No abstract

Unusually high concentrations of NH 4 + (up to 10 lM) were observed in the surface waters of polyhaline Chesapeake Bay during July 2000, supporting elevated rates of simulated in situ integrated primary production (4.6 g C m À2 day À1 ) and chlorophyll-a (chl-a) specific integrated primary production (56 mg C mg chl-a À1 day À1 ). These rates were the highest measured in the polyhaline Bay during a 5-year sampling program. Chl-a and the percent contribution of phytoplankton >20 lm to the total phytoplankton increased after the ammonium pulse. We hypothesize that increased wind-driven mixing and a tilting of the pycnocline caused by northeast winds combined to increase the transport of NH 4 + from below the pycnocline to the surface water. Summer wind and chl-a data collected in the southern Bay between 1984 and 2000 revealed that chl-a was significantly higher 2 weeks after northeast winds than in years when no northeast wind occurred. Episodic peaks in NH 4 + and primary productivity resulting from wind events lasting only a few days are poorly captured by traditional shipboard surveys, but may be detected if sampling is focused on periods when wind forcing favors enhanced NH 4 + transport to the surface waters. This process of introduction of NH 4 + to the surface water from sediments followed by enhanced primary productivity may help explain some of the phytoplankton blooms that are observed in the polyhaline Bay and other estuaries during summer months.