Response of Lower Trophic Levels to Water Level Fluctuation in Natural Lakes Could be Mediated By Alternative Stable States

Fluctuating water levels can influence primary productivity, zooplankton, and macroinvertebrate communities in reservoir systems; however, information is lacking on the relationship between water level and these variables in natural lakes. Therefore, we attempted to link natural lake water levels to these study variables using a temporal and spatial approach in two Nebraska Sandhill lakes (West Long Lake: 1998, 2005-2010 and Pelican Lake: 1998, 2005-2011).  Both lakes are shallow (<1.2 m maximum depth), windswept, and groundwater-connected, and thus are highly vulnerable to changes in water level but exist in different stable states. We hypothesized that a positive relationship would exist between water level and organism abundance at the lower trophic levels as observed in reservoir systems, but our results indicated a negative relationship between water level and chlorophyll-a in Pelican Lake but not in West Long Lake.  The strength of relationship with water level was less apparent in Pelican Lake for zooplankton and macroinvertebrate abundance and essentially absent for West Long Lake for all trophic levels. These patterns suggest that response to water level changes elicited by each system may differ depending on lake state (West Long has more submersed vegetation and higher water transparency than Pelican Lake).  In addition, the role of groundwater may also help explain these patterns. Further experimental studies are necessary to identify specific processes involved to help elucidate functional responses to changes in water level compared to reservoir systems.  This information will be imperative for critical management decisions as drastic water level changes are expected globally in freshwater ecosystems.