Consumer-driven nutrient recycling in arctic Alaskan Lakes: Controls, importance for primary productivity, and influence on nutrient limitation

In lakes, fish and zooplankton can be both sources and sinks of nitrogen (N) and phosphorus (P) through the consumption of organic N and P, and subsequent excretion of bioavailable inorganic forms. These source/sink dynamics, known as consumer-driven nutrient recycling (CNR), may, in turn, control the availability of potentially limiting nutrients for algal primary production. In this dissertation I investigate the importance and controls of CNR as a source of inorganic N and P for primary production (Chapter 2). I then examine zooplankton CNR as a mechanism for increasing nutrient mean resident time (MRT) in the mixed layer of lakes (Chapter 3). Finally, I assess whether zooplankton communities dominated by different taxa can affect N versus P deficient conditions for phytoplankton production through differential N and P recycling rates (Chapter 4). Direct excretion of N and P by fish communities was modest in arctic lakes, and accounted for < 4% of the N and P required for primary production. Recycling of N and P by zooplankton communities was relatively high, and the fraction of algal N and P demand supplied by zooplankton CNR ranged from 4--90% for N and 7--107% for P. MRT of 15N, measured in the mixed layer of an arctic lake, was ∼16 days, compared to 14 days predicted by a ecosystem model simulation with zooplankton N recycling and 8 days in a model simulation where zooplankton N recycling was absent. The 75% increase in N MRT between model simulations with and without zooplankton recycling suggests that zooplankton N recycling is an important mechanism for retaining N in lake ecosystems. I observed relatively high negative correlations between precipitation and phytoplankton N (r = -0.33) and P (r = -0.30) deficiencies. I also observed a significant positive correlation (r = 0.42, p = 0.03) between zooplankton communities with higher copepod biomass, relative to cladoceran biomass, and phytoplankton N-deficient conditions. These results suggest that when precipitation is high N and P deficiency is low in the phytoplankton. When precipitation is low, however, zooplankton communities composed primarily of copepods contribute to N-deficient conditions for phytoplankton production...