Brent A. McKee

Climate change decreases aquatic ecosystem productivity of Lake Tanganyika, Africa

Although the effects of climate warming on the chemical and physical properties of lakes have been documented, biotic and ecosystem-scale responses to climate change have been only estimated or predicted by manipulations and models. Here we present evidence that climate warming is diminishing productivity in Lake Tanganyika, East Africa. This lake has historically supported a highly productive pelagic fishery that currently provides 25–40% of the animal protein supply for the populations of the surrounding countries. In parallel with regional warming patterns since the beginning of the twentieth century, a rise in surface-water temperature has increased the stability of the water column. A regional decrease in wind velocity has contributed to reduced mixing, decreasing deep-water nutrient upwelling and entrainment into surface waters. Carbon isotope records in sediment cores suggest that primary productivity may have decreased by about 20%, implying a roughly 30% decrease in fish yields. Our study provides evidence that the impact of regional effects of global climate change on aquatic ecosystem functions and services can be larger than that of local anthropogenic activity or overfishing.

Sources of terrestrially-derived organic carbon in lower Mississippi River and Louisiana shelf sediments: Implications for differential sedimentation and transport at the coastal margin

In this study, we examined the temporal and spatial variability of terrestrial organic carbon sources in lower Mississippi River and Louisiana shelf sediments (during 11 cruises over a 22-month period) to further understand the sorting dynamics and selective transport of vascular plant materials within the primary dispersal system of the river. Bulk d 13 C values in lower river sediments ranged from 21.90xto 24.64x(mean=23.20F1.09x), these values were generally more depleted than those found in shelf sediments (22.5xto 21.2x). The L8 (L8=sum of vanillyl, syringyl and cinnamyl phenols produced from the oxidation of 100 mg of organic carbon) values in the lower river ranged from 0.71 to 3.74 (mean=1.78F0.23). While there was no significant relationship between L8 and river discharge (p>0.05), the highest value occurred during peak discharge in April 1999—which corresponded to the highest observed C/N value of 17.41. The L8 values on the shelf ranged from 0.68 to 1.36 (mean=0.54F0.30) and were significantly lower (p<0.05) than the average value for lower river sediments. The range of S/V (syringyl/vanillyl) and C/V (cinnamyl/vanillyl) ratios on the shelf, 0.11 to 0.95 and 0.01 to 0.08, respectively, were similar to that found in the lower river. These low C/V ratios are indicative a mixture of woody and non-woody carbon sources. Recent work by Goni et al. [Nature 389 (1997) 275; Geochim. Cosmochim. Acta 62 (1998) 3055], which did not include sampling transects within the primary dispersal system of the Mississippi River, showed a non-woody vascular plant signature on the Louisiana shelf. This suggests that riverine-derived woody tissues preferentially settle out of the water column, in the lower river and inner shelf, prior to the selective dispersal of C3 versus C4 non-woody materials in other regions the shelf and slope. This works further demonstrates the importance of differential settlement of particles, sampling location within the dispersal system, and river discharge, when examining biogeochemical cycles in river-dominated margins. D 2002 Elsevier Science B.V. All rights reserved.

Breathing metals as a way of life: Geobiology in action

Many microbes have the ability to reduce transition metals, coupling this reduction to the oxidation of energy sources in a dissimilatory fashion. Because of their abundance, iron and manganese have been extensively studied, and it is well established that reduction of Mn and Fe account for significant turnover of organic carbon in many environments. In addition, many of the dissimilatory metal reducing bacteria (DMRB) also reduce other metals, including toxic metals like Cr(VI), and radioactive contaminants like U(VI), raising the expectations that these processes can be used for bioremediation. The processes involved in metal reduction remain mysterious, and often progress is slow, as nearly all iron and manganese oxides are solids, which offer particular challenges with regard to imaging and chemical measurements. In particular, the interactions that occur at the bacteria-mineral interfaces are not yet clearly elucidated. One DMRB, Shewanella oneidensis MR-1 offers the advantage that its genome has recently been sequenced, and with the availability of its genomic sequence, several aspects of its metal reducing abilities are now beginning to be seen. As these studies progress, it should be possible to separate several processes involved with metal reduction, including surface recognition, attachment, metal destabilization and reduction, and secondary mineral formation.

Recent accretion in mangrove ecosystems based on137Cs and210Pb

Accretion rates were measured in fringe and basin mangrove forests in river and tidally dominated sites in Terminos Lagoon, Mexico, and a basin mangrove forest in Rookery Bay, Florida, USA. Accretion rates were determined using the radionuclides210Pb and137Cs. Consolidation-corrected accretion rates for the Rookery Bay cores, ranged from 1.4 to 1.7 mm yr−1, with an average rate of 1.6 mm yr−1. Rates at the Mexico sites ranged from 1.0 to 4.4 mm yr−1, with an average of 2.4 mm yr−1. Determination of rates in these mangrove forests was greatly affected by the consolidation corrections which decreased the apparent accretion rate by over 50% in one case. Accretion rates at basin sites compare favorably with a reported 1.4 to 1.6 mm yr−1 rate of sea-level rise, indicating little or no subsidence at inland locations. Accretion rates in fringe sites are generally greater than basin sites, indicating greater subsidence rates in these sediments over longer time intervals.

Records of nutrient-enhanced coastal ocean productivity in sediments from the Louisiana continental shelf

Shelf sediments from near the mouth of the Mississippi River were collected and analyzed to examine whether records of the consequences of anthropogenic nutrient loading are preserved. Cores representing approximately 100 yr of accumulation have increasing concentrations of organic matter over this period, indicating increased accumulation of organic carbon, rapid early diagenesis, or a combination of these processes. Stable carbon isotopes and organic tracers show that virtually all of this increase is of marine origin. Evidence from two cores near the river mouth, one within the region of chronic seasonal hypoxia and one nearby but outside the hypoxic region, indicate that changes consistent with increased productivity began by approximately the mid-1950s when the inorganic carbon in benthic forams rapidly became isotopically lighter at both stations. Beginning in the mid-1960s, the accumulation of organic matter, organic δ13C, and δ15N all show large changes in a direction consistent with increased productivity. This last period coincides with a doubling of the load of nutrients from the Mississippi River, which levelled off in the mid-1980s. These data support the hypothesis that anthropogenic nutrient loading has had a significant impact on the Louisiana shelf.

Historical trends of hypoxia on the Louisiana shelf: application of pigments as biomarkers

Increases in the deposition of phytoplankton-derived organic carbon resulting from increases in nutrient inputs through the Mississippi–Atchafalaya system since the early 1950s has been speculated as the primary reason for the occurrence of hypoxic events in this region (Rabalais, N.N., Wiseman, W.J., Turner, R.E., Sen Gupta, B.K., Dortch, Q., 1996. Nutrient changes in the Mississippi river and system responses on the adjacent continental shelf. Estuaries 19(2B), 386–407). However, due to the lack of long-term measurements of oxygen in this region it is unclear if hypoxia events occurred prior to anthropogenic inputs of nutrients from the Mississippi river. In this study, we used naturally occurring radionuclides and plant pigment biomarkers to document changes in hypoxia events over the past 100 years. Specifically, we used pigments derived from the anoxygenic phototrophic brown-pigmented green sulfur bacteria Chlorobium phaeovibroides and C. phaeobacteroides. In sediments, at a hypoxic site west of the Mississippi plume, we observed high concentrations (52 nmol/g OC) of bacteriochlorophyll-e along with the specific decay product homologues of bacteriopheophytin-e (15 nmol/g OC). The down-core distribution of bacteriochlorophyll-e and bacteriopheophytin-e homologues (in particular the more stable bacteriopheohytin-e) indicated that the highest concentrations occurred between 1960 and the present, coinciding with increased nutrient loading from the Mississippi river. These bacteriopigments were not detected prior to the early 1900s. These results are consistent with the view that increases in riverine nutrient loadings is likely the major cause of increasing trends in hypoxic events along the Louisiana coast over the past 50 years.

Wind-induced bottom sediment resuspension in a microtidal coastal environment

Bottom sediment resuspension frequency, duration, and extent (i.e. areal percent of bottom sediments a!ected) were characterized for the 15 month period from September 1995 to January 1997 for select water bodies within the Barataria Basin, Louisiana, USA. An empirical model of sediment resuspension as a function of water depth, wind speed, direction, and fetch was derived from surface gravity wave theory. The model was validated with water-column turbidity imagery obtained from visible and near-IR AVHRR data. Based on model predictions, wind-induced resuspension occurred during all seasons of this study. Model predictions of the critical wind speed required to induce resuspension indicate that winds of 4 m s~1 (averaged over all wind directions) resuspend approximately 50% of bottom sediments in the water bodies examined. Winds of this magnitude (4 m s~1) occurred for 80% of the time during the late fall, winter, and early spring and for approximately 30% of the time during the summer. More than 50% of the bottom sediments, on an areal basis, are resuspended throughout the year, indicating the importance of resuspension as a process a!ecting sediment and biogeochemical #uxes in the Barataria Basin. ( 2000 Elsevier Science Ltd. All rights reserved.

Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters

Seagrass meadows are highly productive habitats that provide important ecosystem services in the coastal zone, including carbon and nutrient sequestration. Organic carbon in seagrass sediment, known as “blue carbon,” accumulates from both in situ production and sedimentation of particulate carbon from the water column. Using a large-scale restoration (>1700 ha) in the Virginia coastal bays as a model system, we evaluated the role of seagrass, Zostera marina , restoration in carbon storage in sediments of shallow coastal ecosystems. Sediments of replicate seagrass meadows representing different age treatments (as time since seeding: 0, 4, and 10 years), were analyzed for % carbon, % nitrogen, bulk density, organic matter content, and 210Pb for dating at 1-cm increments to a depth of 10 cm. Sediment nutrient and organic content, and carbon accumulation rates were higher in 10-year seagrass meadows relative to 4-year and bare sediment. These differences were consistent with higher shoot density in the older meadow. Carbon accumulation rates determined for the 10-year restored seagrass meadows were 36.68 g C m-2 yr-1. Within 12 years of seeding, the restored seagrass meadows are expected to accumulate carbon at a rate that is comparable to measured ranges in natural seagrass meadows. This the first study to provide evidence of the potential of seagrass habitat restoration to enhance carbon sequestration in the coastal zone.

Uranium geochemistry on the Amazon shelf: Chemical phase partitioning and cycling across a salinity gradient

Abstract The size distribution of U was examined in surface waters of the Amazon shelf. Water samples were collected during a low discharge river stage across a broad salinity gradient (0.3–35.4%) and fractionated by planar filtration and tangential-flow ultrafiltration into (1) solution (U s , c 10,000 MW-0.4 μm), (3) dissolved (U d , p , >0.4 μm) phases. Concentrations of colloidal U comprise up to 92% of the dissolved U fraction at the river mouth and attain highest values (∼0.45 μg/L) in the productive, biogenic region of the Amazon shelf (salinities above ∼20%). U d and U c distributions are highly nonconservative relative to ideal dilution of river water and seawater, indicating extensive removal at salinities below ∼10%. The distribution of U s also shows some nonconservative behavior, yet removal, if any, is minimal. Saltwater-induced precipitation and aggregation of riverine colloidal material is most likely the dominant mechanism of U removal in the low salinity, terrigenous region of the Amazon shelf. There is evidence of a substantial colloidal U input (∼245% of the riverine U c flux into surface waters above 5%. Such U c enrichment most likely is the result of colloidal U-rich porewater diffusion/advection from the seabed and fluid muds or shelf-wide particle-colloid disaggregation. Removal of solution and dissolved phase U via a colloidal intermediate and U c aggregation was examined in terms of coagulation theory. The highly reactive nature of all U phases on the Amazon shelf suggests that remobilization and fractionation of U may also occur in other river-influenced coastal environments.

Time-based correlation of biogenic, lithogenic and authigenic sediment components with anthropogenic inputs in the Gulf of Mexico NECOP study area

Hypotheses related to variability in seasonal hypoxic conditions, coastal nutrient enhancement, and off-shelf transport of carbon on the Louisiana continental shelf were tested by characterization of biogenic, lithogenic, and authigenic components from two shelf and one Mississippi Canyon sediment cores. The authigenic-phase glauconite occurs above detection limits only in the core from the hypoxic area. A major increase in glauconite concentration was coincident with the onset (≈1940) of the increased use of commercial fertilizers in the United States. In the same hypoxic-area core, benthic foraminifera species diversity decreases upcore from approximately the turn of the century to the present in a manner concurrent with glauconite and fertilizer increases. A subset of opportunistic benthic foraminifera species, known to become more prominent in stressed environments (i.e., hypoxic), increased upcore from ∼52% of the total population at core bottom to ≈90% at core top. These benthic foraminifera population and diversity changes were not apparent in a “control” core outside the area of documented hypoxia. Seaward of the shelf, in the Mississippi Canyon, coincident increases in sediment accumulation rate, percentages of coarse fraction and of organic carbon at core top indicate increased offshelf transport of carbon and other components. Quartz percentages indicate that episodic down-canyon transport has been active to core bottom (prior to the mid 1800s).