Evaluation of 24-h screen deployments as a standardized platform to monitor Gambierdiscus populations in the Florida Keys and U.S. Virgin Islands

Anchored mesh screens have been suggested as a standardized approach to monitor the cell abundances of epiphytic dinoflagellates in benthic habitats, including toxigenic members of the Gambierdiscus genus responsible for ciguatera poisoning (CP). Here we deployed screens for 24h at eight sites in the Florida Keys and St. Thomas (US Virgin Islands) to evaluate their performance relative to the traditional method of assessing Gambierdiscus abundance in which cell counts are normalized to wet weight of host algae. The 30-month study (April 2013 – August 2015) involved monthly sampling at sites where screens were suspended at near-bottom locations for a 24h period and retrieved, with concurrent collections of macrophytes; including Halimeda, Laurencia, and Thalassia in the Florida Keys, and Dictyota in both regions. Gambierdiscus cells were identified and enumerated in the screen and macrophyte samples, and several regression techniques were evaluated (linear regression using untransformed and log-transformed data; negative binomial distribution (NBD) regression) to determine how well the screen-derived data could estimate algal cell concentrations on the host algae. In all cases, the NBD models performed the best based on Akaike Information Criteria values, although 38% of the regressions were not statistically-significant, including all of the St. Thomas sites. The r(2) values were all < 0.75 and averaged 0.36, indicating relatively poor fit of the screen data. False negative results (regression models underestimating actual cell abundances) were common occurrences, ranging from 5 to 74% of the scenarios tested. In summary, these results indicate that 24h screen deployments do not appear to be consistent in all situations. Caution is therefore needed when considering 24h screens as a standardized monitoring approach for quantifying Gambierdiscus population dynamics across geography and ecosystems. Furthermore, neutral (artificial) substrates may not adequately capture either the host preference or palatability that likely influence the initial vector of toxin incorporation in the food web via herbivory on these macrophytes.