Ecological stoichiometry: Contrasts across the marine and freshwater divide

Background/Question/Methods Ecological Stoichiometry examines how the nutrient content of organisms shapes their ecology.  The C:N:P at the base of the food web is a crucial parameter in consumer growth, nutrient cycling, and other kinds of dynamics.  C:N:P ratios differ between freshwater and marine systems, but there are many unanswered questions.   Are stoichiometric principles in freshwater and marine systems different in degree or in kind?  Are there fundamentally different processes at work or do these systems represent ends of a continuum?  What are the main factors causing contrasts between these systems; are they salinity, water residence time, light:nutrient ratios, contrasting biological communities, etc.?  To examine these questions we will use a meta-analysis of seston C:N:P relationships in small lakes, large lakes, nearshore oceans and offshore oceans.  We will also look in detail at the stoichiometric relationships within an “inland sea”, Lake Superior, Earth’s largest lake by area.

Results/Conclusions

Previous work has indicated that freshwater systems typically show greater variability in stoichiometric ratios than does the global ocean.  A recent metanalysis (Sterner et al, Limnology and Oceanography 53(3), in press) reinforces that conclusion but at the same time suggests that the functional coupling between C and P as well as C and N is perhaps just as flexible in offshore marine systems as in freshwater systems, but that marine systems express less of this variability due to narrower ranges of particle abundance. 

Lake Superior, as a large freshwater system with long hydraulic residence time and low productivity is a test bed for some of these questions.  Seston C:N:P ratios in Lake Superior are often similar to marine values but also can exhibit modestly large departures from Redfield Ratios.  A great contrast to marine systems in stoichiometric ratios in Lake Superior is in the nitrate to phosphate relationships.  Nitrate:phosphate ratios in Lake Superior exceed 10,000 (molar) and seem to have been on a continuous, steady rise for a century.  These ratios are driven by within-lake biogeochemical processes.  In this way, this lake is processing nutrients in an “anti-Redfield” manner. 

The Biogeochemical Mosaic Hypothesis brings all these observations together.  Biogeochemical balancing mechanisms, steadying C:N:P values around some biologically driven value, operate over large spatial and temporal scale principally because larger scale encompasses a greater heterogeneity of environments, promoting different portions of the biogeochemical cycles.