Hypoxia Interrupts the Secondary Production Service Provided by Oyster Reef Macrofauna in Mississippi Sound, USA

Production by macrofauna associated with oyster reefs offers a multi-purpose indicator of ecosystem function, ecological integrity, and restoration success. We examined responses by oyster-reef-associated macrofauna to a hypoxic event across three subtidal oyster-reef complexes in western Mississippi Sound as part of a large-scale restoration program. Four hypotheses within the context of a Linear mixed model examined differences in the following: (1) the macrofauna relative to the hypoxic event; (2) subregions relative to the severity of hypoxia; (3) responses by key macrofaunal taxa; and (4) short-term macrofaunal recovery. Densities and production potential of the entire oyster-reef macrofaunal community as well as for six key taxa all abruptly decreased in connection with the hypoxic event. Overall, total production potential and total density were four-fold lower in the summer of 2015 during the hypoxic event than in the summer of 2014. The degree of macrofaunal impact corresponded with the severity of hypoxia on the landscape scale. Total density and total production averaged nearly three-fold lower and more than eight-fold lower respectively across two critically hypoxic reef complexes compared to the least affected reef complex. Taxon-specific responses depended on tolerances to hypoxia as well as modes of reproduction and dispersal. Short-term macrofaunal recovery indicated surprising ecological resilience relative to the hypoxic interruption. Detrimental macrofaunal effects ameliorated substantially within several months when total production potential was comparable to that of the previous year. However, the short-term recovery of key taxa varied with respect to the hypoxic event. Hypoxia-induced seasonal directionality reversed between summer 2015 and fall 2015, as evidenced by lower values for total density, total production, and the density and production of the larger-bodied crustaceans, Eurypanopeus and Palaemonetes . Secondary production offers a key functional indicator of multiple ecosystem services and an informative gauge of ecological disturbance. Associated macrofauna should be used more widely to assess the ecological function and recovery of oyster reefs. This study expands our understanding of a productive oyster reef ecosystem that is threatened by multiple environmental challenges.