TitleEcological and biogeochemical interactions constrain planktonic nitrogen fixation in estuaries
Publication TypeJournal Article
Year of Publication2002
AuthorsMarino, R, Chan, F, Howarth, RW, Pace, M, Likens, GE
Type of ArticleJournal Article

Many types of ecosystems have little or no N-2 fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N-2-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N-2 fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N-2 fixation are seldom observed in estuaries where the salinity exceeds 8-10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N-2 when zoo-plankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N-2 fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO42-), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom-up) factors that slow the growth rate of cyanobacteria and ecological (top-down) factors such as grazing are likely to be important regulators excluding planktonic N-2 fixation from most Temperate Zone estuaries.

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