ECOLOGICAL STOICHIOMETRY OF N AND P IN PELAGIC ECOSYSTEMS - COMPARISON OF LAKES AND OCEANS WITH EMPHASIS ON THE ZOOPLANKTON-PHYTOPLANKTON INTERACTION

By using an elemental-stoichiometry approach to zooplankton-phytoplank ton interactions, we compare elemental composition and aspects of nutr ient deficiency across a variety of marine and freshwater ecosystems. During 1992 and 1993 we sampled a total of 31 lakes (in northern Wisco nsin and Michigan and the Experimental Lakes Area of northern Ontario) and 21 marine stations (at seven estuarine, coastal, and open-ocean s ites in the Atlantic and Pacific) for elemental composition of zooplan kton, seston, and dissolved components. Relative degree of nutrient de ficiency was assessed by phytoplankton dark uptake of ammonia and phos phate, as well as growth response of phytoplankton to N and P addition . Marine and freshwater systems differed greatly in N and P concentrat ions, N:P stoichiometry, and the distribution of N and P within dissol ved, seston, and zooplankton pools. Particularly notable was the high proportion of N and, especially, P that was incorporated in the partic ulate fraction (seston + zooplankton) of lakes compared to marine site s. Tn freshwater systems, Daphnia spp., which have low body N:P, domin ated zooplankton communities when seston C:P and N:P were also low, an d calanoids that tend to have high body N:P dominated when seston C:P and N:P was high. This relationship between zooplankton community comp osition and seston elemental stoichiometry supports arguments for the importance of food quality constraints on zooplankton growth in freshw ater systems. Such patterns were not seen in marine systems.