Biomass, size structure and trophic compartments of the metazooplankton in the Sontecomapan lagoon (Veracruz, Mexico)

Abstract : In this chapter, based on re-analysis of previous published data, we study the biomass, trophic component and size structure of the zooplankton in the Sontecomapan Lagoon, in order to better evaluate its trophic status and assess its possible incidence on food transfers, eutrophication and terminal productivity in this lagoon. Our results show very low values of zooplankton/phytoplankton ratio, compared to literature data, revealing an overall low food-transfer at the basis of the food chain. The size-structure is in average dominated by small organisms which suggests a predation impact from fish on larger organisms (calanoid copepods). This impact was particularly important during the north wind season perhaps in relation with the breeding cycles of important fish species such as Cathorops aguadulce. However, tidal inputs of large-sized coastal marine organisms (e.g. Paracalanus aculeatus, Temora turbinata and Centropages velificatus) seems to compensate this "erosion" of large zooplankton in the region close to the communication channel. Our data analyses confirm the top-down effect on zooplankton, but also show that bottom-up forces affect phytoplankton (NH4 limitation) and zooplankton (phytoplankton limitation), meanwhile the top-down control by herbivorous zooplankton on phytoplankton is very low (inf. 10% of the phytoplankton stock day).