Whole-Ecosystem Experiment Illustrates Short Timescale Hydrodynamic, Light, and Nutrient Control of Primary Production in a Terminal Slough

Abstract

Estuaries are among the most productive of aquatic ecosystems. Yet the collective understanding of patterns and drivers of primary production in estuaries is incomplete, in part due to complex hydrodynamics and multiple controlling factors that vary at a range of temporal and spatial scales. A whole-ecosystem experiment was conducted in a deep, pelagically dominated terminal channel of the Sacramento-San Joaquin Delta (California, USA) that seasonally appears to become nitrogen limited, to test whether adding calcium nitrate would stimulate primary productivity or increase phytoplankton density. Production did not respond consistently to fertilization, in part because nitrate and phytoplankton were dispersed away from the manipulated area within 1–3 days. Temporal and spatial patterns of gross primary production were more strongly related to stratification and light availability (i.e., turbidity) than nitrogen, highlighting the role of hydrodynamics in regulating system production. Similarly, chlorophyll was positively related not only to stratification but also to nitrogen—with a positive interaction—suggesting stratification may trigger nutrient limitation. The average rate of primary production (4.3 g O2 m−2 d−1), metabolic N demand (0.023 mg N L−1 d−1), and ambient dissolved inorganic nitrogen concentration (0.03 mg N L−1) indicate that nitrogen can become limiting in time and space, especially during episodic stratification events when phytoplankton are isolated within the photic zone, or farther upstream where water clarity increases, dispersive flux decreases, and stratification is stronger and more frequent. The role of hydrodynamics in organizing habitat connectivity and regulating physical and chemical processes at multiple temporal and spatial scales is critical for determining resource availability and evaluating biogeochemical processes in estuaries.

Hydrodynamics structure plankton communities and interactions in a freshwater tidal estuary

“…Previous work in the SFE has demonstrated light limitation of phytoplankton productivity due to high turbidity (Alpine & Cloern, 1988; Cloern, 1987), though field studies have shown discrepancies between productivity predicted by light models compared to in situ measurements in certain habitats (Parker et al, 2012; Stumpner, Bergamaschi, et al, 2020). Given this context, we expected to see negative effects of turbidity on phytoplankton, and only weak effects of nitrate in the landward reaches of the DWSC where nitrogen concentrations can become limiting in summer (Loken et al, 2022). However, only cryptophytes showed the expected negative relationship with turbidity (Figure 4), whereas bacillariophyte biomass was positively associated with turbidity in all three zones, and chlorophytes were positively associated in the HE zone.…”

“…Positive associations between nitrate and chlorophytes may reflect the fact that chlorophyte biovolume in the landward reaches is highest in summer, when dissolved inorganic nitrogen concentrations are lower. In situ measurements and incubation experiments suggest nitrogen‐limitation in the landward reaches of the DWSC during stratified periods of summer (Loken et al, 2022), but monthly sampling is insufficient to capture rapid phytoplankton blooms in response to nutrient inputs (Dugdale et al, 2007) and short‐time scale variation in hydrodynamics (Lenoch et al, 2021). Overall, the associations of nitrate and phosphate with phytoplankton were small and frequently non‐significant, suggesting that at seasonal and inter‐annual time scales other drivers were more important.…”

“…While our models supported the hypothesis of both taxon‐specific and habitat‐specific drivers of phytoplankton, the combined set of abiotic drivers and trophic interactions explained little temporal variation in phytoplankton biovolume (Appendix S1: Table S6, Figure S6), particularly for bacillariophytes. This is likely due to the coarse timescale of our sampling relative to phytoplankton doubling rates (Loken et al, 2022). While other studies in the SFE did not show substantial variation in primary production rates or community composition at weekly to biweekly time scales (Kimmerer et al, 2012), monthly sampling is insufficient to capture certain bottom‐up processes, such as bloom formation in response to nitrogen inputs or other physical dynamics such as stratification (Dahm et al, 2016; Dugdale et al, 2007; Lenoch et al, 2021).…”

“…For example, estuarine turbidity maxima created by density-driven currents are often sites of chlorophyll a accumulation and high zooplankton productivity, despite light-limitation of in situ primary production (Lapierre & Frenette, 2008;Lee et al, 2012). In estuarine terminal channels and sloughs, variation in tidal forcing and tidal excursion length creates distinct hydrodynamic zones with different water exchange rates (Stumpner, Burau, & Forrest, 2020), while tidal asymmetry produces variation in turbidity (e.g., light availability) and nutrient concentrations (Feyrer et al, 2017;Loken et al, 2022), all of which can potentially influence phytoplankton productivity.…”

“…Understanding these dynamics in the SFE is necessary to quantify food availability for threatened native fishes such as the Delta Smelt (Hypomesus transpacificus), the health and fitness of which are positively correlated with chlorophyll a and zooplankton biomass (Hammock et al, 2021). We conducted longitudinal sampling along a terminal channel in the north Delta (Figure 1), characterized by spatial gradients in water residence time (Downing et al, 2016;Stumpner, Burau, & Forrest, 2020), turbidity (Feyrer et al, 2017), and nutrient concentrations (Loken et al, 2022). The morphological simplicity and stable spatial gradients present within the study channel allowed us to test for the effects of water exchange and residence time on drivers of plankton biomass.…”