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Using a , 1000 lake data set that spans the entire continental United States, we applied empirical modeling approaches to quantify the relative strength of nutrients and water temperature as predictors of cyanobacterial biomass (CBB). Given that cyanobacteria possess numerous traits providing competitive advantage under warmer conditions, we hypothesized that water temperature, in addition to nutrients, is a significant predictor of CBB. Total nitrogen (TN), water temperature, and total phosphorus were all significant predictors of CBB, with TN explaining the most variance. Using multiple linear regression analysis, we found that TN and water temperature provided the best model and explained 25% of the variance in CBB. However, when the data set was divided according to basin type, these same variables explained a higher amount of the variation in deep natural lakes (33%, n 5 253), whereas the least amount of variation was explained by these variables in shallow reservoirs (12%, n 5 307). Competing path models on the full data set using the best variables selected by multiple linear regression show that nitrogen and temperature are indirectly linked to cyanobacteria by association with total algal biomass, which likely reflects changes in light climate and other secondary factors. Our models also indicated that temperature was linked to cyanobacteria by a direct pathway. Under a scenario of atmospheric CO2 doubling from 1990 levels (resulting in an estimated 3.3uC increase of the maximum lake surface water), we predict on average a doubling of CBB.

By measuring Chl a, zooplankton biomass, nutrient concentrations, and water residence time in 31 rivers in eastern Canada, we tested the following hypotheses: Chl a is positively related to nutrient concentrations and water residence time, and zooplankton biomass is positively related to Chl a and water residence time. Chl a ranged from 1.8 to 27.6 μg liter -1 , total phosphorus (TP) from 7.3 3 to 212.3 μg liter -1 and water residence time from 3.3 to 19.4 d. There was a positive relationship between Chl a and TP (R 2 = 0.76) and no relationship between Chl a and residence time. In contrast, zooplankton biomass was positively related to water residence time (R 2 = 0.33). There was only a weak positive relationship between Chl a and zooplankton biomass. Mean river zooplankton biomass was low (11.3 μg liter -1 dry mass) compared to lakes of similar Chl a concentration.

A metalimnetic chlorophyll peak (40-70 pg.liter-*) in Jacks Lake, Ontario, was due primarily to a colonial chrysophyte, Chrysosphaerella longispina. The peak was not the result of in situ growth, nor of a passive and progressive accumulation of epilimnetic cells. Instead, a sudden bloom of the algae which occurred in the epilimnion at the end of June, possibly due to excystment, descended rapidly and within no more than 4 days concentrated in a narrow band below the thermocline. There was no net increase in chlorophyll a or cell biomass with time once the population was established in this region. No dividing cells were observed. After about 2 weeks the Chl a concentration at the peak began to decline and the peak disappeared by the end of August. The importance of metalimnetic peaks is greatly exaggerated by the measurement of Chl a.

The nutrient state of the chrysophyte, Chrysosphaerella longispina, forming a large metalimnetic chlorophyll a peak in Jacks Lake, Ontario, was examined over several summers. Turnover times of phosphate were fast in the epilimnion (minutes) but slow (hours) at the depth of the peak. Long turnover times, high concentrations of polyphosphate, low P debt, low V,,,, and high K, values for phosphorus all indicated that initially the peak was not more P-deficient than epilimnetic phytoplankton. However, with time it became P-deficient and this factor coincided with its demise in August. At no time was the peak population N-deficient. The peak population was not supplied by nutrients from the hypolimnion nor from the mineralization of sedimenting epilimnetic material but instead seemed to have brought cellular reserves of phosphorus when it migrated down from the epilimnion.

Measurements of carbon uptake by lake plankton at different levels of added phosphorus reveal some phytoplankton strategies that contribute to their survival in low-nutrient waters. Nutrient-deficient populations seem to temporarily reduce, rather than enhance, photosynthesis when phosphorus becomes available. This contradicts the basic hypothesis of the 14C bioassay used to determine “limiting” nutrients in natural waters. The reduction of carbon uptake rate continues until either the nutrient addition is taken up or no further storage is possible; at this time carbon upake may be enhanced. Concurrent measurements of phosphorus uptake showed no light response unless the plankton were exposed to significant phosphorus concentrations in the dark for several hours. Phytoplankton seem to be adapted for and dependent upon intermittent events exposing them to nutrient-enriched water. The ability of phosphate-depleted plankton to take up phosphate in excess of their growth requirements was used to develop an index for phosphate deficiency. To normalize for different biomass levels, the optimum photosynthetic rate was divided by the maximum uptake velocity giving a range for the C:P maximum uptake from 1.2 to 206 for plankton samples of varying phosphate deficiency.