Growth and mortality of oysters (Crassostrea virginica) on constructed intertidal reefs: effects of tidal height and substrate level

Intertidal oyster reefs, 3-dimensional structures created by years of successive settlement of larval oysters on adult oyster shells, provide levels of surface and interstitial heterogeneity that are rare in marine ecosystems. Surprisingly, little is known about the ecological benefits for oysters (Crassostrea virginica) in these aerially exposed, structurally complex systems. In this study a 210 m 3 30 m intertidal reef was constructed. During three, 28-day sampling periods in the summer and early fall, growth and mortality of two size classes of oysters placed in cages at three tidal heights (25 cm above mean low water (MLW), MLW, and 90 cm below MLW) and at two substrate levels (reef surface and 10 cm below the reef surface) within the reef setting were examined. Mid-intertidal oysters residing within the reef interstices grew faster and enjoyed greater survival than mid-intertidal oysters at the reef surface during certain times of the year. Conversely, subtidal oysters inhabiting subsurface environments did not grow faster than surface dwelling subtidal oysters, but some oysters within the reef fabric (larger size class) experienced significantly higher survival. Along a tidal continuum, oysters at the reef surface grew best in subtidal locations, yet experienced the lowest mortalities in the low intertidal zone (MLW). Reef heterogeneity, which allows for residence along both interstitial and tidal gradients, may provide physical and biological refugia for oysters and furnish ideal spatial platforms for growth. Therefore, location within the reef habitat has a significant impact on the biology and ecology of C. virginica.  1999 Elsevier Science B.V. All rights reserved.