Experimental determination of the effects of light limitation from suspended bag oyster (Crassostrea virginica) aquaculture on the structure and photosynthesis of eelgrass (Zostera marina)

- Decreased light levels caused reduced eelgrass structure and photosynthesis.
- Responses were detected within 67 days of exposure to 26% sub-surface irradiance.
- Response variables remained reduced 253 days after treatment removal.
- Our findings support field results of reduced eelgrass at oyster aquaculture sites.

Recent studies have hypothesized reduced eelgrass distribution in areas exposed to suspended bag oyster aquaculture of Crassostrea virginica in Eastern Canada is related to shading from aquaculture stock and equipment. The results of a 359-day manipulative field experiment support this hypothesis. Reductions in underwater light, at levels comparable to those found at suspended oyster operations, caused reduced eelgrass structure, morphometrics, and photosynthesis. Increased organic matter deposition under suspended bags neither led to biologically relevant declines in eelgrass metrics, nor mitigated the effects of light limitation. Shoot density, above-ground biomass, below-ground biomass, canopy height, leaf area index, leaf width, and photosynthetic capacity were all significantly reduced. These variables declined along a gradient of increased shading, with significant responses detected in as few as 67 days after exposure to 26% subsurface irradiance. Subsequent sampling 253 days after the removal of experimental treatments documented the potential for recovery in the form of seedling recruitment to the plots of heaviest impact. However, eelgrass response variables remained significantly reduced relative to controls, indicating previous assumptions of a rapid recovery potential for eelgrass exposed to suspended bag oyster aquaculture were incorrect.