Nitrogen fixation is mediated by a variety of autotrophic and heterotrophic bacteria. Cyanobacteria appear responsible for most planktonic fixation in aquatic ecosystems, and rates of fixation are high only when these organisms make up a major percentage of the planktonic biomass. Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes ≪ 0.1 g N m−2 yr−1) but is often high in eutrophic lakes (0.2–9.2 g N m−2 yr−1). We found no data on planktonic nitrogen fixation in estuaries or coastal seas except for the Baltic Sea and for the Peel‐Harvey estuary in Western Australia. Fixation rates are quite high in the Peel‐Harvey estuary; rates are low offshore in Baltic waters but can be high near shore. As in lakes, fixation in these systems is associated with major blooms of planktonic, heterocystic cyanobacteria. However, nitrogen‐fixing cyanobacteria are much more abundant in the Baltic Sea and in the Peel‐Harvey estuary than in other estuaries or coastal waters. In most estuaries and coastal waters, species of nitrogen‐fixing cyanobacteria are absent or make up a very small percentage of the phytoplankton biomass (< 1%), suggesting insignificant amounts of nitrogen fixation. Annual average rates of nitrogen fixation reported for blooms of Trichodesmium and for Rhizosolenia mats in oceanic waters are also low. Unlike nitrogen fixation by planktonic organisms, there appear to be no major differences between freshwater and marine ecosystems with regard to fixation by benthic bacteria. Rates of nitrogen fixation in the sediments of most lakes and estuaries are low to moderate, generally <0.25 g N m−2 yr−1 except in extremely organic‐rich estuarine sediments. In estuarine sediments which are organic‐rich, nitrogen fixation rates range from 0.4 to 1.6 g N m−2 yr−1. Most benthic fixation in mesotrophic and eutrophic lakes and estuaries is mediated by heterotrophic (and perhaps chemoautotrophic) bacteria, but benthic fixation in oligotrophic ecosystems is often dominated by cyanobacteria. Rates of nitrogen fixation in freshwater and marine wetlands and seagrass beds appear similar to or somewhat greater than those in nonvegetated, organic‐rich sediments. Rates of fixation in cyanobacterial mats are high to very high (1.3–76 g N m−2 yr−1), but these mats usually cover only a small area of the ecosystems in which they reside, limiting the importance of fixation in the mats to the mats themselves. The importance of nitrogen fixation to the nitrogen economy of aquatic ecosystems is quite variable. For example, fixation by planktonic organisms appears unimportant as a nitrogen source to most oligotrophic and mesotrophic lakes (generally <1% of total nitrogen inputs) but accounts for 6–82% of the nitrogen inputs to eutrophic lakes. Planktonic fixation provides ≪ 1% of the nitrogen inputs to surface waters of the world’s oceans and is probably also of little importance in most estuaries, including eutrophic estuaries. However, planktonic fixation provides >20% of the nitrogen input to the Asko region of the...
Blooms of nitrogen (N)-fixing cyanobacteria are common in freshwater lakes of moderate to high productivity. In contrast, blooms of N-fixing cyanobacteria are largely absent from the water columns of N-limited estuaries. In a companion study, we reported that the abundance and N-fixation rates of planktonic filamentous heterocystous cyanobacteria are strongly suppressed by the presence of zooplankton consumers in saline estuarine mesocosms. Here, we show that bloom formation in these estuarine cyanobacterial N-fixers (Anabaena sp.) is strongly dependent on their ability to grow into sufficiently large filamentous colonies that are capable of developing heterocysts and thereby fixing N. This basic physiological constraint on N fixation results in a sensitivity of N-fixer bloom initiation to suppression by herbivorous zooplankton. In the presence of zooplankton consumers, small populations of heterocystous cyanobacteria exhibited reduced colonial filament size, decreased heterocyst frequencies, and suppressed growth rates. In short-term grazing assays, estuarine zooplankton (Acartia tonsa) directly consumed heterocystous cyanobacteria. This consumption occurred at high rates and resulted, at times, in marked reductions in colonial filament size. These results provide a mechanistic understanding of how trophic interactions can mediate the functional composition of phytoplankton communities and thereby constrain the biogeochemical response of estuaries to N limitation.
In this study, we present data to support the hypothesis that removal of epiphytes by grazers is an important control of nitrogen fixation in temperate seagrass meadows during the summer. Previous work in West Falmouth Harbor, Massachusetts, USA, found highest rates of epiphytic nitrogen fixation in the part of the harbor (Snug Harbor) with the greatest nitrogen load and the lowest phosphate concentrations, a somewhat paradoxical result suggesting that biogeochemical controls are not the major factor regulating this nitrogen fixation. Here we report that the density of invertebrate grazers on epiphytic algae (predominantly Bittiolum alternatum) was least in Snug Harbor, where nitrogen fixation rates were greatest. Reciprocal transplant experiments showed that seagrass shoots transplanted into Snug Harbor from the part of the harbor (Outer Harbor) where external nitrogen loading was lower but grazer densities were 4-fold higher, had a more than 5-fold increase in epiphytic nitrogen fixation after a 12 d incubation period. Shoots transplanted from Snug Harbor to Outer Harbor showed a large, rapid reduction in epiphytic nitrogen fixation rates after only 6 d, likely due to consumption of epiphytes. Our results suggest that trophic control is a potentially important determinant of epiphytic nitrogen fixation rates in temperate seagrass meadows.
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