Bacterial activities and abundance were measured seasonally in the water column of meromictic Big Soda Lake which is divided into three chemically distinct zones: aerobic mixolimnion, anaerobic mixolimnion, and anaerobic monimolimnion. Bacterial abundance ranged between 5 and 52 x 10h cells ml-', with highest biomass at the interfaces between these zones: 2-4 mg C liter' in the photosynthetic bacterial layer (oxycline) and 0.8-2.0 mg C liter' in the chemocline. Bacterial cell size and morphology also varied with depth: small coccoid cells were dominant in the aerobic mixolimnion, whereas the monimolimnion had a more diverse population that included cocci, rods, and large filaments. Heterotrophic activity was measured by [methyl-3H]thymidine incorporation and [14C]glutamate uptake. Highest uptake rates were at or just below the photosynthetic bacterial layer and were attributable to small (< 1 pm) heterotrophs rather than the larger photosynthetic bacteria. These high rates of heterotrophic uptake were apparently linked with fermentation; rates of other mineralization processes (e.g. sulfate reduction, methanogenesis, denitrification) in the anoxic mixolimnion were insignificant. Heterotrophic activity in the highly reduced monimolimnion was generally much lower than elsewhere in the water column. Therefore, although the monimolimnion contained most of the bacterial abundance and biomass (-60%), most of the cells there were inactive.
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...
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