Nutrient limitation of phytoplankton growth in Arctic lakes of the lower Mackenzie River Basin, northern Canada

Water chemistry and the nutrients limiting phytoplankton growth were examined in lakes located in the Northwest Territories, Canada, along a transect from the Mackenzie River Delta south to Travaillant Lake. This 230 km stretch spans gradients in vegetation types (tundra to boreal forest) and elevation (flooded by the Mackenzie River versus higher elevation lakes). Total phosphorus concentrations generally were typical of oligotrophic lakes (<10 mu g.L-1), while nitrate and orthophosphate concentrations were below detection limits. Although some growth was noted in bioassays receiving single additions of nitrogen (N) or phosphorus (P), the greatest growth occurred in those bioassays in which nitrogen and phosphorus were added. This suggests that phytoplankton in the majority of study lakes were strongly co-limited by N and P irrespective of watershed type and Mackenzie River influence. Furthermore, it suggests that algal communities will be highly responsive to anthropogenic disturbances, which result in increased N and P loading to these lakes. Experimental results from March bioassays suggest that anthropogenic eutrophication would be excerbated by increased temperatures (e.g., those associated with global warming).