An experimental study of the role of phosphorus, molybdenum, and grazing as interacting controls on planktonic nitrogen fixation in estuaries

Heterocystic cyanobacteria respond to low nitrogen (N) availability in nutrient-enriched lakes by fixing N₂, but these organisms are rare in the plankton of nearly all estuaries worldwide under similarly N-depleted conditions. Due partially to the absence of these cyanobacteria from saline estuaries, little is known about the factors which might differentially exclude them. In this dissertation, an experimental system was developed in which heterocystic, N₂-fixing cyanobacteria were grown under saline estuarine conditions in 3000-liter tanks. These mesocosms were used for three experiments exploring the interaction of biogeochemical and ecological controls on cyanobacterial N₂ fixation.; The hypothesis tested was that planktonic cyanobacterial growth in seawater slow, due to sulfate inhibiting the availability of molybdenum, a trace metal required for N₂ fixation; slow growth leaves a developing population susceptible to control by grazing. Mesocosms were enriched with phosphorus (P) to provide a low N:P ratio, known to stimulate planktonic N₂ fixation in freshwaters, and in one experiment with molybdenum. Grazing pressure was manipulated using zooplanktivorous fish or a benthic filter-feeder. Grazing suppressed both heterocyst abundance and total N increase. The cyanobacterial response to P availability, both absolute and relative to N, was not as strong as observed in freshwater systems, and was insufficient to overcome the grazing effect and ecosystem N limitation, indicating another constraint on growth and N₂ fixation.; Molybdenum additions (10X seawater) affected heterocyst abundance or N₂ fixation inputs less than expected, and were insufficient to stimulate rapid enough growth to overcome grazing control. A microcosm experiment with freshwater Anabaena confirmed a strong inhibition of N₂ fixation at seawater sulfate and molybdenum levels, and showed a similarly small reversal of that effect at higher molybdenum. A re-examination of published kinetic data used to design these experiments suggested that the influence of sulfate on molybdenum assimilation is not entirely reversible, as assumed using a simple, single-enzyme competitive inhibition model. Nonetheless, these experiments support the hypothesis that a bottom-up constraint on growth of heterocystic cyanobacteria (molybdenum), coupled with an ecological interaction (grazing), can significantly reduce the ability of heterocystic, N₂-fixing cyanobacteria to eliminate N deficits in nutrient-enriched, saline estuarine ecosystems.