High Cyanobacterial Abundance in Three Northeastern Gulf of Mexico Estuaries

Abstract

Aquatic phytoplankton comprise a wide variety of taxa spanning more than 2 orders of magnitude in size, yet studies of estuarine phytoplankton often overlook the picoplankton, particularly chroococcoid cyanobacteria (cf. Synechococcus). Three Gulf of Mexico estuaries (Apalachicola Bay, FL; Pensacola Bay, FL; Weeks Bay, AL) were sampled during summer and fall 2001 to quantify cyanobacterial abundance, to examine how cyanobacterial abundance varied with hydrographic and nutrient distributions, and to estimate the contribution of cyanobacteria to the bulk phytoplankton community. Cyanobacterial abundances in all 3 estuaries were high, averaging 0.59 ± 0.76 X 109 L–1 in Apalachicola Bay, 1.7 ± 1.2 X 109 L–1 in Pensacola Bay and 2.4 ± 1.9 X 109 L–1 in Weeks Bay (mean ± standard deviation). Peak abundances typically occurred in the oligohaline zone (low salinity estuarine zone) during the summer. Freshwater sites had nearly undetectable abundances, and marine sites had abundances several-fold lower than the oligohaline zone. When converted to equivalent chlorophyll a concentrations, cyanobacteria comprised a large fraction of the total phytoplankton biomass, at times approaching 100% in all 3 systems. These observations clearly indicate a cyanobacterial community of estuarine origin that can make up a large proportion of phytoplankton biomass.