INTERACTIONS OF TOP-DOWN AND BOTTOM-UP CONTROL IN PLANKTONIC NITROGENCYCLING

Although our understanding of the complexity of the plankton and micro bial food webs has increased substantially over the past decade or two , there has been little appreciation to date of the interactions betwe en top-down (grazing) control and bottom-up (nutrient supply) control on the structure and nutrient cycling processes within these webs. The quality of nutrient supply, both in terms of the relative proportion of inorganic: organic nitrogen, as well as the relative proportion of inorganic nitrogen substrates has a direct impact on rates of nitrogen uptake, and ultimately on the relative composition of phytoplankton a nd bacteria. At the same time, grazing by microzooplankton and macrozo oplankton also influences both the composition of the food web and the rate of supply of nitrogen. The impact of macrozooplankton on rates o f nitrogen cycling in a microbial community is complex: macrozooplankt on release NH4+, urea, and amino acids by direct excretion and by 'slo ppy feeding', but they also control both the rates of nitrogen regener ation and uptake within the community by grazing the microzooplankton, the primary regenerators of NH4+, and the phytoplankton, the primary consumers of nitrogen. Thus, grazing and nitrogen recycling are intric ately connected: the presence of large zoooplankton simultaneously pro vides top-down control of biomass and bottom-up nutrient supply. These relationships vary depending on the scale of interest, and have impor tant consequences for how we measure and model total nitrogen cycling in a natural food web.