David N. Edgington

History and timing of human impact on Lake Victoria, East Africa

Lake Victoria, the largest tropical lake in the world, suffers from severe eutrophication and the probable extinction of up to half of its 500+ species of endemic cichlid fishes. The continuing degradation of Lake Victorias ecological functions has serious long–term consequences for the ecosystem services it provides, and may threaten social welfare in the countries bordering its shores. Evaluation of recent ecological changes in the context of aquatic food–web alterations, catchment disturbance and natural ecosystem variability has been hampered by the scarcity of historical monitoring data. Here, we present high–resolution palaeolimnological data, which show that increases in phytoplankton production developed from the 1930s onwards, which parallels human–population growth and agricultural activity in the Lake Victoria drainage basin. Dominance of bloom–forming cyanobacteria since the late 1980s coincided with a relative decline in diatom growth, which can be attributed to the seasonal depletion of dissolved silica resulting from 50 years of enhanced diatom growth and burial. Eutrophication–induced loss of deep–water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for certain deep–water cichlids. Conservation of Lake Victoria as a functioning ecosystem is contingent upon large–scale implementation of improved land–use practices.

Impact of fossil fuel combustion on the sediments of Lake Michigan

A study was conducted to determine the relationships between metal and mineral contents of the Lake Michigan sediments and fluxes of charcoal, whose morphologies and surface characteristics are indicative of different combustion processes. Sediments deposited during 1930-1968 showed increasing concentrations of chromium, nickel, iron, aluminum, zinc, cobalt, lead, cadamium, and copper. After 1968, these concentrations decreased slightly. A similar profile was found for total charcoal concentration.

Particle Transport, Nutrient Cycling, and Algal Community Structure Associated with a Major Winter-Spring Sediment Resuspension Event in Southern Lake Michigan

Abstract Over the past decade, intermittent satellite imagery revealed the presence of an extensive plume of resuspended sediments in late winter-early spring with a clear offshore projection coinciding with the region of maximum sediment accumulation in the lake. The large scale of the plume implied that this process was important in sediment, and associated constituent, cycling and transport, but it had never been sampled due to severe conditions. The onset of the 1996 event coincided with a major March storm. Within a few days the plume was approximately 10 km wide and over 300 km in length, implying that the source of the reflective materials was widely distributed. An estimate of the total mass of resuspended sediment, 12 days after the storm, was similar to the annual external load of (sand-free) particulate material to the southern basin. The high turbidity plume persisted for over a month, progressing northward along the eastern shore with a major offshore transport feature. Sediment traps within this feature recorded a major mass flux event. The plume was sampled on two occasions and was found to contain 5 to 10 times as much suspended matter as open-lake locations outside the visible plume. Total particulate phosphorus was high within the plume making this episodic process important in sedimentwater exchange. The diatom community structure within the plume was significantly different from outside the plume and was characteristic of more eutrophic waters. Abundance of non-diatom phytoplankton and microbial food web organisms were highest at the plume edge. The episodic nature of this process makes it difficult to sample, but the scale makes it important in designing monitoring programs and massbalance modeling efforts.

Silica Depletion in Lake Victoria: Sedimentary Signals at Offshore Stations

Abstract Six short sediment cores from offshore stations in Lake Victoria (East Africa) were analyzed for evidence of recent change in the lakes pelagic ecosystem. Three stations were located on a NW-SE transect between 48 m water depth, near the present upper limit of seasonal hypolimnetic oxygen depletion, and the deepest point of Lake Victoria at 68 m. Four stations formed a NE-SW transect across the east-central zone of maximum Holocene sediment accumulation below 64 m water depth. 2I0 Pb dating of two cores from deepwater stations established average recent sediment-accumulation rates of 0.032xa0±xa00.001 g/cm 2 /yr and 0.028xa0±xa00.001 g/cm 2 /yr. Although the deepest part of the basin has been subject to an event of possibly widespread sediment erosion dated to the mid-1920s, core correlation based on the stratigraphy of biogenic Si above this unconformity indicates that deepwater stations have accumulated representative high-resolution archives of lake history over the past 70 years. The sedimentary record of biogenic-Si accumulation in deepwater cores reflects a sequence of events in which progressive enrichment of Lake Victoria with essential nutrients other than Si first led to increased diatom production, until the combination of excess Si demand and greater burial losses of diatom-Si resulted in depletion of the dissolved-Si reservoir and a transition to Si-limited diatom growth. Available sediment chronologies infer that increased diatom production in offshore areas started between the 1930s and early 1950s, and that the recently documented phytoplankton transition to year-round dominance by cyanobacteria started in the late 1980s. Excess diatom production over the past half century has led to significantly higher burial losses of biogenic Si only in the depositional center of the basin at water depths below 60 m.

Measuring low concentrations of 234Th in water and sediment

Th/ 238 U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle- reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of 234 Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of 234 Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a 229 Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low 234 Th/ 228 Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure 234 Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity. D 2002 Elsevier Science B.V. All rights reserved.

Patterns of Mass Sedimentation and of Deposition of Sediment Contaminated by PCBs in Green Bay

Almost one thousand sediment samples were collected from many sediment depths at sixty-four sampling stations in Green Bay as part of the Green Bay Mass Balance Study. The samples were analyzed for Pb-210 and Cs-137, and PCB congeners. Mass sedimentation rates and mixed depths were calculated from radionuclide profiles constructed for each sampling station. Most of the mass sedimentation in Green Bay occurs in the southern bay. Maximum sedimentation rates reach levels above 150 mg/cm2/yr. Three distinct sedimentation zones exist. The main sedimentation zone, located between the Peshtigo River and Sturgeon Bay, receives sediment from throughout the southern bay. South of this zone, the Oconto River helps to create a second smaller zone of sedimentation. The Fox River supports a much larger zone which is focused at its southern end, located along the eastern shore of Green Bay about 35 Km from the mouth of the Fox River. Maximum depth-averaged sediment PCB concentrations are about 1500 ng/g dry. By combining sedimentation data and measured sediment PCB concentrations, it was possible to calculate the mass of PCBs in Green Bay sediment. Green Bay contains about 8500 Kg of PCBs, and PCB-contaminated sediment is not uniformly distributed over the bay. Instead, the most contaminated sediments, as well as the greatest mass of sediment-bound PCBs, are located in the Fox River deposition zone. Sediment from the other three major rivers entering the bay dilutes the contaminated Fox River sediment. Highly contaminated sediment is found only in the Fox River deposition zone. About 50% of the total mass of PCBs in Green Bay sediments is found within 10 Km of this zone, in an area equal to about 3% of the total area of the bay. Despite this fact, eliminating PCBs from Green Bay would require the removal of large amounts of sediments.

Seasonal and spatial dynamics of 234Th/238U disequilibria in southern Lake Michigan

[1]xa0Measurements of 234Th/238U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km2). The mean activity of 238U in Lake Michigan measured 230 ± 20 dpm m−3, approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (<0.45 μm) and excess particle-bound 234Th were 8.0 and 60.6 dpm m−3, respectively. Using a simple one-dimensional model, median residence times for dissolved and particle-bound 234Th were ∼1 and ∼14 days, respectively. 234Th-based particle settling velocities had a median value of 0.4 m d−1 (range: ∼0.0–1.4 m d−1), and instantaneous 234Th-based estimates of the net vertical mass flux had a median value of 0.4 g m−2 d−1 (range: ∼0.0–5.2 g m−2 d−1). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind-induced wave height. Using a simple two-box model, the 234Th-based cross-margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 × 109 kg yr−1, which is in excellent agreement with a 210Pb − 137Cs based, basin-wide mass sedimentation rate of 1.28 × 109 kg yr−1.

Mercury in Green Bay, Lake Michigan Surficial Sediments Collected between 1987 and 1990

Abstract Sediments were collected by gravity core from 74 locations in Green Bay, Lake Michigan. Surficial sediments (0 to 1 cm depth) from these stations were analyzed for total mercury for the purpose of describing the horizontal variation of mercury in Green Bay sediments, estimating mercury fluxes to surficial sediments, and identifying potential sources of mercury. With concentrations ranging between 0.006 and 1.0 mg/kg, the surficial sediments had a mean mercury concentration of 0.36 mg/kg. The median concentration was 0.28 mg/kg. Surficial sediment mercury concentrations were highest along the eastern shore between the cities of Green Bay and Sturgeon Bay. Elevated concentrations were also found near Escanaba and west of Chambers Island. Lowest concentrations were found north, south, and west of Washington Island and near the Little Suamico River. Mercury fluxes ranged between 0.03 and 100 ng/cm 2 /y, with a mean of 20 ng/cm 2 /y. Only 14% of this flux is delivered by regional atmospheric sources. The Fox River is the dominant contributor, and the majority of the total mercury flux to the bay is delivered by the rivers that are tributary to the bay.

Sulfate Reduction in Sediment Interstitial Fluids in Lakes Michigan and Erie

Sulfate disappears from the interstitial fluids of recent sediments in Lakes Michigan and Erie within a zone extending no more than a few centimeters below the sediment-water interface. The results of concomitant analyses of the fluids for dissolved carbon dioxide, pH, total ammonia nitrogen, and redox potential suggest that sulfate is undergoing reduction in this zone. Stoichiometrically consistent increases in dissolved ammonia and carbon dioxide accompany sulfate reduction. Application of a simplified steady state reaction-diffusion model to the observed sulfate profiles yielded inferred rates of sulfate reduction (l0−11 to 10−9 mol L−1s−1) that are similar to those reported for marine muds.