Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably NH 4 that promote vs. repress NO -3 uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a "generic" eukaryotic cell, and the contrasting metabolic pathways and regulation of NH 1 4 and NO 2 3 when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory NO 2 3 / NO 2 2 reduction to NH 1 4 and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in NH 1 4 and NO 2 3 metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are presented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing.

Lake Victoria, the largest tropical lake in the world, suffers from severe eutrophication and the probable extinction of up to half of its 500+ species of endemic cichlid fishes. The continuing degradation of Lake Victoria's ecological functions has serious long-term consequences for the ecosystem services it provides, and may threaten social welfare in the countries bordering its shores. Evaluation of recent ecological changes in the context of aquatic food-web alterations, catchment disturbance and natural ecosystem variability has been hampered by the scarcity of historical monitoring data. Here, we present highresolution palaeolimnological data, which show that increases in phytoplankton production developed from the 1930s onwards, which parallels human-population growth and agricultural activity in the Lake Victoria drainage basin. Dominance of bloom-forming cyanobacteria since the late 1980s coincided with a relative decline in diatom growth, which can be attributed to the seasonal depletion of dissolved silica resulting from 50 years of enhanced diatom growth and burial. Eutrophication-induced loss of deep-water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for certain deep-water cichlids. Conservation of Lake Victoria as a functioning ecosystem is contingent upon large-scale implementation of improved land-use practices.

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Dissolved nitrogen (N) as urea ([NH 2 ] 2 CO), nitrate (NO { 3 ), and ammonium (NH z 4 ) was added to naturally phosphorus (P)-rich lake water (up to 175 mg P L 21 ) to test the hypotheses that pollution of hypereutrophic lakes with N increases total algal abundance, alters community composition, and favors toxic cyanobacteria that do not fix atmospheric N 2 . Monthly experiments were conducted in triplicate in polymictic Wascana Lake, Saskatchewan, Canada, during July, August, and September 2008 using large (. 3140 liters) enclosures. Addition of all forms of N added at 6 mg N L 21 increased total algal abundance (as chlorophyll a) by up to 350% relative to controls during August and September, when soluble reactive P (SRP) was . 50 mg P L 21 and dissolved N : P was , 20 : 1 by mass. In particular, NH z 4 and urea favored non-heterocystous cyanobacteria and chlorophytes and NO { 3 , urea promoted chlorophytes, some cyanobacteria, and transient blooms of siliceous algae, whereas N 2 -fixing cyanobacteria and dinoflagellates exhibited little response to added N. Added N also increased microcystin production by up to 13-fold in August and September, although the magnitude of response varied with N species and predominant algal taxon (Planktothrix agardhii, Microcystis spp.). These findings demonstrate that pollution with N intensifies eutrophication and algal toxicity in lakes with elevated concentrations of SRP and low N : P, and that the magnitude of these effects depends on the chemical form, and hence source, of N.