Near-shore Cores Research Articles

Seasonal variation in activity and nearshore habitat use of Lake Trout in a subarctic lake

BackgroundIn lake ecosystems, predatory fish can move and forage across both nearshore and offshore habitats. This coupling of sub-habitats, which is important in stabilizing lake food webs, has largely been assessed from a dietary perspective and has not included movement data. As such, empirical estimates of the seasonal dynamics of these coupling movements by fish are rarely quantified, especially for northern lakes. Here we collect fine-scale fish movement data on Lake Trout (Salvelinus namaycush), a predatory cold-water fish known to link nearshore and offshore habitats, to test for seasonal drivers of activity, habitat use and diet in a subarctic lake.MethodsWe used an acoustic telemetry positioning array to track the depth and spatial movements of 43 Lake Trout in a subarctic lake over two years. From these data we estimated seasonal 50% home ranges, movements rates, tail beat activity, depth use, and nearshore habitat use. Additionally, we examined stomach contents to quantify seasonal diet. Data from water temperature and light loggers were used to monitor abiotic lake conditions and compare to telemetry data.ResultsLake Trout showed repeatable seasonal patterns of nearshore habitat use that peaked each spring and fall, were lower throughout the long winter, and least in summer when this habitat was above preferred temperatures. Stomach content data showed that Lake Trout acquired the most nearshore prey during the brief spring season, followed by fall, and winter, supporting telemetry results. Activity rates were highest in spring when feeding on invertebrates and least in summer when foraging offshore, presumably on large-bodied prey fish. High rates of nearshore activity in fall were associated with spawning. Nearshore habitat use was widespread and not localized to specific regions of the lake, although there was high overlap of winter nearshore core areas between years.ConclusionsWe provide empirical demonstrations of the seasonal extent to which a mobile top predator links nearshore and offshore habitats in a subarctic lake. Our findings suggest that the nearshore is an important foraging area for Lake Trout for much of the year, and the role of this zone for feeding should be considered in addition to its traditional importance as spawning habitat.

Small Muddy Paleochannels and Implications for Submarine Groundwater Discharge near Charleston, South Carolina, USA

The spatial variations in Quaternary sediments on the inner continental shelf are produced by the progression of depositional environments during the latest sea-level rise, and this sedimentary architecture plays a fundamental role in controlling groundwater discharge. However, coincident seismic mapping, sediment cores, and hydrological studies are rare. Here, we combine high-resolution, 0.5–10 kHz, high-frequency seismic profiles with sediment cores to examine the nature of the sediment deposits, including paleochannels, where submarine groundwater discharge has also been studied in a 150 km2 area of the inner shelf north of Charleston, South Carolina. We used high-frequency seismic reflection to interpret seismic facies boundaries, including 16 paleochannel crossings, to 20 km offshore. From 13 vibracores taken at the intersections of the seismic lines, we defined seven lithofacies representative of specific depositional environments. The paleochannels that we cored contain thick layers of structureless mud sometimes interbedded with silt, and mud is common in several of the nearshore cores. Our results indicate that paleochannels are often mud-lined or filled in this area and were most likely former estuarine channels. Neither the paleochannels nor a mud layer were found farther than 11 km off the present shoreline. This offshore distance coincides with the strongest pulses of groundwater discharge, emerging just beyond the paleochannels. This suggests that the muddy paleochannel system acts as a confining layer for submarine groundwater flow.

Bridging the Gap Between Conservation Paleobiology and Resource Management: Recognizing the Past is the Key to the Future

A persistent challenge for conservation paleobiologists is communication of information on past environmental changes to resource managers in a way that allows them to apply these data to current restoration efforts. USGS scientists have learned a few lessons over 25 years of conducting applied paleoecology research in collaboration with the Greater Everglades Ecosystem Restoration (GEER) project. The first step is to engage resource managers in conversations prior to beginning research. What are their goals and information needs? Participation in GEER with teams of scientists and decision-makers working together to develop measures of success for Everglades restoration allowed us to overcome this first hurdle. Our initial research showed changes in salinity and freshwater influx over time, but how to use this information was not immediately apparent to management, so continued communication was critical. Through participation in meetings and presentation of our preliminary findings, the management team provided feedback that led us to develop a modern analog-based method to estimate past salinity, which was then used to adjust system-wide hydrologic models to reflect past conditions. Recently, we realized another management information gap — a set of indicator species for nearshore estuarine zones to monitor the effectiveness of upstream changes in flow. Again, by working with managers to determine needs, we combined distribution data of mollusk species in nearshore cores with our modern analog dataset to develop a suite of indicator species. These are a few examples of positive impacts from our long-term collaboration. We believe the key to advancing the use of conservation paleobiologic research in resource management is to communicate frequently and often, listen closely to management, discuss how paleo data can be applied, and be persistent. It is essential that we bridge these gaps because the past is our window to anticipating and planning for future change.

Wind‐Modulated Western Maine Coastal Current and Its Connectivity With the Eastern Maine Coastal Current

AbstractUsing a high‐resolution circulation model and an offline particle tracking model, we investigated variations of the Western Maine Coastal Current (WMCC) and its connectivity with the Eastern Maine Coastal Current (EMCC). The models showed that the weak, broad, and sinuous WMCC is generally southwestward with an offshore and a nearshore core, fed by the extension of the EMCC and runoff from the Penobscot and Kennebec–Androscoggin Rivers, respectively. A sea‐level dome can form offshore of Casco Bay in late fall and early winter as the northeastward alongshore wind sets up a seaward sea‐level gradient from the coast to meet the shoreward sea‐level gradient from Wilkinson Basin. Consequently, northeastward flows (i.e., the counter‐WMCC) emerge on the inshore side of the dome. Both the circulation and particle tracking models suggested that the connectivity generally peaks twice annually, highest in winter and then secondarily in late spring or early summer. The former is concurrent with the most southwest offshore veering of the EMCC, while the latter is concurrent with the strongest EMCC. Moreover, the counter‐WMCC can reduce the connectivity and result in year‐to‐year variations.

Seasonal Variations and Driving Factors of the Eastern Maine Coastal Current

AbstractTo investigate the coastal current in the Gulf of Maine (GoME) and its relation to forcing from outside of the gulf, a high‐resolution circulation model was developed and validated. Our model shows that the Eastern Maine Coastal Current (EMCC) possesses two cores, an offshore and a nearshore core that peak in summer and spring, respectively. The two cores can be traced back to outflows from the Bay of Fundy from opposite sides of Grand Manan Island, and both cores are deeper and slightly more onshore in summer and fall in response to tidal mixing, surface thermal stratification and wind. The two cores merge south of Pleasant Bay, then split into two branches again east of Mount Desert Rock, where the nearshore branch flows along the coast, while the offshore branch turns southward to recirculate in the eastern GoME. Subject to variations of Scotian Shelf Water and Slope Water (SW) inflows, the offshore veering occurs further upstream (northeastward) in late winter and summer, but gradually shifts downstream (southwestward) from summer to winter. Diagnosis of momentum balance demonstrates that the EMCC is primarily driven by the pressure gradient force (PG), of which the barotropic PG is dominant and offshore, while the baroclinic PG is onshore and increases with depth. The large baroclinic PG at depths, modulated by SW, that is, blended by tidal mixing, offsets the barotropic PG. Near the surface, the barotropic PG is nearly balanced by the Coriolis force, forming the geostrophic EMCC.

Mercury Inputs to Chinese Marginal Seas: Impact of Industrialization and Development of China

AbstractIn the past decades, China has experienced substantial economic growth and industrialization. However, the effects of vast development of China on Hg input to the nearby oceans are still unclear. In this study, the influx and isotopic compositions of Hg in four 210Pb‐dated sediment cores were examined to investigate changes in Hg deposition to the marginal seas off China over a century. Nearshore cores had higher Hg influxes than offshore cores. Increases of Hg influxes started since the 1950s, which coincides with China's economic development. Dramatic historical changes in Hg isotopic composition were observed in the cores (δ202Hg: −2.01‰ to −0.69‰; Δ199Hg: −0.16‰ to 0.31‰; n = 106). δ202Hg increased from the deep to the surface layers of the cores. The offshore cores mainly showed positive Δ199Hg values, but the pre‐1950 samples had more positive Δ199Hg values than the younger samples. The nearshore cores mainly showed negative Δ199Hg values in the pre‐1950s samples, but the younger samples showed Δ199Hg values close to zero. A triple‐mixing isotope model, used to quantify the contribution of potential Hg inputs (e.g., direct discharge of industrial Hg, soil Hg, and precipitation‐derived Hg), showed clear evidence of enhanced industrial Hg inputs to the ocean margin, but slightly decreased watershed soil Hg inputs in the last few decades. The variations of watershed‐derived Hg were likely caused by the construction of impoundments in major rivers of China. This study demonstrates that mercury inputs to Chinese marginal seas have been largely altered due to the industrialization and economic development of China.

Re-Evaluating the Geological Evidence for Late Holocene Marine Incursion Events along the Guerrero Seismic Gap on the Pacific Coast of Mexico

Despite the large number of tsunamis that impact Mexico’s Pacific coast, stratigraphic studies focusing on geological impacts are scanty, making it difficult to assess the long-term risks for this vulnerable region. Surface samples and six cores were taken from Laguna Mitla near Acapulco to examine sedimentological and geochemical evidence for marine incursion events. Sediment cores collected from behind the beach barrier are dominated by intercalated layers of peat and inorganic sediments, mostly silt and clay, with little or no sand. Sand- and shell-rich clastic layers with high levels of sulfur, calcium, and strontium only occur adjacent to the relict beach ridge remnants near the center of the lagoon. With the exception of one thin fine sand layer, the absence of sand in the near-shore cores and the predominance of the terrigenous element titanium in the inorganic layers, evidently eroded from the surrounding hillslopes, suggests that these large-grained intervals do not represent episodic marine incursions, but rather were likely formed by the erosion and redeposition of older marine deposits derived from the beach ridge remnants when water levels were high. These results do not support the occurrence of a large tsunami event at Laguna Mitla during the Late Holocene.

Hydrologic changes in Colorado during the mid-Holocene and Younger Dryas

The stratigraphy of Upper Big Creek Lake (UBC), an overflowing lake in northern Colorado, indicates that the lake responded to a series of hydroclimatic changes over the past ~13,000years. Modern sediments in eight comparator lakes from across Wyoming and Colorado facilitate the stratigraphic interpretation by showing that littoral sediments have exponentially higher sand content, lower loss-on-ignition (LOI), and lower net accumulation rates than off-shore sediments. Sand layers in two near-shore cores from UBC share the characteristics of the modern littoral sediments (>80% sand, <5% LOI, and <10cm/ka accumulation rates), and indicate that UBC was >1.9m below its outlet from 12,500 to 11,300 and from 7500 to 2100calyr BP. Ground-penetrating radar (GPR) profiles consistent with the low-water episodes show correlative sand layers and submerged wave-cut scarps around the lake margin. The stratigraphic features correlate with those formed in the comparator lakes during extensive droughts at >11,300, 6900–6000, and 4800–3400calyr BP as well as a series of north-south anti-phased droughts between 5800 and 1200calyr BP. Lake closure at UBC amplified the local changes, and demonstrates that climate changes can increase the extent of intermittent streamflow in the Rocky Mountains relative to today.

Assessment of drought over the past two millennia using near-shore sediment cores from a Canadian boreal lake

Lack of long-term studies on drought in the boreal region of northwest Ontario limits our ability to assess the vulnerability of this region to climate change. We investigated changes in diatoms, scaled chrysophytes, and sedimentary pigments in two near-shore cores from Gall Lake to infer limnological and water-level changes over the last two millennia. The two coring locations, at modern water depths of 7.5 and 11.5 m, were selected to contrast inferences for past lake level based on distance from the modern water-depth boundary between predominantly benthic and planktonic diatom assemblages in surface sediments (6.0 m). Diatom-inferred depth inferences were more variable in the core from 7.5-m water depth, consistent with the hypothesis that the greatest changes occurred at water depths closest to the benthic:planktonic boundary. Both sites revealed a pronounced drought from ~AD 950 to 1300, synchronous with the medieval climate anomaly (MCA). This finding suggests a northeast expansion of the arid MCA into northwest Ontario, extending the known spatial extent of this megadrought. Scaled chrysophytes and sedimentary pigments suggest a recent increase in thermal stratification. Our findings indicate this region is more susceptible to changes in moisture than was previously suspected based on the instrumental record for the past century.

Diatom-inferred changes in effective moisture during the late Holocene from nearshore cores in the southeastern region of the Winnipeg River Drainage Basin (Canada)

The Winnipeg River Drainage Basin (WRDB), within the boreal forest region of northwest Ontario, is a region that is expected to be negatively affected by climate warming. Inferences of droughts over the past two millennia from Little Raleigh Lake were based on two nearshore sediment cores. The core locations were from depths of ~12 and 15 m and were based on sufficient nearshore sediment accumulation and distance from the modern benthic-to-planktonic diatom boundary, where a distinct shift from dominance of benthic taxa changed to dominance of planktonic taxa in surficial sediments at ~11.8 m. Diatom-inferred depth was based on a model developed from 60 surficial sediments within the study lake. Depth inferences indicate that prolonged periods of aridity occurred from ~ad 950 to 1300 (corresponds to ‘Medieval Climate Anomaly’) and from ~ad 1625 to 1750 (aridity during ‘Little Ice Age’). We found that the core collected from a depth closer to the benthic-to-planktonic diatom boundary was more sensitive to changes in lake level than the deeper core where planktonic diatoms dominated the assemblage. The inferred low-water stands of the past two millennia are well outside of the range of the past ~100 years, suggesting that recent drought history may not be a good estimate of future extremes.

The Impacts of The Churchill Barriers on the Coastal Environment

This research investigated the impact of The Churchill Barriers on the coastal and marine environment through analysis of nearshore and terrestrial cores using geochemical and diatom analyses, 210Pb and 137Cs dating and field surveying. Analyses of a core taken from near barrier 1 showed that an eutrophic environment dominated throughout the sedimentary sequence. An increase in calcium carbonate, combined with increases in strontium and sodium and a small change in diatom composition, is thought to relate to barrier construction around 1940–1945. A similar pattern is revealed from a core taken near barrier 4 and also from Echna Loch (Burray). On the east side of barrier 4, a large area of sand deposition has occurred as a result of the sheltered environment offered by the barrier. Sediment has been transported northwards by longshore drift and moved onshore from offshore sand bars, forming an extensive sand dune environment. The sediment deposition can be seen to a lesser extent on the east side of the barriers 1, 2 and 3. These results suggest tentatively that barrier formation may have impacted on water flow patterns, and hence algal communities, leading to changes in floral composition. Furthermore, changes in sediment patterns, coastal geomorphology are evident from barrier emplacement.

Diatom-inferred depth models in 8 Canadian boreal lakes: inferred changes in the benthic:planktonic depth boundary and implications for assessment of past droughts

Assessment of past drought in boreal regions, a region predicted to be strongly affected by climate warming, can provide insights into future availability of water. However, limited instrumental data and paleoclimatic data are available for this assessment. To address this lack of data in the boreal region of northwest Ontario, a regional study of lakes in the Winnipeg River Drainage Basin was initiated. Diatom-inferred (D-I) depth models were developed based on surface samples collected along a depth gradient within 8 small boreal lakes. Weighted-averaging and modern analog approaches provided robust within-lake depth models for each of the study lakes, with bootstrapped r 2 values ranging from 0.90 to 0.98, and root-mean-squared-errors of prediction (RMSEP) between 1.1 and 2.5 m. Large differences in the estimated depth optima for three representative, but common diatom species across our 8 study lakes suggested that within-lake calibration datasets are more appropriate for inferring past drought based in depth models than a regional multi-lake calibration dataset, and that light and related variables are controlling factors governing the maximum depth of benthic taxa. A down-core application of the D-I depth models on a near-shore core from Meekin Lake, retrieved near the present-day ecotone between the benthic and planktonic diatom assemblages indicated highly similar trends in inferred depth ( r = 0.96). The models have significant correlations with other metrics of changes in depth including diatom species richness ( r = 0.74–0.78) and evenness ( r = 0.76–0.8), thereby allowing a check on the strength and direction of the depth inferences down-core. Near-shore cores located near the benthic:planktonic transition is a sensitive region that can provide estimates of past droughts in lakes where such inferences have been difficult to estimate.

A late Holocene paleoclimatic history of Lake Tanganyika, East Africa

A nearshore core (LT03-05) from the north basin of Lake Tanganyika provides diatom, pollen, and sedimentary time series covering the last ca. 3800 yr at 15–36 yr resolution. A chronology supported by 21 AMS dates on terrestrial and lacustrine materials allows us to account for ancient carbon effects on 14C ages and to propose refinements of the region's climatic history. Conditions drier than those of today were followed after ca. 3.30 ka by an overall wetting trend. Several century-scale climate variations were superimposed upon that trend, with exceptionally rainy conditions occurring 1.70–1.40 ka, 1.15–0.90 ka, 0.70–0.55 ka, and 0.35–0.20 ka. Around 0.55–0.35 ka, during the Spörer sunspot minimum, drier conditions developed in the northern Tanganyika basin while more humid conditions were registered at Lakes Victoria and Naivasha. This indicates significant variability in the nature and distribution of near-equatorial rainfall anomalies during much of the Little Ice Age.

Climate-related eutrophication of a small boreal lake in northwestern Ontario: a palaeolimnological perspective

A palaeolimnological investigation of changes in water quantity and quality in Lake 239, Experimental Lakes Area, in northwestern Ontario indicates marked changes in limnological conditions during the Holocene. Water quantity changes are based on the analysis of diatoms and inferences of lake-level changes from a nearshore sediment core. Changes in water quality are based on the analysis of diatom assemblages and associated quantitative inferences of total phosphorus (TP) from a deep central core, as well as changes in diatom accumulation rates, the proportion of chrysophyte scales to diatoms and changes to organic matter content. Lake levels at least 8 m lower than today were inferred from the nearshore core and were concurrent with an increase in nutrient-rich diatom assemblages, an increase in diatom accumulation and a decrease in the chrysophytes relative to diatoms in the central core. Fragilaria crotonensis and Aulacoseira subarctica were two of the dominant nutrient-rich taxa that contributed to an increase in diatom-inferred TP during the mid Holocene. Results from this study provide strong empirical evidence that water levels much lower than today can dramatically change trophic status, as occurred during the dry mid Holocene, and may provide a good analogue for understanding future impacts of climatic changes due to anthropogenic activities.

Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area, northwestern Ontario, Canada

We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.

Late Pleistocene desiccation of Lake Tana, source of the Blue Nile

High-resolution seismic data from Lake Tana, the source of the Blue Nile in northern Ethiopia, reveal a deep sedimentary sequence divided by four strong reflectors. Data from nearshore cores show that the uppermost strong reflector represents a stiff silt unit, interpreted as a desiccation surface. Channel cuts in this surface, bordered by levee-like structures, are apparent in the seismic data from near the lake margin, suggesting fluvial downcutting and over-bank deposition during seasonal flood events. Periphytic diatoms and peat at the base of a core from the deepest part of the lake overlie compacted sediments, indicating that desiccation was followed by development of shallow-water environments and papyrus swamp in the central basin between 16,700 and 15,100cal BP. As the lake level rose, open-water evaporation from the closed lake caused it to become slightly saline, as indicated by halophytic diatoms. An abrupt return to freshwater conditions occurred at 14,750cal BP, when the lake overflowed into the Blue Nile. Further reflection surfaces with downcut structures are identifiable in seismic images of the overlying sediments, suggesting at least two lesser lake-level falls, tentatively dated to about 12,000 and 8000cal BP. Since Lake Victoria, the source of the White Nile, was also dry until 15,000cal BP, and did not reach overflow until 14,500cal BP, the entire Nile system must have been reduced to intermittent seasonal flow until about 14,500cal BP, when baseflow was re-established with almost simultaneous overflow of the headwater lakes of both the White and Blue Nile rivers. Desiccation of the Nile sources coincides with Heinrich event 1, when cessation of northward heat transport from the tropical Atlantic disrupted the Atlantic monsoon, causing drought in north tropical Africa. The strong reflectors at deeper levels in the seismic sequence of Lake Tana may represent earlier desiccation events, possibly contemporaneous with previous Late Pleistocene Heinrich events.

Middle Holocene Hydrologic Change and Hypolimnion Formation in Lake Erie

Data from a nearshore sediment core and a deep-water sediment core from the central basin of Lake Erie reveal shifts in sediment properties and stable isotope composition of shell carbonate between ca. 4,600 and 3,500 14C yrs BP. Radiocarbon dates are corrected for the hardwater effect by subtracting 670 years based on a modern calibration for the central basin. Silt content increased in the deep water core at 4,600 and again, slightly, at 3,900 14C yrs BP. Sand size increased in the nearshore core at 3,500 14C yrs BP. δ 18O of shell carbonate increased and δ 13C decreased in both cores between about 4,200 and 3,500 14C years BP. Magnetic susceptibility and percent calcite decreased sharply and percent organic carbon increased slightly in the deep water core beginning at 4,000 14C yrs BP. Most of the changes in sediment properties and stable isotope composition of shell carbonate occurred between 4,200 and 3,900 14C yrs BP. They coincide in time with the Nipissing II highstand of Lake Nipissing, in the Huron and Michigan basins, and with evidence for higher-than-present lake levels in Lake Erie. The changes in proxy data are interpreted as evidence for an influx of surface water as drainage from the Upper Great Lakes was rerouted through Lake Erie. There is little evidence in the sediment proxy record for changes in Lake Erie during the earlier Nipissing I highstand or the middle Holocene transition in regional climate. A 9,000-year composite stable isotope record for the central basin shows that the sediment cores document a transformation in drainage that established the modern hydrologic system for the lake. High lake level induced a seasonal hypolimnion, setting the stage for the low pH, oxygen-depleted bottom waters of today.

TMAH thermochemolysis characterization of marine sedimentary organic matter in a Newfoundland fjord

Marine sedimentary organic matter of Trinity Bay and its associated fjord, the Northwest Arm (Newfoundland), was characterized by “batch-wise” tetramethyl ammonium hydroxide (TMAH) thermochemolysis. TMAH thermochemolysis products were characterized using GC/MS, and they included aromatic and aliphatic compounds determined as methyl esters and ethers. The dominant aliphatic compounds are fatty acids that ranged from C 14 to C 28, with a predominance of even carbon numbered species, and are either saturated, mono-unsaturated or branched. The aromatic compounds are phenols, of which the most abundant is 3,4-dimethoxybenzoic acid methyl ester, an important chemical marker in evaluating the terrestrial contribution to the organic carbon pool. The spatial variation of the thermochemolysates shows expected higher abundances of fatty acids and phenols in the near-shore sediments of the Northwest Arm compared to those of the offshore sediment cores. 210Pb dates of sediments of two near-shore cores enabled calculation of annual preserved fluxes of organic carbon, total fatty acids and phenols. The temporal and spatial input of phenols appear to be significantly influenced by saw-milling and logging activities of the local communities in the surrounding watershed.

Variation and distribution of total mercury in water, sediment and soil from northern Lake Victoria, East Africa

Lake Victoria, the world's largest tropical freshwater lake, is an important resource, ecologi- cally and economically. THg distribution in the northern parts of the lake are not well known, so to answer this gap, patterns in total mercury (THg) in water, soil and two dated sediment cores from north- ern Lake Victoria were determined. Water THg concentrations ranged from 0.7 to 5.8 ng/L, and there were no apparent differences observed between Napoleon and Winam Gulfs. Two precipitation samples had Hg concentrations of 7 and 31 ng/L. Surface soil samples collected from various agricultural sites around Jinja, Napoleon Gulf, have THg concentrations between 12.7 and 48.4 ng/g dry weight; they were correlated with organic carbon, total phosphorus and % clay. A near-shore core taken in Itome Bay in Napoleon Gulf, and an offshore core collected from the deepest part of the lake had similar THg concentrations and profiles (78 to 458 ng/g dry weight). The increase in THg concentration in the pro- files of both cores began around 1960 and peaked around 1980. The similar sedimentary THg profiles and fluxes in the cores suggest that the THg sources to L. Victoria are primarily atmospheric, with some erosion inputs, and that equatorial African ecosystems are not exempt from the global increase in base- line THg concentrations.

Characterization and FATE of PAH-contaminated sediments at the Wyckoff/Eagle Harbor Superfund Site.

Eagle Harbor, a shallow marine embayment of Bainbridge Island, WA approximately 10 miles west of Seattle, WA), was formerly the site of the Wyckoff wood-treatment facility. The facility used large quantities of creosote in its wood-treating processes from the early 1900s to 1988. Historical creosote seepage into the harbor resulted in substantial accumulation of polycyclic aromatic hydrocarbon (PAH) contamination in the harbor sediments over time. This investigation focused on the distribution and fate of the PAH-contaminated harbor sediments. Analyses of 10 sediment cores using total petroleum hydrocarbon (TPH) fingerprinting, the distribution of 50 PAH analytes, and sediment age dating revealed the contributions of three distinct sources of PAHs to sediment contamination in the harbor during various periods over the past 100 years; namely, creosote, urban runoff, and natural background. Surface sediments (upper 20-30 cm) in the cores closestto the Wyckoff wood-treatment facility and southeast of an existing cap were dominated by urban runoff and weathered creosote; the deeper sediments (> 30 cm) were heavily contaminated with relatively unweathered creosote and some pure-phase creosote. Cores located the furthest from the area of contamination, in the center of the harbor, were dominated by urban runoff, showed no signs of creosote contamination, and had much lower PAH and TPH concentrations than those adjacent to the facility. In the four cores in the center of the Harbor, farthest from the former Wyckoff facility, PAH concentrations increased significantly (p < 0.01) with proximity to the northern shore of the harbor, which is more heavily developed than the southern shore and is where all automobile traffic enters and exits the island through the Bainbridge Island ferry terminal. Deeper portions of these cores were contaminated primarily with natural background PAHs, likely representing preurbanization sediments. Sedimentation rates ranged from 0.54 to 1.10 gm/ cm2 in the four cores located in the middle of the harbor, and for the single nearshore core that could be used to calculate sedimentation rates. Recognition that urban runoff has been a fairly consistent and ongoing source of PAHs to the harbor's sediments for the past 50-70 years may influence future sediment management decisions for this site with respect to long-term monitoring of surface sediments to assess cap performance. The results provided information on the ability of Eagle Harbor sediments to recover under natural conditions, identified the occurrence of creosote-derived PAH weathering in off-cap surface sediments, and distinguished between these distinct PAH sources in the harbor (creosote, urban runoff, and natural background).