Author(s): Maria Angeles Serrano; Manuel Diez-Minguito; Isabel Caballero; Gabriel Navarro

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Keywords: Exchange; Suspended sediments; Water density; Chlorophyll fluorescence; Dissolved oxygen

Abstract: Shelf seas and estuaries are coastal zones that are subject to an intense exchange of sediments, nutrients, and organic matter, thereby making them to be among the most productive areas worldwide. The Guadalquivir estuary plays a critical role in the fertilization and close control of biological productivity of the continental shelf of the Gulf of Cádiz (SW Iberian Peninsula). The work presented here was undertaken to analyze the temporal variability of the exchanges of biogeochemical properties driven by tides, freshwater discharges, and wind, between the estuary and its adjacent inner shelf. To achieve this aim, observations recorded by a real-time monitoring system during three years near the estuary mouth were analyzed at seasonal, event, and tidal scales. At seasonal scale, turbidity was positively correlated with high freshwater discharges and represented the most important limiting factor for productivity. This agrees with previous studies in the Gulf of Cádiz. High-resolution observations further revealed that the development of phytoplanktonic growth close to the mouth and its adjacent coastal waters occurred several months later than that in more open waters, which occurs in late winter. This difference in timing suggests that this increase of Chla is due to higher turbidity conditions near the Guadalquivir estuary mouth, which delays in the year the phytoplanktonic growth occurrence due to higher light limitations. The effects of upwelling- and downwelling-favorable wind events on the structure of the water column was evidenced by the tilting of isothermals and changes in phytoplankton growth rates and oxygen production, which responded rapidly to wind changes, as expected in shallow waters. A moderate discharge event during winter promoted high suspended sediment concentrations near the mouth. The discharge was followed by strong depletion of dissolved oxygen and (somehow weaker) chlorophyll fluorescence. One month later, once sediment concentrations decayed, chlorophyll and dissolved oxygen values notably increased. This behavior might be also induced by particular meteorological and oceanographic conditions or even by an expansion of the chlorophyll patch that develops in late winter due to the use of nutrients supplied by the discharge by photosynthetic organisms. Regarding the spring-neap and tidal variability during summer conditions, observed turbidity levels were greater during spring tides due to enhanced tidal shear stress. During floods (ebbs), turbidity decreased (increased), and higher (lower) values of salt, dissolved oxygen and chlorophyll fluorescence concentrations were observed. This is due to the M2 advection of the mean concentrations of these biogeochemical properties. Depth-time patterns of dissolved oxygen and chlorophyll were similar, i.e., positively correlated (and thus anti-correlated with turbidity patterns), which points to a photoautotrophic growth. Semidiurnal maxima of dissolved oxygen and chlorophyll are modulated at diel scale due to the control exerted by the solar incoming radiation on the primary production.

DOI: https://doi.org/10.3850/IAHR-39WC25217119202259

Year: 2022