Evaluating chlorophyll depletion in mitigation mussel cultivation at multiple scales

Implementation of mussel culture for mitigating nutrient enrichment of coastal waters aims to optimize growth of mussels at production carrying capacity for maximum nutrient extraction. Simplified approaches for describing reductions in chlorophyll concentrations for the strategic deployment of mitigation units with regards to limited space and biophysical conditions can facilitate the proliferation of mitigation culture as a tool. Here we report data on measured chl a depletion at different scales in and around a mussel culture unit in western Sweden. A monitoring program was set up for a production period of 15 mo to characterize patterns of chl a depletion. The water column was almost permanently stratified, and concentrations of chl a were higher outside of the culture unit and autocorrelated within the unit by vertical and horizontal dimensions. Depletion of chl a within the culture unit during a focused sampling campaign period was negatively correlated with current velocity, greatest in the center of the unit (ca. 60% of ambient chl a concentrations) and lowest at the unit borders. Depletion was observed at the micro-scale within the boundary layer around the mussel aggregates, corresponding to a depletion gradient from the mussel aggregate surface of 0.074 µg l-1 cm-1. Shell length increment ranged between 0.04 and 0.29 mm d-1 and specific growth rate ranged between 0 and 4.6% of dry weight d-1. A simple model was applied to evaluate scenarios of alternate farm configurations and fit to measured depletion. Model runs showed that the angle of incoming flow is important in a low-current regime (i.e. <10 cm s-1), and orientating the culture unit in line with predominant current directions would allow for enhanced reduction of chl a.