Seasonal and spatial variation of species toxicity in Mediterranean seaweed communities: correlation to biotic and abiotic factors

The toxicity of crude extracts of 32 seaweed species from the western Mediterranean was analysed by Microtox® assay in spring and autumn of 1996 and 1997. The species analysed represented more than 76% of seaweed coverage in the 3 algal communities studied: photophilic and sciaphilic communities from the Cabrera Archipelago (Balearic Islands), and a hemisciaphilic community from the Medes Archipelago (northeastern Iberian Peninsula). Most species showed seasonal variation of toxicity, which was greater in species from Cabrera than in those from Medes. Both, intra and interspecies variation of toxicity were found. Moreover, comparison of mean toxicity of these communities showed that toxicity was higher in November than in June in all cases, and that the photophilic community had both the highest number and the most toxic species. To make an ecologically relevant interpretation of the toxicity detected by Microtox®, we compared the toxicity of extracts analysed by the Microtox® test and those analysed by the commonly used sea urchin embryo assay. In addition to seaweeds, some species of invertebrates (sponges and ascidians) were compared to ascertain whether the relationship between the 2 tests was applicable to species belonging to different phyla. These comparisons allowed us to establish that 0.5 gamma units in Microtox® assay is the threshold value between toxic and non-toxic species. Following a light gradient from the photophilic to the sciaphilic communities, the seaweed species that were occasionally toxic increased while the always-toxic seaweeds decreased. Rhodophyta and Phaeophyta had a higher percentage of toxic species than Chlorophyta. Non-encrusting seaweed forms were more toxic than the encrusting ones, and in contrast to most other seaweeds, the non-encrusting calcareous species that increased coverage from June to November simultaneously displayed a marked decrease in toxicity. We conclude that the temporal variation of toxicity observed in the seaweeds studied may be partially explained by intrinsic factors of the species (growth rates and growth shapes).