A comparative study of recurrent blooms of Alexandrium catenella in two Mediterranean coastal environments over an 11-year period

Abstract

Blooms of the toxic dinoflagellate Alexandrium catenella (temperate Asian clade) develop every year in two coastal environments in the NW Mediterranean Sea: Tarragona harbour and Thau lagoon. Using an 11-year data series (2000¿2010) from these localities, we studied the annual pattern, seasonal variability and trends in A. catenella cell abundance. Samples from Thau (2000¿2010) were re-examined with the calcofluor technique, revealing that A. catenella, not A. tamerense, is the species that produce annual recurrent blooms in Thau. We also studied the seasonal variability in relationship with biological, physico-chemical and meteorological variables occurring during the bloom events at the two sites. The occurrence of rhythms in the two A. catenella time series was investigated by Fourier spectral analyses, which showed two different frequency patterns. In addition to the annual periodicity, the spectrogram showed a periodicity of 4 months in Tarragona, and 6 months in Thau. None of the frequency patterns at either site showed periods longer than 1 year. There were no relevant trends in A. catenella cell abundances in both stations. The seasonality of A. catenella differed at the two sites, although both were subject to two temporally distinct blooming periods per year. Inorganic nutrients at both sites were clearly different, suggesting that blooms are not related to conventional nutrients. A common feature is the need of periods of calm wind required for bloom development at both sites. Wind stress previous to bloom developement is not required in Tarragona harbour in contrast to Thau lagoon. All together indicates that local factors are the main drivers of the different A. catenella seasonality at both locations