Geoscience Reference
In-Depth Information
Do nutrients structure ecosystems in different ways under
current and anticipated future climatic conditions?
There is a clear change in trophic structure in lakes along the climate gradient
from simple, often elongated, food webs in cold climates to truncated webs in
warm ones with a higher degree of omnivory. Preliminary results indicate that
this conclusion, well supported by data for lakes, also holds for streams.
The effect of increasing nutrient supply also differs among climate regions. In
temperate lakes, a shift often occurs from high proportions of potential piscivorous
fish, few and large plankti-benthivorous fish, high abundance of zooplankton
and clear water often with macrophytes to a turbid state with dominance of small
plankti-benthivorous fish and phytoplankton. This shift is associated with
increased nutrient supply, although additional factors may be needed to trigger
the switch and to maintain one state or the other. In the subtropics, lakes subject
to both low and high nutrient loading are typically dominated by numerous small
omnivorous fish that exert a high predation pressure on zooplankton. They leave
little scope for daytime hiding of zooplankton in vegetation because the fish
aggregate in the vegetation. Such systems can have clear water when nutrient
loading is low. However, they are very vulnerable to increases in nutrient levels
because the top-down effect of zooplankton is weaker than in temperate lakes
due to the higher predation on zooplankton by fish.
Although less well studied than in lakes, the same probably holds true for
streams and rivers, with a higher likelihood that benthic filamentous algae will
grow abundantly on hard substrata at the bottom of small streams and dense
phytoplankton communities develop in large rivers. There remains a big gap in
knowledge about how wetlands will change in response to climate change and
eutrophication. As many have a strong terrestrial character, analyses have focused
on vegetation rather than whole food webs, and although animal communities,
particularly birds, have also been extensively described in wetlands, overall
trophic structures and their potential changes are not well understood.
Will changing climate interact with increased nutrient supply
to alter ecosystem processes?
Whether changing climate interacts with increased nutrient supply to alter
ecosystem processes is more uncertain at the moment than change of trophic
structure. There is evidence that processes like deoxygenation, decomposition and
denitrification are influenced by both nutrients and warming, but the interaction
among factors appears to be complex and variable, and there are discrepancies
about how system components and whole systems are affected. In lakes, we can
expect (i) higher internal loading of phosphorus in response to higher temperatures,
more prolonged stratification in deep lakes and higher sedimentation rates as
phytoplankton becomes more abundant and grazing by zooplankton is reduced;
(ii) possibly a higher likelihood of losing submerged macrophytes and thereby
shifting shallow lakes from benthic- to pelagic-dominated systems and reducing
biodiversity (although different pictures emerge from space-for-time studies and
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