Hydrodynamics in a shallow seasonally low-inflow estuary following eelgrass collapse

Abstract

Hydrodynamics play a critical role in mediating biological and ecological processes and can have major impacts on the distribution of habitat-forming species. Low-inflow estuaries are widespread in arid regions and during the dry season in Mediterranean climates. There is a growing need to evaluate dynamics and exchange processes in these systems and the resultant ecological linkages. We investigate the role that hydrodynamics play in shaping environmental gradients in a short and seasonally low-inflow estuary located along the central California coast. Since 2007, eelgrass meadows in Morro Bay have declined by more than 90%, representing the collapse of the major biogenic habitat. Despite the large-scale decline, eelgrass beds near the mouth of the bay remain resilient, suggesting that conditions in certain areas of the bay might allow or impede eelgrass retention and recovery. Oceanographic moorings were deployed throughout the bay during the summer dry season to assess spatial differences in environmental conditions and hydrodynamics across gradients in eelgrass survival. Relative to the mouth of the bay, the back bay water mass was significantly warmer (hyperthermal), more saline (hypersaline), less oxygenated, and more turbid, with longer flushing times, all of which have been identified as significant stressors on seagrasses. Moreover, there is weak exchange between the mouth and the back bay that effectively decouples the two water masses during most periods. Though the causes of the decline are not clear, gradients in environmental conditions driven by bay hydrodynamics appear to be preventing eelgrass recovery and restoration attempts in the back bay and keeping this region in an alternative state dominated by unvegetated intertidal mudflats. Ecosystems in low-inflow estuaries may be especially prone to ecological regime shifts or collapse and may require precautionary monitoring and management. This system and the dramatic ecological change that it has experienced, demonstrate the critical role that hydrodynamics play in ecosystem health and habitat suitability.

Northeast Pacific eelgrass dynamics: interannual expansion distances and meadow area variation over time

“…Heat stress exacerbates effects of low light on photosynthesis (Krumhansl et al 2021) and diminishes disease resistance (Graham et al 2021). The effects of runoff, eutrophication, sedimentation, and heat stress may be exacerbated by water withdrawals, drought, or landform changes that decrease estuarine water turnover (Walter et al 2018). Overall, sites varied in natural (e.g.…”

Wave, light, and dissolved oxygen exposures drive novel coastal eelgrass (Zostera pacifica) transplant performance

“…DO values in donor bed sites did not drop below 6.24 mg O 2 L -1 , but transplant sites, primarily Dockweiler and Malaga Cove, underwent multiple periods of extended hypoxia, and at Dockweiler, even values approaching anoxia (Figure 6). Low dissolved oxygen concentrations are reported as contributors of Zostera decline in other systems in the USA, such as the lower Chesapeake Bay (Moore and Jarvis, 2008), an Oregon estuary (Magel et al, 2023), and two separate California estuaries (Walter et al, 2018;Beheshti et al, 2022). A buildup in concentration of sulfides in the rhizome can occur during periods of hypoxia (Goodman et al, 1995), and may have significantly impacted transplant performance, though a quantification of sediment composition in our study is absent.…”

Application of Deep Learning for Classification of Intertidal Eelgrass from Drone-Acquired Imagery

“…Morro Bay is a relatively small (~930 hectares) semi-enclosed shallow estuarine ecosystem, located south of Monterey Bay and north of Point Conception State Marine Reserve, along an underexamined stretch of the California coast [56]. Morro Bay not only supports a wide variety of bird, fish, mammalian, and invertebrate communities [57], but is also a popular tourist destination, home to a fishing port for local fisheries [47], and supports two commercial shellfish aquaculture operations.…”