Sediment Transport Model Quantifies Plume Length and Light Conditions From Mussel Dredging

Abstract

Mussel dredging causes resuspension of sediment particles that reduce water clarity and potentially leads to reduced eelgrass growth. In order to study the impact of resuspension from mussel dredging on light conditions in the water column, field experiments were conducted at two sites in the Limfjorden. Light loggers were placed in two circular arrays around the dredge area. Vertical profiles of current velocity were measured by an ADCP and the sediment particle size composition was obtained from sediment core samples. The field data was used to force, calibrate and validate a sediment transport model developed in the FlexSem model system. Changes in sediment concentrations during and after mussel dredging were modeled for the two sites and for seven scenarios. We found that the distance and direction of the plume in the model was in good agreement with light logger data. The plume duration was less than 1 h, and the impact range was between 260-540 m. The scenarios showed that fishing intensity and current speeds were most important for shaping the sediment plumes. Changes in suspended sediment concentrations were 0.62-1.79 mg l −1 on median average and 1.22-11.61 mg l −1 for the upper quantile of the plume, which were on the same order of magnitude as background values in the Limfjorden. The amount of fishing days during the eelgrass growth season was 6-8% in Lovns Bredning and 16-35% in Løgstør Bredning and less than 1-2% of the total area was dredged per season. Even though there are substantial changes in the light conditions from the sediment plumes, the overall spatio-temporal impact in the study area is considered low. We recommend that management plans in other areas could sustain a shellfish fishery by limiting fishing intensity and frequency near eelgrass beds. The presented approach combines observational data, sediment transport modeling and reported fishing activity. It is a step forward within sediment transport modeling and could be incorporated into environmental impact assessments. The results have recently been used as scientific background for recommendations to improve the management plans according to the Danish Mussel Policy and relevant EU Directives.

Ocean Circulation Model Applications for the Estuary-Coastal-Open Sea Continuum

“…ADCP observations of currents at two mussel dredging sites near Lovns and Løgstør have been collected during two measurement campaigns in spring 2017, as part of the TASSEEF project. The two deployments covered 5 days (27th February-3th March 2017, Lovns Bredning; 6th-10th March 2017, Løgstør Bredning), using a 600 kHz ADCP (RDI Workhorse Sentinel) as part of a larger mooring array (Pastor et al, 2020). The ADCP was deployed in a bottom-mounted, upward-looking configuration and recorded ensemble averaged profiles of 3D current velocities every 30 s. The vertical bin size was set to of 0.5 m and the first bin was collected at 1.59 m above the bottom (blanking distance: 0.88 m).…”

“…Currents, mixing and stratification in Limfjord are mainly driven by wind induced mixing and density changes (Hofmeister et al, 2009). Recent measurements in different broads of the Limfjord indicated rapidly changing currents of both magnitude and direction over the course of hours to days (Pastor et al, 2020). Model simulations by Pastor et al (2021) predict the strongest time-averaged currents and highest variability of current velocities across the fjord at the western boundary in Nissum Bredning and inside narrow channels connecting the broads further east.…”

“…Thus, safeguarding the presence and recovery of eelgrass beds in protected areas of the fjord is a central aspect of local marine management efforts (Krause-Jensen et al, 2005). Part of the management strategies involve a coordinated effort of mussel fishing and eelgrass protection interests to limit possible effects of suspended sediment plumes from mussel dredging on light conditions for seagrasses (Holmer et al, 2003;Pastor et al, 2020). The implementation of local or fjord wide management plans such as the delineation of fixed buffer zones around mussel dredging areas requires information of local environmental conditions on fine temporal and spatial scales.…”

“…Previous modeling efforts in the Limfjord involved mechanistic models for a variety of management and research applications. The modeling studies investigated the functioning and dynamics of benthic filter feeders (e.g., Maar et al, 2010), estimated the effects of mussel fishing activities on the sediment dynamics (Holmer et al, 2003;Pastor et al, 2020), studied the species connectivity (Pastor et al, 2021) and investigated physical drivers of stratification and de-stratification (Wiles et al, 2006;Hofmeister et al, 2009). Other modeling applications were dedicated to estimate changes in the frequency and magnitude of storm surges and water levels as a consequence of climate change (Knudsen et al, 2012).…”

“…The recent need for these parameters for coastal aquaculture support has sparked new model developments as part of two recent national and international research initiatives, TASSEEF and FORCOAST, dedicated to the management of the coastal zone in Danish and European waters. The main goal of the TASSEEF project was to develop new tools and methods for estimating the environmental impacts of mussel fishing in the Limfjord, in particular the generation and propagation of sediment plumes from mussel dredging (Pastor et al, 2020). The EU H2020 FORCOAST aims to develop novel CMEMS-based downstream services in different case study areas along European coastal zones including the Limfjord.…”