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Higher taxon richness and significantly greater densities of littoral invertebrates
occurred in the temperate lakes with an about eightfold higher densities of predators,
tenfold of grazers and twofold of collectors. Cladoceran species richness, density and
body size were also significantly higher in the temperate lakes. However, periphyton
biomass in the subtropical lakes was much lower than expected from the lower density
of grazing invertebrates and the more positive environmental conditions for periphyton
growth (more light and higher temperature). The observed fourfold reduction in
periphyton biomass in the subtropical lakes seems likely to have been the result of
periphyton feeding by fish and shrimps.
These results help to explain why the buffers that the plants provide in the
temperate zone against increasing external nutrient load apparently are less
strong in warmer lakes (Romo et al . 2005; Blenckner et al . 2007; Jeppesen et al .
2009). Danish lakes (as in most Northern European lakes) show a clear positive
effect of submerged macrophytes on water clarity, but no clear difference with
plant cover was found in the chlorophyll:TP or Secchi depth:TP relationships in
shallow lakes in Florida (USA), irrespective of plant cover (Bachmann et al . 2002;
Jeppesen et al . 2007a, E. Jeppesen et al. unpublished data). It has been argued
that macrophyte growth will be stimulated by climate warming (Scheffer et al .
2001) due to higher temperature and, in the Mediterranean region, due also to
reduced water depth and therefore greater light penetration to the bottom (Coops
et al . 2003; Beklioglu et al . 2006). However, an analysis of data from lakes from
the temperate zone to the tropics suggests a lower probability of macrophyte
dominance in warm lakes (Kosten et al . 2009).
Moreover, even when submerged macrophytes are abundant, the positive
effect of the plants on water clarity is much less pronounced at higher
temperatures (Jeppesen et al . 2007a), probably reflecting much lower
zooplankton grazing on phytoplankton due to a higher fish predation on
zooplankton (Jeppesen et al . 2007b) by small omnivorous fish. In addition,
higher dominance of benthivorous fish foraging in the sediment may enhance
internal nutrient loading when lake temperature increases. This may accelerate
eutrophication and lead to higher and prolonged dominance of nuisance algae,
especially cyanobacteria. Cyanobacteria, most notably the N-fixing forms, are
highly sensitive to increases in temperature (Fig. 6.1). Likewise, dinophytes,
which may also cause problems for human water supplies, may become more
important with higher temperatures, whereas diatom proportion, in particular,
diminishes (Jeppesen et al . 2009; Fig. 6.1).
Mesocosm experiments
The question of whether warming intensifies symptoms of eutrophication was
also addressed using controlled mesocosm experiments. These included studies
in tanks in the United Kingdom, a flow-through pond system in Denmark and in
situ enclosure experiments in a littoral reed stand of a lake in Switzerland (Fig.
6.4). In such experiments, temperature was controlled at desired values above
ambient, and additional treatments included differential nutrient loading or a
change in the fish community or both.
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