Estuarine phytoplankton response to annual and manipulated river inputs

Abstract

River water entering estuaries affects the physical and chemical environment at irregular intervals creating a highly dynamic aquatic habitat. Phytoplankton are important primary producers in estuaries that respond quickly to their changing environment. Since 1991, Breton Sound estuary in southeast Louisiana has been directly influenced by Mississippi River water through the Caernarvon Freshwater Diversion structure. Over a 12 month period, the phytoplankton response was examined, in terms of biomass, abundance, community composition and potential phycotoxin production to seasonal changes in river input into the estuary. Within this 12 month period, a short pulse study was also carried out to examine the immediate response of phytoplankton to pulsed river input. Chlorophyll a (chl a) measurements estimated phytoplankton biomass and light microscopy identified phytoplankton abundance and community composition. Phycotoxins were measured using ultra-sensitive ELISA. During the seasonal study, chl a and cell abundance concentrations exhibited an inverse relationship with river input rates. Mean chl a concentrations were 79.1+38.7 and 55.2+48.5 µg chl a l-1 for low and high river input, respectively. Phytoplankton cell abundance concentrations averaged 7.5x105+6.7x105 cells l-1 and 1.2x105+2.5x105 cells l-1 during low and high river input, respectively. The community was dominated by cyanobacteria for most of the year, corresponding to higher temperatures. For the rest of the year, cyanobacteria decreased while chlorophytes and centric diatoms increased to approximately equal contributions. Chlorophytes dominated during the entire pulse study, however, cyanobacteria increased during high river input. Over both studies, the phytoplankton community composition was most commonly moderated by salinity and nutrient availability. Salinity, temperature and nutrient availability primarily influenced phytoplankton biomass and abundance during the seasonal study. The distance from the diversion and phosphate availability were the most important factors influencing phytoplankton biomass and abundance during the pulse study. Microcystins (MCs) were detected throughout the seasonal and pulse studies, ranged from below detection to 2.92 µg MCs l-1 and were highest during low river input and toward the outer estuary. The detection of MCs in Breton Sound estuary illustrates a potential risk to human health and economically important estuarine food webs.