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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California II. Eelgrass A. BACKGROUND Seagrasses are important to estuarine and coastal lagoonal ecosystems because they provide food for some herbivores, notably black brant (Branta bernicla nigrans) on the Pacific coast of the United States and Canada, generate detritus to feed deposit-feeding invertebrates, and form structured habitat for fish and invertebrates in what would otherwise be a plain of soft sediments (Jackson, E.L. et al., 2001; Williams and Heck, Jr., 2001; Heck et al., 2003; Bostrom et al., 2006). Seagrasses are susceptible to multiple anthropogenic disturbances, which have been shown to be at least partly responsible for a general worldwide decline in their abundance (Orth et al., 2006). Although several species are found along the west coast of the United States, eelgrass (Zostera marina) is the dominant native species and apparently the only one found in Drakes Estero. The now common nonnative eelgrass, Zostera japonica, has not been documented in Drakes Estero, although Ruppia maritime may occur there (S. Williams, personal communication). Zostera marina typically occurs from about 1 m above to 1.5 m below mean low water (MLW) in estuaries along the West Coast of the United States with its upper limit determined primarily by desiccation (Boese et al., 2005) and the lower limit determined by light. Consequently, its distribution varies by location and extends to almost 10-m depth where water clarity is high (Phillips, 1984; Thom et al., 2003). The Z. marina distribution, thus, overlaps directly with the depth range across which most oysters are cultured (Conte et al., 1994). The structured habitat formed by Z. marina in West Coast estuaries has been shown to influence the abundance and diversity of everything from small
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California epibenthic invertebrates to large fish and birds (Simenstad and Fresh, 1995; Moore et al., 2004; Dumbauld et al., 2005; Hosack et al., 2006; Ferraro and Cole, 2007). While seagrass abundance has declined worldwide (Orth et al., 2006), at least 24% of seagrass populations along the west coast have shown increasing trends in abundance (S. Williams, personal communication), including Drakes Estero, although, in some estuaries, this increase may be attributed to expansion of the introduced nonnative eelgrass Z. japonica. B. WHAT IS THE BODY OF SCIENTIFIC STUDIES ON THE IMPACT OF THE OYSTER FARM ON DRAKES ESTERO? No study has comprehensively evaluated the impacts of shellfish mariculture on eelgrass in Drakes Estero. Eelgrass information is limited to some observations of eelgrass distribution relative to oyster mariculture racks in research examining potential impacts of the oyster mariculture on eelgrass invertebrate and fish communities (Harbin-Ireland, 2004; Wechsler, 2004), a visual analysis of the extent of tracks of boat propeller damage by NPS scientists and apparent eelgrass displacement by oyster racks (NPS Trip Report of March 13, 2007), and a monitoring of both eelgrass abundance and persistence over 18 months from April 1996 to October 1997, which included some structural parameters like densities of blades and turions at two sites in Drakes Estero and one in Estero de Limantour using six plots per site (Applied Marine Sciences, 2002). C. WHAT EFFECTS CAN BE DIRECTLY DEMONSTRATED BY RESEARCH CONDUCTED IN DRAKES ESTERO ITSELF? The limited scope and effort of the studies that infer impacts of shellfish mariculture on eelgrass of Drakes Estero prevent any definitive conclusions. The Applied Marine Sciences (2002) study did not control for tidal elevation in establishing its sites in Drakes Estero and Estero de Limantour. Reasonable inferences can be drawn, however, from the observations of propeller damage in the NPS GIS map of July 2007 and from the Harbin-Ireland (2004) and Wechsler (2004) observations, consistent with research conducted elsewhere along the west coast as presented below. D. WHAT EFFECTS CAN REASONABLY BE INFERRED FROM RESEARCH CONDUCTED IN SIMILAR ECOSYSTEMS? Shellfish mariculture and eelgrass compete directly for space; however, they also interact indirectly via changes each makes to the immediate environment like altering water flow, sediment structure, light pen-
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California etration, and nutrient supply. Other environmental changes arising from mariculture come from the addition of structures (e.g., bags, racks, and lines) and disturbances of transportation and culture operations. Whereas no directed research has been undertaken on these interactions in Drakes Estero, a fairly substantial amount of applicable information is available from elsewhere along the west coast of the United States (Rumrill and Poulton, 2004; Dumbauld et al., 2005). Oysters and other bivalve molluscs feed by extracting particulates from the water column, which can locally increase water clarity, thereby promoting spread of eelgrass, especially to depths where light would otherwise be limiting (Dennison et al., 1993; Peterson and Heck, 2001; Newell and Koch, 2004). Competition for space has been noted, particularly for on-bottom shellfish culture, with an apparent threshold loading function observed in Willapa Bay, Washington, above which eelgrass can “under-yield” or decline by more than the percent cover of oysters present; however, eelgrass can also over-yield or increase at lower levels of oyster cover (Dumbauld et al., 2005, [unpublished data]). Part of the under-yield response has been attributed to eelgrass blades rubbing across the sharp edges of growing oysters and being cut off (Schreffler and Griffen, 2000). Perhaps the most relevant to off-bottom rack-and-line culture—the dominant form of oyster culture in Drakes Estero—is work conducted by Everett et al. (1995) in Coos Bay, Oregon. This study demonstrated complete absence of eelgrass directly under oyster racks and lines, presumably due to shading and sediment erosion (10–15 cm at the base of the structure). The absence of eelgrass immediately beneath racks in Drakes Estero (as reported by Harbin-Ireland  and Wechsler ) can therefore be reasonably attributed to mariculture. Small reductions in eelgrass cover and density have been documented with other forms of off-bottom culture, such as long-lines and stakes, but losses tended to scale with density or spacing and were restricted primarily to the area beneath lines and stakes where there is shading or sedimentation (Everett et al., 1995; Rumrill and Poulton, 2004; Tallis et al., in press). Nonetheless, all culture methods were shown to result in decreased production of eelgrass in Willapa Bay (Tallis et al., in press). Recovery of eelgrass from areas disturbed by mariculture can be fairly rapid, either by rhizome spread or from seed dispersal. Pregnall (1993) found that eelgrass density remained depressed five months after removal of oyster stake culture in Coos Bay, Oregon. Recovery was from vegetative propagation and related to the density of eelgrass plants present before oysters were added. Wisehart et al. (2007), who examined recovery over a longer period, found enhanced seedling survival following disturbance caused by on-bottom culture and dredge harvest versus long-line oyster culture in Willapa Bay, Washington. They speculated that dredging
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California had removed more adult eelgrass plants as competitors and found that remaining plants in dredge areas also produced more seeds, suggesting that the mechanism for recovery is more complex and likely depends on surrounding eelgrass beds and other culture areas as well. Eelgrass may recruit to some areas where seeds are deposited or sediment has been stabilized by some mariculture practices. In Drakes Estero, the mariculture footprint is roughly 8 acres for racks predominantly in areas of eelgrass and perhaps an additional 10 acres of bottom bag culture, most of which occurs on intertidal flats with no eelgrass (Brown and Becker, 2007: Figures 2 and 3). A secondary impact to eelgrass arises from damage by boat propellers; scars or disturbance tracks are visually documented in aerial photos of Drakes Estero (total area with scars loosely quantified to be about 50 acres; NPS GIS Map, July 27, 2007). The committee infers that these scars were caused by DBOC boats because the scars are located near the site of rack deployment and are aligned in the direction that leads from or to those racks. In addition, all other motorboats were excluded from Drakes Estero upon passage of the Point Reyes Wilderness Act of 1976. This photograph was thus taken in 2007 and is therefore indicative of current impacts of mariculture boating activities. In past years, such as 2000, when shellfish culturing activities were dramatically lower (Figure 6) as the Johnson Oyster Company became less active and before the sale to DBOC, eelgrass scarring by boats may have differed. Oyster production levels varied by more than an order of magnitude over the decades, shown in Figure 6, for a variety of reasons, and the past impacts of the oysters, clams, and mariculture activities doubtlessly varied as well. Recovery from scars has been shown to take up to four years in other areas for turtle grass, Thalassia testudinum, but this set of observations was made on a different species, only from small disturbance tracks, and in a very different system; recovery rate on a larger scale is unknown (Dawes et al., 1997). Based on existing data on growth and recovery of Zostera marina in Willapa Bay and elsewhere on the West Coast, recovery from propeller scars should be rapid (weeks) for this species, unless the rhizomes were removed from the sediment (still less than two years based on above studies) or there was repeated scarring on a regularly travelled route. While bivalves have been shown in other systems to enhance eelgrass production via secondary mechanisms such as water clarification and fertilization of the sediments (Peterson and Heck, 2001; Newell and Koch 2004), the relatively small culture footprint in Drakes Estero suggests that these effects would be localized. Areal coverage of eelgrass in Drakes Estero has expanded from 368 acres in 1991 to about 740 acres in 2007 (Brown and Becker, 2007). At the 2007 level of mariculture activity in Drakes Estero, the estimates for potential eelgrass lost to rack culture (8 acres) and partially degraded by propeller scars (50
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California acres, likely an overestimate as a consequence of the spatial resolution of images used to estimate eelgrass loss from propeller tracks), represent less than 8% of the total eelgrass cover (NPS, 2007e; Brown and Becker, 2007) in the estuary. Changes in spatial scale of eelgrass cover at the estuarine landscape scale rarely have been assessed in areas with aquaculture, but a decline was attributed to storm events and not the presence of aquaculture in Bahia de San Quentin, despite a large increase in the number of oyster racks placed in that estuary between 1987 and 2000 (Ward et al., 2003).