Observations and modelling of the variability of the Solent-Southampton Water estuarine system

Abstract

Understanding the effect of physical forcing on estuarine functioning is of major importance to determine the rate of exchange of water, sediments, pollutants and nutrients between the continent and the ocean. The combination of numerical models and discrete datasets is used to describe and investigate processes of natural variability in the partially-mixed, non-turbid, macrotidal Solent-Southampton Water estuarine system (UK).
The estuarine circulation and the response of wind forcing is examined using a three dimensional, free surface, finite volume and finite element grid model. Results from short-term (three months) simulations have been compared against data (ADCP measurements, tidal elevations and salinity distributions) collected in spring 2001 in Southampton Water and the Solent. The model reproduces the unique tidal curve of Southampton Water and the partially-mixed conditions prevailing in the upper estuary. The contribution of the local wind forcing (wind intensity · 12 m/s) to changes in water level is estimated to be up to 6 cm in Southampton Water in the model. The modelled salinity stratification varies over a semi-diurnal cycle with the highest stratification occurring at mid-ebb. Wind forcing is more efficient in altering stratification at ebb than flood. The temporal and spatial variability of light attenuation is also investigated.
Turbidity is demonstrated to be the major contribution to light attenuation using a time-series of discrete data collected in 2001, 2002 and 2003. A typical seasonal cycle of the coefficient of light attenuation is revealed, with a minimum in May-June and a maximum occurring in September-October. A second dataset of continuous measurements (10-minute interval) demonstrates the spring-neap modulation of the turbidity. The mouth of Southampton Water is more exposed to tidal mixing and therefore more turbid than the mid-estuary.
A five-compartment zero-dimensional pelagic ecosystem model including a sediment compartment has been developed to assess the impact of the variability of the light attenuation on the timing and the magnitude of the spring phytoplankton bloom. Using high resolution irradiance forcing and a constant coefficient of attenuation k set to the minimum May-June value, simulations compare well with discrete data of chlorophyll a, and less successfully with zooplankton and Dissolved Inorganic Nitrogen. A sensitivity analysis indicates that interranual variability in the phytoplankton spring bloom originates in order of importance from 1) parameterization of k 2) the variation of the seasonal cycle of surface irradiance 3) the intrinsic dynamics determined by the combination of fixed parameters of the ecosystem model.

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ORCID for Duncan A. Purdie: orcid.org/0000-0001-6672-1722

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