*WINNER* Effects of Lake Depth on Hypoxia in Agricultural Lakes

This won best graduate poster for Biology. Lake depth in the Lower Mississippi River Alluvial Plain is heavily affected by agricultural irrigation, flood-control structures, and large-scale pumping projects.  Small oxbow lakes are widespread in this region, and commonly experience hypoxia that leads to fish-kills.  We examined how lake depth affects dissolved oxygen (DO) availability and lake hypoxia potential.  Specifically, we measured water column and sediment primary production and respiration relationships with water depth prior to, during, and after a whole-lake drawdown to measure changes during a single drying and refilling event. Sediment net primary productivity (NPP) was predominately negative regardless of lake depth and location and ranged from -0.594 to 0.251 g O2/m2/h.  Results were used to model whole-lake metabolism. Increasing depth resulted in lower whole-lake NPP due to the decreased photic zone for phytoplankton production.  In the turbid conditions that are common after precipitation, greater lake depths expand the heterotrophic dark zone of the water column. This reduction in water column NPP combined with permanent DO uptake by sediments can increase whole lake hypoxia during lake refilling.