Issues in Ecology Number 7 Fall 2000 increase in population served by the sewage treatment plant. results obtained from earlier studies.most bioassay data from Finally.N removal technology was gradually introduced to estuaries and coastal marine systems indicates that these are the se veen 1988 and 1993.even N limited.This is supported by the tallreducing theNad to the value originally seen in 1976.Throughout the 17 years of observation.both the e hehe ner.this body of clarity of the water and the abundance of phytoplankton were clearly related to the total N concentration in the estu- primary regulator of eutrophication in most coastal systems. ary.In contrast.total P was a poor predictor of phytoplank- ton abundances. Mechanisms That Lead to Nitrogen Control of Eutrophica- A third study explored long-term changes in Laholm tion in Estuaries Bay.an estuary on the southwestern coast of Sweden (Fig Whether primary production by phytoplankton is ure 5).Early signs of eutrophication appeared there in the limited by Nor P depends on the relative availability of each 1950s and 1960s and steadily increased over time.The of these nutrients in the water.Algal growth will slow when earliest reported indicator of eutrophication was a change in the concentration of the scarcest nutrient drops.Phytoplank the seaweed community,and filamentous or sheet-like sea ton require approximately 16 moles of N for every mole of F weed species typical of eutrophic conditions have gradually they take in.This N:P ratio of 16:I is called the Redfield become more prevalent.Harmful algal blooms also ratio.If the ratio of avai ble N to available P in an aquati 980s gostcmssthanl6al,aealgownhwlim he ratio is higher.primary production will tend to creasing.From the late 1960s th inp The relat eavailability of N and Ptothe phytoplank nputs ton is det cating his perio into the ecosystem from that N controls ·ho recycled,or os ett and how much N is "fixed" erted from aseous with cor clusions drawn from shor term hio studios and Nin the air directly into biologically from ratios of dissolved inorganic N to P in these ecosystems. within the ecosystem. As a result.these three ec svstem studies add credence These three factors interact in several ways to make to coasta waters, directly rough trogen9 Issues in Ecology Number 7 Fall 2000 increase in population served by the sewage treatment plant. Finally, N removal technology was gradually introduced to the sewage treatment plant between 1988 and 1993, even￾tually reducing the N load to the value originally seen in 1976. Throughout the 17 years of observation, both the clarity of the water and the abundance of phytoplankton were clearly related to the total N concentration in the estu￾ary. In contrast, total P was a poor predictor of phytoplank￾ton abundances. A third study explored long-term changes in Laholm Bay, an estuary on the southwestern coast of Sweden (Fig￾ure 5). Early signs of eutrophication appeared there in the 1950s and 1960s and steadily increased over time. The earliest reported indicator of eutrophication was a change in the seaweed community, and filamentous or sheet-like sea￾weed species typical of eutrophic conditions have gradually become more prevalent. Harmful algal blooms also increased in frequency, particularly in the 1980s. During the early stages of eutrophication in Laholm Bay, inputs of both P and N were increasing. From the late 1960s through the 1980s, however, P inputs declined by a factor of almost two, while N inputs more than doubled. Plankton blooms continued and eutrophication worsened during this period, clearly indi￾cating that N was controlling eutrophication in the bay. Although these three large-scale studies show only that N controls eutrophication in Narragansett Bay, Himmerfjarden, and Laholm Bay, the findings are consistent with conclusions drawn from short-term bioassay studies and from ratios of dissolved inorganic N to P in these ecosystems. As a result, these three ecosystem studies add credence to results obtained from earlier studies. Most bioassay data from estuaries and coastal marine systems indicates that these are N limited. This is supported by the generally low inorganic N to P ratio found in most estuaries when they are at the peak of primary production. Thus, taken together, this body of evidence leads to the conclusion that N availability is the primary regulator of eutrophication in most coastal systems. Mechanisms That Lead to Nitrogen Control of Eutrophica￾tion in Estuaries Whether primary production by phytoplankton is limited by N or P depends on the relative availability of each of these nutrients in the water. Algal growth will slow when the concentration of the scarcest nutrient drops. Phytoplank￾ton require approximately 16 moles of N for every mole of P they take in. This N:P ratio of 16:1 is called the Redfield ratio. If the ratio of available N to available P in an aquatic ecosystem is less than 16:1, algal growth will tend to be N limited. If the ratio is higher, primary production will tend to be P limited. The relative availability of N and P to the phytoplank￾ton is determined by three factors: l the ratio of N to P that comes into the ecosystem from both natural and human-derived sources; l how each nutrient is handled stored, recycled, or lost in the ecosystem; l and how much N is fixed converted from gaseous N in the air directly into biologically useable forms within the ecosystem. These three factors interact in several ways to make Figure 7 - Animal wastes may be the largest single source of N that moves from agricultural production to coastal waters, either directly through runoff or indirectly through volatilization and deposition of atmospheric ni￾trogen. Photo by Larry Rana, USDA. Photo by Gene Alexander, USDA.