Cold Lakes

Shallow lakes and ponds are extremely frequent in the Arctic landscape. These systems are tightly linked to climate, both directly and via catchment inputs, with comparatively small or highly localized anthropogenic disturbances. The shallow lakes in Antarctica are simple, fishless ecosystems ranging in productivity from extremely low productive melt-water lakes to highly productive guano lakes. Shallow systems located in cold and very cold regions are very prone to anoxic conditions and fish kills or are fishless because they freeze solid in winter, while typically have a very poorly developed plant littoral zone. In systems where fish are present, the zooplankton community is usually very depressed and dominated by small taxa. The thresholds of fish density required to lose Daphnia and other large-bodied cladocerans are much lower in these systems due to several co-occurring factors. The usual oligotrophic conditions and high water transparency make visual-feeding fish extremely efficient. Large-bodied cladocerans are thus easier to spot, not least because they often present dark pigmentation as a protection against the enhanced UV radiation. The resulting predation pressure is higher than in comparative temperate systems, also because fish biomass is sustained by the usually high benthic production (benthic facilitation) in these nutrient poor systems. This also results in a much lower biomass of benthic invertebrates than in lakes without fish.

The cascading effect of fish in phytoplankton is, however, much weaker than in other lakes as nutrient limitation of algae growth in the water is often strong. High water clarity allows dominance of benthic production, enhanced by the nutrients in the sediments.

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