Influence of environmental stressors and grazer immigration on ecosystem properties of an experimental eelgrass community

• Anthropogenic stressors are increasingly changing conditions and impacting important coastal habitats.
• We manipulated light, water temperature, and crustacean grazer propagule supply in an experimental eelgrass system.
• Shading reduced primary producer biomass, and had the largest effect overall.
• Warming reduced epiphytic algal biomass in the absence of grazers, and increased sessile invertebrate biomass overall.
• Propagule supply had no effect on crustacean grazer diversity, relative species abundance, and primary producer biomass.

Anthropogenic stressors associated with climate change and shoreline development are increasingly impacting important habitats. But when multiple stressors act simultaneously, the results are often difficult to predict. Effects of multiple anthropogenic stressors and their interactions with community processes are of particular concern in estuarine habitats because of their vulnerability to anthropogenic impacts and their provision of valuable ecosystem services such as fisheries production and sediment stabilization. Two stressors, warming temperatures and reduced light availability due to suspended materials, are expected to simultaneously affect important habitats such as seagrass systems, but the impacts of these stressors may be influenced by community processes such as immigration. Immigration of propagules can alter diversity and relative species abundances, rescue populations from extinction, and modify the capacity of communities to respond to environmental conditions. Using an experimental eelgrass (Zostera marina) system, we examined how warming, reduced light availability (shading), and periodic immigration of crustacean grazers influenced diversity, biomass, and structure of this community. Shading generally had large effects, and reduced biomass of all primary producers but did not affect grazer biomass. Warming reduced epiphytic algal biomass but only in the absence of grazers, while it increased sessile invertebrate biomass overall. Immigration of crustacean grazers did not interact significantly with stressors, and had little effect overall. Immigration did not influence diversity of grazers, relative abundance of grazer species, or biomass of primary producers and sessile invertebrates, however, it did increase biomass of grazers. Overall, reduced light availability by shading had strong effects on primary producers, whereas warming generally had weaker effects that differed with grazer presence. Thus, our results show that the impacts of reduced light availability and warming were independent and context specific, while crustacean grazer immigration effects were minimal in this model eelgrass system.