Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 131

As for the UK experiments, the Danish mesocosm results suggest that warming
exacerbates symptoms of eutrophication, in this case with substantially higher
phytoplankton biomass occurring both at low nutrient level and in the A2 scenario
with high nutrient levels. A shift to stronger dominance of filamentous algae or
macrophytes was observed in the A2 scenario with high nutrient levels. The season
with macrophyte growth was, however, prolonged at higher temperature, and the
biomass of submerged macrophytes did not decline at elevated temperature in these
mesocosms. In fact, higher temperature gave P. crispus , commonly a dominant
species in spring, a window of opportunity that led to a higher average biomass,
whereas no clear effect was seen for E. canadensis (T.L. Lauridsen et al . unpublished
data). The preliminary results on the oxygen balance indicate a daily GPP and ER
that were higher overall with warming but hint at lower NEP when 24-hour periods
are considered. The long-term responses of these variables are yet to be evaluated.
Of great interest is the fact that although warming noticeably increased many
symptoms of eutrophication in both the Danish and the UK experiments, it
sometimes had no effects and occasionally reduced them. There was great variety
in the details of the responses, even under the well-controlled conditions in the
experimental ponds compared with natural ponds and lakes. Starting conditions,
intrinsic chaotic dynamics and interactions of nutrient loading and temperature
increase combine to make precise predictions of the future a challenging task.
Mesocosms in a littoral reed stand in Switzerland
Experiments with mesocosms in Switzerland (Fig. 6.4) addressed impacts of
warming and eutrophication on heterotrophic processes of the carbon cycle in
freshwater marshes, an important wetland type with large potential for feedback
effects on climate (e.g. Brix et al . 2001). In contrast to the experiments conducted
in Denmark and the United Kingdom, the mesocosms were not isolated tanks but
were placed within the littoral zone of a lake and included unfenced control plots
(Flury et al . 2010). Emphasis of the Swiss experiments was on plant litter
decomposition, although responses of whole-system metabolism and several
other variables were also assessed (Flury 2008; Hammrich 2008). The mesocosms
were arranged in a randomized block design in a monospecific stand of common
reed, Phragmites australis . Half of them were continuously heated to a target
temperature of 4°C above ambient (typically >2°C achieved) and/or periodically
fertilized with Ca(NO 3 ) 2 to increase dissolved nitrogen concentration fivefold (in
practice threefold to ninefold) above the ambient concentration (annual load of
approximately 10 g N m −2 ).
Experiments conducted in these mesocosms produced notable and sometimes
unexpected results. Raised temperature stimulated microbial respiration associated
with decomposing leaves in the short term, as anticipated, but had no effect in the
long run, that is, respiration rates on litter from heated and control mesocosms
did not differ when measured at a standard temperature in the laboratory (Fig.
6.8). Nitrogen enrichment, in contrast, tended to curtail respiration both in the
long and in the short term, and leaf litter mass loss was also slowed in mesocosms
receiving extra nitrogen. This counterintuitive inhibition of decomposition by
increased NO - supply has previously been observed in forests where it was