Total fish biomass per unit area was positively correlated with total phosphorus, total nitrogen, chlorophyll a, and inversely correlated with Secchi disk transparency in 65 Florida (U.S.A.) lakes selected to range from oligotrophic to hypereutrophic. Species numbers were positively related to lake surface area but not trophic state. There were some shifts in species composition with changes in trophic state, though only a few species showed significant changes in their standing crops. In particular the recreationally important centrarchids did not show important changes with trophic state, and there were no critical points on the trophic spectrum where there were dramatic changes in fish abundance or standing crops. The facts that Florida lakes do not have deep, cold hypolimnia, do not have salmonid species, and have no ice in the winter are among the possible reasons that the more eutrophic Florida lakes do not show the same changes in fish populations often described for northern lakes.Résumé : Il y avait une corrélation positive entre la biomasse totale de poissons par unité de surface et le phosphore total, l'azote total et la chlorophylle a, et une corrélation inverse entre cette biomasse et la transparence mesurée à l'aide du disque de Secchi, dans 65 lacs de Floride (É.-U.) allant d'oligotrophes à hypereutrophes. Les nombres d'espèces étaient liés positivement à la superficie des lacs, mais non à leur état trophique. La composition des espèces variait quelque peu avec l'état trophique, mais seules quelques espèces accusaient des différences importantes dans le nombre d'individus. Plus particulièrement, les centrarchidés importants sur le plan récréatif n'accusaient pas d'importants changements avec l'état trophique, et le spectre trophique ne présentait aucun point critique correspondant à des changements dramatiques de l'abondance ou du nombre d'individus. L'absence d'un hypolimnion froid et profond, l'absence de salmonidés et l'absence de glace en hiver comptent parmi les raisons pouvant expliquer pourquoi les lacs de Floride plus eutrophes n'accusent pas les mêmes variations de population de poissons dont on a souvent fait état dans les lacs nordiques.[Traduit par la Rédaction]

A model for the prediction of total phosphorus was developed and tested using data on 704 nautral and artificial lakes including 626 lakes in the U.S. Environmental Protection Agency (EPA) National Eutrophication Survey. A statistical analysis showed that the best estimate for the sedimentation coefficient (σ) in the Vollenweider equation was[Formula: see text]for artificial lakes where L is the areal phosphorus loading rate (mg∙m−2∙yr−1) and z is the mean depth (m). The model yields unbiased estimates of phosphorus concentrations over a wide range of lake types and has a 95% confidence interval of 31–288% of the calculated total phosphorus concentration. Other models are less precise. Though total phosphorus concentrations can be predicted equally well in natural and artificial lakes, predictions of algal densities and water transparency are less reliable in artificial lakes, as the phosphorus–chlorophyll and chlorophyll–Secchi depth relationships are less precise. This seems to be due to the influence of nonalgal particulate materials.Key words: phosphorus models, eutrophication, lake trophic state

We measured levels of inorganic suspended solids in Little Wall Lake, Iowa, and concluded that sediment resuspension occurred when wind velocities exceeded critical velocities as calculated from wave theory. The percentages of the lake bed subject to resuspension for winds of given velocities were calculated, as were the percentages of time that winds of such velocities could be expected. We concluded that only a small percentage of the lake bed is subject to resuspension most of the time. The techniques to calculate wind effects and to summarize the data on frequency of wind mixing used in this study should be generally applicable to problems of sediment resuspension in other shallow lakes.

It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (430% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2 mg L À1 . Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1-2 mg L À1 , except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes' ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered.

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