Seasonal Variation of Phytoplankton and Primary Production in the Thames River, Southeastern Connecticut

A year-long study was performed to investigate seasonal changes in the phytoplankton biomass and primary production in the Thames River, a salt wedge estuary that empties into Long Island Sound in southeastern CT. Chlorophyll measurements were made on discrete filtered samples collected above and below the 1-3 m deep pycnocline at a 5-meter deep station. Surface chlorophyll concentrations, primarily from diatoms, averaged approx. 2 mg m^-3, with maxima (up to 10 mg m^-3) during summer months (Jun to Aug) and minima during October through March (as low as 0.3 mg m^-3). The lower water layer had nearly the same annual average but a smaller range (0.7-3.3 mg m^-3) and a winter/spring bloom (Jan-Apr) that was not seen in surface water. During most of the winter, chlorophyll concentrations were higher in the lower layer. Primary production, as measured by ¹³C uptake in bottle incubations, averaged 67 mgC m^-3 h^-1 in surface water [range 0.1 (Jan 2012) to 800 mgC m^-3 h^-1 (Aug 2011)], and 3 mgC m^-3 h^-1 [range 0.04 (Jan 2012) to 17 mgC m^-3 h^-1 (Aug 2011)] in the lower layer. On most occasions, deep water incubated near the surface had a higher primary production rate than surface water incubated at the surface; apparently the light-limited phytoplankton in the lower layer were released from light-limitation during these incubations. During the study period there were over a dozen heavy wind or heavy rain events, including Hurricane Irene in August and a freak Nor’easter snow storm in October 2011. Hurricane Irene was associated with a large decline in phytoplankton biomass and primary production. With significant storms as frequent as the rate of sampling, it is difficult to separate a “storm effect” from a background seasonal pattern. The study reveals that phytoplankton, especially those in the lower layer, are light-limited in the Thames River estuary, and that the effects of significant storm events are superimposed on significant seasonal variation.