The supply of nitrogen (N) determines fundamental aspects of ecosystem structure and dynamics. In aquatic systems such as freshwater lakes, heterocystic N-fixing cyanobacteria can be sufficiently abundant such that shortages of N at the ecosystem scale are alleviated by N-inputs from biological nitrogen fixation. In surprising contrast, heterocystic cyanobacteria are largely absent from the water columns of estuaries, even where N strongly limits the rate of net primary production. The factors that constrain the success of heterocystic cyanobacteria are thus central to the maintenance of persistent N-limitation in estuaries. The disparate responses of freshwater and estuarine systems to N-limitation further indicate that estuarine heterocystic cyanobacteria must face unique constraints relative to their cyanobacterial counterparts in freshwaters and to all other estuarine phytoplankton taxa that are able to form seasonal blooms.

The ability of heterocystic cyanobacteria to produce and fix N in specialized heterocystic cells is central to their success in N-limited freshwaters. In estuaries, trace-element limitation specific to N-fixers have been proposed to limit the ability of heterocystic cyanobacteria to produce heterocysts and thereby increase their vulnerability to growth suppression by herbivore consumers. In particular, reductions in colonial filament size by consumers are proposed to suppress the ability of heterocystic cyanobacteria to fix nitrogen and form blooms that are of significance to the N-economy of estuaries. I experimentally manipulated the abundance of zooplankton herbivores and benthic filter feeders in high salinity pelagic estuarine mesocosms to (1) test whether estuarine heterocystic cyanobacteria are disproportionately vulnerable to herbivore control and (2) identify the mechanisms that are responsible for this vulnerability. Estuarine heterocystic cyanobacteria populations were disproportionately sensitive to grazing control by estuarine zooplankton. This sensitivity stemmed from size-structure dependent growth in heterocystic cyanobacteria, where reductions in filament colony size by grazers strongly limited the ability of heterocystic cyanobacteria to produce heterocysts and fix N. Size-structure dependent growth was also clearly evident in in situ populations of heterocystic cyanobacteria in freshwater ponds. Temporally-detailed sampling indicated that heterocystic cyanobacteria initially recruit to the water column at low densities as short and poorly heterocysted filaments. Rapid growth and bloom formation was possible only after cyanobacterial populations were able to grow in to long filaments and support the development and N-fixing activities of heterocysts. In addition, comparisons of potential growth rate in freshwater and estuarine heterocystic cyanobacteria suggest that the growth of N-fixers in estuaries face further physiological constraints from associated with growth in saline environments. A comparative synthesis of phytoplankton biomass turnover rates also indicated that estuarine phytoplankton communities are subject to biomass-specific mortality rates that are ca. 3-fold higher than those experienced by their counterparts in freshwaters. The exclusion of heterocystic cyanobacteria from estuarine phytoplankton communities is likely to reflect the interactive effects of trace-element limitation, size-structure dependent growth, and the effects of consumer controls on estuarine N-fixer populations and phytoplankton community structures.

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