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Effects of nutrient patchiness and N:P supply ratios on the ecology and physiology of freshwater phytoplankton Suttle, Curtis Arnold


Laboratory and field experiments examined several aspects of the interaction of freshwater phytoplankton species and plankton communities with nitrogen and phosphorus nutrient resources. The laboratory studies focused on the following three main areas: 1) effects of nutrient 'patchiness' on phytoplankton community structure; 2) kinetics of phosphate (PO₄⁻³ ) and ammonium (NH₄⁺) uptake of phytoplankton grown under non-steady-state but limiting rates of nutrient supply; 3) the effect of different N:P supply ratios on phytoplankton NH₄⁺ and PO₄⁻³ uptake kinetics and community structure. Nutrient 'patchiness' was simulated by altering the frequency of nutrient addition to cultures. Under conditions of infrequent addition (once per 18 days) dominance shifted to a larger species, and the average cell size of another species increased. Observations of PO₄⁻³ uptake kinetics were not consistent with most other studies where kinetics were determined under steady-state conditions. With respect to PO₄⁻³, the duration over which maximum uptake rates were sustained was species specific. There was a short lag before maximum uptake rates were realized, and whether maximum uptake rates occurred at the lowest or at intermediate dilution rates depended on the time scale over which the uptake measurements were made. NH₄⁺ uptake rates were found to be greatly enhanced during the first few minutes of uptake. When natural plankton assemblages were grown under N:P supply ratios of 5:1, 15:1 and 45:1 (by atoms), the treatments selected for different competitive dominants. An N:P ratio of 45:1 resulted in total dominance by Synechococcus sp.; cultures grown under 5:1 and 15:1 supply ratios were dominated by Synedra radians, Nitzschia holsatica and Scenedesmus sp. NH₄⁺ and PO₄⁻³ uptake kinetics were not the same in cultures grown under different supply ratios, and ratios of saturated PO₄⁻³ to NH₄⁺ uptake rates were a good indicator of the N:P supply ratio under which the cultures were grown. This relationship was used to derive an index termed the Relative Field investigations were conducted on an oligotrophic coastal lake. NH₄⁺ and PO₄⁻³ uptake rates of size fractionated plankton (< and > 3 um), at a range of substrate concentrations, revealed that a large portion of the total uptake (50-90 % and 65-85 % for NH₄⁺ and PO₄⁻³, respectively) was attributable to cells in the < 3 um fraction. In addition, saturating PO₄⁻³, uptake rates of the > 3 um cells were less sensitive to incubation time than smaller cells. The ratio of saturated PO₄⁻³ to NH₄⁺ uptake rates were consistent with nutrient bioassay experiments, and indicated that N:P supply ratios in the lake were in the range where both N and P could be limiting to phytoplankton growth.

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