Landscape Transformation and Variation in Invasive Species Abundance Drive Change in Primary Production of Aquatic Vegetation in the Sacramento–San Joaquin Delta

Abstract

Conversion of wetlands in the Sacramento–San Joaquin Delta beginning in the mid-1800s resulted in a pronounced shift from a wetland-dominated food web to one driven by open-water primary producers. Submersed and floating aquatic vegetation (SAV and FAV) now rank highest in potential net primary production (NPP) among producer groups, and provide a comparable amount of carbon to the detrital food web as marshes. However, important details of this contribution that relate to shifts in species composition and habitat extent were not understood. Here, we review how changes in aquatic vegetation influence NPP and trophic support from the historical to modern periods, within the modern period (the last 2 decades), and under future management and climate scenarios. We estimate that NPP of SAV and FAV during the historical period was approximately half that of today, before increases in open water and introduction of the highly productive water primrose. During the modern period (the last 20 years), high interannual variability in the extent and relative composition of aquatic vegetation species has driven significant variation in total NPP. This recent temporal variation is 6 to 13 times larger than projected changes in production from the potential future scenarios we modeled, including a reduction in FAV by 20% through control measures, substantial wetland restoration (and thus increased channel area that could support SAV and FAV), and increased salinity intrusion in the western Delta with climate warming, which favors native species with greater salinity tolerance. Large temporal swings in NPP of SAV and FAV cascade to influence the degree of carbon that flows to consumers through detrital pathways and herbivory. This volatility and interannual inconsistency in aquatic vegetation support of food webs make achieving wetland restoration goals for the Delta—which could lead to recovery of a portion of the NPP lost since historical times—even more imperative.

Estimating Biogeochemical Rates Using a Computationally Efficient Lagrangian Approach

“…The first-order loss formulation in Eqs. 11 and 12 then imply an annual DIN uptake of 34 g N m −2 y −1 , roughly consistent with the carbon uptake estimate of Boyer et al (2023). This rough calculation indicates that uptake by vegetation growth alone could plausibly account for the estimated DIN loss in the model results.…”

“…While rooted vegetation may take up nutrients from sediment (Novick et al 2015;Boyer and Sutula 2016), Egeria densa, a species of submerged aquatic vegetation that is widely distributed through the Delta (Ta et al 2017;Khanna et al 2022), has been shown to take up most of its nitrogen from the water column (Feijóo 2002). Carbon uptake by Egeria Densa is estimated to be 464 g C m −2 y −1 (Boyer et al 2023). Assuming a carbon to nitrate ratio of 11.1 (Cloern et al 2002), this corresponds to a nitrate uptake of 41.8 g N m −2 y −1 .…”

“…These SVAs are widely distributed in the Delta and contain submerged aquatic vegetation species, such as the invasive Brazilian waterweed, Egeria densa, and floating aquatic vegetation species including the invasive water hyacinth, Pontederia crassipes (Ta et al 2017;Khanna et al 2022). The quantitative effect of SVAs on DIN cycling in the Delta is unknown, but SVAs are biogeochemically complex and can be highly productive (Boyer et al 2023).…”

Regional Diversity Trends of Nearshore Fish Assemblages of the Upper San Francisco Estuary

“…Severe, prolonged, and frequent drought conditions create cascading ecological effects throughout the Delta that are driven by elevated temperatures, changes to water residence time and primary productivity, and facilitation and establishment of invasive aquatic vegetation that can all detrimentally affect native species (Winder and Jassby 2011;Kimmerer et al 2019;Boyer et al 2023). These effects may affect environmental filtering processes and trophic interactions that limit the distribution and abundance of some species while allowing others to proliferate.…”