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Identification of Important Habitats in the Lower Casco Bay (Maine) Watershed

Chapter 4. Eelgrass, Cordgrass Habitats

GENERAL: Eelgrass (Zostera marina) and smooth cordgrass (Spartina alterniflora) are highly ranked species on the Gulf of Maine Council's Species List for Identifying Regionally Significant Habitats. Both are of major ecological importance as structure for marine and estuarine vertebrates and invertebrates, and as primary producers of organic matter for coastal food chains. In the current context their habitats are appraised in purely horticultural terms, their suitability for growth of these plants; other aspects are considered in the analyses for fish and wildlife species which share their "community". Accordingly, the greatest observed density of plant growth is regarded as indicating the highest value habitat for that species.

SOURCES OF BIOLOGICAL AND SPATIAL DATA: The analysis relies on National Wetland Inventory (NWI) maps for cordgrass and maps of eelgrass created by Seth Barker, Maine Department of Marine Resources (DMR). Eelgrass beds had been identified from true color 1:12,000 aerial photos, field verified, and digitized from mylar overlays produced from the photos. Areal coverage of eelgrass (crown densities) were estimated and assigned to four classes: 0-10%, 10-40%, 40-70%, and 70-100%.

MAPPING OF HABITATS: The cordgrass coverage (CORDGRS4) was created by selecting all areas from NWI digital maps designated estuarine intertidal emergent and converting them to grid cell format. Corrections to the NWI information were made as noted in Chapter 2. We also found that most of the tidal marshes had pronounced zonation. In marshes having freshwater tributaries the lowest band was smooth cordgrass, the next higher a band of saltmeadow hay (Spartina patens), and the highest was cattail. When such areas of mixed vegetation were dominated by S. alterniflora, we included the entire NWI polygon in our coverage.

In lieu of information on vigor or biomass, as indicating relative habitat quality, all areas of cordgrass were assigned an "intermediate" score of 4 (out of a possible 8, before adjustments for species rank, habitat abundance, etc.). We did not establish a sensitivity or buffer zone for this coverage.

The DMR eelgrass coverage was converted to grid cell format (CASEELG6). Cells having eelgrass were scored 2 to 8 for habitat quality, corresponding to the density classes originally assigned by Seth Barker.