Physical-biological coupling on oyster reefs: How habitat structure influences individual performance

A large-scale field experiment was conducted to test whether the physical s tructure of biogenic reef habitat controls physical conditions (hydrodynami cs and hydrographics) with subsequent influence on the performance (recruit ment, growth, and survival) of a benthic suspension feeder. The experimenta l system consisted of restored subtidal oyster reefs inhabited by the easte rn oyster Crassostrea virginica. To determine whether the size of reefs inf luences the flow environment and oyster performance, reefs of four heights- tall (2 m), short (1 m), dredged (0.6 m), and low (0.1 m)-were constructed at 3-m water depth in the Neuse River estuary, North Carolina, USA. To test whether oyster performance varies with water depth and hydrographic condit ions, tall and short reefs were also constructed at 6-m water depth. Flow s peed, sedimentation, temperature, salinity, dissolved oxygen, and the perfo rmance of oysters were measured as a function of reef height, position on r eef, and water depth over a 10-mo period. Flow speed was found to increase on reefs with reef height and elevation on reefs. Rates of sediment deposit ion were seasonally highest where flow speed was lowest, at the bases of re efs, and seasonally decreased with increasing water depth. More than 90% of the surface area of low reefs was buried after only 16 mo of exposure in t he estuary, indicating that reef height controls habitat quality (and quant ity) indirectly through its effect on flow. Short reefs and the bases of ta ll reefs at 6-m depth were exposed to a total of 26 d of hypoxia/anoxia. No other reef treatment was exposed to >5 d of hypoxia. Physical conditions on experimental reefs had a profound influence on the p erformance of oysters as the flow environment alone explained 81% of variab ility in oyster growth and mortality. Recruitment of oysters over a 2-mo pe riod was slightly higher on the front bases than the crests of reefs, but d id not vary with reef height or water depth. After 10 mo, the shell growth and condition index of genetically similar, hatchery-raised oysters were gr eatest on the crests of tall and short reefs, where flow speed and quality of suspended food material were highest, and sediment deposition was lowest . Growth was greatest overall at the crests of tall reefs located at 6-m wa ter depth where flow speed was high, and the numbers of days exposed to hyp oxia/anoxia and variation in salinity were lowest. Total percentage mortali ty of oysters after 10 mo was greater on low reefs located at 3-m depth tha n on all other reef types and was greater on the bases than crests of tall, short, and dredged reefs. Predation by crabs and fishes accounted for 4-20 % of total oyster mortality and showed no pattern across reef treatments. R esults of this experiment indicate (1) that the physical structure and loca tion of biogenic habitat controls local physical variables and (2) that, in turn, physical variables, especially flow speed, have a profound influence on the performance of a resident species. Realization that an ecological f unction of habitat is to indirectly control local population production thr ough physical-biological coupling should improve our ability to conserve, r estore, and manage habitat and associated species diversity. Better ecologi cal engineering of restored oyster reef habitat is likely to improve fisher y production and help maintain estuarine biodiversity.