Bio-morphodynamics of the Choked Passage seagrass meadow on Calvert Island, British Columbia, Canada

Abstract

Seagrasses are ecosystem engineers, forming extensive meadows that provide critical habitat and modulate local morphodynamics. Their canopies induce drag on flow to attenuate mean flow and reduce near-bed flow velocities, which can shield the bed from erosion and sediment suspension. Alternatively, seagrass loss can enhance erosion and sediment suspension, which can be initiated through short-lived extreme events, or chronic long-term disturbances. Physical process and disturbances can govern the evolution of seagrass meadow ecosystems. In two separate chapters, this research examined 1) the influence of climate variability and storms on seagrass loss and erosion at a high spatial resolution, and 2) how flow attenuation by seagrass varies across tidal cycles and at different locations in the Choked Passage meadow, on the Central Coast of British Columbia. We used high resolution multibeam echosounder (MBES) bathymetry and backscatter data from 2018 to 2021, drone mapped seagrass delineations from 2014 to 2021, and wind and wave data from 2014 to 2021. Flow data (i.e. velocity magnitude, velocity direction, and acoustic backscatter) above the seagrass canopy was collected with an Acoustic Doppler Current Profiler (ADCP) along transects and moored to the seafloor over a tidal cycle. Sediment samples were collected from the bed to estimate critical shear stress and verify sediment classes from an acoustic backscatter analysis. From 2018 to 2021, the meadow experienced significant erosion (net surface lowering of -18,768 m3) and loss of seagrass (10% reduction), which we attribute to the preceding winter storm activity driven by moderate La Niña conditions. The spatial patterns of erosion and seagrass loss was non-uniform across the meadow. Coupled erosion and seagrass loss resulted in the generation and/or expansion of blowouts. We observed a trend of a reduction in seagrass coverage following winters with a high number of storm events and/or high recorded storm intensity from 2014 to 2021. We believe the Choked Passage seagrass meadow undergoes cyclic behaviour with reduction in seagrass coverage during energetic ENSO years, followed by a recovery period during weak years. The ADCP was used to detect the seagrass canopy height, measure flow, and estimate shear stress. Overall, flow is fastest in the northern section of the main meadow, particularly in the north-west corner where the meadow is patchy. Moreover, flow appears to accelerate through the meadow interior, which suggests that topographic steering and the strength of incoming currents exceeds the ability of seagrass to dampen flow velocity. During the transition from peak flood to ebb, flow velocity remained heightened for longer above the southern meadow and lagged the other sections. Shear stress results indicate that sediment can be transported as bedload and in suspension under peak flow velocities at some of the sites examined within the meadow. Shear stress is largest in the meadow center and lower towards the southern margin of the main meadow. Based on our results, when sediment transport is initiated under peak tidal and/or extreme conditions, sediment is likely primarily transported as bedload, creating the observed sand wave and blowout bedforms. This research demonstrated linkages between extreme storms (during ENSO years), seabed morphology, and seagrass coverage, and examined the variability in the interaction between flow, seagrass, and sediment transport. Geomorphic processes and disturbances have an important influence on ecosystem structure and function over time, therefore, it is important to understand how these processes operate and are modified by external drivers. The results of this study have significant implications on seagrass conservation, restoration, and the evolution of coastal landscapes.