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|Title:||Impact of open-water disposal of Black Rock Harbor dredged material on benthic recolonization at the FVP site|
|Authors:||Science Applications International Corporation|
Computer Sciences Corporation
University of Rhode Island.
Environmental Research Laboratory (Narragansett, R.I.)
United States. Environmental Protection Agency.
Field Verification Program (Aquatic Disposal)
Scott, K. John.
Rhoads, Donald C.
Pratt, Sheldon D.
Gentile, J. H. (John H.)
Black Rock Harbor
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: This report describes the effects of the disposal of dredged Black Rock Harbor (BRH) sediments on the ambient (predisposal) community and the mode and pattern of recolonization at the Field Verification Program (FVP) site. The recolonization process was measured by documenting the rate of recruitment of the FVP site by the dominant species and comparing this with the ambient (reference) community. The parameters used to describe recolonization and convergence with the ambient system are species numbers, abundances of numerically dominant species, degree of infaunalization (successional stage), and depth of biogenic mixing of the bottom sediments (another measure of infaunalization). Both quantitative grab samples and the REMOTS interface camera were used to assess recolonization. The predisposal sampling indicated that the FVP site was characteristic of the silt-clay facies common to Central Long Island Sound. Extensive surveys to examine the benthic infauna and organism-sediment properties indicated a very homogeneous environment. The patterns of species dominance and organism-sediment indices were also relatively consistent over time. The only differences noted between the disposal site and the reference (REFS) station were mean numbers of individuals per quadrat. As a result of the disposal operation, a dredged material mound of BRH sediments approximately 1.5 to 2.0 m deep was deposited at the center (CNTR) station. The apron of the mound (depths <10 cm) extended to ca. 200 m in each direction. The species numbers at the 200E station had returned to background levels by December 1983, whereas the recruitment at CNTR was lagging behind what was found at 400E, 1000E, and REFS. By mid-1984, species numbers were similar at CNTR and REFS, with a more diverse assemblage present on the mound. In the samples that were analyzed, two periods of significant recruitment at CNTR were noted, December 1983 and October 1985. In December 1983, there was significant recruitment of Mulinia, Polydora (two species), and Streblospio, which were not similarly recruited at REFS. In October 1985, recruitment patterns at the two stations were more alike, except for higher abundances of Mediomastus and Tellina at CNTR and high densities of Nucula at REFS, with no significant Nucula recruitment to the CNTR station. REMOTS surveys indicated a significant change in the physical properties of the dredged material disposal mound as compared with the surrounding ambient stations and REFS. A sandy surface was found soon after disposal and continued throughout the survey period. The biological mixing depths (BMD) on the mound (CNTR and other mound stations) increased at a rate of 200 to 400 μm/day, returned to a unimodal condition, and appeared to converge with the BMD at REFS by January 1984. Following this date, however, physical scouring arrested the BMD at the CNTR to levels significantly shallower than those at the REFS station. The Organism-Sediment Index (OSI) over all of the mound stations developed a bimodal frequency distribution that persisted throughout the study period. In addition, OSI values at the disposal site CNTR were consistently lower than at REFS. The FVP site as a whole showed a successional retrograde in June 1985, in conjunction with the appearance of black (anoxic) sediment at or near the surface in 80 percent of the stations surveyed. Following Hurricane Gloria in early October 1985, all stations sampled (including REFS) experienced a successional retrograde as the result of severe erosion and bottom resuspension. These erosional effects were comparable, in terms of REMOTS parameters, with those measured at the FVP site immediately following the disposal of BRH sediments.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|