Hydrodynamics structure plankton communities and interactions in a freshwater tidal estuary

Abstract

Drivers of phytoplankton and zooplankton dynamics vary spatially and temporally in estuaries due to variation in hydrodynamic exchange and residence time, complicating efforts to understand controls on food web productivity. We conducted approximately monthly (2012–2019; n = 74) longitudinal sampling at 10 fixed stations along a freshwater tidal terminal channel in the San Francisco Estuary, California, characterized by seaward to landward gradients in water residence time, turbidity, nutrient concentrations, and plankton community composition. We used multivariate autoregressive state space (MARSS) models to quantify environmental (abiotic) and biotic controls on phytoplankton and mesozooplankton biomass. The importance of specific abiotic drivers (e.g., water temperature, turbidity, nutrients) and trophic interactions differed significantly among hydrodynamic exchange zones with different mean residence times. Abiotic drivers explained more variation in phytoplankton and zooplankton dynamics than a model including only trophic interactions, but individual phytoplankton–zooplankton interactions explained more variation than individual abiotic drivers. Interactions between zooplankton and phytoplankton were strongest in landward reaches with the longest residence times and the highest zooplankton biomass. Interactions between cryptophytes and both copepods and cladocerans were stronger than interactions between bacillariophytes (diatoms) and zooplankton taxa, despite contributing less biovolume in all but the most landward reaches. Our results demonstrate that trophic interactions and their relative strengths vary in a hydrodynamic context, contributing to food web heterogeneity within estuaries at spatial scales smaller than the freshwater to marine transition.

Evaluating top‐down, bottom‐up, and environmental drivers of pelagic food web dynamics along an estuarine gradient

“…However, we found net biotic and abiotic effects to be of comparable magnitude. We also found that driver strength varied by region and timescale, in agreement with prior studies that found variability in the relative importance of biotic and abiotic drivers with spatiotemporal context (Guinder et al, 2017; Smits et al, 2023). From a management perspective, the relative importance of abiotic drivers is important because environmental factors (i.e., flow and turbidity) can often be manipulated directly, for example, through freshwater flow manipulation (Hemraj et al, 2017; Sommer, 2020).…”

Influences of zonation on water fertility and structure communities of phytoplankton and benthos in Batukaras Mangrove Forest, Pangandaran District, Indonesia

“…Landward zone transitions from the more marineinfluenced seaward areas to the terrestrial realm; it experiences unique conditions that shape the phytoplankton dynamics. Furthermore, the hydrology of the landward zone, shaped by tides and freshwater inflow, plays a role in determining phytoplankton distribution (Smits et al 2023). One of the defining features of the landward zone is its proximity to terrestrial inputs, which can bring freshwater and nutrients into the ecosystem (Ward et al 2020).…”