Why so Little Planktonic Nitrogen Fixation in Estuaries-- Limitation by Trace Metals and Grazing?

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9307337 Howarth In the temperate zone, primary production is limited by Nitrogen in many if not most estuaries and by Phosphorus in most lakes. One major reason for these difference is a difference in Nitrogen fixation by planktonic cyanobacteria (blue-green algae). In lakes, temporary shortages of Nitrogen relative to Phosphorus result in blooms of cyanobacteria which fix Nitrogen and help alleviate the shortage. This does not occur in most estuaries, where planktonic cyanobacteria capable of Nitrogen fixation are rare and rates of Nitrogen fixation low. The work proposed here addresses this difference with a series of mesocosm experiments using water from Narragansett Bay. The central hypothesis of the proposal is that low availability's of Iron and/or Molibdium in estuaries lead to slow growth rates of planktonic, Nitrogen-fixing cyanobacteria. This leaves the cyanobacteria vulnerable to consumption by generalized grazing animals, leading to low population numbers and rates of Nitrogen fixation. The hypothesis does not state that herbivores in estuaries specifically feed on cyanobacteria. Rather, it is hypothesized that estuarine ecosystems may contain high numbers of generalized grazers which are capable of feeding on fine cyanobacterial filaments and are likely to do so incidentally to their feeding on other particles when such filaments are present. Zooplankton, ctenophores, and a variety of bivalves (mussels, oysters, clams) are examples of such filter feeding animals common in estuaries which may fed on cyanobacterial filaments. With sufficiently high growth rates, cyanobacteria might bloom and fix Nitrogen even with high rates of grazing, as can happen in freshwater. Iron and/or Molibdium availability's are viewed as central to the growth rate of cyanobacteria based on previous research; the requirement for both in Nitrogen fixation is high. %%% Mesocosm experiments form the core of the proposed research. Estuarine water and plankton assemblages will be subje ct to different additions of Iron, Molibdium, and Phosphorus, and levels of grazing will be controlled by adding predators on herbivorous zooplankton. Silversides (Menidia beryllina) will likely be the predator used. Interactions of metal additions with the fish additions in replicated experiments are of particular interest. Response variables to be measured include rates of Nitrogen fixation, abundance's of cyanobacteria and heterocysts, composition of the zooplankton community, assimilation, of Molimdium, Sultan, and Iron, and concentrations of nutrients, Molibdium, and Iron. A recently published method using oxine to complex labile forms of Iron will be used to estimate Iron availability to phytoplankton. The proposed research should lead to a better understanding of the factors which exclude Nitrogen-fixing cyanobacteria from the plankton of most temperate estuaries and coastal seas, helping to cause Nitrogen limitation of net primary production in these systems. The research should also lead to a better understanding of the controls on Nitrogen fixation by plankton in lakes, and therefore on the maintenance of Phosphorus limitation even in lakes receiving nutrient loading with low Nitrogen:Phosphorus. Thus, the research relates to the practical issue of management strategies to control eutrophication in both estuaries and lakes.