Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean

•The sample period was influenced by a strong El Niño event, decreasing rainfall in the sampling area.•Differences in species composition and density of the dinoflagellates were strongly influenced by the different characteristic of the sampling sites.•Highest cell densities were recorded in Bahía Chengue during the “rainy season”.•Salinity, temperature and nutrients were related to the variation in potentially toxic epiphytic dinoflagellates densities.•Bahía Chengue is considered favorable site for the occurrence of benthic harmful algae blooms. Some benthic dinoflagellates produce toxins that can affect other organisms including humans, and their proliferation seems to be related to the environmental variability. For this reason, the present study aims to compare the structural variation of potentially toxic dinoflagellates associated with the seagrass Thalassia testudinum from two nearby systems, with different environmental characteristics in Colombian Caribbean, corresponding to a brackish water coastal lagoon and an adjacent bay. Between January 2014 and December 2015, leaves of T. testudinum were collected monthly to obtain the dinoflagellates. Salinity, temperature, dissolved oxygen, pH, nutrients and total suspended solids (TSS) were measured, and precipitation data and the Oceanic Niño Index (ONI) were obtained. Dinoflagellates were detached from the leaves, morphologically identified by analyzing their thecal plates arrangements, and quantified using a Sedgewick-Rafter chamber. The information was analyzed using standard statistics and regression models. Fourteen species of potentially toxic epiphytic dinoflagellate belonging to four genera were recorded, being Prorocentrum the most representative in number of species. The maximum density, dominated by P. lima, were found in Bahía Chengue during the rainy season of 2014 (18452 and 20109 cells g−1 w.w.), with salinity of 35.50, high temperatures (>29.60 °C), dissolved oxygen >6 mg L−1, pH close to 8 and TSS >85 mg L−1. Densities at the Lagoon were lower than 80 cells g−1 w.w. with the highest values of Prorocentrum sp.1 under different environmental conditions. With the statistical relationships between the most abundant species and the main environmental variables, fundamental niche models were proposed in which cells could proliferate. The degree of risk to human health due to the presence of these potentially toxic epiphytic dinoflagellates will not be resolved until their toxicity discarded.