Using bottom trawls to monitor subsurface water clarity in marine ecosystems

Biophysical processes that affect subsurface water clarity play a key role in ecosystem function. However, is poorly monitored marine ecosystems because doing so requires in-situ sampling logistically difficult to conduct and sustain. Novel solutions are thus needed improve monitoring of clarity. To end, we developed method data processing algorithm enable the use bottom trawl fishing gear as platform for conducting using trawl-mounted irradiance sensors without disruption operations. The applies quality control checks measurements calculates downwelling diffuse attenuation coefficient, Kd, optical depth, ζ– apparent properties (AOPs) characterize rate decrease relative transmission respectively. We applied our measurements, obtained bottom-trawl-mounted archival tags equipped with photodiode collected during NOAA’s Alaska Fisheries Science Center annual summer surveys eastern Bering Sea continental shelf from 2004 2018. validated AOPs by quantitatively comparing surface-weighted Kd multi-sensor Kd(490) product Ocean Colour Climate Change Initiative project (OC-CCI) qualitatively evaluating whether tag was consistent patterns chlorophyll-a concentrations predicted coupled regional physical-biological model (Bering10K-BESTNPZ). additionally examined trends Sea. Key findings are: 1) decreased significantly 2018; 2) recurrent, pycnocline-associated, maximum occurred over much northwestern shelf, putatively due chlorophyll maximum; 3) turbid layer (nepheloid layer) present large portion shelf. Our study demonstrates trawls can provide useful clarity, especially when trawling conducted part systematic stock assessment survey.