Basal food web support of blue crab fisheries in the upper Barataria Estuary, Louisiana with a
focus on toxic cyanobacteria: a combined approach of natural abundance stable isotopes (13C and
15N) and toxin analysis (ELISA)
Kari Galván, Sibel Bargu, Ana Cristina Garcia
Department of Oceanography and Coastal Studies, Louisiana State University, Baton Rouge, LA. USA
In Louisiana, the upper regions of the Barataria Estuary support a substantial blue crab fishery.
While gut contents of blue crab have long been studied, little is known about the basal food resources
(primary producers) that ultimately support this fishery in Louisiana waters. Cyanobacteria including
toxic species are often among the dominant phytoplankton species in these waters and as such have the
potential to contribute to the food web. Furthermore, we found cyanotoxins in blue crab collected from
the upper Barataria Estuary indicating toxic cyanobacteria may be an important basal resource. For these
reasons, we investigated basal food resource contributions and food web pathways of primary producers
in two lakes, Lac des Allemands and Lake Salvador, in the upper Barataria Estuary, Louisiana with a
focus on toxic cyanobacteria. We used multiple, natural abundance stable isotopes (15N and 13C) and
gut content analysis to determine food web interactions. To better understand the full extent of toxic
cyanobacteria on estuarine food webs, we also examined the transmission of cyanotoxins through
multiple trophic levels using a relatively new, highly sensitive toxin assay, enzyme-linked
immunosorbent assay (ELISA). While the combined use of stable isotopes and ELISA allowed us to
determine food web impacts and pathways of harmful algae, these techniques have rarely been used in
tandem. Water quality characteristics; such as NO3-, NH4+, PO43-, salinity, temperature and total
suspended solids (TSS); phytoplankton community composition and Chl a were also determined. We
found abundant toxic cyanobacteria including Microcystis and Anabaena in Lac des Allemands only. In
Lake Salvador, centric and chain diatoms dominated communities. Chl a and nutrients were higher in
Lac des Allemands relative to Lake Salvador. Natural abundance stable isotopes indicate that the nonselective, filter feeding clam, Rangia cuneata, were an important intermediate food resource for blue crab
and were a pathway in which cyanotoxins may reach blue crab in Lac des Allemands. Because these
waters are important sites for recreational and commercial fisheries, a better understanding of trophic
support and food web pathways of toxic cyanobacteria are essential.