Nitrogen-fixing planktonic

Not all chemical reactions proceed at rates fast enough to create significant deviations from NAECs. For example, although N2 gas is consumed by nitrogen-fixing plankton and produced by denitrifying bacteria, these processes are too slow to affect the relatively high seawater concentrations established by equilibration with the atmosphere. 

Howarth, R.W., Chan, F., and Marino, R. (1999) Do top-down and bottom-up controls interact to exclude nitrogen-fixing cyanobacteria from the plankton of estuaries explorations with a simulation model. Biogeochemistry 46, 203-231. 

Howarth, R. W., Marino, R. (1990). Nitrogen-fixing cyanobacteria in the plankton of lakes and estuaries A reply to the comment by Smith. Limno/. Oceanogr., 35, 1859-1863. 

The problem with this fixed nitrogen is that its use as a fertilizer requires land application. While some of the nitrogen is retained by the plants, much is carried off the land as stormwater runoff This increases the concentration of DIN in groundwater and river-water. Drainage of these waters into the coastal ocean supplies nutrients that stimulate plankton growth. Remineralization of the plankton biomass can lead to development of hypoxic and anoxic conditions in coastal waters. 

Table 8.30 shows the chemistry of seawater compiled by Turekian (1969) for major, minor, and trace constituents, expressed in parts per billion (ppb) at a mean salinity of 35. The listed values are estimates of mean amounts in solution, whereas elemental concentrations actually vary with depth. The most conspicuous variations are observed in the first 200 m from the surface, where photosynthetic processes are dominant and phosphorus and nitrogen are fixed by plankton and benthos, as well as silica and calcium, which constitute, respectively, the skeletons of planktonic algae (diatom) and the shells of foraminifera and mollusks. 

Benthic Food Web. In contrast to the planktonic community, there was a much smaller enrichment of 15N in plants and animals of the benthic food web. The moss Calliergon showed an enrichment in 15N up to about 10(/,o, which indicates that these plants are drawing fixed nitrogen directly from the water column (Table I natural-abundance 815N values of Calliergon are around l-2%o). Periphyton scraped from the control side of the curtain... 

Both of the models presented here are based on the flow of nitrogen through ecosystems in one case a nearshore kelp-bed system and in the other a general offshore plankton community. The klep bed model was developed to explore the hypothesis that nitrogen flow is affected by horizontal water transport in shallow water marine systems here wave action or mixing associated with horizontal transport are likely to retain nitrogen in the photic zone and the benthic community is of fixed location so that boundaries of the system can be easily defined. In pelagic systems, on the other hand, the community tends to move horizontally with water in the mixed layer, and vertical transport into and out of the mixed layer is an important feature of the system dynamics. 

Nitrification is the process whereby ammonium (NH4+) is oxidized to nitrite (N02 ) and then to nitrate (N03 ). It thus links the most oxidized and most reduced components of the nitrogen (N) redox cycle and helps determine the overall distributions of these important nutrients. Ammonium rarely occurs at significant concentrations in oxygenated habitats. It is recycled rapidly between heterotrophic and N2 fixing organisms (which excrete NH4+ directly or release organic N that is microbiaUy degraded to NH4+) and many heterotrophic and photosynthetic plankton (which utilize NH4+ as a N source) in the surface ocean. 

In the ocean interior, the ratio of fixed inorganic nitrogen (in the form of N03 ) to PO4 in the dissolved phase is remarkably close to the ratios of the two elements in living plankton. Hence, it seemed reasonable to assume that the ratio of the two elements in the dissolved phase was the result of the sinking and subsequent remineralization... 

Under the low nutrient summer conditions that follow the spring plankton bloom, the single largest pools of fixed nitrogen and phosphorus in coastal... 

---