Impacts of hydrodynamic regimes on the resuspension of particulate matter within a seagrass restoration site: Jennycliff Bay, Plymouth Sound, UK

Abstract

Seagrass habitats are ecosystem engineers, providing a multitude of valuable ecosystem services that benefit the environment and human populations. Seagrass meadows are declining at an enhanced rate; therefore, in recent years restoration efforts have increased. However, many projects have had limited rates of success. Site suitability assessments are conducted ahead of plantation efforts; however, some regimes are seldom considered despite them providing important insights into a site’s environmental conditions. For example, light availability to the benthos is a key component of restoration success and this is affected by hydrodynamic processes impacting the local resuspension regime of suspended particulate matter (SPM), and therefore water clarity. This study assesses the local SPM resuspension regime within the active seagrass restoration site known as Jennycliff Bay, Plymouth Sound, UK. Multiple surveys were conducted to assess the temporal variation in SPM concentrations. An oceanographic mooring provided a 7-week time series of current velocity and echo intensity (EI) measurements. Water samples’ SPM concentrations were determined using optical backscatter (OBS) data. OBS measurements were converted into mg/l values and used to validate the use of EI as a proxy measurement of SPM concentrations. A background resuspension regime was observed within Jennycliff Bay with SPM concentrations influenced by tidal cycles. Maximum SPM concentrations were observed ~2m above the seabed during an ebbing tidal phase. Peak in-situ EI measurements confirmed the occurrence of SPM re-settlement. Current velocities measured throughout the study were relatively modest, with a maximum current velocity of 0.28m/s observed throughout a flooding tide; an in-situ bed shear stress (BSS) values measured 0.15N/m2, only 0.01N/m2 below the maximum observed value, throughout the 7-week dataset. A clear correlation between current velocity, BSS and EI confirmed the relationship between hydrodynamic processes and SPM concentration. Furthermore, these results identified the influence of storm forcing, amplifying all the considered variables. This study can not only be used to advise seagrass restoration efforts within Jennycliff Bay, but also highlights the importance of assessing hydrodynamic regimes and local SPM resuspension within a proposed restoration site; therefore, the study can be carried forward and used as guidance for future research.