Phytoplankton cell size in an urban tropical estuarine system in Northeast Brazil

Abstract

The variability of the phytoplankton community structure and cell size classes was quantitatively studied at an urban hypereutrophic tropical estuarine system to assess responses to inorganic nutrient levels. Samplings were carried out in three stations during the dry and wet seasons in 2015. Seasonal variability of temperature, salinity, phosphate, and ammonia levels was statistically significant, with higher values found in the dry season. Total chlorophyll a varied from 4.76 to 79.69 mg m−3 and the fractionated chlorophyll a (pico–nanophytoplankton) from 1.28 to 28.35 mg m−3. Cell density ranged from 172 x 10 3 to 8.1 × 10 6 cells L −1 with higher values during the dry season. The community structure was dominated by the nanophytoplankton (<20μm fraction) with cyanobacteria being particularly abundant. Pico–nanophytoplankton​ contributed to 47% of the total biomass, representing 65% at point 1, 55% at point 3, and 38% at point 2. Thalassiosira sp 1, Melosira sp. and Chroococcus sp. were the dominant species in number of cells. The largest cell size variation occurred in the cyanobacteria and diatoms, which dominated the area and were favored by the increase of temperature, salinity, ammonia and phosphate during the dry season. The high values of nutrients demonstrated that the environment is hypereutrophic as a consequence of the release of domestic effluents from a highly urbanized area. Salinity and nutrients (nitrate and ammonia) were influential factors for the quantity and size of phytoplankton cells, leading to changes in community structure. The availability of ammonia in the study area favored the selection and dominance of species of the fraction <20μm. However, the fraction >20μm had a greater contribution to total chlorophyll a and biomass in the studied region.