What is stoichiometry?

You may have learned the term stoichiometry in a chemistry class, referring to the relative quantities of elements in a molecule, or the relative amounts of reactants and products in a chemical reaction.  Stoichiometry, in its simplest definition, merely refers to the relative proportion of components.  For example, the stoichiometric relationship of steering wheels to tires in a normal automobile is 1:4.

Why do we care about stoichiometry?

Biotic factors such as predation are often critical determinants of community composition.  Environmental inputs of nutrients are known to be important for limiting primary production and influencing plant species composition, but elemental nutrients rarely have been evaluated as controllers of herbivore species distribution and abundance.  In aquatic systems, the stoichiometry of ambient available nutrients, such as nitrogen and phosphorus, has been used to predict phytoplankton species composition (e.g. Tilman 1982, Tilman et al. 1982, Smith 1983, Sommer 1993), and models that predict species composition based on nutrient loading ratios (and the associated elemental stoichiometries of phytoplankton) recently have been extended to zooplankton (e.g. Sterner 1990, Sterner and Hessen 1994).  These approaches are exciting, because they have the potential to unite predictive models of populations with environmental processes determining large scale patterns of nitrogen and phosphorus limitation.

Enclosures in a lake (Storvatn) in the Norwegian arctic
Bags used to manipulate food elemental stoichiometry and determine the effects on zooplankton communities.  This experiment was run in the Norwegian arctic in 1998 in collaboration with Professor Dag Hessen and Anne Lyche Solheim, Ph.D.

The goal of our research on stoichiometry is to examine the potential for nutrient and light limitation to control zooplankton herbivore composition.  We are studying zooplankton herbivores, in part because their life cycles are short and amenable to experimental time scales.  Furthermore, the elemental stoichiometry of phytoplankton and zooplankton is well studied, allowing for specific predictions about the effects of nutrient loading on zooplankton herbivore communities.  We predict that some large zooplankton species require particular ratios of nitrogen and phosphorus in the phytoplankton they consume.  Because these large species are preferred prey for fish and their presence in zooplankton communities results in more efficient transfer of energy from the lower food web to fish, nutrient ratios in phytoplankton actually may affect the energetic efficiency of aquatic communities.


One area of research that I am pursuing involves experimentally evaluating the importance of the stoichiometry of nutrients in phytoplankton (the food of zooplankton) on zooplankton community composition. Individual herbivorous zooplankton species maintain relatively stable N:P (nitrogen:phosphorus) ratios in their bodies, despite the widely varying algal stoichiometries often found in freshwater aquatic systems.  For example, Daphnia has a high P content, Bosmina has a somewhat lower P content, and copepods contain an even lower P content (Andersen and Hessen 1991).  These P contents may be constrained by life-history traits, where high growth strategies require high RNA and high P levels (Elser et al. 1996).  In fact, Daphnia growth, reproduction and fecundity are actually limited by low P content in its phytoplankton food (Sterner et al. 1993, Urabe et al. 1996).
This is a photograph of a Bosmina, an organism that is less sensitive to phosphorus content in its food than are some other zooplankters.  This Bosmina is approximately 0.2 mm in length.  No common name is listed for Bosmina, but I'd vote for elephant water flea.

Daphnia is an important component in aquatic ecosystems because of its ability to reduce the standing stock of phytoplankton by grazing (e.g., Edmondson and Litt 1982; Lampert et al. 1986).  Bio-control efforts to mediate excessive phytoplankton biomass by addition of piscivores often break down when either Daphnia populations do not develop to control algal growth after zooplanktivorous fish reduction, or the algae are resistant to Daphnia grazing (McQueen et al. 1989).  Two of the major hypotheses proposed to explain zooplankton community composition under conditions of low fish planktivory are predation by invertebrates and competition for algal food.  The discovery that different species have different stoichiometric requirements leads to a third hypothesis: that algal elemental composition (influenced by environmental nutrient loading) may constrain zooplankton community structure.  I have been running a series of laboratory and field experiments to differentiate between these hypotheses and to evaluate the strength of the stoichiometric constraints in nature.  Currently I am focusing on the possibility that nutrient limitation may an important life history constraint for copepods -- affecting nauplii but not adults.

The second line of mineral limitation research I am involved with is using laboratory and model results to predict both energetic and nutrient limitations on zooplankton populations.  An extensive database containing data collected from many lakes in North America and Norway has been used to test the predictions made in this modeling effort.  Xinli Ji, a doctoral student in the laboratory, is constructing a model to predict phytoplankton stoichiometry from some basic light and nutrient data.  She is validating this model with a long-term data set from Oneida Lake, N.Y.  She is also running zooplankton competition experiments under differing light and nutrient regimes.

Selected Publications by K.L. Schulz on Stoichiometry and Zooplankton Food Quality:

Current research on elemental stoichiometry in the Schulz lab is funded by the National Science Foundation and the Great Lakes Research Consortium.

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