Fig. 3. Experimental design for in situ nutrient loading and sampling for light experiment. (A) Layout of the experimental array within eelgrass meadows (green rectangle) from a bird’s eye view. Each square represents an experimental quadrat. White circles represent PVC poles, present in every quadrat, and orange circles represent nutrient addition treatments present in half the quadrats. Insets show profile views of (B) control and (C) treatment quadrats. Sun icons indicate examples of random sampling locations for eelgrass shoots brought back to the research facility for production–irradiance measurements
ABSTRACT
Eelgrass Zostera marina meadows provide valuable ecosystem services to coastal communities. These shallow-water ecosystems in Eeyou Istchee (eastern James Bay, Quebec, Canada) support Cree ways of life by providing waterfowl foraging habitat, fish nurseries, and natural storm buffers. In 2019-2021, Eeyou Istchee eelgrass extent and shoot size remained well below historical baseline levels following a major decline in the late 1990s. We experimentally tested the potential roles of present-day nutrient and light conditions in limiting eelgrass productivity during the growing season. We tested the hypothesis that eelgrass growth is limited by water column nutrients using in situ nutrient additions in 2 eelgrass meadows. Eelgrass growth rate did not respond to nutrient addition at either site. We then assessed the ability of eelgrass to grow in low light conditions by producing ex situ production-irradiance curves. Eelgrass at both sites showed no evidence of low light acclimatization with saturating irradiances of 224 and 260 µmol photons m-2 s-1 and compensation points of 31 and 61 µmol photons m-2 s-1. We observed eelgrass growth rates of about 3.8% areal growth per day during the peak growth period, which are high when compared to other rates globally. Together, our results suggest that Eeyou Istchee eelgrass is growing under sufficient water column nutrient levels, but suboptimal growing season water column light conditions. Because Eeyou Istchee eelgrass meadows must endure long periods of seasonal ice-cover, light limitation during the short growth season may have longer-lasting impacts on these meadows than in more temperate ones.