NO3) and

SO42)).

Primary producers

Ten cobbles were collected randomly from each reach

and upon return to the laboratory placed in separate

containers, covered with 96% ethanol and stored for

extraction of chlorophyll at 5 �C in a refrigerator for

12–18 h. After extraction, the volume of ethanol in

which each stone was immersed was determined and

the chlorophyll concentration was measured using a

Shimadzu uv-160 spectrophotometer without correc-

tion for phaeophytin. The projected area of each stone

was measured to calculate the amount of mg chloro-

phyll per m2.

Macrophyte coverage and taxa composition were

estimated within a 25 · 25 cm2 in each of 40 points

within a reach used to assess depth and substrate

composition.

Structure and function in streams contrasting in temperature 2053


Macroinvertebrates

Nine Surber samples (sampler area 500 cm2; mesh

size 200 lm) were taken at each reach using stratified

random sampling by sub-dividing the reach into two

25 m sections in which five and four samples,

respectively, were taken using randomly generated

x-y positions. Samples were preserved in 70% ethanol

in the field and transported to the laboratory for

sorting and identification to the lowest possible

taxonomic level which was predominantly to species

in Gastropoda, Hirundinea, Plecoptera, Trichoptera,

Simulidae and Chironomidae. Other taxa were iden-

tified to family or order. Chironomidae was mounted

on slides using Hoyer’s medium (Anderson, 1954)

and identified using 400–1000· magnification. Other

taxa were identified using 50· magnification. Identi-

fications were based on Gislason (1979), Cranston

(1982), Wiederholm (1983), Dall & Lindegaard (1995),

Merritt & Cummins (1996), Nilsson (1996, 1997) and

Thorp & Covich (2001). In the present study, we use a

limited amount of the available data. More details on

macroinvertebrate community structure are provided

in Woodward et al. (G. Woodward, J.B. Dybkjær,

J. Olafsson, G. Gıslason, E. Hannesdottir & N. Friberg,

unpubl. data) covering 15 sites of which the 10 sites

reported here are a subset.

Fish

Species composition and population density in each

reach were estimated by electrofishing at 230 V using

a Honda EX 500 Inverter generator. Most reaches

were sampled using two independent runs with

successive removals. Reaches with no fish present

were sampled only once and reaches with low catch

efficiency were sampled using three runs. Population

densities were calculated using formulae from Serber

& Le Cren (1967).

Leaf bags

Arctic downy birch (Betula pubescens) is the only tree

native to Iceland that forms woodlands and forests

(Steindorsson, 1964). Leaves were hand picked from

Arctic downy birch trees in early August and subse-

quently dried at 60 �C to a constant weight. After

drying, fine mesh (200 lm) and coarse mesh (10 mm)

leaf bags containing 2.00 g DW each (weighed using a

Vibra CG-3000 to nearest 0.01 g) were made and

leaves were rewetted (1 h in tap water) prior to

packing to avoid breakage ⁄ leaf fragmentation. All leaf

bags (50 fine and 50 coarse mesh bags) were con-

structed on the same day using an identical proce-

dure. The leaf bags were stored cold (5 �C) and dry

before they were introduced to each of the 10 streams

the next day. Five fine and five coarse mesh leaf bags

were placed randomly throughout the reach and

attached to stream bed using tent pegs. After retrieval,

leaves were dried to a constant weight at 60 �C and

weighed with an accuracy of 0.01 mg.

Diffusion substrates

Nutrient diffusion substrates consisted of 90 mL plas-

tic pots containing either 2% agar (controls) or agar to

which N, P or N + P were added. Nitrogen was added

as 1 MM NaNO3 and phosphorus as 0.06 MM KH2PO4. The

colonisation surface of each pot covered an area of

20 cm2 and consisted of 200 lm nylon mesh. Twenty

pots (five replicates of each treatment and controls)

were placed in all 10 streams (200 in total) in the

downstream end of each 50 m reach, fitted in random

order into two stainless steel frames (10 pots in each)

which were anchored to the stream bed using large

stones. Immediately after substrates were taken from a

stream, the colonisation surfaces were removed from

pots and placed into individual plastic containers. The

surfaces were kept dark and cool (5 �C) until they were

extracted for chlorophyll in 96% ethanol using the

same procedure as for algal biomass on stones.

Stable isotope analysis

Samples for stable isotope analyses (SIA) were taken

in each 50 m reach covering all trophic levels and kept

in cooler boxes upon returning to the laboratory. In

the study, we focus on the relationship between

isotope signal (d13C or d15N) and temperature. More

detailed analysis of food web structure using SIA will

be published elsewhere.

Stream bed POM was collected using a Kajak corer

at deposition zones on the stream bed. At least 10

separate POM aggregations were sampled and pooled

in one container. In the laboratory, samples were wet

sieved through 1 mm and 0.063 mm mesh. All the

POM found was fine (FPOM) as no material was

retrained on the 1 mm sieve. Any fresh material or



visible macroinvertebrates were removed before three

subsamples were frozen for subsequent analysis.

Biofilm was sampled from 10 to 20 stones randomly

collected along the length of the reach. Stones were

gently rinsed in stream water after retrieval to remove

excess organic matter and subsequently brushed

vigorously to dislodge biofilm. The dislodged mate-

rial was collected in one container and subsequently

filtered in the laboratory on 20 lm filters to remove

excess water. Any macroinvertebrates present on the

filters were removed prior to freezing three subsam-

ples.

Plant material without roots was collected from

several different plants ⁄stands and transferred to

plastic bags, one for each species. Any dead tissue

was removed before freezing three subsamples of

each species. Between one and three species were

analysed from the streams with sufficient macrophyte

coverage.

Macroinvertebrates were sampled along the entire

reach by both kick-sampling using a handnet

and brushing collected stones. Excess plant material

and inorganic substrates were removed in the field

and one container from each stream was brought back

to laboratory. Samples were kept cool (5 �C) and

sorted within 24 h after collection and separated by

taxon. Macroinvertebrates were placed in Petri dishes

filled with tap water and kept over night (c. 12 h) at

5 �C to allow gut evacuation before samples of the

individual taxa were frozen. If possible three subs-

amples of each taxon were taken but in many cases it

was possible to get only enough material for one

sample or two subsamples. All sufficiently abundant

taxa were analysed, ranging from 2 to 7 taxa across the

stream sites studied.

All fish caught by electrofishing was killed and

retained for SIA analysis. In the laboratory dorsal

muscle was removed and subsequently frozen.

Between 1 and 27 individuals were analysed from

the individual streams depending on trout densities.

All material was freeze-dried and homogenised

before it was analysed at the Stable Isotope Facility,

University of Davis, California, U.S.A., using a Europe

Hydra 20 ⁄20 continuous flow isotope ratio mass

spectrometer (CFIRMS) (PDZ Europe, Cheshire,

U.K.) coupled with an element analyser. Standards

were injected directly into the CFIRMS before and

after sample peaks. Replicate laboratory standards

(sucrose 13C = )23.83) were analysed before and after

every 12 samples to determine the accuracy of 13C

values. Measurements errors averaged ±0.04o ⁄ oo.

Data treatment and statistical analysis

Evenness was calculated by dividing the Shannon–

Wiener value by the natural logarithm of the number

of taxa reported (Krebs, 1989). Species overlap (b-

diversity) between stream communities (Sørensen

similarity index; Sørensen, 1948) was calculated using

the equation: b ¼ 2C=ðS1 þ S2Þ with S1 and S2 repre-

sent the total number of taxa recorded in the first and

second community respectively, and C equals the

number of taxa common to both communities. The

Sørensen index is a very simple measure of beta

diversity, ranging from a value of 0 between commu-

nities with no faunal overlap to a value of 1, when

exactly the same taxa are found in both communities.

In the analysis we have a priori classified macroin-

vertebrates into functional feeding groups (FFGs)

using Merritt & Cummins (1996) and Thorp & Covich

(2001). Functional evenness was calculated in the

same manner as described above but using FFGs

instead of taxa.

Principal components analysis (PCA) based on a

correlation matrix was used to investigate relation-

ships between temperature and the other physical and

chemical variables measured (McCune & Mefford,

1999). In addition to temperature, PCA axis scores

were related to structural and functional variables as

were nutrient concentrations and ratios from the

diffusion substrate experiments. Relationships be-

tween temperature and the other physical–chemical

variables, structural and functional variables were

analysed statistically by linear or polynomial regres-

sion (Snedecor & Cochran, 1989). Pearson correlation

coefficients were calculated to test if statistically

significant relationships existed. Replicate samples

within a reach (Surber samples, stones for algal

biomass, leaf bags and nutrient diffusion substrates)

were averaged before applying the correlation anal-

ysis giving n = 10 as the streams studied were

independent of each other. In the SIA samples, results

from more than one macrophyte or macroinvertebrate

taxon (including FFGs) could occur per stream in

which case they would each enter the analysis as

separate data values i.e. n > 10. Effects of treatment

and temperature with regard to diffusion substrates

and leaf bags were tested using a randomised block



design with covariance (Littell et al., 1996) where

streams were randomised blocks and temperature

covariate. Pairwise tests between treatment means

were undertaken using a t-test for independent

samples in accordance with Littell et al. (1996). If data

were not normal distributed they were log(x + 1)

transformed before analysis.

Results

Temperature regime

Mean water temperature ranged from 6.8 to 23.5 �C in

the 10 streams investigated during the study period in

August ⁄September (Table 1). Mean diurnal amplitude

ranged from 0.8 to 6.2 �C. Some streams showed 10 �C

or more difference in minimum and maximum tem-

perature during the study period. Differences in

diurnal amplitudes could be attributed to distance to

source and hence the degree of contact with atmo-

spheric conditions. IS 13 and IS 8 are close to springs

(<20 m) and both exhibit limited fluctuations in

temperature. In contrast, IS14 and IS1 have larger

catchments and exhibit more pronounced tempera-

ture variation as a result of factors including influx of

water from surface run-off and solar irradiation.

Importantly, however, there was no consistent pattern

between mean diurnal amplitudes, temperatures

ranges recorded in the study period and mean stream

temperatures. This reflects the fact that the reaches

investigated were fed by either cold or warm springs.

In the following analyses mean temperature in the

study period was used as the temperature variable to

which physical–chemical variables and structural–

functional attributes of the streams were compared.

Physical and chemical variables

The PCA of the physical and chemical variables of the

10 streams (Fig. 1, Table 2) revealed that axis 1,

explaining 29.4% of the variability, was primarily

correlated temperature, N : P ratios, phosphorous

concentrations and K+. Warm and P-rich streams

were found on one side of the ordination but

temperature and TP ⁄PO43) were clearly separated in

the ordination space. Axis 2 explained 20.5% of the

variability, and was correlated with depth and slope

as well as NO3) and Ca2+ (Fig. 1).

The streams were all small and shallow (Table 2).

Variations in slope and substrate reflected the sur-

rounding topography which was either in the valley

floor (lower slopes, finer substrates) or valley slopes

(steeper slopes, coarser substrates). There was no

significant correlation between any of the habitat

features recorded and temperature (P > 0.05; Table 3).

Nutrient levels were generally low (Table 2). There

was no significant relationship between either Total-P

Table 1 Mean temperature, mean diurnal amplitude, minimum

and maximum temperature values recorded during the sam-

pling period (9 August to 6 September 2004) in the 10 streams

studied. Values are based on a raw data set of 1344 continuous

measurements (30-min interval) from each stream and are in �C.

The streams are listed with increasing mean temperature

Stream

Mean

temperature

Mean

diurnal

amplitude Minimum Maximum

IS 13 6.8 1.5 6.4 7.7

IS 7 7.7 2.5 5.8 10.2

IS 14 9.5 2.8 5.9 17.1

IS 11 11.2 4.4 7.5 16.3

IS 12 14.8 6.2 9.1 23.2

IS 9 14.9 4.2 10.6 20.0

IS 6 19.5 2.6 13.8 22.3

IS 5 20.5 2.0 17.4 22.2

IS 1 20.9 6.1 12.9 26.2

IS 8 23.5 0.8 22.0 23.9

Axis 1

0

0.2

0.4

0.6

0.8

Axi

s 2

1 5

6

7

8

9

11 13

14

12

Temp.

TN:TP Habitat width

Depth

Area

Slope % Cobble

% Mud

Oxygen

% Sat

PO4

NH4

NO3

SO4

Cl

TP

TFe

TN

Na+ K+

Ca2+

Mg2+

DIN:DIP

–0.6 –0.6

–0.4

–0.4

–0.2

–0.2

0 0.2 0.4 0.6 0.8

Fig. 1 Principal component analysis (PCA) of physical and

chemical variables (see Table 2) in the 10 streams investigated.

Position of streams in ordination space is indicated by their

numbers.



or Total-N and temperature (Table 3). TN : TP ratios

were significantly negatively related to temperature.

Inorganic P constituted the main part of Total-P.

Concentrations of inorganic N were very low compared

to Total-N (Table 2). Consequently, all DIN : DIP

ratios were low, ranging from 1.24 to 2.99. There

were no significant relationships between inorganic P

(PO43)) or N (NH4

+ and NO3)) and temperature

(Table 3) while the DIN : DIP ratio was significantly

negatively related to temperature (r = )0.736;

P = 0.015).

There was no significant relationship between any

of the other chemical variables measured and tem-

perature with the exception of K+ (Table 3) and

oxygen concentration that, as expected, was nega-

tively correlated to temperature.

Structural attributes

Algal biomass ranged from 35 to 377 mg Chl. m)2.

There was no significant relationship with tempera-

ture or PCA axis 1 or 2 (P > 0.05). Macrophyte cover

varied considerably among stream sites but was

unrelated to temperature (P > 0.05) as the coldest (IS

13) and the three warmest streams (IS1, IS 5 and IS 8)

had the highest coverage. Common macrophyte taxaTa

ble

2K

eyp

hy

sica

lan

dch

emic

alfe

atu

res

of

the

ten

stre

ams

inv

esti

gat

ed

Str

eam

Wid

th

(m)

Dep

th

(m)

Slo

pe

(cm

m)

1)

% Co

bb

les

Ox

yg

en

(mg

L)

1)

To

tal-

P

(mg

L)

1)

To

tal-

N

(mg

L)

1)

TN

:T

P

(mo

lar)

PO

43)

(mg

L)

1)

NO

3)

(mg

L)

1)

NH

4+

(mg

L)

1)

DIN

:DIP

(mo

lar)

SO

42)

(mg

L)

1)

Cl)

(mg

L)

1)

To

tal-

Fe

(mg

L)

1)

Na+

(mg

L)

1)

K+

(mg

L)

1)

Ca2

+

(mg

L)

1)

Mg

2+

(mg

L)

1)

IS13

0.83

0.16

3.1

3410

.10.

016

0.30

41.4

60.

018

0.01

50.

009

2.95

15.4

5.01

0.25

9.1

0.53

13.6

4.5

IS7

1.61

0.04

19.6

4510

.55

0.02

60.

1714

.46

0.02

50.

016

0.01

32.

574.

575.

635

0.00

97.

20.

507.

72.

1

IS14

1.47

0.06

0.5

6711

.06

0.01

00.

1328

.75

0.01

70.

009

0.00

92.

349.

953.

740.

047.

90.

3111

.53.

8

IS11

1.38

0.05

6.4

3610

.83

0.01

10.

1122

.11

0.02

00.

009

0.00

91.

992.

114.

610.

0333

.31.

1813

.57.

9

IS12

5.32

0.17

5.6

110

.10.

017

0.14

19.3

50.

017

0.00

90.

014

2.99

9.24

4.43

50.

5711

.20.

9511

.94.

0

IS9

0.63

0.04

19.7

419.

920.

037

0.13

7.77

0.03

20.

012

0.01

21.

667.

324.

680.

3814

.10.

939.

32.

6

IS6

0.95

0.07

6.9

479.

020.

044

0.24

12.0

60.

032

0.00

90.

009

1.24

10.4

4.07

50.

2620

.51.

539.

02.

7

IS5

1.05

0.10

38

9.06

0.03

60.

127.

370.

029

0.00

90.

009

1.37

12.6

3.95

0.41

18.3

1.33

11.1

2.6

IS1

1.08

0.12

1.8

89.

350.

024

0.14

12.9

00.

024

0.00

90.

009

1.66

15.2

5.38

0.10

19.1

1.21

13.1

3.9

IS8

1.75

0.10

14.5

497.

450.

027

0.15

12.2

90.

027

0.01

30.

009

1.80

7.62

3.68

0.00

921

.81.

6710

.72.

8

Table 3 Pearson correlation coefficients between temperature

and physical and chemical variables

Variable r P-value

Width ⁄ area )0.008 0.9819

Depth 0.152 0.675

Slope )0.030 0.935

%Cobbles )0.325 0.359

%Mud )0.133 0.714

Oxygen sat. 0.583 0.077

Oxygen conc. )0.853 0.002

Total-P 0.591 0.072

Total-N )0.285 0.425

TN : TP )0.710 0.021

PO43) 0.597 0.068

NO3) )0.454 0.188

NH4+ )0.277 0.439

DIN : DIP )0.736 0.015

SO42) 0.224 0.534

Cl) )0.424 0.223

Total-Fe 0.151 0.676

Na+ 0.464 0.177

K+ 0.886 <0.001

Ca2+ )0.072 0.843

Mg2+ )0.323 0.363

Significant values (P < 0.05) in bold.



in both cold and warm streams were Fontinalis

antipyretica and Callitriche sp. and there was no

significant relationship between macrophyte taxa

richness and temperature or PCA axis 1 or 2

(P > 0.05).

Macroinvertebrate density ranged from 3000 to

16 000 individuals m)2 (Fig. 2a) and was significantly

correlated to temperature. Between 12 and 22 macr-

oinvertebrate taxa were found, dominated by Chiro-

nomidae (16 species out of 35 taxa in total).

Chironomidae were especially abundant in the colder

streams in some of which Eukiefferiella minor consti-

tuted up to 50% of the individuals recorded. Density

of the only blackfly species found, Simulium vittatum,

was highest in the warmer streams with only low

densities recorded in colder streams. Even more

pronounced was the distribution of the snail Lymnaea

peregra, which was found only in streams with a

temperature above 14 �C. They dominated five out of

six warm streams sampled, constituting up to 63% of

the total number of individuals. Taxa richness showed

a tendency for a unimodal response with the highest

number of taxa in the mid-range temperatures

(Fig. 2b) whereas evenness and species overlap were

strongly negatively related to temperature (Figs 2c

and 3). There were no significant relationships

between any of the macroinvertebrate structural

attributes and PCA axis 1 or 2 (P > 0.05).

The only fish found in the streams was Salmo trutta

in densities up to 0.3 individuals m)2. Highest den-

sities were found in the warmer streams with the

exception of IS 8 (the warmest) in which densities

were very low (0.01 individuals m)2). Three streams

were without fish altogether (IS 7, IS 9 & IS 11) and

there was a tendency for fish densities to be positively

correlated with temperature (r = 0.59; P = 0.074).

However, fish density was negatively correlated to

PCA axis 2 (r = )0.655; P = 0.040) reflecting impor-

tance of habitat with more fish found in deeper and

less steep streams.

Fig. 2 Macroinvertebrate (a) density (ind m)2) (r = 0.65;

P = 0.043), (b) richness (r = 0.64; P = 0.14) and (c) evenness

(r = )0.92; P < 0.001) correlated with temperature in the 10

streams investigated.

Fig. 3 Macroinvertebrate species overlap calculated using

Sørensen (1948) correlated with the pairwise temperature

differences between the ten streams investigated (r = 0.79;

P < 0.0001).



Indirect functional attributes: feeding groups and

isotope ratios

The streams investigated were completely dominated

by filter-feeders and scrapers. Densities of filter-

feeders ranged from 147 to 10 447 individuals m)2

with significantly higher numbers found at the high-

est temperatures (Fig. 4a). Scraper densities ranged

from 1100 to 7400 individuals m)2 and appeared

unrelated to temperature (Fig. 4b). The other func-

tional feeding groups were found in much lower

densities. Density of collector–gatherers increased,

albeit not significantly, with temperature (r = 0.62;

P = 0.055). Almost no shredders (<10 individu-

als m)2) were found in the four warmest streams.

Densities of predators as well as functional evenness

were not related to temperature (P > 0.05). There were

no significant relationships between functional feed-

ing groups and PCA axis 1 or 2.

From d15N values three or four trophic levels were

identified depending on the presence of fish. Average

and ranges of d15N ratios in macroinvertebrate scrap-

ers were: )1.92 ()4.01 to 0.42), in macroinvertebrate

predators: )0.049 ()2.09 to 2.62) and in fish muscle:

3.35 (1.80 to 4.32). There was a clear tendency for a

positive relationship between d15N and temperature

for several of the taxa sampled across the trophic

levels identified. IS 8, the warmest stream, showed

consistently lower d15N levels than the other warm

streams thereby weakening this relationship. Biofilm

dN content showed a non-significant positive rela-

tionship with temperature which became significant if

IS8 was removed from the analysis (r = 0.67;

P = 0.047). Similarly, the relationship between scraper

d15N content and temperature became significant

(r = 0.72; P = 0.01) when IS8 was omitted. In contrast,

a positive, significant relationship was found between

macroinvertebrate predator d15N content and temper-

ature using all data (Fig. 5) and this relationship was

considerably improved when IS8 was removed. A

similar positive trend between temperature and both

fish muscle and macrophytes was found, but the small

number of samples limited the power of the test.

There was no significant relationship between d15N

content and temperature for either POM or detriti-

vores (P > 0.05) and between d15N content at each

trophic level and PCA axis 1 or 2. Furthermore, d13C

content for all elements investigated was unrelated to

temperature (P > 0.05).

Fig. 4 Densities of the numerically most important

macroinvertebrate functional feeding groups (a) filter feeders

(r = 0.81; P = 0.005) and (b) scrapers (P > 0.05) correlated with

temperature in the 10 streams investigated.

Temperature (°C)

δ15

N

4 6 8 10 12 14 16 18 20 22 24 26

–2

–1

0

1

2

Fig. 5 d15N ratios in macroinvertebrate predators correlated

with temperature in the 10 streams investigated (r = 0.68;

P = 0.004). Note that number of points is >10, reflecting that

some streams had more than one predatory species.



Direct measures of function: litter decomposition and

algal biomass accrual

Decomposition rates in both fine and coarse litter bags

were significantly correlated with temperature (Fig. 6-

a,b). However, leaf mass lost was significantly differ-

ent between the two types of bags [F(1,9) = 26.38;

P < 0.001] with a significant temperature effect

[F(1,9) = 9.38; P = 0.0135]. Loss in coarse bags over

28 days ranged between 0.53 and 1.26 g DW which

was significantly higher than loss in fine mesh bags

(t = )5.16; P < 0.001), ranging from 0.39 to 0.71 g DW.

Decomposition rates of both fine and coarse litter

were negatively related to PCA axis 1 (r = )0.723;

P = 0.018 and r = )0.766; P = 0.010 respec-

tively).There were no other significant relationships

between leaf litter decomposition and PCA axis 2.

Algal biomass accrual on the diffusion substrates in

the 10 streams showed a significant response to

nutrient additions [F(3,24) = 4.38; P = 0.014] and a

weak suggestion of a temperature effect

[F(4,24) = 2.57; P = 0.064]. Algal biomass accrual was

highest in the N + P treatment followed by N whereas

P additions did not have any effect when compared

with controls (Fig. 7). Consequently, algal biomass

accrual was significantly higher in N + P treatment

compared with N alone (t = )3.20; P = 0.004) and the

N treatment had significantly higher algal biomass

than both P treatment and controls (t = 2.70; P = 0.013

and t = )2.99; P = 0.006 respectively). There were no

significant linear relationships between algal biomass

in any of the nutrient treatments and controls with

temperature. However, by subtracting algal biomas-

ses in the N treatments from those found in N + P

treatments, a clear significant effect with temperature

was found (r = 0.75; P = 0.01, Fig. 8). This indicated

that the additional effect of adding P in the N + P

treatment was temperature-related.

Algal biomass accrual in most treatments was

unrelated to TN : TP and DIN : DIP ratios with the

exception of the N treatment that was positively

related to the TN : TP ratio (r = 0.817; P < 0.01) and

N + P–N which was negatively related to both ratios

Fig. 6 Leaf litter breakdown rates during the 28-day

experimental period in (a) fine (200 lm) mesh bags (r = 0.79:

P = 0.007) and (b) coarse (10 mm) mesh bags (r = 0.67;

P = 0.034) correlated with temperature in the 10 streams

investigated.

Fig. 7 Mean algal biomass accrual (±1SE, n = 10 streams) on

nutrient diffusion substrates to which nitrogen (N), phospho-

rous (P) or nitrogen and phosphorous (N + P) were added

compared to controls (C). Accrual was significantly higher in

N + P treatment compared with N alone (t = )3.20; P = 0.004)

and the N treatment had significantly higher algal biomass than

both P treatment and controls (t = 2.70; P = 0.013 and t = )2.99;

P = 0.006 respectively).



(r = )0.632; P = 0.049 and r = )0.725; P = 0.018

respectively).

Algal biomass accrual in the N treatment was

positively related to PCA axis 1 (r = 0.674; P = 0.033)

while there was a negative relationship with N + P–N

and PCA axis 1 (r = )0.739; P = 0.015). There were no

other significant relationships between algal biomass

accrual and PCA axis 1 or 2.

Discussion

We found clear changes in both macroinvertebrate

community structure and diversity with temperature.

Jacobsen et al. (1997) found an increase in insect

species richness with increasing maximum tempera-

ture in temperate and tropical streams. This is in

contrast to our findings that taxa richness was fairly

constant (with a tendency for maximum at mid-

temperatures) across the temperature gradient with a

significant decline in evenness and species overlap

with increasing temperature between any two-stream

comparisons. However, the streams investigated were

very species-poor with only 35 taxa found including

10 families of insects. This can be attributed to island

biogeography and evolutionary history. Iceland is

isolated in the North Atlantic and relatively young

(Olafsson et al., 2002; Denk, Grımsson & Kvacek, 2005;

Gislason, 2005). As the overall climate on Iceland is

sub-arctic, a possible reason that we did not find an

increase in species richness with temperature was the

very limited species pool available. Adaptations to

high temperatures are unlikely as both dispersal from

the continents and speciation within the Icelandic

fauna are minimal (Lindegaard, 1992; Sadler, 1999;

Gordon, Covich & Brasher, 2003; Gislason, 2005).

Indirect functional attributes: feeding groups and

isotope ratios

We found that number of filter-feeders increased

exponentially with temperature, which primarily

reflected the increased number of Simulium vittatum.

Scrapers were the dominant functional feeding group

but did not show a significant positive relationship

with temperature as the two coldest and three

warmest streams had very similar numbers (Fig. 4b).

In the cold streams, however, all scrapers were

chironomids whereas in the warmest streams

Lymnaea peregra was completely dominant. In this

study we did not measure biomass but given the

much lower average individual weight of chironomid

species compared with that of Lymnaea (J. Olafsson,

unpubl. data), it can be predicted that scraper

biomass would increase markedly with temperature.

From a functional perspective this could indicate

increased quantities of algae ⁄biofilm available to

consumers with increasing temperature. The lack of

relationship between algae biomass on stones and

temperature suggests that grazing rates, and conse-

quently algal productivity, is higher in the warm

streams.

We investigated if isotopic ratios of d13C and d15N

showed a pattern that could be related to temperature

in a similar manner to what has been done along

gradients in nutrient enrichment (see below). The

increase in d15N with temperature across trophic

levels could both reflect dietary intake and tempera-

ture per se. d15N content of consumers such as

macroinvertebrates and fish reflects the content of

their diet (Olive et al., 2003) while an alternative

explanation could be changes in growth rates and

turnover of nitrogen with temperature as shown for

fish (Koskela, Pirhonen & Jobling, 1997; Vøllestad,

Olsen & Forseth, 2002; Sweeting et al., 2007). Increased

growth can lead to faster incorporation of dietary d15N

into fish tissue (Miller, 2000) although Sweeting et al.

(2007) found the opposite result. They explained this

by better use of nitrogenous compounds and reduced

Fig. 8 Difference in algal biomass accrual on nutrient diffusion

substrates between N + P substrates and N-alone substrates

as a function of temperature in the 10 streams investigated

(r = 0.75; P = 0.01).



excretion. Furthermore, McIntyre & Flecker (2006)

found high nitrogen turnover rates in four tropical

primary consumer species which they attributed to

temperature. Clearly, there is some controversy as to

how temperature influences d15N. We found d15N to

increase with temperature in most trophic levels,

especially if the warmest stream was excluded from

the analysis. We cannot determine if this increase is

caused by direct temperature-induced isotope frac-

tionation or indirectly mediated through the resource

base. There was a relationship, which became signif-

icant when the warmest stream was excluded from

the analysis, between increased d15N and temperature

in biofilm samples. Biofilm is likely to be important

food base in the food webs we studied because dead

organic matter is very limited and macrophytes are

rarely eaten by lotic macroinvertebrates (Giller &

Malmquist, 1998). This was supported by the macro-

phyte d13C signals, which in most cases were very

different from the signal found in primary consumers

(N. Friberg, unpubl. data). Fagan et al. (2002) found

that nitrogen limitation could induce consumers to

preferentially feed on more nutrient-rich food sources.

Given that N limitation increased with temperature

when considering N : P ratios this might explain our

findings. However, the direct response to N addition

in the diffusion substrates appeared not to be strongly

temperature dependent, suggesting that the differ-

ences in fractionation are less likely to reflect N

limitation. More work is needed to elucidate if this is

the case, including measurements of d15N content in

the stream water. Likewise, the present data do not

provide any insight to why d15N content is decreasing

in IS8, the warmest stream. Other studies have shown

that d15N content could potentially be a sensitive

indicator for nutrient enrichment (Harrington et al.,

1998; Vander Zanden et al., 2005) and our findings

suggest that it might also be an indicator for temper-

ature effects but a larger data set, covering more

systems and larger temperature range, is needed to

test if this is the case.

Direct measures of function: litter decomposition and

algal biomass accrual

As would be expected from previous studies, leaf

litter decomposition in both fine and coarse mesh bags

was significantly correlated with temperature. The

increased decomposition rates in fine mesh bags at

higher temperatures reflects higher microbial activity.

This has been reported previously when comparing

litter decomposition between temperate and tropical

climates (e.g. Irons et al., 1994; Abelho, Cressa &

Graca, 2005; Bergfur et al., 2007). Leaf mass loss was

almost twice as high in coarse mesh bags as in fine

mesh bags but the linear relationship was weaker,

indicating a larger inter-stream variability than found

for fine mesh bags. Increased breakdown of litter has

often been attributed to shredding activities of macr-

oinvertebrates (e.g. Benfield & Webster, 1985). We

found very few shredders in our streams and hardly

any in the four warmest streams. However,

Lymnaea peregra was frequently found in leaf bags in

the warmest streams and appear to be efficient in

scraping off leaf tissue only leaving the veins (N.

Friberg, pers. obs.). The increased breakdown rates in

the coarse mesh bags can at least partly be attributed

to macroinvertebrate activity although physical abra-

sion could also have played a part and is the most

likely reason for the increased variability in inter-

stream breakdown rates when comparing breakdown

in coarse and fine mesh bags. Leaf litter breakdown

was significantly related to PCA axis 1 which was a

combined temperature and nutrient axis. Previous

studies have shown that nutrients promote leaf litter

breakdown rates (e.g. Elwood et al., 1981; Pascoal

et al., 2003) so the higher P concentrations found in the

warmer streams might have had an additional effect

on leaf litter breakdown rates.

Results from diffusion substrate experiments

showed that the streams were limited primarily by

N independently of temperature. This was not sur-

prising given that the DIN : DIP ratio ranged from

1.24 to 2.99 (molar) in water samples and a ratio of

18 : 1 (molar) is considered optimal for benthic algae

(Kahlert, 1998). N limitation has been reported from

several other freshwater ecosystems although P is

often considered the main limiting nutrient (Moss

et al., 2005; Demars & Edwards, 2007). The low N : P

ratio found in the study streams is likely to reflect

volcanic origin of the underlying rock (Gislason,

Arnorsson & Armannsson, 1996; Gıslason &

Eirıksdottir, 2004). In a nutrient diffusion substrate

experiment similar to our own, Tank & Dodds (2003)

found that streams along a long latitudinal gradient

investigated were primarily N-limited. They also

found a negative relationship between algal biomass

and DIN : SRP ratios which were generally much



higher than in our studied streams. Surprisingly, we

found a positive relationship between algal biomass

accrual in N treatments and DIN : DIP and a negative

relationship with the additional effect of P in the

N + P treatment and DIN : DIP. In both cases, the

opposite relationship would have been expected if the

DIN : DIP ratio was the main driver of algal biomass

accrual. However, a number of studies have also

found none of the predictable effects of nutrient

additions even in streams where ambient concentra-

tion and ratios of nutrients suggested limitation (e.g.

Snyder et al., 2002; Tank & Dodds, 2003). A meta-

analysis of 85 nutrient addition experiments showed

that periphyton is both highly controlled by top-down

(grazing) and bottom-up (nutrients) mechanisms

(Hillebrand, 2002) so one explanation for the lack of

a relationship with temperature could be a difference

in grazing pressure. When interpreting the nutrient

results we need to consider uncertainty in the analyt-

ical results. While Total-P and Total-N are fairly stable

over time the inorganic nutrients can react or get

adsorbed to particles prior to analysis. Because we

were working in a remote area our water samples had

to be frozen and transported by ship to Denmark for

analysis. This might have influenced the results.

Indeed we found that ortho-phosphate in a few cases

were slightly higher than total-P but this is more likely

to reflect general analytical problems (Neal, Neal &

Wickham, 2000) as the opposite (i.e. ortho-phosphate

entering the Total-P pool) would have been expected

if the main issue was storage time. However, there is

no reason to believe that the order of magnitude

measured in our study is not correct and that a

generally very high proportion of water column

phosphorous was bioavailable and that DIN : DIP

(and most TN : TP) ratios were low.

Highest algal biomass accrued in the N + P treat-

ment which is in accordance with previous findings.

Whole-stream N and P additions have been shown to

increase periphyton accrual rates (Perrin, Bothwell &

Slaney, 1987). A large scale meta-analysis illustrated

that freshwater, marine and terrestrial ecosystem are

similar in terms of N and P limitation and simulta-

neous addition of both N and P produced synergistic

responses in all three ecosystem types (Elser et al.,

2007). Francoeur (2001) found in a meta-analysis of

lotic nutrient amendment experiments that multispe-

cies communities are unlikely to be limited by a single

nutrient. Single species have been shown to respond

to specific N and ⁄or P concentrations or molar N : P

ratios (Tate, 1990). It is therefore very likely that the

different species constituting the periphyton commu-

nity in our streams have different nutrient demands

and that species composition is influenced by tem-

perature. This contention is supported by the finding

that when the response to nitrogen addition alone was

subtracted from the N + P treatment response, the

influence of temperature became highly significant

reflecting that colder streams primarily responded to

the added nitrogen in the combined N + P treatment

whereas warmer streams could use the phosphorous

as well. These findings suggest that productivity

potential of algae is higher in the warmer streams

making P the limiting nutrient when N is added. This

is unlikely to be an effect of differences in release rates

from the nutrient diffusion substrates with tempera-

ture. Rugenski et al. (2008) found a <3% increase in

release from nutrient diffusion substrates with tem-

perature (4–21 �C) compared with an expected (from

published studies, Op. cit. Rugenski et al., 2008)

increase in periphyton growth rates of c. 300% within

the same temperature range. The temperature range is

very similar to the range found in our study,

supporting the notion that differences found is not

an artefact of changed release rates from the diffusion

substrates. The higher productivity is thus most likely

to be an effect of temperature directly, affecting

periphyton growth rates. Furthermore, at the reach

scale the warmer temperature would be expected to

promote increased rates of mineralisation and possi-

bly a tighter spiralling of nutrients because of

increased uptake by periphyton. We found no effect

of temperature on algal biomass on stones but this

might reflect grazing pressure as discussed above.

Interpretation of results and confounding variables

Our study showed both structural and functional

ecosystem responses to the gradient in temperature

which appeared to be largely not confounded by other

environmental variables. Physical–chemical variables

measured were overall not related to temperature and

hence they appear not to confound interpretation of

results. However, it has to be acknowledged that the

data set only comprised 10 streams which limit the

ability to statistically tease apart more complicated

relationships among variables. Furthermore, the PCA

only explained 50% of variability in the abiotic data set.



K+ concentration was strongly correlated to tem-

perature which is a result of increased temperature-

induced weathering of rocks (Arnorsson, 1983). It is,

however, unlikely that the K+ concentrations mea-

sured [ranging from 0.31 to 1.67 mg L)1, which is well

within the concentration range found in undisturbed

streams elsewhere (e.g. Giller & Malmquist, 1998)]

would have had any impact on the biota.

Relationship between structure and function

The structural and functional attributes that re-

sponded to the temperature gradient gave comple-

mentary information about the stream ecosystems

with both sets of attributes responding to temperature

in a similar manner to what has been observed along

enrichment gradients (e.g. Pascoal et al., 2003; Hering

et al., 2006). The macroinvertebrate community

showed clear changes in structure along the temper-

ature gradient, corresponding to what has previously

been reported in streams moderately influenced by

organic pollution (e.g. Rosenberg & Resh, 1993), with

an increase in density and decrease in diversity,

driven by increased density of a few dominant

species. The change in structure was mirrored in the

functional feeding groups with especially filter-feed-

ers becoming important with increased temperature

which is also similar to findings in enriched systems

as exemplified by lake outlets (e.g. Malmquist &

Eriksson, 1995). Decomposition rates, response to

nutrient additions and d15N increased with tempera-

ture, which have also been observed along enrichment

gradients (Perrin et al., 1987; Vander Zanden et al.,

2005; Gulis, Ferreira & Graca, 2006). Loss of diversity

is essential to assess in a conservation context whereas

the functional attributes give us more insight into

important ecosystem processes linked to rates and

fluxes of matter and energy. Knowledge of the latter is

essential when predicting ecosystem responses to

anthropogenic pressures and the possible loss of

ecosystem services. In conclusion, the question if we

should use structural or functional attributes when

assessing ecosystem integrity is not choosing one or

the other but include both measures, a contention also

supported by Gessner & Chauvet (2002).

What about the attributes that did not respond? Some

structural attributes appeared to be more confounded

by habitat characteristics as exemplified by both

macrophytes and fish. Occurrence of macrophytes

was restricted by substrate and depth whereas fish

were excluded from certain streams by a combination

of barriers to migration and water depth. However,

with regard to densities of S. trutta there appeared to be

a positive effect of the increased productivity with

temperature in all but the warmest stream. In this

study, we did not include community composition of

algae but only biomass which did not show any

relationship with temperature. However, preliminary

results have shown that species diversity of diatoms

show a decline similar to that of macroinvertebrates

with increasing temperature (R. Guðmundsdottir,

unpubl. data) which might also explain the tempera-

ture-dependent response to nutrient additions.

Climate change context

The streams at Hengill are an almost ideal model

systems in which to investigate impacts of climate

change on aspects of lotic ecosystem structure and

function. However, extrapolating our results to other

climatic zones and more species rich systems should

be done with care given the special characteristics of

the Icelandic streams (low diversity, spatial isolation).

They are simple systems providing a relevant tem-

perature gradient that is not to any large degree

confounded by other environmental variables. If our

results are interpreted in a climate change context we

can predict similar changes to what is often found in

moderately polluted freshwater ecosystem: domi-

nance of a few macroinvertebrate taxa and increased

productivity. Our nutrient additions, albeit on a very

small scale, furthermore suggest that the response to

nutrient loading could increase with temperature

thereby making freshwater ecosystems more suscep-

tible to eutrophication.

Acknowledgments

We wish to thank G.V. Ingimundardottir for excellent

field and laboratory assistance on Iceland, D. Nederg-

aard at NERI for assistance with chemical analyses and

D. Harris at University of California for SIA analysis.

We furthermore wish to thank J. Bergfur, B. Demars, M.

Futter and two anonymous referees for constructive

criticism on earlier drafts of this manuscript. The study

was supported by the EU Eurolimpacs project (GOCE-

CT-2003-505540), the Macaulay Institute and the Scot-

tish Government Rural and Environment Research



Directorate (Waters Objective, Programme 3), contrib-

uting to the CCT on climate change.

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(Manuscript accepted 18 March 2009)



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