Water flow mediates microscale hypoxia within mussel aggregations

Foundational species, such as bivalves, corals, seagrasses are well known for their ability to modify their local flow environment. This form of habitat amelioration is important for providing refuge habitat for a diversity of species with low tolerance for high flow conditions. For foundational species such as mussels, the benefits of reduced flow include reduced risk of dislodgment and energetic costs for producing byssal attachments. However, reduced flow in aggregations can also reduce water exchange with the environment, which can limit access to food resources (phytoplankton) and alter the local, ‘microscale’ chemical environment within the aggregations. Motivated by recent laboratory findings that mussel byssal attachment is weakened by low pH (< 7.6) and low dissolved oxygen (<1 mg/l), and field observations that these conditions can occur within mussel aggregations in coastal conditions, we conducted laboratory flume and field assays to quantify flow effects on the microscale chemical environment where the byssal threads are produced. Specifically, we placed small aggregations of mussels (Mytilus trossulus, M. galloprovincialis or M. californianus) in a flume and measured the dissolved oxygen (DO) concentration within the aggregation relative to free stream as a function of flow speed. Mussels in aggregations that mimicked monolayer beds or aquaculture long lines reduced microscale DO conditions at flow speeds < 5 cm/s. The magnitude of the reduction in DO, up to 2.5 mg/L below ambient after 30 minutes, depended on flow speed, water temperature, mussel size and species. Preliminary field trials yielded similar results, with microscale DO dropping below 4 mg/l. We hypothesize that more extensive reductions in DO would occur in the ‘mature’ aggregations that are commonly found in wild and farmed mussel populations, which accumulate microorganisms and sediments. It is likely that similar flow-mediated microscale processes affect many other foundational species, and that traditional environmental monitoring may not adequately represent the chemical environment at biologically relevant scales.