Identification of seasonal and interannual drivers of primary production in two temperate estuaries using high-frequency environmental data

Gomez Castillo, Africa Paulina (2022) Identification of seasonal and interannual drivers of primary production in two temperate estuaries using high-frequency environmental data. University of Southampton, Doctoral Thesis, 281pp.

Record type: Thesis (Doctoral)

Abstract

Estuaries are one of the most productive coastal regions and, within them, phytoplankton is the most important primary producer. Net community production, the balance between production and consumption, provides an integrated measure of the trophic state, determining if the system is accumulating or depleting organic matter and whether is a net sink or source of O₂ and CO₂. Given that interactions between physical and biological processes within estuaries tends to vary over diurnal and semi-diurnal timescales, this study used high-frequency environmental data to estimate interannual and seasonal drivers of primary production of two contrasting temperate estuaries, the Southampton Water estuary (2014–2020) and Christchurch Harbour estuary (2014–2018). An analysis of the correlation between phytoplankton blooms (from chlorophyll ‘a’) and environmental conditions, demonstrated correlations between the spring bloom initiation and week-long periods with >800 Wh m^-2 d^-1 peaks in surface light availability. Christchurch displayed the typical dynamics described for coastal temperate estuaries, displaying spring and autumn blooms, whereas Southampton presented blooms in spring and summer. Blooms in Southampton developed during neap tides and dissipated on the following spring tide, suggesting reduced estuarine flushing and possible stratification enhancing phytoplankton biomass growth during neap tides. Application of the open water diel oxygen method proved to be a reliable technique to integrate daily estimations of ecosystem production and respiration rates in both estuaries, as long as assumptions are appropriately addressed. Results from this method suggested Southampton Water had become more net heterotrophic (-1.3 to -48.7 mmol O₂ m^-2 d^-1), while Christchurch Harbour seemed had turnes net autotrophic (-11.7 to 19.8 mmol O₂ m^-2 d^-1). An examination of the metabolic balance (GPP:ER) classified both estuaries between oligotrophic and mesotrophic states, with Southampton leaning towards mesotrophic conditions. Estimations of the estuarine carbonate system parameters were performed in Southampton (2019-2020). The estuary was identified to be a source of CO₂ to the atmosphere, agreeing with the net heterotrophic classification from dissolved oxygen derived net community production measurements. Inconsistencies among estimations were attributed to possible seasonal stratification, underestimation of wind speed and estuarine flushing times. Frequency and magnitude of riverine inputs influenced the distribution of carbonate system parameters. Biological processes were identified as a major factor controlling the pH/oxygen saturation dynamics and CO₂ fluxes were observed to follow the pattern of dissolved inorganic carbon concentration. The heterogeneity of coastal zones is of global concern and results from this study will allow a better understanding of local and regional primary production dynamics as well as provide a baseline to assess future anthropogenic impacts and climate change alterations to the aquatic trophic state of the Southampton Water and Christchurch Harbour estuaries.