Assessing in situ tolerances of eastern oysters (Crassostrea virginica) under moderate hypoxic regimes: implications for restoration.(Report)

ABSTRACT Increases in the frequency and duration of hypoxia and the loss of biogenic reefs are two of the most prominent environmental insults to estuaries. We investigated the interaction between moderate hypoxia and habitat restoration activities on estuarine ecosystems by measuring population growth and somatic growth for newly settled Eastern oysters (Crassostrea virginica). Experiments were conducted at a site that historically experiences moderate hypoxia (2.0 mg [l.sup.-1]

KEY WORDS: Hypoxia, Crassostrea virginica, Eastern oyster, population dynamics, oxygen

INTRODUCTION

Success of habitat restoration in the marine environment has proven to be one of the most difficult tasks confronting habitat managers, despite the large investment of both time and money (Mann & Powell 2007). A reason for this lack of success may be that location and design of restoration projects are chosen without accounting for the complex interactions that are possible when working in situ. Historical data and surveys of the biological and physical conditions are often the only tools used in the decision making process (see Brumbaugh et al. 2006 and may fail to provide adequate data to ensure success. Integration of ecologically focused empirical testing into restoration design may help increase the odds of success by determining if an animal or habitat "can survive" rather than "should survive" (Hare et al. 2006, Kennedy et al. 2008). The advantage of integrating experimental ecology with restoration would be the successful identification of survival bottlenecks and the design criteria to mitigate such problems.

Among the myriad of environmental issues that may create such survival bottlenecks in estuarine ecosystems are increasing frequency and severity of low oxygen conditions and wide-scale loss of biogenic habitats (Diaz & Rosenberg 1995, Jackson et al. 2001, Orth et al. 2006). Increases in severity and frequency of low oxygen events in coastal marine systems have made it important to understand the role that hypoxia has in structuring coastal ecosystems (Diaz & Rosenberg 1995, Lenihan & Peterson 1998). For bivalves, a low oxygen event can be operationally classified into three discrete levels, moderate hypoxia (4 mg. [L.sup.-1] [greater than or equal to] [O.sub.2] [greater than or equal to] 2 mg [L.sup.-1]), severe hypoxia (2 mg. [L.sup.-1] [greater than or equal to] [O.sub.2] [greater than or equal to] 0.5 mg [L.sup.-1]), and anoxia ([O.sub.2]

For benthic organisms, experimental research has concentrated on identifying and describing an organism's physical or physiological responses to hypoxia, then relating this back to that organism's ecology (e.g., de Zwaan 1977, Mistri 2004). Experiments are often conducted under highly controlled laboratory conditions (e.g., Seitz et al. 2003) and conducted at temperatures that often do not reflect conditions when hypoxic events are common, especially for locations in subtropical climates (NSTC 2003). Because of the lack of confounding factors and variability in oxygen concentrations for these laboratory experiments, such trials rarely replicate natural conditions (Diaz & Rosenberg 1995). The result is that few of the conclusions reached under laboratory conditions are robust enough to be applied to field conditions; however, the results from laboratory experiments are often accepted as prima facie for the observed patterns in the field.

Information obtained from the field can also result in data that is just as limiting as that obtained in the laboratory. Because the costs associated with restoring estuarine habitats are often high, field studies rather than habitat restoring pilot programs are often used to guide restoration. These studies are often limited to intermittent information about oxygen levels (e.g., Powers et al. 2005) and descriptions of organism settlement, recruitment, behavioral changes, and colonization during or after hypoxic events (Sagasti et al. 2000, Lenihan et al. 2001, Bell et al. 2003a, Bell et al. 2003b). The consequences are that projects are often designed without in situ data is that high-quality and biologically relevant for a specific location. To mitigate this, historical locations are often chosen with the assumption that simply returning the habitat to an earlier state will be enough to allow for the habitat to become stabile and thrive (Brumbaugh et al. 2006). One problem is that the conditions today (e.g., water quality, food supply, surrounding habitats, etc ...) are most likely different than they were before the habitat was damaged. As a result, only returning the benthic habitat to a previous site or condition may not be adequate to ensure success. Simple and inexpensive techniques are needed to assess the biological tolerances of benthic organisms to ensure that habitats are constructed in the most effective manner possible.