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

The seasonal distribution of plankton in a Mediterranean hypersalinecoastal lagoon has been studied through a dataset, comprisingthe taxonomic composition and the size–abundance distributionof both phyto- and zooplankton, measured by image analysis techniquesduring a one-year time series of weekly samplings. The studiedorganisms ranged from small nanoplanktonic heterotrophic flagellates(2 µm diameter) to fish larvae (>2 µm). The phytoplanktonannual succession was characterized by a winter period dominatedby 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 monospecificblooms of other diatoms (mainly of Chaetoceros sp.), a summerphase characterized by diatoms with blooms of Niztschia closterium,and a post-summer phase where dinoflagellates increased withpeaks of Ceratium furca. High densities of the microbial foodweb elements, flagellates and ciliates, indicate the importanceof the microbial loop in the ecosystem. Meroplankton contributedwidely to the seasonal character of the zooplankton distribution.Copepods, represented by Oithona nana, Centropages ponticusand Acartia spp. (mainly latisetosa), remained relatively constantthroughout the year, exhibiting a lower density in the warmerwater period (July–September). At the end of the samplingperiod, a massive proliferation of copepods (>1000 ind l^–1), mainly due to O. nana, took place. The autotrophsto heterotrophs biovolume ratio (A:H) remained lower than 1throughout the year except when, occasionally, large phytoplanktoncells bloomed. Persistent very low values of A:H suggest thatadditional sources of energy, such as the microbial loop ordetrital pathways, would be needed to sustain the high heterotrophicbiovolume found in the lagoon.