We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day(-1)) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.

A strong, positive empirical relationship was found between bacterial abundance and chlorophyll concentration in fresh and marine waters. Freshwater and marine linear regression equations are statistically indistinguishable. The overall equation is log AODC = 5.867 + 0.776 log chl a, r2 = 0.88, where AODC (acridine orange direct count) is the number of bacteria per millilitre and chl a is micrograms of chlorophyll a per litre. It is apparent that planktonic bacteria and algae are tightly linked in lakes and the sea. The slope of the regression line, however, shows that bacterial numbers do not increase as rapidly as algal biomass with an increase in nutrient concentration. We suggest that this disproportionately smaller increase in bacterial numbers need not signify a smaller role for bacteria in lake metabolism with increasing nutrient availability, if bacterial productivity per unit bacterial biomass increases as total bacterial biomass increases between systems.

Attached algae play a minor role in conceptual and empirical models of lake ecosystem function but paradoxically form the energetic base of food webs that support a wide variety of fishes. To explore the apparent mismatch between perceived limits on contributions of periphyton to whole-lake primary production and its importance to consumers, we modeled the contribution of periphyton to whole-ecosystem primary production across lake size, shape, and nutrient gradients. The distribution of available benthic habitat for periphyton is influenced by the ratio of mean depth to maximum depth (DR = z/ z(max)). We modeled total phytoplankton production from water-column nutrient availability, z, and light. Periphyton production was a function of light-saturated photosynthesis (BPmax) and light availability at depth. The model demonstrated that depth ratio (DR) and light attenuation strongly determined the maximum possible contribution of benthic algae to lake production, and the benthic proportion of whole-lake primary production (BPf) declined with increasing nutrients. Shallow lakes (z < or =5 m) were insensitive to DR and were dominated by either benthic or pelagic primary productivity depending on trophic status. Moderately deep oligotrophic lakes had substantial contributions by benthic primary productivity at low depth ratios and when maximum benthic photosynthesis was moderate or high. Extremely large, deep lakes always had low fractional contributions of benthic primary production. An analysis of the world's largest lakes showed that the shapes of natural lakes shift increasingly toward lower depth ratios with increasing depth, maximizing the potential importance of littoral primary production in large-lake food webs. The repeatedly demonstrated importance of periphyton to lake food webs may reflect the combination of low depth ratios and high light penetration characteristic of large, oligotrophic lakes that in turn lead to substantial contributions of periphyton to autochthonous production.

Nine common species of aquatic macrophytes took all their phosphorus from the sediments when grown in situ in both a mesotrophic and a mildly eutrophic bay. Even under hypertrophic conditions, the sediments contributed an average of 72 percent of all the phosphorus taken up during growth. These experiments unambiguously demonstrate for the first time that submergent macrophytes in nature over-whelmingly depend on the sediments for their phosphorus supply and characterize them as potential nutrient pumps to the open water.

Extant physiological and in situ data provide contradictory answers to the question of whether the bacterioplankton can outcompete the phytoplankton for phosphorus at natural concentrations. Two phytoplankton species and three bacterial spccics isolated from the epilimnion of Lake Memphremagog, Quebec, were grown in phosphorus-limited continuous cultures and their abilities to take up and retain phosphorus compared. The algae showed orthophosphate uptake kinetics comparable to those reported elsewhere. In contrast, the specific orthophosphate uptake rates (i.c. uptake per unit cell P) by the bacteria were variable, but much higher than the algal rates at the low P concentrations characteristic of natural waters. Algal and bacterial alkaline phosphatase activities were similar and provided no evidence that the algae could utilize organic P more efficiently than the bacteria. The bacteria showed some tendency to excrete P more readily than the algae but mixed culture experiments indicated that excretion did little to alter the long term partitioning of phosphorus between the algae and the bacteria. The kinetic data suggest that planktonic bacteria are unlikely to be limited by phosphorus in situ. Furthermore, our results are consistent with the hypothesis that the bacteria should bc markedly superior competitors at natural phosphate concentrations.