Estrutura da comunidade zooplanctônica e disponibilidade de presas para peixes zooplanctívoros em um estuário antropizado no Nordeste do Brasil

Abstract

Zooplankton is available in estuaries as a link between plant energy sources and animal energy sources. This group is present as a food resource for fish communities, especially juveniles, who need these small items for their development. The zooplankton community is drived by hydrological processes, productivity, variations in the physical and chemical conditions of water and biotic factors. For this reason, this group is widely used as a model for studies in impacted environments due to its great sensitivity to environmental changes. The aim of this study was verify the structure of the zooplankton community in the Paraíba do Norte river estuary, an urbanized estuary, and to compare the predation on these organisms by juvenile fish belonging to the zooplankton guild along the estuarine zones. Sampling was carried out on spring tides, high tide and low tide, in six excursions throughout 2018 along three estuarine zones, divided by the salinity gradient. For study of zooplankton structure of the zooplankton community, three subsurface trawls were carried out at high tides, using a conical-cylindrical plankton net. For the study of the availability of zooplankton for fish, sampling was carried out with a plankton net to collect the available zooplankton organisms and simultaneously a haul was carried out with a beach seine net to collect the ichthyofauna. In each sampling point, environmental parameters of salinity, water temperature (ºC), transparency (cm), depth (m), chlorophyll a (µg/l) and total phosphorus (μg/l) were recorded. The biological material was taken to the laboratory for counting and identification at the lowest possible taxonomic level. The most abundant taxa that contributed to the similarity in the three estuarine zones were Calanoida and Cyclopoida, always presenting at least 35% contribution. The lowest taxon contributions and highest densities occurred in the intermediate zone due to the greater influence of the concentration of nutrients in that zone, mainly the Calanoida and Cyclopoida taxa. Although there is a greater predominance of these taxa, the prey selectivity values for zooplanktivorous fish were higher for other food items, indicating the preference for less abundant items. The selectivity is also related to the morphological characteristics of the fish, in which they showed correlations with the sizes of the items, indicating that the species select the food according to their ability to detect and capture prey, following the Theory of Foraging Optimum. These results emphasize the importance of studying estuarine communities in the face of environmental changes in a scenario of increasing urbanization. The study of food availability for estuarine fish, especially juveniles, allows to understand the dynamics of the food chain in the pelagic segment and to predict possible changes in the energy flow. The availability of food resources for the initial stages of fish species influences the stock of future fish and, thus, the study of availability acts as a tool for alternatives for the conservation and management of fishing resources in these coastal ecosystems.