Assessing the impact of seasonal variations on the density structure of a weak freshwater plume

Abstract

Freshwater plumes are features of periodically intense salinity stratification created by an ebbing tide. The dynamics of these features are important for biological events, such as the spring bloom. Using an undulating towed Conductivity-Temperature-Depth (CTD) probe with a fluorometer attached, repeated sections were completed beyond Plymouth breakwater to a moored scientific station 9.6km offshore. These data were then gridded using linear interpolation and a density ratio was calculated at 3 positions: behind the plume, plume front and in front of the plume. The ratios of influence which thermohaline properties contribute to the density structure of a water column under seasonal variations were investigated for the first time. A greater thermal input during spring induced strong thermal stratification in the shelf sea environment, but had little influence upon estuarine outflow. The thermohaline gradient between these two water bodies determines the rate of exchange experienced at the front. The spatial extent of the freshwater plume showed a strong negative correlation (-0.986, P-value<0.001) compared to the distribution of phytoplankton estimated using the fluorometer data. Therefore it was hypothesised that: the weak freshwater front is too weak to induce sufficient nutrient upwelling; therefore it cannot host high concentrations of phytoplankton in comparison to fresh plume waters.