Water currents and light intensity data for estimating sediment plume size and light conditions from mussel dredging in two areas of the Limfjorden, Denmark

As part of a research project supported by the European Marine Fishery Fund and the Danish Fisheries Agency (grant no. 33113-I-16-011) and the Danish Centre for Environment and Energy at Aarhus University, field studies were conducted in two different areas of the Limfjorden, in Lovns Bredning (26th February to 4th March 2017) and in Løgstør Bredning (6-10th March 2017). In each sampling area, 16 fixed-position moorings were deployed in two circular sensor arrays around a mussel dredging area to investigate possible effects of dredging related sediment plumes (Pastor et al, 2020). The inner array (stations 1 - 8) had a diameter of 200 m and the outer array (stations 9 - 16) was set up with a diameter of 600 m. Integrated temperature and light loggers (HOBO Pendant Temperature/Light 64K Data Logger) were mounted on each mooring in the two circular arrays at three different water depths (0.5, 1.5 and 3.0 m above seabed). The 48 loggers (3 depths and 16 stations) were deployed in each area over a period of four days and light intensity time series were collected with a sampling interval of 30 seconds. Five loggers in the Lovns area did not provide any data due to technical problems (two near-surface loggers and three loggers at 1.5 m above the seabed). However, only one bottom logger at Løgstør Bredning failed to provide data. Each time series was filtered using a 3-minute moving average for removing the largest outliers, but retaining sharp changes in light intensity associated with short-lived sediment plumes caused by experimental mussel dredging. In addition, a 600 kHz ADCP (RDI Workhorse Sentinel) was deployed at each site (station 1 at the Lovns Bredning mooring, station 3 at the Løgstør Bredning mooring) in a bottom-mounted, upward-looking configuration. 30-second ensembles of 3D velocity components were collected along with corresponding records of beam correlation and error velocity. Each vertical profile had a bin size of 0.5 m and the first bin was at 1.59 m above the bottom (blanking distance: 0.88 m). The velocity data time series at each bin were filtered using a 30 minute moving average.