Estrutura do microfitoplâncton em um estuário em duas condições contrastantes: regime hiposalino x regime hipersalino

Abstract

Estuarine microphytoplankton is relevant for maintaining the diversity, richness and composition of all resident biota and estuarine environmental services. It is important that changes in the structure of this community - in response to environmental variations - are well understood, as they may have repercussions throughout the trophic web, creating risks to estuarine environmental health and codependent human activities. In this sense, this study aims to analyze the variations in the microphytoplankton community structure of the Piranji River estuary under two contrasting environmental conditions: hyposaline regime, where the saline gradient grows in the estuary towards the sea; and hypersaline regime, where there is a decreasing saline gradient in the estuary towards the sea. The data were collected in two oceanographic campaigns, the first in May (hyposaline regime) and the other in November (hypersaline regime) 2015, at a fixed point near the mouth of the estuary. In each campaign, eight plankton samplings were performed over 21 hours during the spring tide, with intervals of 3 hours between them, in order to encompass the influence of the tides on the microphytoplankton composition. Samples were collected by subsurface horizontal trawls with 65 μm plankton net, coupled with a mechanical flowmeter. The abiotic data (pH, dissolved oxygen, temperature and salinity) were obtained by multiparametric probe, while the microphytoplankton was quantified in a Sedgewick-Rafter chamber. The average microphytoplankton density was higher in the hypersaline regime (5319 ± 6287 org / L) than in the hyposaline regime (291 ± 187 org / L). In both regimes, diatoms dominated the estuarine environment with relative abundance greater than 98% in all samples. Four diatom taxa were the most abundant: Lithodesmium undulatum, Bellerochea spp., Asterionelopsis tropicalis and cf. Lithodesmiodes sp. While L. undulatum dominates the condition in the hyposaline regime (> 80%) (Figure 6a), cf. Lithodesmioides sp. dominates in the hypersaline regime (> 95%). Both species are markers of estuarine waters. Coastal diatoms Bellerochea spp. show excellent osmoregulation potential, being present in the estuary in both salinity regimes. However, the genus occurs in higher density in the hypersaline regime. The RDA analysis pointed out that the main environmental variables that determine the differences in the structure of the microphytoplankton are salinity, total suspended solids (TSS), and orthophosphate and total phosphorus concentrations. It is warned that the excessive increase in microphytoplankton density in the estuary under the hypersaline regime may pose risks to the environmental health of the site and may cause undesirable consequences for the environment such as eutrophication. In addition, monitoring the structural fluctuations of microphytoplankton in semi-arid estuaries such as the Piranji River against extreme conditions such as the hypersaline regime may contribute to the projection of scenarios affected by global climate change.