Bio-optical variability at a Vancouver Island aquaculture site

Variability of the particulate beam attenuation, scattering, and backscattering coefficients, along with discrete measurements of HPLC derived pigments and particulate organic carbon, is investigated within and beside a finfish aquaculture cage on the west coast of Vancouver Island, Canada. Large bio-optical variability was observed over three seasons: fall, spring, and summer. The use of multiple optical proxies for organic particle concentrations, bulk particle-size distributions, and compositions allowed for the characterization of the particle assemblage over the dynamic conditions. Specifically, in fall, low biological productivity and surface influences from runoff were observed. In the lower-cage, after feeding the fish, optical proxies suggest the dominance of large particles with high indices of refraction, possibly due to the influence of fish fecal particles. Optical variability in spring was driven by diatom bloom conditions (Chaetoceros and Skeletonema). Strong bio-optical relationships were observed and the optical proxies suggest a particle-size distribution (PSD) dominated by large particles with low indices of refraction. In turn, summer conditions displayed noticeably high and persistent particulate backscattering in the surface waters, suggesting the presence of an Emiliania huxleyi bloom. Neither spring nor summer showed distinct influences by aquaculture wastes indicating that optical characterization of particulate waste dispersal is likely constrained to low ambient seston conditions. Outside of these conditions bio-optical analysis would be beneficial for environmental monitoring of ambient particles moving through aquaculture systems.