Application of extended full resolution MERIS imagery to assist coastal management of the area adjacent to the Guadalquivir estuary

Abstract

High concentrations of suspended solids in coastal waters directly affect numerous processes. In the case of the marine region influenced by the Guadalquivir estuary, turbidity is one of the primary factors affecting this complex ecosystem and its adjacent coastal region. Ten-year (2002–2012) ocean color observations from the MEdium Resolution Imaging Spectrometer (MERIS) at 300 m are used to assess the variability of turbidity and chlorophyll-a concentration (Chl) in this region. The combination of high-frequency data from automated sensors in addition to synoptic imagery results in an excellent complement to better study water quality parameters. Turbidity plumes detected using the backscattering characteristics of the surface waters with Remote Sensing Reflectance at wavelength 665 nm (Rrs665) reveal the effects of physical processes governing both the spatial and temporal turbidity distribution. The results exhibit seasonal and interannual fluctuations driven by rainfall, freshwater discharges from Alcalá del Río dam, and the North Atlantic Oscillation index (NAO), with maximum turbidity levels in late autumn and winter. In addition, the temporal 1-month lag of Chl maximum and peak turbidity suggests a coupling between sediment resuspension and blooms, whereas extreme turbidity is limiting light availability for photosynthesis. Furthermore, turbidity varies on fortnightly scales, presenting lower intensity during neap tide and vice versa during spring tide. An asymmetric behavior of plume signals in connection with the semidiurnal tidal cycle is found, with maximum values during low tide. Both fortnightly and semidiurnal regimes appear to be superimposed on the large time-scale variability and occur throughout the year. Chl concentrations are dominated by the fortnightly-driven turbidity features throughout the year with minimum Chl during spring tides. This study evidences the main mechanisms influencing plume patterns and Chl, characterizing for the first time their variability in relation with tidal forcing, and therefore advancing the understanding of sediment dynamics and its relevant impact on the regulation of the pelagic ecosystem of the basin. The information obtained is critical for supporting scientists, stakeholders and ecosystem policy-makers for the challenging sustainable coastal management of the Eastern Gulf of Cádiz and its resources.