Limitation of zooplankton Production: Beyond Stoichiometry

Stoichiometry is a commonly adopted concept in studying food-related limitation of zooplankton production in both marine and freshwater systems (Sterner 1990, Hessen 1992, Elser and Hassett 1994, Sterner and Hessen 1994, Hassett et al. 1997). Stoichiometric studies typically place substrates into three major elemental categories: carbon, nitrogen and phosphorus. By employing stoichiometric arguments, some researchers have concluded that production of zooplankton in the ocean is limited by nitrogen whereas freshwater zooplankton are limited by phosphorus (Hessen 1992, Elser and Hassett 1994). Some commonly quoted evidence includes: 1) The C:N or P:N ratio of in situ marine zooplankton biomass is lower than the C:N or P:N ratio of suspended particulate matter (Redfield et al. 1966, Elser and Hassett 1994); thus nitrogen is always in relatively shorter supply than carbon and phosphorus and becomes limiting (sensu Liebig). On the other hand, higher P:N ratio in freshwater zooplankton relative to particulate matter is interpreted as evidence of phosphorus limitation among freshwater zooplankton (Elser and Hassett 1994). 2) In laboratory studies, the gross-growth efficiency (growth/ingestion) in terms of carbon of marine calanoid copepods decreases with an increase in the C:N ratio (range 5-30) of food of monospecific algal diets, but the grossgrowth efficiency in terms of nitrogen remains constant over the same C:N ratio of food (see Fig. 11 in Checkley 1980 and Fig. 3 in Ki0rboe 1989). This has been interpreted as the animals maximizing the uptake of the limiting element for growth: nitrogen (Ki0rboe 1989; similar argument by Sterner and Hessen 1994). Other researchers have applied stoichiometric arguments implicitly by quoting the lower C:N ratio of protozoa, when compared to other diets, as evidence of their better nutritional quality for metazoan zooplankton (Stoecker and Capuzzo 1990, Gifford 1991, Sanders and Wickham 1993). Here we will argue that the concept of stoichiometry, although powerful and useful, is not sufficient for understanding food-related limitation of zooplankton production. We will discuss the confusion created by some assumptions in stoichiometric arguments and how stoichiometric theory can benefit from biochemical and physiological studies. One possible drawback of stoichiometric arguments is that they ignore the biochemical characteristics of specific groups of substrates (see debate among Brett 1993, Hessen 1993 and Urabe and Watanabe 1993). In nature, food substrates do not exist as individual elements, but always as compounds. Therefore, how an element is processed by the consumer is dictated by the biochemistry of the compounds in which that element is present. If we take biochemistry and physiology into consideration, nitrogen limitation in marine zooplankton is not always warranted by simple stoichiometric