University of California Department of Environmental
Science and Policy
Determining the factors which control food web
interactions is a key issue in ecology. The empirical relationship between
nutrient loading (TP) and phytopl ankton standing stock (chlorophyll-a)
in lakes was revealed more than 30 years ago and is central for managing
surface water quality. However, the efficiency with which biomass and energy
is transferred through the food web and sustains higher trophic level (e.g.,
fish) production declines with nutrient loading and s ystem productivity
but mechanisms behind this phenomenon are poorly unders tood. Here we show
that in summer specific w3-polyunsaturated fatty acids (w3-PU FAs), important
for zooplankton, are significantly correlated to lake trophic status. The
w3-PUFAs octadecatetraenoic acid (OCT), eicosapentaenoic acid (EP A), and
docosahexaenoic acid (DHA), but not a-linolenic acid (ALA), decrease double-logarithmically
with increasing lake total phosphorus [TP] concentrations. By combining
the empirical relationship between EPA*C-1 and TP with functional mo dels
relating EPA*C-1 to daphnid growth and egg production, we predict secondary
production for this keystone consumer. This shows that decreasing energy
transfer efficiencies with increasing lake productivity can be explained
by w3-PUFA associated food quality differences at the plant-animal interface.
*Present address, Institute for Hydrobiology and Fishery Science, University of Hamburg