USGS Studies in Long Island Sound:
Geology, Contaminants, and Environmental Issues

Regional Processes, Conditions, and Characteristics of the Long Island Sound Sea Floor. Harley J. Knebel, Ralph S. Lewis, Johan C. Varekamp, Guest Editors, pages 519-521.

A Review of the Geologic Framework of the Long Island Sound Basin, With Some Observations Relating To Postglacial Sedimentation. Ralph S. Lewis, Mary DiGiacomo-Cohen, pages 522-532.

Sea-Floor Environments Within Long Island Sound: A Regional Overview. Harley J. Knebel, Lawrence J. Poppe, pages 533-550.

Bottom Currents and Sediment Transport in Long Island Sound: A Modeling Study. Richard P. Signell, Jeffrey H. List, Amy S. Farris, pages 551-566.

Distribution of Surficial Sediment in Long Island Sound and Adjacent Waters: Texture and Total Organic Carbon. Lawrence J. Poppe, Harley J. Knebel, Zofia J. Mlodzinska, Mary E. Hastings, Barbara A. Seekins, pages 567-574.

Contaminant Distribution and Accumulation in the Surface Sediments of Long Island Sound. Ellen L. Mecray, Marilyn R. Buchholtz ten Brink, pages 575-590.

Clostridium perfringens in Long Island Sound Sediments: An Urban Sedimentary Record. Marilyn R. Buchholtz ten Brink, Ellen L. Mecray, Erin L. Galvin, pages 591-612.

Mercury in Long Island Sound Sediments. J.C. Varekamp, M.R. Buchholtz ten Brink, E.L. Mecray, B. Kreulen, pages 613-626.

Relationships Among Sea-Floor Structure and Benthic Communities in Long Island Sound at Regional and Benthoscape Scales. Roman N. Zajac, Ralph S. Lewis, Larry J. Poppe, David C. Twichell, Joseph Vozarik, Mary L. DiGiacomo-Cohen, pages 627-640.

Benthic Foraminifera and Environmental Changes in Long Island Sound. E. Thomas, T. Gapotchenko, J.C. Varekamp, E.L. Mecray, M.R. Buchholtz ten Brink, pages 641-655.

Modern Pollen Deposition in Long Island Sound. Kristina R. M. Beuning, Lindsey Fransen, Berna Nakityo, Ellen L. Mecray, Marilyn R. Buchholtz ten Brink, pages 656-662.

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No Abstract-KNEBEL, H.J.; LEWIS, R.S., and VAREKAMP, J.C., 2000. Regional Processes, Conditions, and Characteristics of the Long Island Sound Sea Floor. Journal of Coastal Research, 16(3), 519-521. Royal Palm Beach (Florida), ISSN 0749-0208.

Abstract-LEWIS, R.S. and DIGIACOMO-COHEN, M., 2000. A Review of the Geologic Framework of the Long Island Sound Basin, With Some Observations Relating To Postglacial Sedimentation. Journal of Coastal Research, 16(3), 522-532. Royal Palm Beach (Florida), ISSN 0749-0208.

Most of the papers in this thematic section present regional perspectives that build on more than 100 years of geologic investigation in Long Island Sound. When viewed collectively, a common theme emerges in these works. The major geologic components of the Long Island Sound basin (bedrock, buried coastal-plain strata, recessional moraines, glacial-lake deposits, and the remains of a large marine delta) interact with the water body to affect the way the modern sedimentary system functions.

Previous work, along with our present knowledge of the geologic framework of the Long Island Sound basin, is comprehensively reviewed with this theme in mind. Aspects of the crystalline bedrock, and the deltaic deposits associated with glacial Lake Connecticut, are examined with respect to their influence on sedimentation along the Connecticut coast and in the northern and western Sound. We also discuss the influence of the glacial drift that mantles the coastal-plain remnant along the north shore of Long Island and in the southern Sound.

A total of approximately 22.7 billion m³ of marine sediment has accumulated in the Long Island Sound basin. A significant portion (44%) of the fine-grained marine section in the central and western basins was redistributed there from the eastern Sound, as tidal scour removed slightly over 5 billion m³ (5.3 x 10¹² kg) of fine material from glacial-lake and early-marine deposits east of the Connecticut River. The remainder of the estimated 1.2 x 10¹³ kg of fine-grained marine sediment that now resides in the central and western Sound can be accounted for by riverine input over the past 13.5 ka.

Abstract-KNEBEL, H.J. and POPPE, L.J., 2000. Sea-floor Environments Within Long Island Sound: A Regional Overview. Journal of Coastal Research, 16(3), 533-550. Royal Palm Beach (Florida), ISSN 0749-0208.

Modern sea-floor sedimentary environments within the glaciated, topographically complex Long Island Sound estuary have been interpreted and mapped from an extensive collection of sidescan sonographs, bottom samples, and videocamera observations together with supplemental bathymetric, marine-geologic, and bottom-current data. Four categories of environments are present that reflect the dominant long-term processes of: erosion or nondeposition; coarse-grained bedload transport; sediment sorting and reworking; and fine-grained deposition. (1) Environments of erosion or nondeposition contain exposures of glacial drift, coarse lag deposits, and possibly bedrock and include sediments which range from boulder fields to gravelly coarse-to-medium sands. (2) Environments of coarse-grained bedload transport are mantled by sand ribbons and sand waves and contain mostly coarse-to-fine sands with only small amounts of mud. (3) Environments of sediment sorting and reworking comprise both uniform and heterogeneous sediment types and contain variable amounts of fine sand and mud. (4) Environments of fine-grained deposition are blanketed by muds and sandy muds.

The patchy distribution of sedimentary environments within Long Island Sound reflects both regional and local changes in bottom processes. Regional changes are primarily the result of a strong, east-to-west decreasing gradient of bottom tidal-current speeds, coupled with the net (westward) estuarine bottom drift. The regional current regime has produced a westward succession of environments along the basin floor beginning with erosion or nondeposition at the narrow eastern entrance to the Sound, changing to an extensive area of coarse-grained bedload transport, passing into a contiguous band of sediment sorting, and ending with broad areas of fine-grained deposition in the central and western Sound. However, local changes in processes are superimposed on the regional conditions within the central and western parts of the basin and along the nearshore margins. Within the central and western basin, localized sedimentary environments are produced where the bottom flow is enhanced by, and interacts with, the bottom topography, whereas along the nearshore margins, they variously reflect wave-produced currents, the irregular bathymetry, the indented shoreline, and the proximal supply of sediments.

Results from this study (1) confirm the high trapping efficiency of fine-grained sediments in the Sound, (2) suggest that fine-grained sediments accumulate at an average (regional) rate of 0.08 g/cm2/y, and (3) indicate that the postglacial delta in the eastern Sound was a significant source of fine-grained sediments now buried beneath depositional areas.

Abstract-SIGNELL, R.P.; LIST, J.H, and FARRIS, A.S., 2000. Physical processes affecting the sea-floor environments of Long Island Sound. Journal of Coastal Research, 16(3), 551-566. Royal Palm Beach (Florida), ISSN 0749-0208.

A high resolution (300-400 m grid spacing), process oriented modeling study was undertaken to elucidate the physical processes affecting the characteristics and distribution of sea-floor sedimentary environments in Long Island Sound. Simulations using idealized forcing and high-resolution bathymetry were performed using a three-dimensional circulation model ECOM (Blumberg and Mellor, 1987) and a stationary shallow water wave model HISWA (Holthuijsen et al., 1989). The relative contributions of tide-, density-, wind- and wave-driven bottom currents are assessed and related to observed characteristics of the sea-floor environments, and simple bedload sediment transport simulations are performed. The fine grid spacing allows features with scales of several kilometers to be resolved.

The simulations clearly show physical processes that affect the observed sea-floor characteristics at both regional and local scales. Simulations of near-bottom tidal currents reveal a strong gradient in the funnel-shaped eastern part of the Sound, which parallels an observed gradient in sedimentary environments from erosion or nondeposition, through bedload transport and sediment sorting, to fine-grained deposition. A simulation of estuarine flow driven by the along-axis gradient in salinity shows generally westward bottom currents of 2-4 cm/s that are locally enhanced to 6-8 cm/s along the axial depression of the Sound. Bottom wind-driven currents flow downwind along the shallow margins of the basin, but flow against the wind in the deeper regions. These bottom flows (in opposition to the wind) are strongest in the axial depression and add to the estuarine flow when winds are from the west. The combination of enhanced bottom currents due to both estuarine circulation and the prevailing westerly winds provide an explanation for the relatively coarse sediments found along parts of the axial depression. Climatological simulations of wave-driven bottom currents show that frequent high-energy events occur along the shallow margins of the Sound, explaining the occurrence of relatively coarse sediments in these regions. Bedload sediment transport calculations show that the estuarine circulation coupled with the oscillatory tidal currents result in a net westward transport of sand in much of the eastern Sound. Local departures from this regional westward trend occur around topographic and shoreline irregularities, and there is strong predicted convergence of bedload transport over most of the large, linear sand ridges in the eastern Sound, providing a mechanism which prevents their decay. The strong correlation between the near-bottom current intensity based on the model results and the sediment response, as indicated by the distribution of sedimentary environments, provides a framework for predicting the long-term effects of anthropogenic activities.

Abstract-POPPE, L.J.; KNEBEL, H.J.; MLODZINSKA, Z.J.; HASTINGS, M.E., and SEEKINS, B.A, 2000. Distribution Of Surficial Sediment In Long Island Sound and Adjacent Waters: Texture And Total Organic Carbon. Journal of Coastal Research, 16(3), 567-574. Royal Palm Beach (Florida), ISSN 0749-0208.

The surficial sediment distribution within Long Island Sound has been mapped and described using bottom samples, photography, and sidescan sonar, combined with information from the geologic literature. The distributions of sediment type and total organic carbon (TOC) reveal several broad trends that are largely related to the sea-floor of modern tidal- and wind-driven currents.

Sediment types are most heterogeneous in bathymetrically complex and shallow nearshore areas; the heterogeneity diminishes and the texture fines with decreasing bottom-current energy. Lag deposits of gravel and gravelly sand dominate the surficial sediment texture in areas where bottom currents are the strongest (such as where tidal flow is constricted) and where glacial till crops out at the sea floor. Sand is the dominant sediment type in areas characterized by active sediment transport and in shallow areas affected by fine-grained winnowing. Silty sand and sand-silt-clay mark transitions within the basin from higher- to lower-energy environments, suggesting a diminished hydraulic ability to sort and transport sediment. Clayey silt and silty clay are the dominant sediment types accumulating in the central and western basins and in other areas characterized by long-term depositional environments.

The amount of TOC in the sediments of Long Island Sound varies inversely with sediment grain size. Concentrations average more than 1.9% (dry weight) in clayey silt, but are less than 0.4% in sand. Generally, values for TOC increase both toward the west in the Sound and from the shallow margins to the deeper parts of the basin floor. Our data also suggest that TOC concentrations can vary seasonally.

Abstract-MECRAY, E.L. and BUCHHOLTZ TEN BRINK, M.R., 2000. Contaminant Distribution and Accumulation in the Surface Sediments of Long Island Sound. Journal of Coastal Research, 16(3), 575-590. Royal Palm Beach (Florida), ISSN 0749-0208.

The distribution of contaminants in surface sediments has been measured and mapped as part of a U.S. Geological Survey study of the sediment quality and dynamics of Long Island Sound. Surface samples from 219 stations were analyzed for trace (Ag, Ba, Cd, Cr, Cu, Hg, Ni, Pb, V, Zn and Zr) and major (Al, Fe, Mn, Ca, and Ti) elements, grain size, and Clostridium perfringens spores. Principal Components Analysis was used to identify metals that may co-vary as a function of common sources or geochemistry. The metallic elements generally have higher concentrations in fine-grained deposits, and their transport and depositional patterns mimic those of small particles. Fine-grained particles are remobilized and transported from areas of high bottom energy and deposited in less dynamic regions of the Sound. Metal concentrations in bottom sediments are high in the western part of the Sound and low in the bottom-scoured regions of the eastern Sound. The sediment chemistry was compared to model results (Signell et al., 1998) and maps of sedimentary environments (Knebel et al., 1999) to better understand the processes responsible for contaminant distribution across the Sound. Metal concentrations were normalized to grain-size and the resulting ratios are uniform in the depositional basins of the Sound and show residual signals in the eastern end as well as in some local areas. The preferential transport of fine-grained material from regions of high bottom stress is probably the dominant factor controlling the metal concentrations in different regions of Long Island Sound. This physical redistribution has implications for environmental management in the region.

Abstract-BUCHHOLTZ TEN BRINK, M.R.; MECRAY, E.L., and GALVIN, E.L., 2000. Clostridium perfringens in Long Island Sound Sediments: An Urban Sedimentary Record. Journal of Coastal Research, 16(3), 591-612. Royal Palm Beach (Florida), ISSN 0749-0208.

Clostridium perfringens is a conservative tracer and an indicator of sewage-derived pollution in the marine environment. The distribution of Clostridium perfringens spores was measured in sediments from Long Island Sound, USA, as part of a regional study designed to: (1) map the distribution of contaminated sediments; (2) determine transport and dispersal paths; (3) identify the locations of sediment and contaminant focusing; and (4) constrain predictive models. In 1996, sediment cores were collected at 58 stations, and surface sediments were collected at 219 locations throughout the Sound. Elevated concentrations of Clostridium perfringens in the sediments indicate that sewage pollution is present throughout Long Island Sound and has persisted for more than a century. Concentrations range from undetectable amounts to 15,000 spores/g dry sediment and are above background levels in the upper 30 cm at nearly all core locations. Sediment focusing strongly impacts the accumulation of Clostridium perfringens spores. Inventories in the cores range from 28 to 70,000 spores/cm2, and elevated concentrations can extend to depths of 50 cm. The steep gradients in Clostridium perfringens profiles in muddier cores contrast with concentrations that are generally constant with depth in sandier cores. Clostridium perfringens concentrations rarely decrease in the uppermost sediment, unlike those reported for metal contaminants. Concentrations in surface sediments are highest in the western end of the Sound, very low in the eastern region, and intermediate in the central part. This pattern reflects winnowing and focusing of Clostridium perfringens spores and fine-grained sediment by the hydrodynamic regime; however, the proximity of sewage sources to the westernmost Sound locally enhances the Clostridium perfringens signals.

Abstract-VAREKAMP, J. C.; BUCHHOLTZ TEN BRINK, M. R., MECRAY, E. L., and KREULEN, B., 2000. Mercury in Long Island Sound sediments. Journal of Coastal Research, 16(3), 613-626. Royal Palm Beach (Florida), ISSN 0749-0208.

Mercury (Hg) concentrations were measured in 394 surface and core samples from Long Island Sound (LIS). The surface sediment Hg concentration data show a wide spread, ranging from <50 ppb Hg in eastern LIS to >600 ppb Hg in westernmost LIS. Part of the observed range is related to variations in the bottom sedimentary environments, with higher Hg concentrations in the muddy depositional areas of central and western LIS. A strong residual trend of higher Hg values to the west remains when the data are normalized to grain size. Relationships between a tracer for sewage effluents (C. perfringens) and Hg concentrations indicate that between 0-50 % of the Hg is derived from sewage sources for most samples from the western and central basins. A higher percentage of sewage-derived Hg is found in samples from the westernmost section of LIS and in some local spots near urban centers. The remainder of the Hg is carried into the Sound with contaminated sediments from the watersheds and a small fraction enters the Sound as in situ atmospheric deposition. The Hg-depth profiles of several cores have well-defined contamination profiles that extend to pre-industrial background values. These data indicate that the Hg levels in the Sound have increased by a factor of 5-6 over the last few centuries, but Hg levels in LIS sediments have declined in modern times by up to 30 %. The concentrations of C. perfringens increased exponentially in the top core sections which had declining Hg concentrations, suggesting a recent decline in Hg fluxes that are unrelated to sewage effluents. The observed spatial and historical trends show Hg fluxes to LIS from sewage effluents, contaminated sediment input from the Connecticut River, point source inputs of strongly contaminated sediment from the Housatonic River, variations in the abundance of Hg carrier phases such as TOC and Fe, and focusing of sediment-bound Hg in association with westward sediment transport within the Sound.

Abstract-ZAJAC, R.N., LEWIS, R.S., POPPE, L.J., TWICHELL, D.C., VOZARIK, J. and DIGIACOMO-COHEN, M.L. 2000. Relationships Among Sea-Floor Structure and Benthic Communities in Long Island Sound: a Benthoscape Prospectus. Journal of Coastal Research, 16(3), 627-640. Royal Palm Beach (Florida), ISSN 0749-0208.

Long Island Sound is comprised of a rich and spatially heterogeneous mix of sea-floor environments which provide habitat for an equally diverse set of assemblages of soft-sediment communities. Information from recent research on the geomorphological and chemical attributes of these environments, as well as from studies of the hydrodynamics of the Sound, provide the opportunity to develop a landscape, or "benthoscape" framework for understanding the soft-sediment ecology of this estuary and for guiding future research focusing on structure and function at multiple spatial scales. This contribution reviews past research on benthic communities in Long Island Sound and addresses how they may be shaped by sea-floor characteristics at regional and benthoscape scales. At the regional scale (i.e. the entire Sound), differences in benthic community composition correspond to the distribution of general sedimentary environments. However, significant variation in community structure also occurs at the benthoscape scale (within regions) related to local variations in sediment properties, and physical and biogenic topographic features. Several topical areas in particular need further research in Long Island Sound, including temporal dynamics of benthic communities relative to sea-floor structure and the interaction between the dynamics of benthoscapes and hydrologic seascapes.

Abstract-THOMAS, E.; GAPOTCHENKO, T.; VAREKAMP, J.C.; MECRAY, E.L., and BUCHHOLZ TEN BRINK, M.R., 2000. Benthic Foraminifera and Environmental Changes in Long Island Sound. Journal of Coastal Research, 16(3), 641-655. Royal Palm Beach (Florida), ISSN 0749-0208.

Benthic foraminiferal faunas in Long Island Sound (LIS) in the 1940s and 1960s were of low diversity, and dominated by species of the genus Elphidium, mainly Elphidium excavatum clavatum, with common Buccella frigida and Eggerella advena. The distribution of these species was dominantly correlated with depth, but it was not clear which depth-related environmental variable was most important. Differences between faunas collected in 1996 and 1997, and in the 1940s and 1960s include a strong decrease in relative abundance of Eggerella advena over all LIS, an increase in relative abundance of Ammonia beccarii in western LIS, and a decrease in species diversity.

The decreased diversity suggests that environmental stress caused the faunal changes. Oxygen isotope data for E. excavatum clavatum indicate that a change in salinity is not a probable cause. Carbon isotope data suggest that the supply of organic matter to the benthos increased since the early 1960s, with a stronger increase in western LIS where algal blooms have occurred since the early 1970s, possibly as a result of nutrient input by waste water treatment plants. These blooms or the resulting episodes of anoxia/hypoxia may have played a role in the increased relative abundance of A. beccarii. There is no clear explanation for the decreased abundance of E. advena, but changes in the phytoplankton composition (thus food supply) are a possible cause. Benthic foraminiferal faunal and stable isotope data have excellent potential as indicators of physicochemical environmental changes and their effects on the biota in LIS.

Abstract-BEUNING, K.R.M.; FRANSEN, L.; NAKITYO, B.; MECRAY, E.L., and BUCHHOLTZ TEN BRINK, M.R., 2000. Modern pollen deposition in Long Island Sound. Journal of Coastal Research, 16(3), 656-662. Royal Palm Beach (Florida), ISSN 0749-0208.

Palynological analyses of 20 surface sediment samples collected from Long Island Sound show a pollen assemblage dominated by Carya, Betula, Pinus, Quercus, Tsuga, and ambrosia, as is consistent with the regional vegetation. No trends in relative abundance of these pollen types occur either from west to east or associated with modern riverine inputs throughout the basin. Despite the large-scale, long-term removal of fine-grained sediment from winnowed portions of the eastern Sound, the composition of the pollen and spore component of the sedimentary matrix conforms to a basin-wide homogeneous signal. These results strongly support the use of select regional palynological boundaries as chronostratigraphic tools to provide a framework for interpretation of the late glacial and Holocene history of the Long Island Sound basin sediments.