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NOAA Technical Memorandum NMFS-AFSC-240

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Changes in Eelgrass Habitat and Faunal Assemblages
Associated with Coastal Development in Juneau, Alaska


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 fishes 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.
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.
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