Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 137

Hering et al . 2006). Warm streams under similar nutrient loading showed
changes similar to those found in moderately eutrophic streams: dominance of
a few macro-invertebrate taxa and increased productivity. Experimental nutrient
additions suggested, furthermore, that the response to higher nutrient loading
could increase with temperature, thereby making freshwater ecosystems more
susceptible to eutrophication, similar to the responses in lakes described above.
Macro-invertebrate densities ranged from 3,000 to 16,000 individuals m −2
and were positively correlated with temperature. Communities had between 12
and 22 macro-invertebrate taxa and were dominated by Chironomidae (16
species out of 35 taxa in total). Chironomidae were especially abundant in the
colder streams; in some, Eukiefferiella minor constituted up to 50% of the
individuals recorded. The 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 . It was only found in streams with a temperature above 14 °C. There,
however, it dominated the macro-invertebrate community in five of the six
streams sampled, constituting up to 63% of the total number of individuals.
Taxon richness tended to be unimodal with the highest number of taxa occurring
at mid-range temperatures, whereas evenness was strongly negatively related to
temperature (Fig. 6.10). The invertebrate species pool is limited in Iceland.
Therefore, warming in the more species-rich streams of mainland Europe may
show stronger effects, which may further exacerbate eutrophication symptoms.
Litter decomposition rates of Arctic downy birch ( Betula pubescens ) were used
as an indicator of ecosystem functioning. Decomposition in both fine (mesh size:
200

μ

m) and coarse (mesh size: 1 cm) litter bags placed for 28 days in the streams
was significantly correlated with temperature (Fig. 6.10). The leaf mass lost (initial
weight 2.00 g DW) also differed significantly between the two types of bags, and
a significant temperature effect was observed. Loss in coarse bags ranged between
0.53 (26.5%) and 1.26 g (64.5%) DW (28 days) −1 , being higher than the loss in
fine mesh bags (0.39 (19.5%) to 0.71 g (35.5%) DW (28 days) −1 ).
Nutrient-diffusing substrata consisting of plastic pots (surface area 20 cm 2 ,
covered with 200-

μ

m nylon mesh) containing either 2% agar (controls) or agar
with N, P or both added were used in the same 10 streams to investigate the
combined effects of temperature and nutrient additions. Algal biomass that grew
on the pots was significantly higher in the N + P treatment (mean: 93 mg chl
m −2 ) compared with N alone (mean: 53 mg chl m −2 ), and the N treatment had
significantly higher algal biomass than both the P treatment and the controls,
which did not differ. There were no significant relationships between temperature
and algal biomass in any of the nutrient treatments or controls. However,
compared with the other cold streams, the coldest stream investigated showed a
deviant pattern, with high algal biomass on all substrates, including the controls,
and a particularly strong response to both N and N + P additions. If this stream
was excluded from the analysis, there was a clear trend in the N + P treatment
towards higher algal biomass at raised temperature. Subtracting algal biomasses
in the N treatments from those in the N + P treatments also revealed a clear
temperature effect (Fig. 6.10), indicating that the additional effect of adding P in
the N + P treatment was temperature sensitive.