Nutrient-toxicant interactions in natural and constructed phytoplankton communities: results of experiments in semi-continuous and batch culture.

Natural communities and clonal cultures of phytoplankton derived from large lakes in Yellowstone National Park (WY, USA) were employed to explore the effects of interactions between resource availability and toxic metals (divalent Cu and Cd) on pelagic plant communities. Results of semi-continuous competition experiments demonstrated strong direct and interactive effects of resource availability and additions of toxic metals (25-150 nmol l(-1)) in both natural and laboratory constructed four-species communities. Both endpoint community composition and population dynamics of individual species elicited responses to interaction between limiting resources and toxic metals. N limited growth in low-density batch cultures was suppressed by addition of Cu and these effects were specific to both algal species and level of available nitrate. Measures of semi-continuous culture effluent pH and pCu demonstrated that high levels of productivity can allow phytoplankton to selectively alter bulk water chemistry in the presence of elevated levels of metals to counteract their toxic effects. Concentrations of both limiting nutrients and toxicants employed were within the range of field-measured levels, suggesting that these types of interactions may be commonplace in natural environments. As such, environmental assessments considering potential impacts of toxic agents should take into account the nutrient status of the aquatic environment, and the interactions among stressors as demonstrated here.