Issues in Ecology Number 7 Fall 2000 differ in their level of nitrogen fixation.and these differences are a major reason why so many es tuaries are N limited while lakes are P limited.If a lake of moderate productivity begins to expe rience a shortage of N.blooms of nitrogen-fix ing cyanobacteria(formerly called blue-green al gae)occur,and the cyanobacteria tend to fix enough N to alleviate the shortage.Primar productivity of the lake remains limited by avail able P.This has been demonstrated in whole e experiments by fertilizing a lake amount ver everal yea ake also re r so d nt of N h the Re atio appea and the N dei Estuaries and eutrophic casta waters pr vide a striking con rast ith this behavior.Wth only a few exceptions anywhere in the world. nitrogen fixation by planktonic cyanobacteria is immeasurably low in these waters.even when they are quite N limited,and this allows N short. ages to persist.There is a growing consensus that nitrooen fixation in marine systems-estu aries,coastal seas,as well as oceanic waters age nitrogen is ed from the Missi probably is regulated by complex interactions of IS i om op he chemical,biological,and physical factors. Re cent evidence indicates that cyanobacteria in coastal bays and rivers,for instance.are con strained by slow growth rates caused by short xports astal wat Miss ages of trace metals such as iron or molybde mby by and bot ts besides N and Pas ca N limitatio es and P limitatio Other e more have a on the omm eive nu rient inpu ar e their resp se to stal water n. sources utriante for a the northe ast of the United of the maio an N:P ratio asses lgae the ratio thanks to the phy h ria on the toplankton down nit Supplies of biologically available silica entering tain oxvgen.a process that releases N to the atmosph terways come largely from weathe ring of soils and sediments Thus.given similar nutrient inputs from land,estuaries are Human activities over the past few decades have tended to more likely to be N limited than are lakes. decrease the delivery of useable silica to coastal marine sys Freshwater lakes and coastal marine ecosystems also tems by spurring eutrophication upstream.which tends to 10 Issues in Ecology Number 7 Fall 2000 N limitation more likely in estuaries and P limitation more likely in lakes. For instance, lakes receive nutrient inputs from upstream watersheds and from the atmosphere, while estuaries and coastal waters receive nutrients from these sources and neighboring ocean waters. The ocean-water sources of nutrients for estuaries such as those along the northeastern coast of the United States tend to have an N:P ratio well below the Redfield ratio thanks to the activity of denitrifying bacteria on the continental shelf. These bacteria break down nitrate to ob￾tain oxygen, a process that releases N to the atmosphere. Thus, given similar nutrient inputs from land, estuaries are more likely to be N limited than are lakes. Freshwater lakes and coastal marine ecosystems also differ in their level of nitrogen fixation, and these differences are a major reason why so many es￾tuaries are N limited while lakes are P limited. If a lake of moderate productivity begins to expe￾rience a shortage of N, blooms of nitrogen-fix￾ing cyanobacteria (formerly called blue-green al￾gae) occur, and the cyanobacteria tend to fix enough N to alleviate the shortage. Primary productivity of the lake remains limited by avail￾able P. This has been demonstrated in whole￾lake experiments by fertilizing a lake with a con￾stant amount of P over several years. For the first few years, the lake also received relatively high levels of N fertilizer so that the ratio of N:P was above the Redfield ratio. Under these con￾ditions, no nitrogen fixation occurred in the lake. The experimental treatment was then altered so that the lake received the same amount of P, but a reduced amount of N so that the input was below the Redfield ratio. Nitrogen-fixing organ￾isms quickly appeared and made up the N defi￾cit. Estuaries and eutrophic coastal waters pro￾vide a striking contrast with this behavior. With only a few exceptions anywhere in the world, nitrogen fixation by planktonic cyanobacteria is immeasurably low in these waters, even when they are quite N limited, and this allows N short￾ages to persist. There is a growing consensus that nitrogen fixation in marine systems estu￾aries, coastal seas, as well as oceanic waters probably is regulated by complex interactions of chemical, biological, and physical factors. Re￾cent evidence indicates that cyanobacteria in coastal bays and rivers, for instance, are con￾strained by slow growth rates caused by short￾ages of trace metals such as iron or molybde￾num and by grazing by zooplankton and bot￾tom-dwelling animals. Other elements besides N and P also can have a major influence on the structure of aquatic communi￾ties and can affect the nature of their response to eutrophi￾cation. As noted above, a key element in this regard is silica, an element required by diatoms. The availability of silica has little or no influence on the overall rate of algal growth in an aquatic system, but when silica is abundant, diatoms are one of the major components of the phytoplankton. When silica is in low supply, other classes of algae dominate the phy￾toplankton community. Supplies of biologically available silica entering wa￾terways come largely from weathering of soils and sediments. Human activities over the past few decades have tended to decrease the delivery of useable silica to coastal marine sys￾tems by spurring eutrophication upstream, which tends to Figure 8 - On average, nitrogen is exported from the Mississippi River basin and imported to the northeastern US in the form of food crops. The flux of N down rivers to the coast is greater per area of watershed in the northeast￾ern US than in the Mississippi basin. In the Northeast, atmospheric deposi￾tion from fossil fuel combustion, food imports, and fertilizer all contribute to these N exports to coastal waters. In the Mississippi basin, fertilizer and N fixation by agricultural crops contribute most to the flow of N into the Gulf of Mexico. (reprinted from NRC 2000).