Abstract
The sea-surface microlayer (SSML) has been shown to be a concentration point for a variety of anthropogenic contaminants. Even in small amounts these contaminants can be toxic to sensitive early life-stages of ecologically and economically important species of fishes and invertebrates that spend time in the microlayer. To determine whether this was occurring off the Florida Keys, a survey of microlayer toxicity was initiated in early 1993. Samples were collected inside the reef tract and offshore (out to 16 km) of both Key Largo and Long Key. Ex situ bioassays, based on the development of the sea urchin, Lytechinus variegatus, and the spotted seatrout, Cynoscion nebulosus, were carried out on 48 SSML and 48 subsurface water samples collected during 26 trips to the Florida Keys from 1993-1995. Initial screening of the SSML demonstrated that 39.6% ($\rm n=48$) of the samples overall, and 61.5% ($\rm n=26$) of the samples taken from naturally slicked areas were toxic compared to collected subsurface water. SSML samples from non-slicked areas were generally not toxic. The severity of toxic responses ranged from mildly retarded growth to 100% embryolethality. Based on the results of a toxicity identification and evaluation (TIE) procedure, the toxicant was suspected to be an organic compound with a non-polar functional group. Preliminary tests, done by an outside laboratory, indicated that two of the most toxic SSML samples contained brevetoxin, a biotoxin produced by Ptycodiscus brevis. Pending a completed TIE, including a Phase III-toxicity confirmation procedure, the identification of any toxic agent remains speculative. Although this study has demonstrated that samples of SSML collected off the Florida Keys were toxic in laboratory bioassays, extrapolations to field settings, to other taxa, and to population-level responses remain open questions that warrant further investigation.