The effects of seagrass edges on hydrodynamic conditions, sediment transport, and bivalve recruitment

Coastal seagrass environments provide an extensive array of ecosystem services to the areas they occupy including carbon sequestration, shoreline protection, economic support of fishery industries, and enhanced quality of water and habitat. An increasingly warming climate and anthropogenic development have directly threatened these valuable environments and spurred accelerated decline of seagrass ecosystems, leading to overall loss and fragmentation. This advancing degradation has prompted new and expanded efforts in protection and restoration, and one of the world’s largest seagrass restoration endeavors exists at the Virginia Coast Reserve Long Term Ecological Research site. As this project and others like it aim to restore seagrass in new locations while combatting concurrent degradation, questions arise regarding the impact of vegetation on its surrounding environment. Considering the presence or absence of vegetation, seagrass has well-studied and significant influences on hydrodynamic conditions, wave activity, sediment transport, and faunal communities. As loss and fragmentation accelerate while restoration generates new frontiers of growth, considerations of spatial ecology have driven work asking how these well-established effects change over heterogenous landscapes, with implications for the value and consequences of a changing seagrass ecosystem. These factors have motivated the work presented here which asks how hydrodynamic and wave activity change across various edges of seagrass vegetation, and whether there are resulting effects on sediment transport and bivalve recruitment.
A combination of hydrodynamic instrumentation, novel physical sampling techniques, meteorological data, and manipulated seagrass landscapes were used at the VCR LTER during 2021 and 2022 to answer these questions and quantify the response of flow, sediment movement, and bivalve recruitment across variable seagrass vegetation and its edges. Mean flow velocities were consistently and significantly reduced in seagrass vegetation regardless of proximity to edges, with reductions compared to unvegetated areas ranging from 30% to over 75% and corresponding to seasonal increases in shoot density. Recruitment of juvenile bivalves was also significantly elevated in the same locations. Bare and vegetated sampling locations across edges yielded no significant differences in wave activity or sediment resuspension, but significant correlations between these factors revealed the sensitivity of edge-adjacent, low-density areas to sediment transport driven by weather events and changes in flow. This was demonstrated by a tenfold increase in sediment collection within benthic traps following severe storms and indicated that wave heights were a major predictor for sediment transport in this study. These results found across various edge configurations and a heterogenous vegetation landscape reveal direct hydrodynamic responses to meteorological conditions (e. g., winds, storms) and shoot density that alter both sediment transport and bivalve recruitment dynamics. This has implications for the success and influence of restoration attempting to combat ecosystem degradation caused by a changing climate and human development.