Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon: the Mar Menor

The seasonal distribution of plankton in a Mediterranean hypersaline coastal lagoon has been studied through a dataset, comprising the taxonomic composition and the size-abundance distribution of both phyto- and zooplankton, measured by image analysis techniques during a one-year time series of weekly samplings. The studied organisms ranged from small nanoplanktonic heterotrophic flagellates (2 mum diameter) to fish larvae (>2 mum). The phytoplankton annual succession was characterized by a winter period dominated by Rhodomonas spp. and Cryptomonas spp. with Cyclotella spp. as the main diatom represented, a spring phase where diatoms (mainly Cyclotella) were the dominant group with some monospecific blooms of other diatoms (mainly of Chaetoceros sp.), a summer phase characterized by diatoms with blooms of Niztschia closterium, and a post-summer phase where dinoflagellates increased with peaks of Ceratium furca. High densities of the microbial food web elements, flagellates and ciliates, indicate the importance of the microbial loop in the ecosystem. Meroplankton contributed widely to the seasonal character of the zooplankton distribution. Copepods, represented by Oithona nana, Centropages ponticus and Acartia spp. (mainly latisetosa), remained relatively constant throughout the year, exhibiting a lower density in the warmer water period (July-September). At the end of the sampling period, a massive proliferation of copepods (>1000 ind l(-1)), mainly due to O. nana, took place. The autotrophs to heterotrophs biovolume ratio (A:H) remained lower than 1 throughout the year except when, occasionally, large phytoplankton cells bloomed. Persistent very low values of A:H suggest that additional sources of energy, such as the microbial loop or detrital pathways, would be needed to sustain the high heterotrophic biovolume found in the lagoon.