Nutrient input dynamics in the Mar Menor coastal lagoon has recently changed as a consequence of changes in agricultural practises. An interannual comparison of the environmental variables and the planktonic biomass size-spectra was performed between 1988 and 1997. While nitrate concentration was low in 1988, the values in 1997 increased considerably. Since 1995, two alloctonous jellyfish species (Rhyzostoma pulmo and Cotylorhiza tuberculata) occurred in large numbers in summer time and reached peak abundance in summer of 1997. The size-spectra analysis comparison revealed that, in spite of changes in nutrient input that stimulated the growth of larger phytoplankton cells, there were no significant differences in the spectra slope which followed a similar seasonal trend in both years. However, the plankton biovolume considered under the size range compared (between 2 and 1000 µm diameter) was, paradoxically, always lower in 1997. Given that there were higher nutrient levels in 1997, this finding suggest a strong top-down control mechanism of size structure. Gut contents of jellyfishes showed their preference for large diatoms, tintinnids, veliger larvae and copepods, corroborating that size structure in these assemblages can be subject to top-down control. The implication of these results is that the feeding activities of large gelatinous zooplankton (jellyfishes) may play an important role controlling the consequences of eutrophication within the Mar Menor coastal lagoon.

The Mar Menor is a sheltered and hypersaline lagoon, with salinity ranges between 38 and 51 psu. The lagoon is threatened by several pressures and in the last decades detrimental impact on the natural community structure and dynamics have increased. In the watershed, agricultural practices are rapidly evolving from extensive dry crop farming to intensively irrigated crops, with increasing loads of nutrient and pollutants to the lagoon. Hydrological conditions, nutrients and chlorophyll a concentrations were analysed in 1997 and 2002-2003 in a grid of 20 stations in the lagoon. Different time scales, from daily to interannual, were considered. In the considered periods, the dissolved inorganic nitrogen (DIN) increased whilst phosphate decreased significantly. These contrasting patterns depended upon the increased agricultural loading for DIN and were due to the implementation of the wastewater works for phosphates. In 1997 and 2002, the highest nitrate concentrations were usually found on the west coast of the lagoon, close to the mouths of the main watercourses. In parallel, the lowest concentrations were detected at the inner coastline along ''La Manga'' sandy bar and ''El Estacio'' channel. Based on weekly data, correlations between chlorophyll a concentrations and environmental variables disagreed with traditional eutrophication models. Relationships between chlorophyll a and nutrients were negative, suggesting that in the short term phytoplankton controlled nutrient concentrations. Moreover, nitrate and phosphorous seemed to alternate as limiting factors. The relationships between chlorophyll a became positive when considering time lags and analysed at longer time scales (monthly or seasonal means), thus suggesting a very rapid response of primary producers to nutrient enrichment. A significant correlation between chlorophyll a concentration and fish larvae density was also found at all time scales analysed, suggesting a top-down control of the trophic web.

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 μm diameter) to fish larvae (>2 μm). 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.

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