Abundance and vertical flux of Pseudo-nitzschia in the northern Gulf of Mexico

Abstract

Many species of the ubiquitous pennate diatom genus Pseudo-nitzschia have recently been discovered to produce domoic acid, a potent neurotoxin which causes Amnesic Shellfish Poisoning (ASP). Pseudo-nitzschia spp. were extremely abundant (up to 108 cells 1-'; present in 67% of 2195 samples) from 1990 to 1994 on the Louisiana and Texas. USA, continental shelves and moderately abundant (up to 10' cells I-'; present in 18% of 192 samples) over oyster beds in the Terrebonne Bay estuary in Louisiana in 1993 and 1994. On the shelf there was a strong seasonal cycle with maxima every spring for 5 yr and sometimes in the fall, which were probably related to river flow, water column stability, and nutrient availability. In contrast, in the estuary there was no apparent seasonal cycle in abundance, but the time series of data is relatively short and the environment highly variable. At one site on the shelf, where sediment traps were deployed from spring to fall and sampled at frequent intervals in both 1990 and 1991, approximately 50% of the Pseudo-njtzschja spp. cells present in the water sank lnto sediment traps Pseudo-njtzschia spp. were also abundant In surficlal sediments. The species of Pseudo-nitzschla present during this study were not routinely identified with the methods employed. However, toxin-producing P multiserjes has been ident.ified previously from Galveston Bay, Texas, and cells from a bloom on the shelf in June 1993 were identified by scannlng electron microscopy as P. pseudodel~cat~ss~ma, whlch is sometimes toxic. Although there have been no known outbreaks of ASP in this area, hlstoncal data suggests that Pseudo-nltzschia spp. abundance may have Increased on the shelf since the 1950s It is hypothesized that the increase is due to doubling of the nutrlent loading from the Mississippi and Atchafalaya rivers and increased eutrophicat~on on the shelf. KEY W O R D S : Pseudo-n~tzschia Gulf of Mexico Harmful algal bloom