Determination of Ecosystem-Scale N2 Fixation and Zooplankton Assimilation of Fixed N Using d15N Analysis in Six Small Reservoirs

Cyanobacteria capable of fixing atmospheric dinitrogen gas (N₂) can become dominant in phytoplankton assemblages of lakes and reservoirs when sources of bioavailable N are low relative to phosphorus (P).  These N₂-fixing cyanobacteria may affect N cycling dynamics within lakes and reservoirs, especially in regards to the food web by altering the amount of N assimilated into primary consumers (zooplankton).  However, few studies have examined the contributing factors and extent of this effect at the ecosystem scale in small reservoirs.  Our study aimed at quantifying whole-lake N₂ fixation rates and contribution to total N inputs as well as assimilation of fixed N into zooplankton in six small reservoirs in northwest Arkansas differing in nutrient conditions, geomorphology, and watershed land-use.  Depth-integrated whole lake composite seston and zooplankton samples were collected biweekly from May through September in each reservoir.  N₂ fixation and zooplankton assimilation of fixed N were determined by analysis of seston and zooplankton δ¹⁵N and concomitant multi-source mixing models.  Zooplankton were also identified and counted microscopically for several sample dates to determine any shifts in community composition during periods of high N­₂ fixation.  Preliminary data from this study will be presented.