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-phytoplankton interactions, we compare elemental composition and aspects of nutrient deficiency across a variety of marine and freshwater ecosystems. During 1992 and 1993 we sampled a total of 31 lakes (in northern Wisconsin and Michigan and the Experimental Lakes Area of northern Ontario) and 21 marine stations (at seven estuarine, coastal, and open-ocean sites in the Atlantic and Pacific) for elemental composition of zooplankton, seston, and dissolved components. Relative degree of nutrient deficiency was assessed by phytoplankton dark uptake of ammonia and phosphate, as well as growth response of phytoplankton to N and P addition. Marine and freshwater systems differed greatly in N and P concentrations, N: P stoichiometry, and the distribution of N and P within dissolved, seston, and zooplankton pools. Particularly notable was the high proportion of N and, especially, P that was incorporated in the particulate fraction (seston + zooplankton) of lakes compared to marine sites. In freshwater systems, Daphnia spp., which have low body N: P, dominated zooplankton communities when seston C :P and N:P were also low, and calanoids that tend to have high body N : P dominated when seston C : P and N : P was high. This relationship between zooplankton community composition and seston elemental stoichiometry supports arguments for the importance of food quality constraints on zooplankton growth in freshwater systems. Such patterns were not seen in marine systems. Interest in comparative analyses of ecosystems seems to be increasing (Cole et al. 1991). Such a trend is arguably a sign of maturation in the science of ecology, as scientists evaluate the generality of the principles of ecosystem structure and function suggested by intensive study of individual ecosystems or a narrow range of ecosystem types. Peters et al. (1991) delineated a variety of benefits arising from ambitious comparative study of ecosystems, including sizable improvements in scale, breadth, generality, economy, and timeliness, compared to the accumulation of disaggregated information arising from an explosively expanding ecological research literature (Root 1987). Aquatic scientists have also recognized the need for comparative study, emphasizing in particular the need for a more complete understanding of the nature of marine and freshwater ecosystems (Nixon 1988). For example, delineation of similarities or differences in the functioning of pelagic ecosystems in lakes and oceans may be critical in determining whether insights obtained at the more manageable scale of lakes may be extrapolated I Present address: Department of Zoology, Michigan State University, East Lansing, Michigan 48824.