frequently less than for lakes, the sediments are often a more important sink of phosphorus in lakes than in marine ecosystems, and nitrogen fixation is a more prevalent process in the plankton of lakes (Howarth 1988). Each of these is discussed briefly below.

In both freshwater and marine ecosystems, the relative requirements of phytoplankton for nitrogen and phosphorus are fairly constant, with the two elements being assimilated in the approximate molar ratio of 16:1, the Redfield ratio (Redfield 1958). If there were no biogeochemical processes acting within a water body, the ratio of nitrogen to phosphorus in the nutrient inputs to the ecosystem would determine whether the system were nitrogen or phosphorus limited. Ratios below 16:1 would lead to nitrogen limitation and higher ratios would lead to phosphorus limitation (Howarth 1988). In fact, the N:P ratio in nutrient loadings to many (but by no means all) estuaries and coastal seas are below this ratio, while nutrient inputs to temperate lakes tend to have higher N:P ratios (Jaworski 1981, Kelly and Levin 1986, NOAA/EPA 1988). This difference in ratios probably reflects the relative importance of sewage, which tends to have a low N:P ratio, as a nutrient source to coastal waters.

Biogeochemical processes within sediments act to alter the relative abundance of nitrogen and phosphorus in an ecosystem. Denitrification, the bacterial reduction of nitrate to molecular nitrogen, removes nitrogen and tends to make coastal marine ecosystems more nitrogen limited (Nixon et al. 1980, Nixon and Pilson 1983). However, this process appears to be even more important in lakes than in estuaries and coastal seas; a higher percentage of the nitrogen mineralized during decomposition is denitrified in lake sediments than in estuarine sediments (Seitzinger 1988, Gardner et al. 1991, Seitzinger et al. 1991). Of more importance in explaining a tendency for nitrogen limitation in coastal marine ecosystems of the temperate zone, therefore, is the relatively high phosphorus flux from sediments; nutrient fluxes from these sediments have fairly low N:P ratios (Rowe et al. 1975, Boynton et al. 1980, Nixon et al. 1980). In many lakes, phosphorus is bound in the sediments (Schindler et al. 1977), although in others, phosphorus fluxes are comparable to marine sediments (Khalid et al. 1977). Nutrient fluxes from lake sediments can be either enriched or depleted in nitrogen relative to phosphorus (Kamp-Nielsen 1974). Caraco et al. (1989, 1990) have suggested that the abundance of sulfate in an ecosystem partially regulates the sediment flux of phosphorus. Phosphorus binding in sediments is greatest where sulfate concentrations are lowest, which is consistent with variable fluxes in lakes and higher fluxes in coastal marine ecosystems.

When the relative abundance of nitrogen to phosphorus is low in the water column of lakes, nitrogen-fixing species of cyanobacteria are favored since they can convert molecular nitrogen to ammonium or organic nitrogen. Under such nitrogen-depleted conditions in lakes, these cyanobacteria



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