Spatio-temporal variation of the phytoplankton community in a tropical estuarine gradient, under the influence of river damming

Abstract

The Paraguaçu River is one of the most important rivers in Bahia State, Brazil, and the main tributary of the Todos os Santos Bay, the second largest in that country. We evaluated the spatio-temporal variations in the composition and structure of the phytoplankton community in the Paraguaçu River estuary as influenced by rainfall and fluvial discharge from the Pedra do Cavalo dam. Bi-monthly collections were carried out from May 2018 to March 2019 at 12 sampling stations in the estuary. During this period, phytoplankton samples, water samples to determinate the concentration of dissolved inorganic nutrients and data on salinity, temperature, pH, dissolved oxygen, and water transparency were collected. Data on rainfall and dam discharge were also obtained. Abiotic variables and chlorophyll-a showed significant differences in the estuary zones, and silicate concentrations demonstrated differences between the dam discharge periods. A total of 194 phytoplankton taxa were identified, with diatoms showing the highest densities and diversities throughout the study period. Cyclotella meneghiniana, Nitzschia sp., and Scrippsiella acuminata formed blooms in the dry season, whereas blooms of Planktothrix isothrix occurred in the rainy season and during a period of medium dam discharge in the dry season. The influence of dam discharge was quite evident, with increases in nutrient concentrations in the estuary during the months of higher flow that altered the composition and density of the phytoplankton communities in the different system zones. The formation of blooms in response to increased nutrient availability represents an increase in the primary productivity of the system, although blooms of potentially harmful species (such as Planktothrix isothrix) may represent a risk to human activities in the region.