1992 Virginian Province Statistical Summary
Statistical Summary 1992
Strobel, C.J., S.J. Benyi, D.J. Keith, H.W. Buffum, and E.A. Petrocelli
1994. U.S. Environmental Protection Agency, Office of Research and Development, Environmental Research Laboratory, Narragansett, RI. EPA/620/R-94/019
The Environmental Monitoring and Assessment Program (EMAP) is a nationwide program initiated by EPA's Office of Research and Development (ORD). EMAP was developed in response to the demand for information about the degree to which existing pollution control programs and policies protect the nation's ecological resources.
EMAP-Estuaries (EMAP-E) represents EMAP's efforts in near-coastal environments. These efforts are designed to provide a quantitative assessment of the regional extent of coastal environmental problems by measuring status and change in selected indicators of ecological condition. Specific environmental problems investigated include: hypoxia, sediment contamination,coastal eutrophication, and habitat loss.
In 1990, EMAP-E initiated a demonstration project in the estuaries of the Virginian Province. The 1992 field season represents the third year of sampling in the Province, which includes the coastal region of the Northeast United States from Cape Cod south to the mouth of Chesapeake Bay. It is composed of 23,574 km2 of estuarine resources including 11,469 km2 in Chesapeake Bay and 3,344 km2 in Long Island Sound.
Estuarine resources in the Virginian Province were stratified into classes by physical dimension for the purposes of sampling and analysis. Large estuaries in the Virginian Province were defined as those estuaries greater than 260 km2 in surface area and with aspect ratios (i.e., length/average width) of less than 18. The areal extent of large estuaries in the Province was 16,097 km2. Large tidal rivers were defined as that portion of the river that is tidally influenced (i.e., detectable tide > 2.5 cm), greater than 260 km2 in surface area, and with an aspect ratio of greater than 18. Approximately 2,602 km2 were classified as tidal rivers. The third class was the small estuaries and small tidal rivers which included those systems whose surface areas fell between 2.6 km2 and 260 km2. This class represented 4,875 km2 of the Virginian Province.
Three field crews sampled 126 sites in the Virginian Province during the six-week sampling period beginning on July 27, 1992. Of these, 103 were "Base Sampling Sites" (BSS) which were the probability-based sites selected according to the EMAP-E design for assessing the condition of the estuarine resources of the Province. Only data collected at these sites were used in the generation of this report.
The 1992 data reported in this document represent only one year of sampling of a four-year cycle; i.e., the total number of samples needed by EMAP to characterize the Province are sampled over a four-year period (Holland, 1990). Therefore, the reader must use these data carefully, and be aware that the proportion of degraded area calculated for 1992 may differ somewhat from the regional assessment to be generated following the completion of the four-year cycle.
All EMAP-VP data used in the generation of this report were subjected to rigorous quality assurance measures as described in the 1992 Quality Assurance Project Plan (Valente et al., 1992).
Biotic Condition Indicators
Biotic condition indicators are characteristics of the environment that provide quantitative evidence of the status of ecological resources and biological integrity of a sample site from which they are collected (Messer, 1990). Ecosystems with a high degree of biotic integrity (i.e., healthy ecosystems) are composed of balanced populations of indigenous benthic and water column organisms with species compositions, diversity, and functional organization comparable to undisturbed habitats (Karr and Dudley, 1981; Karr et al., 1986).
A benthic index which uses measures of organism health, functionality, and community condition to evaluate the condition of the benthic assemblage was utilized in the assessment of biological resources of the Virginian Province. The index under development was determined from the combined 1990/1991 data and is assumed to represent a combination of ecological measurements that best discriminates between good and poor ecological conditions. The reader should be cautioned that this index has not yet been fully validated with an independent dataset, and therefore, should be used with caution.
A benthic index critical value of zero was determined from the combined 1990/1991 Virginian Province dataset. Fourteen (+- 6) percent of the bottom area of the Virginian Province sampled in 1992 had an index value of < 0, indicating likely impacts on the benthic community (Figure 1). The lowest incidence was found in the large estuaries (7 +- 8%), and the highest in large tidal rivers (37 +- 22%).
A "standard" fish trawl (trawling at a specified speed for a specified time) was performed at each station to collect information on the distribution and abundance of fish. Because many factors influence fish abundance, poor catch may not be an indication of degraded conditions, but simply the natural habitat. Catches of <10 fish/trawl (catch per unit effort) occurred at stations representing approximately 37 +- 12% of the Province, and "high" catches (>100 fish/trawl) were experienced at stations representing approximately 26 +- 11% of the area of the Province (Figure 2).
The incidence of the four gross external pathologies (growths, lumps, ulcers, and fin erosion) among fish collected in the Virginian Province in 1992 was 0.3%. Of the 3,290 fish examined, 10 were identified as having one or more of these pathologies. These individuals were collected at nine of the 103 base stations sampled during the index period.
Abiotic Condition Indicators
Abiotic condition indicators historically have been the mainstay of environmental monitoring programs, because these indicators quantify the levels of stresses to which organisms are exposed.
One potential stress to aquatic organisms is a low concentration of dissolved oxygen (DO). Two and five mg/L are values employed by EMAP to define severe and moderate hypoxia, respectively. Approximately 29 +- 10% of the sampled area of the Province lies in waters with bottom DO concentrations less than or equal to 5 mg/L (Figure 3). "Bottom" is defined as one meter above the sediment-water interface. Approximately 5 +- 5% of the sampled area exhibited bottom DO conditions <= 2.0 mg/L. Dissolved oxygen conditions <= 2.0 mg/l were evident in 7 +- 8% of the area of the large estuaries sampled within the Province and none of the small estuaries or large tidal rivers (Figure 3).
In addition to measuring individual stressors (e.g., individual chemical analytes), sediment toxicity tests were performed on sediments collected at each site to determine if they were toxic to the tube-dwelling amphipod, Ampelisca abdita. Sediments were classified as toxic if amphipod survival in the test sediment was less than 80% of that in the control sediment and statistically different from control survival. Approximately 6 +- 5% of the sampled area of the Virginian Province contained sediments which were toxic to the amphipod during 10-day exposures (Figure 4).
Sediments collected at each station were analyzed for both organic contaminants and metals. Because of the complex nature of sediment geochemistry, the ecological impact of elevated contaminant levels is not well understood. Therefore, no attempt is made to estimate the overall aerial extent of sediment contamination in the Virginian Province.
Figure 5 shows the distribution of the sum of measured polycyclic aromatic hydrocarbons (PAHs) in the Virginian Province. The complete list of analytes included in this summation can be found in Section 3. Approximately 92 +- 7% of the Province has concentrations of PAHs below 4,000 ng/g dry weight, with a maximum measured concentration at any station of 13,219 ng/g.
Draft EPA Sediment Quality Criteria (SQC) are currently available for the PAHs acenaphthene, phenanthrene, and fluoranthene; and the pesticide dieldrin. Draft PAH SQC were not exceeded at any stations within the Province in 1992.
The extent to which polluting activities have affected concentrations of metals in sediments is complicated by the natural variation of metals in sediments. Crustal aluminum concentrations are generally many orders of magnitude higher than anthropogenic inputs; therefore, aluminum can be used to "normalize" for differing crustal abundances of trace metals. Figure 6 presents the results of this normalization. Approximately 31 +- 10% of the area of the Province showed enrichment of sediments with at least one metal. Twenty seven (+- 13), 43 +- 13, and 34 +- 39 percent of the large estuary, small estuary, and large tidal river class areas sampled contained sediments with metals concentrations exceeding predicted background levels. This only shows the percent of the Province with elevated concentrations of metals, and does not indicate the magnitude of enrichment, i.e., this does not imply concentrations are elevated to the point where biological effects might be expected.
Presence of marine debris in fish trawls was documented by field crews as being encountered at stations representing 25 +- 11% of the Virginian Province area (Figure 7). The small estuary class had the largest percent area (42 +- 20%) where trash was found.
Habitat indicators describe the natural physical and chemical conditions of the sites sampled. These parameters are important modifying factors controlling both abiotic and biotic condition indicators.
Figure 8 shows the distribution of water depth in the Virginian Province. The area shallower than 2 m is underestimated because this was the minimum depth sampled.
Based on the sampling design where a single station represents a statistical area (e.g., 70 km2 for large estuary sites), 12.5% of the area of large estuaries could not be sampled due to inadequate water depth. Small systems were considered unsampleable if the water depth did not exceed 2 m anywhere in the system. Such systems account for approximately 0.5% of the area of small systems in the Virginian Province. No large tidal river stations were unsampleable due to water depth in 1992. Overall, 8.5% of the area of the Province was deemed unsampleable in 1992 due to water depth.
Bottom water temperatures in the Virginian Province ranged from 11.8 C to 27.8 C during the summer sampling season.
Figure 9 illustrates the distribution of oligohaline (<5 salinity), mesohaline (5-18 ), and polyhaline (>18 ) water in the Virginian Province and by class.
Vertical density differences (a function of both salinity and temperature) in the waters of the Virginian Province can be large enough to result in a reduction in mixing between surface and bottom waters, potentially allowing the bottom waters to become hypoxic. Degree of stratification in the Virginian Province was measured as the delta sigma t, which is the delta t (sigma-t, a density measurement) difference between surface and bottom waters. Approximately 68 +- 11% of the Province area had a delta sigma-t of <1 unit; thus the majority of the water in the Virginian Province was well-mixed (Figure 10). Only 17 +- 10% of the Province area was strongly stratified ( delta sigma-t >2).
Water clarity was determined from light extinction coefficients, which describe the attenuation of light as it passes vertically through the water column. We are defining low water quality as water in which a diver would not be able to see his/her hand when held at arms length in front. Moderate water clarity, in terms of human vision, is defined as water in which a wader would not be able to see his/her feet in waist deep water.
Water clarity was good in 83 +- 8% of the area of the Virginian Province (Figure 11). Water of low clarity was found in 5 +- 6% of the Province and an additional 12 +- 6% had water of moderate clarity.
The silt-clay (mud) content of sediments (the fraction <63 particle diameter) is an important factor determining the composition of the biological community at a site, and is therefore important in the assessment of the benthic community. The distribution of mud (>= 80% silt-clay) vs sand (<= 20% silt-clay) is illustrated in Figure 12.