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

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. Plain Language Summary Phytoplankton exist at the base of the aquatic food web. These microorganisms harvest light to produce oxygen and biomass through photosynthesis in the presence of nutrients. To elucidate their potential for producing biomass, it is important to understand how environmental conditions control phytoplankton growth. This knowledge is particularly relevant in reduced-growth environments, such as the Guadalquivir estuary, which sustain high-nutrient loads from the catchment. In this study, analyses of observations and idealized model experiments revealed the complex spatiotemporal variability of biomass induced by the joint action of tides, runoff, and radiation. The attenuation of light due to the high levels of suspended sediment concentration (SSC) hindered seasonal specific growth rates of biomass. However, SSC and Chlorophyll-a (Chla) fluorescence, which is widely regarded as a measure of biomass content, did not always show an inverse relationship. Tidal resuspension caused SSC and Chla to peak at maximum ebb and flood. The large sinking velocities of Chla-containing matter that ensued from the results analysis indicate that a substantial amount of biomass is composed of algal species that are attached to sediments. The results will allow a better understanding and evaluation of the biotic response of the estuary to future environmental changes. Key Points Observed Chla concentration exhibited in-phase oscillations with the SSC at the spring-neap and tidal scales, contrary to what occurred at larger time scales Chla variability at these scales is hypothesized to be controlled by tidal resuspension of algal microorganisms, except at apogean neap < id=jgrc23863-li-0004>An idealized 1DV model, which accounted for basic biotic and physical processes, mimics and helps to explain the SSC-Chla relationships