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

Chapter 2. Summary of Environmental Data Layers

Following are summaries of the major environmental data layers acquired or developed for use with models to characterize the habitats of the evaluation species.

Casco Bay Wetlands:

Since most of the evaluation species are closely associated with wetland or open water habitats, we relied on wetland maps as a primary basis for characterizing their habitats. Our main source of wetland locations and types was National Wetland Inventory (NWI) digital maps. We made corrections to several polygons which originally were estuarine but, due to impoundment, are now freshwater marsh, then combined data from the 18 7.5- minute quadrangles into a single coverage. For purposes of display the 26 NWI categories occurring in the study area were condensed into 8 major classes as follows:

TABLE 2. ASSIGNMENT OF NWI DESIGNATIONS TO "CLASS"

Class NWI designation

Flats - (M2US2-4; M2RF; M2AB; E2US2-4; E2RF; E2AB; R1US2-5)

Saltmarshes - (E2EM)

Subtidal waters - (M1 and E1)

Deep Freshwater - (R1,2,3,5 that are UB or RB; L1 - except L1AB)

Coastal Islands - (Upland on islands with Maine Coastal Island Registry number)

Freshwater wetlands -(all other Riverine, Lacustrine, and Palustrine)

Mainland - (All other U)

Rocky Shore - (M2RS; M2US1; E2RS; E2US1)

The combined NWI coverage is depicted in Figure 2.

Landcover of Casco Bay:

Upland areas also are used as wildlife habitats, or affect the use or quality of the adjacent wetlands for fish and wildlife. We required maps of landcover for the habitat analysis, and also for estimating effects from future development. The information was developed from a June 6, 1991, Landsat scene, classified by Earthsat Corp. and by Jed Wright (FWS). We compared and corrected the classification by relating output to aerial photos and previous photo-interpreted landcovers of the area. Landcover is displayed in Figure 3a.

Coastal Marine Geologic Environments:

In 1976, Maine Geological Survey (MGS) published a series of 7.5 minute quadrangles of coastal features titled "Coastal Marine Geologic Environments" (CMGE). Features of supratidal, intertidal and subtidal environments were mapped, and classified by substrate type, salinity, vegetative or animal cover, or hydrodynamics. While some of the more dynamic features have probably changed since that interpretation, the information complements NWI data and is useful for assessing habitat suitability for marine and shoreline dependent wildlife. We used digital versions of CMGE for habitat and landcover interpretation.

Bathymetry:

Water depth is an important habitat parameter for wildlife using coastal resources. We obtained draft 10 m contours of Casco Bay from MGS, then digitized a 6 ft (1.8 m) mean low water (mlw) contour from NOAA charts 13290 and 13293 (Casco Bay and Sheepscot Bay, respectively). Mean low water itself was mapped by selecting the outer boundary of intertidal habitats from NWI or CMGE, whichever was more extensive. This GIS line coverage was converted into a lattice (grid coverage) in ARCINFO. The grid had 30 m sq cells to match the units of the Landsat data (Figure 3b). Depth values were integers, ranging from +3 m (approximately mean high water) down to -60 m mlw. Unless otherwise indicated, depths in this report are referred to mlw.

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