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reduced fish biomass, reduced plant diversity and species richness, an increase in
warm-water exotics such as L. major and predominance of free-floating plants
such as lemnids. One common symptom of eutrophication, increased
phytoplankton biomass, was not observed, probably because of shading by
lemnids. Disappearance of submerged plants, seen by many per se as a symptom
of advanced eutrophication, also did not occur. C. demersum increased with
warming, but L. trisulca decreased, resulting in only a small net reduction of
macrophyte biomass. It may be that C. demersum was more tolerant of the
increased lemnid shading.
The implications of warming/deoxygenation for fish communities are severe,
however, for sticklebacks are resilient fish, more tolerant of reduced oxygen
concentrations than most other European fish species. In the United Kingdom,
only tench ( Tinca tinca ), crucian carp ( Carassius carassius ) and the common carp
( Cyprinus carpio ) are more tolerant. Perhaps even more significantly, an analysis
of the oxygen balance of the tanks in summer 2007 suggested that respiration
rates increased markedly through warming compared with photosynthetic rates,
largely through increased metabolism of heterotrophs. The effect also occurred
in response to nutrient loading and could imply a positive feedback of warming
on carbon dioxide release (Moss 2010).
Experimental ponds in Denmark
A longer-term experiment was carried out in Denmark with the aim of contrasting
the effects of warming on the two alternative states in which shallow lakes
typically occur: clear water, macrophyte dominated, with small zooplanktivorous
fish populations and turbid, phytoplankton dominated, with relatively large
zooplanktivorous fish populations. Twenty-four cylindrical outdoor mesocosms,
each 2.8 m 3 in volume, were used (Fig. 6.4). Groundwater was pumped in above
the sediment and drained through an outlet at the water surface. The theoretical
water retention time was 2.5 months, and the water was heated by electrical
elements and continuously mixed by paddles. The system was run at low and
high nutrient concentrations, the latter obtained by weekly dosing of N and P,
and at three temperatures, ambient and elevated according to the IPPC climate
scenarios A2 and A2 + 50%, downscaled to local 25 × 25 km grid cells. There
were four replicates of each treatment. The modelled temperature difference for
the A2 scenario is generally higher in August to January (max. 4.4 °C in September)
than during the rest of the year (min. 2.5 °C in June).
There was high inter-annual variability within and among mesocosms and
among replicates during the season. Cumulative data for the 4-year period,
however, showed a clear pattern. As expected, chlorophyll a was higher overall
in the high-nutrient mesocosms (Fig. 6.6). However, the effects of warming
differed. At low nutrient loading, warming had a substantial effect on chlorophyll
a , which was approximately 2.5 times higher in the heated mesocosms than
in the controls. Chlorophyll a was also 50% higher in the A2 scenario than in
the control mesocosms at high nutrient loading, while it was much lower than
in the control mesocosms at A2 + 50%, although still double that in the low-
nutrient mesocosms run at the same temperature scenario (Fig. 6.6).
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