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In order to facilitate descriptions of groundfish habitat over a large portion of
the EBS shelf, the NMFS/AFSC has assembled a single comprehensive database of the
point sample data (EBSSED; n=2,587) from all available sources. The database represents
sediment variation over the study area with uncompromized (i.e., original) spatial
detail. Textural data in the database are of two main types: 1) standardized statistics
characterizing sample grain size distribution based on laboratory measurements (granulometric
data), including percent composition by size grades (e.g., gravel, sand, mud) and
size distribution parameters (e.g., mean size); and 2) sample descriptions from less
exacting, more subjective visual/tactile observations, usually made in the field,
establishing size-grade constituents. In addition the EBSSED database includes two
descriptive fields which were each added to characterize sample grain size distribution
by a single, standardized variable based on the original data. These fields classify
samples according to gravel-sand-mud composition using high and low resolution schemes
derived from Folk's (1954) classic ternary diagram. The high resolution scheme classifies
903 samples with detailed granulometric data into 15 textural classes, providing the
greater detail regarding textural variation. The low resolution scheme (7 classes)
is designed to allow unambiguous classification of nearly all samples (n=2457) including
those with subjective visual/tactile descriptions. It represents the maximum number
of samples according to a single common variable and thus provides the most spatially
detailed data for the study area, albeit at the expense of some of the textural detail
for samples analyzed in the laboratory. Overall, the EBSSED database is the most comprehensive
and detailed source of information about surficial sediment textures in the EBS study
area. Patterns observed in the data generally agree with large-scale textural maps
and summaries by previous investigators, particularly a general pattern, with exceptions,
of decreasing average grain size with increasing depth and distance from shore. However,
those previous large-scale works spatially smoothed data for the study area from smaller,
more sparsely distributed sets of samples. The EBSSED database preserves potentially
important fine-scale variation. |