Matteo Cazzanelli:
Energy Pathways in Food Webs of Arctic Ponds

Date: 15-04-2012    Supervisor: Kirsten S. Christoffersen

Ponds are the dominant freshwater habitats in the Arctic. These pristine, shallow ecosystems host a variety of plants, animals and microorganisms, and thereby contribute significantly to arctic biodiversity. The biomass of planktonic consumers in oligotrophic arctic ponds is often surprisingly high, relative to the scarce availability of phytoplankton in the water column. Arctic ponds have a high biomass of benthic algae and can receive substantial amounts of terrestrial organic matter from their catchments, which could potentially support consumers.

The main objective of this thesis is to estimate the contribution of pelagic, benthic and terrestrial energy pathways to food webs in high latitude ponds. Special focus is placed on the effect of dissolved organic carbon (DOC) on the balance between primary and bacterial production and, in turn, on the trophic dynamics of arctic ponds. Stable isotope analyses, combined with measurements of basal (primary and bacterial) production, optical properties of DOC and fatty acid composition of sources and consumers, represent the main research tools.

These studies show that phytoplankton accounts only for a small fraction of the whole-pond basal net production in these oligotrophic systems, and alone may not support the consumers. The benthic and terrestrial environments provide complementary and important food sources for the zooplankton and zoobenthos and, combined, can contribute up to 95% of their diets. The relative importance of terrestrial carbon to the consumers’ diet increases with DOC content. DOC has a negative effect on light penetration and thereby hampers primary production, especially in the benthos, and the transfer of this resource to higher trophic levels. On the other hand, increasing DOC content promotes bacterial production and overall increases system heterotrophy.

Permafrost thawing, soil erosion, plant growth in the catchment and the resulting transfer of organic matter to aquatic systems are all predicted to increase with global warming. Increased temperature can additionally favour the evapoconcentration of humic compounds in the water column. Changes in the sources and amount of DOC can in turn have important consequences on basal production, metabolic balance and food web structure of arctic ponds.