Seasonal analysis of abiotic factors impacting phytoplankton assemblages in Offatts Bayou, Galveston, Texas

The aim of this investigation in Offatts Bayou was to quantify natural shifts in distributions and abundances of algal groups and to examine spatio-temporal patterns of abiotic and biotic characteristics in the water column over a one year period. To accomplish this, hydrological and meteorological parameters were collected and phytoplankton biomass, community composition and chlorophyll a data were examined for significant relationships. Seasonal variations in water temperature, salinity, dissolved oxygen concentrations and pH levels, as well as wind effects, zooplankton grazing, light availability and hydrodynamic restriction were considered as the key controlling factors in phytoplankton dynamics in Offatts Bayou. Surface water samples and water column hydrological data were collected at eleven stations in the Offatts Bayou embayment on a regular basis (2 to 4 times per month), along with phytoplankton tows on a monthly basis. Spatial patterns of phytoplankton abundance generally reflected the degree of circulation in Offatts Bayou with higher abundances observed in the restricted areas and lower abundances in the well mixed regions. Temporally, diatom blooms became more prominent during winter, spring and autumn, which were characterized by cooler temperatures, less light availability, increased dissolved oxygen concentrations and reduced salinities than observed in summer. The most dominant diatoms were Guinardia delicatula, Ditylum brightwelli, Rhizosolenia setigera, Dactyliosolen fragillissimus and numerous Chaetoceros species. During summer, the waters of Offatts Bayou were warmer and more saline, which lead to the haptophyte, Corymbellus aureus, becoming the dominant taxa, with highest standing crops at the circulation restricted stations in Lake Madeline. While the results of this study support the importance of temperature, dissolved oxygen and pH as the critical controlling factors (p < 0.05) of phytoplankton biomass and diversity, it is clear that phytoplankton dynamics in Offatts Bayou must be viewed within the broader context of additional parameters such as salinity, stratification and wind effects. The progressively degrading conditions within Offatts Bayou emphasize the significance of studying and understanding the interrelationships of factors and mechanisms that influence phytoplankton dynamics. Long term monitoring of Offatts Bayou is essential for tracking, recording and assessing various human impacts to phytoplankton distribution, abundance, and productivity as well as impacts to higher trophic levels such as fish and humans.