Relationships Between Chlorophyll-a and Suspended Sediment Concentration in a High-Nutrient Load Estuary: An Observational and Idealized Modeling Approach

Abstract

The Guadalquivir estuary is a high-nutrient load environment, yet it has a reduced primary production because the high suspended sediment concentration (SSC) causes light attenuation. High-resolution observations revealed relationships between the SSC and Chlorophyll-a (Chla) concentration from seasonal to intratidal time scales. Local maxima of time-averaged Chla levels occurred at locations with relatively low SSC. In the upper (lower) part of the estuary, larger Chla concentrations were observed during the wet (dry) season. In contrast to longer time scales, SSC and Chla exhibited in-phase oscillations during the spring-neap cycle. Both SSC and Chla were found to peak at maximum ebb and flood. There was no positive correlation at certain apogean neap tides, and instead, Chla exhibited daily variations. An idealized model was developed, which mimicked and helped to explain SSC-Chla relationships at different time scales. The model accounts for the vertical advection of Chla and SSC, tidal resuspension, radiation-mediated growth, and effects of the change in stratification stemming from tides and SSC. The specific growth rate of the Chla biomass is inhibited due to high SSC throughout the year. The in-phase relationship at spring-neap and tidal scales seems to be due to resuspension of algal microorganisms attached to sediments. Daily variability of Chla during apogean neap tides emerges because tidal shear stress is low and the strong decline in the suspended particulate matter reduces the light attenuation. The light-mediated growth of part of the biomass, still remaining in the upper layers of the water column, is then driven by the day-night cycle.