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Chapter 3 - Benthic HabitatsOrganism ClassificationFor each subregion, we provide a summary of the characteristicspecies and their indicator values (Appendix3-2). This table gives diagnostic species for each organismgroup and shows its distribution across all the organismsgroups. The mean indicator value and the probability ofthis distribution being random chance is calculated foreach species in the group that it is most closely associatedwith. Most species don’t have a common name; Gosner(1979), Weiss (1995) and Pollock (1998) were used to addthem where available. Often, these are common names forthe family or genus, not the species.Relationship of the organism groups to thephysical factorsAcross all subregions, depth was the most importantexplanatory variable, followed by grain size, and thenseabed forms. Seabed forms were less important in theMid-Atlantic Bight than the other regions. Standard deviationof depth was somewhat important in SouthernNew England, but not in the other regions. Basic relationshipsbetween each organism group and its characteristicphysical setting are described below. Charts giving thedistribution of the organism groups across each physicalfactor class, a chi-squared test for significance, and theclass where this group is most likely to be found are givenin Appendix 3-1. Tables of key physical factor values thatcorrespond to ecological thresholds separating the distributionof one benthic habitat from another are provided inthe subregion results (Table 3-6, 3-7, 3-8).Benthic Habitat Types and EcologicalMarine UnitsThe benthic habitat types identified for each subregionare presented in the following section of this document.Because the final results are a product of several steps,e.g. the macrofauna classification; the identification ofrelationships between the organism groups and the factorsof depth, grain size and topography; and the mapping ofbenthic environments, the results and details on each stepare provided separately in the appendices.Two separate, but closely related final maps were created.The Ecological Marine Units (EMU) represent all threewaycombinations of depth, sediment grain size, and seabedforms based on the ecological thresholds revealed bythe benthic-organism relationships (Figure 3-11, 3-12, 3-13,3-14). Benthic Habitats are EMUs clustered into groupsthat contain the same species assemblage (Figure 3-15).The two terms are not synonymous, but they are based onthe same information, and thus, represent two perspectiveson the seafloor. Essentially, the EMU maps show thefull diversity of physical factor combinations, regardless ofwhether a specific habitat type was identified for the combination.The benthic habitat map shows only the combinationsof factors, or groups of combinations, for which abenthic organism group was identified. It should be notedthat the numbers of the EMUs and benthic habitats werederived from the statistical relationships and is completelyarbitrary.The Benthic Habitat map is simpler because a singleorganism group typically occurs across several EMUs,although in some instances a single EMU is synonymouswith a single organism group. For example, in the Mid-Atlantic Bight, EMU 1101 (silty depression centers inwater less than 15 m) is synonymous with organism group768, a community identified by a specific set of amphipods,brittle stars, clams, whelks, and snails. More typicalare organism groups that occur across several closely relatedEMUs such as Southern New England organism group25. It ranges across both high position and mid positionflats, very shallow to shallow water ranging in depth from0-23 m, and medium to coarse sand. This community ofshimmyworms, glass shrimp, hermit crabs, and surf clamsis thus found across a small range of EMUs, and the habitatis mapped as the set of EMUs that define it.Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 3-21