Changes In Eelgrass Habitat And Faunal Assemblages Associated With Coastal Development In Juneau Alaska

Summary Of Findings

We studied three eelgrass (Zostera marina) beds in the City and Borough of Juneau (CBJ), Alaska, to track changes associated with coastal development. These beds were initially sampled as part of a baseline eelgrass inventory from 2004 to 2007. Between 2008 and 2011, beds were remapped and resampled for eelgrass variables (e.g., percent cover, faunal assemblage). Eelgrass area declined at all beds from baseline (2004 to 2007) to post-baseline (2008 to 2011) years. Areal loss of eelgrass was twice as great at the bed with the most recent and intense development (Auke Nu Cove, 61% loss) compared with losses at the previously developed Bay Creek (29%) and undeveloped Bridget Cove (30%) beds. The largest loss of eelgrass at Auke Nu Cove was along the seaward edge of the entire bed and was probably related to increased turbidity. In contrast, the seaward extent of eelgrass at Bay Creek and Bridget Cove remained relatively stable. Differences in eelgrass characteristics between baseline and post-baseline years in developed and undeveloped beds were also apparent. Mean percent eelgrass cover and shoot density declined from baseline to post-baseline years at all beds, but declines at developed beds (42% to 51%) were approximately twice those at the undeveloped bed (23% to 25%). Additionally, biomass declined 45% to 48% at developed beds but increased 17% at the undeveloped bed.

Faunal assemblages changed with eelgrass loss. Coincident with the complete loss of eelgrass at one seine site, mean catch-per-unit-effort of fish declined from 401 to 140, and the number of fish species declined from 19 to 16. The most sensitive species to eelgrass loss was tubesnout (Aulorhynchus flavidus). Seine catch of green sea urchin (Strongylocentrotus droebachiensis) increased from baseline to post-baseline years at Auke Nu Cove; urchin grazing likely accounted for some of the observed loss of eelgrass at the cove. 

Discussion

Monitoring of these three eelgrass beds will provide resource managers with useful information to evaluate possible effects of future coastal development upon this important nearshore habitat. Careful consideration of development is especially warranted because eelgrass distribution is limited in the CBJ. We recommend biennial monitoring of these beds that includes, at a minimum, mapping seaward eelgrass boundaries and sampling percent eelgrass cover by tidal elevation. Coincidently, fauna should be sampled with a beach seine to monitor abundance of indicator fish (e.g., tubesnout) and invertebrate (e.g., green sea urchin) species. Seawater temperature and ambient light dataloggers should also be placed at monitored areas.

Few studies in southeastern Alaska have tracked changes in the health of eelgrass (Zostera marina) beds over time (Johnson and Thedinga 2005), especially changes associated with coastal development. Eelgrass supports a high abundance and diversity of marine fish and invertebrates (Thayer et al. 1978, Heck and Orth 1980, Johnson and Thedinga 2005, Harris et al. 2008, Johnson et al. 2012), and may be an essential habitat for some species. In Alaska, eelgrass is often used as a spawning substrate by Pacific herring (Clupea pallasii) (Blankenbeckler and Larson 1982, Cooney 2007) and provides rearing habitat in spring and summer for many commercial and forage fish species (Murphy et al. 2000, Johnson et al. 2012). Eelgrass also provides other important ecological functions, oxygen production, nutrient recycling, erosion control, and contaminant filtration (Spalding et al. 2003, Waycott et al. 2009). 

Eelgrass is the most widely distributed seagrass in Alaska and is often the dominant vegetation in protected, shallow subtidal and lower intertidal areas. Eelgrass occupies approximately 6,000 linear km (20%) of the southeastern Alaska coast (NMFS 2011). In the City and Borough of Juneau (CBJ), Alaska, however, eelgrass is not widely distributed and is present on only about 3% of the shoreline (Harris et al. 2008). Areas with eelgrass in the CBJ are vulnerable to development because their low gradient and protected shorelines facilitate construction of docks and harbors. Anecdotal evidence suggests that the area of some eelgrass beds has declined in the CBJ, but changes near coastal development have not been documented nor compared to beds not subject to developmental pressure. Furthermore, little is known about what ecological variables should be monitored (e.g., bed area, percent cover, faunal assemblages) or what frequency of monitoring is necessary to detect change. 

Purpose Of Study

Our study was undertaken to track changes associated with coastal development at three eelgrass beds that were previously sampled as part of a CBJ-wide eelgrass baseline inventory (Harris et al. 2008). The primary objective was to identify changes in eelgrass variables (e.g., bed area, percent cover, and faunal assemblages) in the CBJ. Secondary objectives were to recommend the most practical and sensitive variables to monitor at CBJ eelgrass beds and the frequency of monitoring.