Light indirectly mediates bivalve habitat modification and impacts on seagrass

• Light availability and blue mussels were manipulated in eelgrass mesocosms.
• Light changed mussel nutrient enrichment, but eelgrass was not nutrient limited.
• Mussels increased sediment sulfides and reduced net oxygen flux.
• Mussels and low light exacerbated sulfide intrusion of eelgrass tissues.
• Eelgrass resisted harm from sulfide stress, even during light limitation.

Environmental context may influence the sign, strength, and mechanisms of species interactions but few studies have experimentally tested the potential for abiotic conditions to mediate interactions through multiple co-occurring stress pathways. Abiotic conditions may mediate species interactions by directly or indirectly influencing the effects of habitat-modifying organisms that are capable of simultaneously ameliorating and exacerbating multiple stressors. It was hypothesized that light availability changes seagrass metabolism and thereby indirectly regulates bivalve habitat modification and subsequent impacts on seagrasses by shifting net effects between alleviation of nutrient stress and intensification of sulfide stress. To test this hypothesis, manipulations of light availability and blue mussel (Mytilus edulis) abundance were made in eelgrass (Zostera marina) mesocosms and biogeochemical and plant responses were measured. Light modified the effect of mussels on porewater ammonium, but eelgrass was not nutrient limited and, therefore, mussels did not enhance growth. Mussels increased sediment sulfides irrespective of light availability and, by reducing net oxygen flux (production and respiration), mussels and low light availability exacerbated sulfide intrusion of eelgrass tissues. Surprisingly, sulfide stress did not affect plant growth, survival, or energy stores. Thus, habitat modification by mussels may represent a risk to eelgrass, especially during low productivity conditions, but eelgrass can resist harm from short-term stress, even during light limitation. These findings suggest that while small-scale bivalve impacts on seagrasses may be variable in oligotrophic estuaries, they have the potential to be negative in eutrophic systems, which are increasing globally.