| My talk explores how plankton populations in the marine mixed layer depend on sunlight and mixed layer depth, which are the primary environmental factors that affect plankton. To answer this question, I consider a sequence of simple mathematical models that capture the most important physical and ecological processes. I study two sets of parameter values, one used in a large-scale circulation model and the other used to explore parameter space, and a seasonal cycle of sunlight and mixed layer depth derived from standard climatologies for a site in the open North Atlantic Ocean near Bermuda.; The simplest model contains the exchanges of nutrient and plankton between the mixed layer and the lower ocean, photosynthesis, nutrient uptake, and zooplankton grazing. In this model, plankton persist in arbitrarily deep mixed layers, in contrast to observations, and the zooplankton concentration is too large. Additional phytoplankton losses limit the depth of mixed layers where plankton persist. Additional zooplankton losses lower the zooplankton concentration to levels more consistent with observations. The other processes that I consider, grazing inefficiency and nutrient recycling, have only small quantitative effects and do not impact the qualitative behavior of the system.; More sunlight yields more production. In contrast, the mixed layer depth plays a dual role. Dilution, which removes plankton from the mixed layer, and the phytoplankton growth rate are both inversely related to the mixed layer depth. Thus, plankton concentrations are low in shallow and deep mixed layers and peak at an intermediate mixed layer depth for a given irradiance. Plankton concentrations also depend more strongly on mixed layer depth than sunlight.; In the most complete model that I consider, the zooplankton concentration only increases in shallow mixed layers. Zooplankton decline in the deeper mixed layers that characterize most of the year. The lengthy equilibration times of deeper mixed layers allow the zooplankton to survive between periods of growth. Climatic shifts that shorten the zooplankton growing season by increasing the mixed layer depth or that decrease the equilibration time during periods of decline may endanger the zooplankton as well as the fisheries that depend on them. |
