Abstract
Suspended mussel aquaculture has been proposed as a possible mechanism by which to remove excess nutrients from eutrophic marine areas. In this study, seasonal mussel growth and water clarification (through seston and phytoplank- ton depletion) were studied at a commercial-scale nutrient extractive mussel farm in a highly eu - trophic Danish fjord. Spatial variations in mussel biomass were examined throughout the year and no significant differences were detected within the farm. Food depletion by mussels was examined at spatial scales ranging from individuals to the entire farm and surrounding area. Phytoplankton deple- tion on the scale of individual mussel loops, deter- mined using the siphon mimic approach, indicated between 27 and 44% depletion of chlorophyll a (chl a). Farm-scale depletion was detected and visualized based on intensive 3D spatial surveys of the distribution of chl a and total suspended partic- ulate matter concentrations both inside and outside the farmed area. Average reductions in food supply within the farm ranged from 13 to 31%, with some areas showing > 50% food depletion. A food deple- tion model was developed to estimate the optimal mussel density required to maximize removal of excess phytoplankton. The model employed mus- sel clearance rate estimates derived from the ob - served magnitude of food depletion within the farm. Model results indicate that the mussel popu- lation filtration rate could be increased by 80 to 120% without any negative feedback on mussel growth. This could be accomplished by approxi- mately doubling the standing stock of mussels in the farm, hence doubling the amount of nutrients removed at mussel harvest.
Highlights
For decades it has been recognized that dense populations of bivalve filter-feeders possess a huge potential for clearing the water column of phytoplankton and other particulate matter (e.g. CloernAquacult Environ Interact 8: 311–329, 20161982, Officer et al 1982, Dame 1993, 1996)
The density and biomass of mussels, averaged over the 3 sections of the mussel farm are presented in Table 1, along with the measured total amount of nitrogen and phosphorus removed in December 2010, March and May 2011
Mussel biomass increased during the growth season, except during winter when the farm was covered with ice (Fig. 2)
Summary
For decades it has been recognized that dense populations of bivalve filter-feeders possess a huge potential for clearing the water column of phytoplankton and other particulate matter (e.g. CloernAquacult Environ Interact 8: 311–329, 20161982, Officer et al 1982, Dame 1993, 1996). The biofiltration activities of mussel populations can, under some conditions, reduce the occurrence and magnitude of algal blooms and markedly increase water clarity (e.g. Dame 1996). Commercial mussel aquaculture has been predicted and shown to reduce phytoplankton and seston biomass (i.e. depletion) at the scale of coastal ecosystems under intensive culture conditions (Grant et al 2008). The introduction of suspended bivalve farms has been proposed as an eco-engineering approach for removing nutrients from eutrophic marine environments and improving water quality (Haamer 1996, Edebo et al 2000, Newell 2004, Petersen 2004, Lindahl et al 2005, Gren et al 2009, Petersen et al 2012, 2014, 2015, Rose et al 2012, 2015)
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