Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia.

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The anoxic factor (AF, days per year or per season) can be used to quantify anoxia in stratified lakes. AF is calculated from oxygen profiles measured in the stratified season and lake surface area (A,) as AF = (duration of anoxia x anoxic sediment area)/& AF represents the number of days that a sediment area, equal to the whole-lake surface area, is overlain by anoxic water. Average AF for 56 central Ontario lakes, 19 additional eastern North American lakes, including the Laurentian Great Lakes and several acidified lakes, ranged from 0 to 83 d per summer. Of this variation, 65% (54%) is accounted for by average phosphorus (nitrogen) concentration and an indicator of lake shape, ??"00.5 (where t is mean depth). Only 17% on average was explained by annual dissolved organic C (DOC) concentration. Annual variation within lakes was large and partly ascribed to variable annual loading of DOC. AF was not correlated with areal hypolimnetic oxygen depletion, because of the opposing dependency on lake shape, but was correlated with Reckhow's probability of anoxia in eutrophic lakes. AF was also correlated with the redox potential of anoxic water at the end of summer stratification, especially after inclusion of an estimate of iron concentration in the sediment.

Release rates of phosphorus from anoxic sediment surfaces in seven North American lakes were determined from core tube incubations. These rates were compared with several P fractions within the 0–5 and 5–10 cm layers of the corresponding sediment. Regressions of release rates both on total sediment P and on reductant-soluble P were highly significant. Analysis of literature data from lakes worldwide also showed significant relationships between the release rates and total sediment P and citrate dithionite bicarbonate extractable P. Mass balance calculations for individual cores indicated that reductant-soluble P decreases in wet surficial sediments, while total P in the overlying water increases. The release rates of different P fractions in the water — total, soluble reactive, and total reactive P — were very similar, indicating the high biological availability of the released P.

Lakes with anoxic hypolimnia (anoxic lakes) have significantly lower values for phosphorus retention than do lakes with aerobic hypolimnia (oxic lakes). This difference may reflect an increased internal phosphorus load from the anoxic hypolimnia.Two models are given to predict internal phosphorus load (L;,,) in such lakes. The first predicts internal load as the difference between the observed phosphorus retention in anoxic lakes and that predicted (Rpred) by a formula that adequately describes phosphorus retention in oxic lakes. The second predicts internal load as the product of an average rate of phosphorus release from anoxic sediments, the surface area of the anoxic sediment, and the period of anoxia. Predictions of the first model compare favorably with 17 observed values of internal load; further data are required to test the second model. These models suggest that mean phosphorus concentration (TP) in anoxic lakes can be predicted in two ways. One can use whole-lake phosphorus budget models which implicilly incorporate internal phosphorus load, because they include a measurement of phosphorus retention. Alternatively, a term to account for the internal load can be added to current models based on external load (L,,,) and predicted retention (I&J, where qs is areal water load:' A contribution to Formula Rcfcrcncc R, = IO/(10 -t q,) Vollenweidcr 1975 R, = 13.2J13.2 + qJ Dillon and Kirchner 1975 R3 = 16/(16 + qJ Chapra 1975 R, = 24/(30 + q,J Ostrofsky 1978a R, = 0.426 exp(-0.271q.J Kirchner and t-O.574 exp(-0.00949q.J Dillon 1975 R, = l/(1 -I-l/fi) Larsen and Mercier 1976

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Lakes with anoxic hypolimnia (anoxic lakes) have significantly lower values for phosphorus retention than do lakes with aerobic hypolimnia (oxic lakes). This difference may reflect an increased internal phosphorus load from the anoxic hypolimnia.Two models are given to predict internal phosphorus load (L;,,) in such lakes. The first predicts internal load as the difference between the observed phosphorus retention in anoxic lakes and that predicted (Rpred) by a formula that adequately describes phosphorus retention in oxic lakes. The second predicts internal load as the product of an average rate of phosphorus release from anoxic sediments, the surface area of the anoxic sediment, and the period of anoxia. Predictions of the first model compare favorably with 17 observed values of internal load; further data are required to test the second model. These models suggest that mean phosphorus concentration (TP) in anoxic lakes can be predicted in two ways. One can use whole-lake phosphorus budget models which implicilly incorporate internal phosphorus load, because they include a measurement of phosphorus retention. Alternatively, a term to account for the internal load can be added to current models based on external load (L,,,) and predicted retention (I&J, where qs is areal water load:

No abstract