Ichthyoplankton community in tropcal estuaries of the brazilian semiarid cost during extreme events: drought and heavy rainfall

Abstract

Tropical estuaries serve as growing and feeding areas for early life stages of fish that take advantage of mangroves as nursery zones. Despite the socioeconomic importance, studies of fish eggs and larvae (ichthyoplankton) in tropical estuaries are relatively scarce, especially in arid and semiarid regions. In a context of extreme precipitation events (e.g., drought and heavy rainfall) that directly influence the ichthyoplankton community, studies on the structure and dynamics of these organisms are essential to understand the role of these events, as they become severe. Therefore, this master dissertation is structured in papers with the objective of understand the ichthyoplankton communities in tropical estuaries, through systematic review (Chapter 01), and on semiarid Brazilian estuaries, during extreme events: severe drought (Chapter 02) and heavy rainfall (Chapter 03). The scientific literature highlights the relative importance of Brazil (N=19 papers) but warns about the scarcity of studies (N=1 papers) in the Brazilian semiarid coast. Water temperature (N=10 papers) and salinity (N=8 papers) are the key environmental factors that significantly affect the ichthyoplankton community in the estuaries analyzed. However, even though these ecosystems are important nursery zones for species of different origins, further research need to be done to better understand their nursery function and the effect of global environmental changes. Piranji estuary (affected by severe drought) was inverse and hypersaline most of the year (dry season) and the ichthyoplankton community presented low density, following a pattern of increasing density towards the downstream zone, near the river mouth, and low diversity, composed of few species of stress-tolerant estuarine and marine species (Anchoa hepsetus, Bathygobius soporator, Hippocampus reidi, Eucinostomus sp., Diapterus auratus, Caranx latus and Bardiella ronchus., Echeneis naucrates and Haemulon sp.). In addition, the dominance of preflexion larvae may indicate difficulty of development due to the retention in the system. Pacoti estuary (affected by heavy precipitation) was positive throughout the year mainly due to the high input of freshwater, during intense rainfall and dam water release. However, the ichthyoplankton community also presented low densities and the larvae assemblage was composed of estuarine and marine species (Atherinella brasiliensis, Gobionellus oceanicus, Sphoeroides testudineus, Strongylura sp., Hyporhamphus unifasciatus, Eucinostomus sp., Sciaenidae sp1 and Anchovia clupeoides). Freshwater species were absent. In Pacoti, the eggs were concentrated in the downstream zone, near the river mouth, and the larvae, although distributed along the river, were concentrated in the upstream zone. Heavy precipitations events may have flushed these organisms out of the estuary and thus reduced the density of these organisms. The dominance of flexion and postflexion larvae in the upstream zone may indicate that this area is used as a feeding area for the fish larvae. Studies of ichthyoplankton in tropical estuaries can elucidate the influence of global changes and extreme precipitation events on the recruitment of commercial fishes and the nursery function of mangroves. In this research, we present novel and valuable information about these organisms that are sensitive to environmental changes and research directions that can collaborate with the ichthyoplankton monitoring and the management of tropical estuaries.