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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California IV. Benthic Invertebrates in Soft Sediments A. BACKGROUND The benthic infaunal community would be expected to be modified by oyster mariculture through several processes: (1) provision of hard substrates in what is otherwise generally a flat plain of sediments, thereby occupying space and harboring predators of infaunal invertebrates; (2) deposition of oyster feces and pseudofeces, which under conditions of low flushing could induce sediment anoxia intolerable to most infaunal invertebrates; (3) modification of sediment organic content and thereby production of microbial foods for deposit feeders; (4) changes in sediment size, coarsening from erosion around bases of structures and fining where biodeposits settle, which can have large impacts on benthic invertebrate community composition; and (5) physical disturbance associated with culturists tending to and finally harvesting the shellfish. These separate processes are rarely distinguished experimentally; instead, comparisons are made between areas with and without all mariculture activity (Newell, 2004). B. WHAT IS THE BODY OF SCIENTIFIC STUDIES ON THE IMPACT OF THE OYSTER FARM ON DRAKES ESTERO? Only one study has been conducted on the effects of oyster maricul-ture on the benthic invertebrate community in Drakes Estero (Harbin-Ireland, 2004; summarized in Elliott-Fisk et al., 2005). Comparisons were made between benthic macro-invertebrate assemblages directly under
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California oyster culture racks and those at varying distances from the racks at two locations in Schooner Bay during January and October 2001. With the exception of enhanced abundance of amphipods and a decrease in abundance of a tanaid crustacean (Leptochelia dubia) under racks, few differences in community composition or diversity were found. No differences in sediment organic content were detected comparing the areas under and away from the racks, but there were small but significant changes in grain size, with proportionately more sand and less silt found under the racks. C. WHAT EFFECTS CAN BE DIRECTLY DEMONSTRATED BY RESEARCH CONDUCTED IN DRAKES ESTERO ITSELF? Few definitive conclusions can be drawn from the Harbin-Ireland (2004) research described above because of the limited nature of the study. Sampling was done during the winter and fall when invertebrate abundance is typically lower in temperate estuaries. Only eelgrass habitat was sampled and the test involved only oyster rack culture, whereas bottom bag culture on intertidal flats is now also an important part of the oyster operation. Nonetheless, some conclusions can be reached, which are supported by significant parallels to other work (see below) in several U.S. West Coast estuaries and elsewhere. Specifically, the flushing by tidal currents in Drakes Estero is sufficient to induce erosion around the stakes holding the oyster racks in eelgrass beds, but the resulting change in size composition of sediments is minor. These tidal currents also are sufficient to disperse the organic rich oyster bio-deposits sufficiently widely to avoid inducing detectable organic enrichment of the sediments nearby and subsequent mass mortality of benthic macro-invertebrates from sediment anoxia. Any changes in the benthic infaunal communities of the eelgrass habitat induced by flow modifications and biodeposition are subtle. D. WHAT EFFECTS CAN REASONABLY BE INFERRED FROM RESEARCH CONDUCTED IN SIMILAR ECOSYSTEMS? Comparable studies of shellfish mariculture show that sediment enrichment from biodeposits varies depending on the culture practices, species cultured, biomass or stocking density, and the physical environment in which it is conducted (Callier et al., 2006; Nizzoli et al., 2006). When organic enrichment of sediments occurs, the typical response to the resulting decrease in sediment oxygenation involves a change from a diverse benthic community dominated by suspension feeders (mollusks, crustaceans, and some polychaetes) to a less diverse community domi-
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California nated by smaller deposit feeders (usually polychaetes). Such responses are associated with low current flow, very dense shellfish culture, or both (Castel et al., 1989; Nugues et al., 1996; Mirto et al., 2000; Christensen et al., 2003; Forrest and Creese, 2006; Lu and Grant, 2008). Such modifications of the benthos are generally absent where stocking density is low or moderate or where currents are strong enough to disperse the biodeposits (Crawford et al., 2003; Mallet et al., 2006). Tidal currents have previously been shown to enhance erosion around the base of mariculture structures (Pregnall, 1993; Everett et al., 1995), and this process probably explains the slightly coarser substrate found under oyster culture racks in Drakes Estero. Such an explanation is consistent with the conclusion that tidal flows are sufficient to disperse the biodeposits far enough to prevent detectable organic loading at the relatively low oyster stocking densities used in Drakes Estero. The absence of eelgrass underneath the racks also implies faster near-bottom flows than underneath the eelgrass canopy, which baffles flow velocity by friction. Slower flows underneath seagrass canopies induce deposition of fine particles and thus create finer sediment sizes (Madsen et al., 2001). The observed enhancement of amphipods and reduction in tanaids underneath racks might represent a response to (1) sedimentary changes induced by local loss of eelgrass, allowing faster flows under racks; (2) some aspect of oysters and epibiota on racks that influences the soft-sediment benthos below; or (3) secondary effects of rack structure acting on predators of benthic macro-invertebrates. For example, racks might attract predatory fishes that feed on tanaids. This suggestion is supported by the recognition that small benthic crustaceans are often preferred prey by demersal fishes and by the trend of more structure-oriented predators like kelp surfperch near the racks (Wechsler, 2004). On the other hand, amphipods are also preferred prey for many demersal fishes. Some amphipods have been shown to associate with oysters and structures, or else with the macroalgae and fouling organisms found on them, both on the west coast (Eogammarus and Amphithoe: Dumbauld et al., 2000; Dumbauld et al., 2001) and elsewhere (Gammarus: Rodney and Paynter, 2006). Scientific studies of both on-bottom culture (Trianni, 1995; Hosack et al., 2006; Ferraro and Cole, 2007) and off-bottom culture in other west coast estuaries (Pregnall, 1993; Rumrill and Poulton, 2004) generally indicate that the benthic community associated with oyster culture is more diverse than that of unstructured bottom, and either equal to or slightly less diverse than that of eelgrass habitat. Enhanced diversity in structured habitat has also been documented for epibenthic meiofauna, which represent important food items in fish diets (Castel et al., 1989; Simenstad and Fresh, 1995; Hosack et al., 2006) and may respond to oyster racks, but this has not been studied in the estero to date.