Phytoplankton Dynamics in a Low-Inflow Estuary (Baffin Bay, TX) During Drought and High-Rainfall Conditions Associated with an El Niño Event

Low freshwater inflow estuaries are common worldwide, yet our understanding of phytoplankton dynamics in these systems is limited compared to river-influenced estuaries. Combining 3 years of monthly sampling data with analysis of a lower frequency, multidecadal dataset, we examined nutrient-phytoplankton dynamics in Baffin Bay-Upper Laguna Madre, TX (USA), a low-inflow lagoonal estuary. During a low rainfall, high salinity period (corresponding with non-El Nino conditions), phytoplankton community biovolume was high and consisted of a near monoculture of the harmful “brown tide” organism, Aureoumbra lagunensis. With the onset of El Nino conditions, rainfall increased and salinity decreased. Other phytoplankton groups became more prevalent, namely diatoms and the mixotrophic ciliate, Mesodinium sp., while prevalence of A. lagunensis declined. Although El Nino conditions corresponded with increased inorganic nitrogen concentrations, this did not lead to a near-term (weeks-months) increase in phytoplankton biovolume, indicating that the stimulatory effects of nutrient pulses may have been countered by the decreased residence times associated with increased inflow. Overall, results demonstrate that low freshwater inflows, as well as high residence times and salinities, can lead to increased phytoplankton biomass and decreased phytoplankton diversity, despite relatively low external nutrient loadings. With future expansion of arid/semiarid regions and/or increasing human freshwater demands, Baffin Bay and similar systems may experience lower inflows, more pronounced hypersalinity, and presumably less diverse phytoplankton communities, potentially dominated by harmful taxa as seen here.