An investigation into iron-stimulated phytoplankton productivity in epipelagic Lake Erie during thermal stratification using trace metal clean techniques

We tested the hypothesis that phytoplankton productivity in pelagic Lake Er ie is limited by low Fe bioavailability during the period of thermal strati fication. Iron enrichment (20 and 200 nM Fe) of water sampled from the east ern basin surface water (5 m depth) in July 1996 revealed a dramatic 180 an d 30% increase in the standing crop of the picoplankton (0.2-2 mu m) and na noplankton (2-20 mu m) size fractions, respectively. Light-saturated rates of photosynthesis for picoplankton increased 2.8 times that of the controls within 24 h. In contrast, simultaneous P and Fe enrichment experiments car ried out in July 1997 and August 1998 with water samples from three pelagic stations revealed that P enrichment alone stimulated phytoplankton growth, although at several stations, phytoplankton yield was greater in combined P- and Fe-amended experiments relative to P-enriched treatments. Periodogra m analysis of meteorological buoy wave height data and heat budgets calcula ted from thermal profiles in the eastern basin revealed important interyear differences in precruise conditioning of the epilimnion. Low-frequency, tu rbulent mixing events that characterized the 30-day precruise period in the years 1997 and 1998 relative to 1996 may have prevented Fe-stressed condit ions in the phytoplankton community studied in 1997 and 1998.