Temporal variability of phytoplankton biomass and net community production in a macrotidal temperate estuary

Coastal zones play a significant role in Earth's biogeochemical processes. Within these regions, estuaries are particularly important due to their complex ecological interactions and spatial and temporal variability. The aim of this study was to apply a year long high-frequency (15 min) environmental data time series to identify both the timing and factors influencing phytoplankton blooms in the Southampton Water estuary. Dissolved oxygen measurements from an in situ deployed optode were applied to the open diel oxygen method to estimate daily integrated rates of gross primary production (GPP), ecosystem respiration (ER) and net community production (NCP). Additional water quality data including temperature, salinity, chlorophyll concentration and turbidity allowed the relationship between physical and biological processes occurring over different time scales to be investigated. The occurrence of major phytoplankton blooms during the spring-summer period were associated with critical values of estuarine water temperature and mean water column irradiance. In addition, neap tides were found to promote the initiation of phytoplankton blooms in late spring and summer months. Annual daily average NCP for the estuarine ecosystem presented an estimated net heterotrophic state (−0.8 mmol O2 m−2 d−1), although seasonal productivity events shifted this state for several days and sometimes weeks to net autotrophic conditions. The results of this study have demonstrated how high frequency in situ dissolved oxygen measurements from an optode can make a valuable contribution to understanding the key factors influencing bloom events in a temperate macrotidal estuary. This approach if applied more widely to other coastal sites could therefore contribute to consolidating global annual primary production budgets for coastal regions. •Productivity rates and environmental conditions were evaluated in a temperate estuarine system using high-frequency data.•Initiation of phytoplankton bloom were correlated with critical values of temperature and mean water column irradiance.•Phytoplankton biomass growth was enhanced during neap tides due to reduced mixing and weak stratification.•An overall annual heterotrophic state was estimated for the estuary, seasonal events shifted the system to net autotrophy.