Richard A. Bourbonniere

Record of postglacial organic matter delivery and burial in sediments of Lake Ontario

A 12 m piston core obtained from Lake Ontario has allowed us to study the accumulation of organic matter in sediments deposited since the retreat of the Laurentian ice sheet about 12,500 years ago. Discrepancies between radiocarbon dates of disseminated organic matter and ostracod shells emphasize the magnitude of the recycling and retention of organic carbon in the Great Lakes. Concentrations of CaCO3 and organic carbon decrease down core due to the presence of fine grained glaciolacustrine clays at the base of the sedimentary sequence. Increases in sediment grain size indicate periods with enhanced fluxes of terrigenous material. CN ratios, %organic carbon and %CaCO3 vary proportionally with fluctuations in terrigenous input. CN ratios indicate that lacustrine algae have been the main source of organic matter to Lake Ontario sediments. Organic δ13C values become heavier at the bottom of the core, suggesting a shift in carbon sources for the bulk organic matter. Anthropogenic effects are well documented in total hydrocarbon and total fatty acid profiles of modern sediments, whereas postglacial trends representing natural changes of organic matter are relatively undetectable. Sedimentary profiles of terrigenous/aquatic ratios of n-alkane and n-alkanoic acids indicate that early variations in postglacial sedimentation rates may have impacted the preservation of aquatic organic matter. Changes in watershed vegetation and organic matter delivery to Lake Ontario altered these ratios as deglaciation progressed. Organic geochemical properties of modern sediments have recorded how anthropogenic activity has augmented algal productivity in Lake Ontario by increasing nutrient input.

Diagenesis of Vascular Plant Organic Matter Components during Burial in Lake Sediments

Diagenetic changes are difficult to distinguish from variations in sources of organic matter to sediments. Organic geochemical comparisons of samples of wood, bark, and needles from a white spruce (Picea glauca) living today and one buried for 10,000 years in lake sediments have been used to identify the effects of diagenesis on vascular plant matter. Important biogeochemical changes are evident in the aged spruce components, even though the cellular structures of the samples are well preserved. Concentrations of total fatty acids dramatically diminish; unsaturated and shorter chainlength components are preferentially lost from the molecular distributions. Concentrations of total alcohols are similar in the modern and 10,000-year-old wood and bark but markedly lowered in the aged needles. Hydrocarbon concentrations and distributions show little diagenetic change in the 10,000-year-old plant materials. Cellulose components in the wood decrease relative to lignin components, although both types of materials remain in high concentration in comparison to other organic components. Aromatization of abietic acid proceeds more rapidly in buried spruce wood than in bark; retene is the dominant polyaromatic hydrocarbon in the aged wood. In contrast to the variety of changes evident in molecular compositions, neither σ13C values nor C/N ratios differ significantly in the bulk organic matter of modern and aged spruce components.

Microbial Distributions and the Impact of Phosphorus on Bacterial Activity in Lake Erie

Abstract In light of recent suggestions concerning the relaxation of controls on phosphorus loading in Lake Erie, and in consideration of our current lack of baseline information on microbial communities in this system, a two-part analysis of the microbial ecology of the lake has been conducted. A comprehensive survey of the surface waters collected data on phytoplankton abundance, bacterial productivity and abundance, and viral abundance that were previously not available for Lake Erie. In parallel, phosphorus amendment experiments were conducted in each of the three hydrologically distinct basins of the lake to determine the effect of increased phosphorus loading on bacterial abundance and productivity. After 72 hours incubation, the addition of phosphorus had a significant impact on the microbial food web. A proliferation of different size classes of phytoplankton was accompanied by increases in bacterial production, but not necessarily bacterial abundance. Observations confirmed previous studies suggesting that the microbial communities in each of the three basins responded differently to the addition of phosphorus. Accompanying nutrient data suggest that the responses were due to limitation by other factors in the presence of excess phosphorus; namely that phytoplankton became nitrogen limited whereas bacteria became carbon limited. These results confirm the importance of the microbial food web in understanding the impact of phosphorus loading on the biotic carbon flow in the Lake Erie ecosystem.

Chlorinated Contaminants in Surficial Sediments of Lakes Huron, St. Clair, and Erie: Implications Regarding Sources Along the St. Clair and Detroit Rivers

Abstract Surficial sediments from southern Lake Huron, Lake St. Clair, and Lake Erie have been analyzed for a broad spectrum of chlorinated organics including PCBs, chlorobenzenes, and several pesticides. The differences between sediment contaminant concentrations in Lake Huron and Lake St. Clair indicated sources of hexachlorobenzene, hexachlorobutadiene, octachlorostyrene, and several other chlorinated benzenes along the St. Clair River. Similar differences between sediment PCB concentrations in Lakes Huron/St. Clair and Lake Erie indicated major PCB sources along the Detroit River. Specific PCB congener analysis revealed that PCBs discharged to the Detroit River contained especially high concentrations of highly chlorinated hexa-, hepta-, and octachloro-biphenyls which are major constituents of the industrial mixture Aroclor 1260. The analysis of individual PCB congeners made it possible to trace PCBs of Detroit River origin to the central and eastern basins of Lake Erie, and to estimate the contribution of the Detroit River to the PCB burden in sediments of these basins.

Hydrocarbons and fatty acids in two cores of Lake Huron sediments

Abstract Compositions of aliphatic hydrocarbons and of fatty acids were analyzed in two half-meter cores of postglacial Lake Huron sediment. One core represents a continuous record of the past 450 yr of sediment accumulation; the other consists of a surficial layer of modern sediment overlying 40 cm of 11,000–12,000 yr-old sediment. Concentrations of hydrocarbons are higher in the younger core than in the older one. Based upon n-alkane distributions, this reflects a smaller input of terrigenous material to Lake Huron 11,000 yr ago rather than diagenetic losses. Most of the hydrocarbons present in the 450 yr-old core are allochthonous while half are autochthonous in the older core. Fatty acids are primarily of aquatic character in both cores, and their concentrations decrease rapidly with depth. Unsaturated acids disappear more quickly than do their saturated analogs. Fatty acid degradation occurs mostly in the biologically active zone of these sediments, and little further alteration of fatty acids appears to happen over times as long as 12,000 yr.

Seasonal Hypoxia and the Genetic Diversity of Prokaryote Populations in the Central Basin Hypolimnion of Lake Erie: Evidence for Abundant Cyanobacteria and Photosynthesis

ABSTRACT The reoccurring region of seasonal hypoxia in the central basin of Lake Erie (“the dead zone”) has been of significant interest to researchers over the past several years. Surprisingly however, no efforts to characterize the endemic microbial community, responsible for the consumption of oxygen in this system, have been published. To understand how the microbial community may be interacting with this event, we have begun to characterize microbial members by using molecular tools. Phycoerythrin-rich cyanobacteria appear abundant and active in a narrow region (∼ 1.5 m) below the thermocline during hypoxic conditions, reaching abundances of greater than 105mL−1and being the primary agent releasing 1.5 mg O2 L−1above the daytime demands in this region. Sequencing of 16S rDNA amplicons, generated with universal eubacterial primer sets, from the Lake Eries hypolimnion during seasonal oxygen depletion demonstrated that cyanobacteria, most closely related to phycoerythrin-rich Synechococcus spp., dominate during rapid drawdown of oxygen (0.083 mg L−1d−1in 2004) in this region. Analyses of another conserved marker of phylogeny (RuBisCO) has been used to confirm the presence of these cell types. Numerous distinct taxa of heterotrophic bacteria are also represented in the 16S library. The results of this study suggest that novel groups of cyanobacteria may persist within the Lake Erie dead zone during hypoxic conditions and, along with the heterotrophic community, strongly influence system geochemistry.

Southern Lake Michigan Sediments: Changes in Accumulation Rate, Mineralogy, and Organic Content

Abstract Data combined from three one-meter gravity cores raised from a single site in southeastern Lake Michigan permit reconstruction of the last 3,550 years of sedimentation at that location. Sediment has been accumulating at about 50 g m −2 y −l through most of the upper 50 cm of the core and nearly 900 g m −2 y −1 below that depth. Several of the physical and chemical properties of the sediments change when the sedimentation rates change; more rapidly accumulating sediments have a finer grain size, more carbonate minerals and more inorganic carbon, less organic carbon, and, among the organic fractions, relatively more humin and less fulvic acid. All these rate and abundance changes apparently occurred about 3,300 years ago and seem to be associated with the lowering of lake levels since the Nipissing high stand which ended about 4,000 years ago.

Methane dynamics of a northern boreal beaver pond

The production, oxidation, and transport of methane (CH4) from a boreal beaver pond were measured during the ice-free period in order to explain the observed flux of CH4 between the pond and the at...

Distribution patterns of dissolved organic matter fractions in natural waters from eastern Canada

Abstract Dissolved organic matter (DOM) in natural waters from eastern Canada, when fractionated into humic acid (HA) and six fulvic sub-fractions—hydrophobic acid, base and neutral (HPOA, HPOB, and HPON), and hydrophilic acid, base, and neutral (HPIA, HPIB, and HPIN)—shows a high degree of variability in the distribution of fractions. On average the fractions are ranked by concentration (mgC/1): HPOA > HPON > HA > HPIN > HPIA > HPIB > HPOB, and 59.7% of the dissolved organic carbon (DOC) is contained in fractions defined as acidic (HA + HPOA + HPIA). Localized conditions are more important in determining the character of DOM than are seasonal fluctuations. Fractionation of DOM yields information on its biogeochemistry which is not obtainable from DOC measurements alone, but further characterization is required to fully understand the influence of DOM character on natural acidity and metal complexation.

Molecular Enumeration of an Ecologically Important Cyanophage in a Laurentian Great Lake

ABSTRACT Considerable research has shown that cyanobacteria and the viruses that infect them (cyanophage) are pervasive and diverse in global lake populations. Few studies have seasonally analyzed freshwater systems, and little is known about the bacterial and viral communities that coexist during the harsh winters of the Laurentian Great Lakes. Here, we employed quantitative PCR to estimate the abundance of cyanomyoviruses in this system, using the portal vertex g20 gene as a proxy for cyanophage abundance and to determine the potential ecological relevance of these viruses. Cyanomyoviruses were abundant in both the summer and the winter observations, with up to 3.1 × 106 copies of g20 genes ml−1 found at several stations and depths in both seasons, representing up to 4.6% of the total virus community. Lake Erie was productive during both our observations, with high chlorophyll a concentrations in the summer (up to 10.3 μg liter−1) and winter (up to 5.2 μg liter−1). Both bacterial and viral abundances were significantly higher during the summer than during the winter (P < 0.05). Summer bacterial abundances ranged from 3.3 × 106 to 1.6 × 107 ml−1 while winter abundances ranged between ∼3.4 × 105 and 1.2 × 106 ml−1. Total virus abundances were high during both months, with summer abundances significantly higher at most stations, ranging from 6.5 × 107 to 8.8 × 107 ml−1, and with winter abundances ranging from 3.4 × 107 to 6.6 × 107 ml−1. This work confirms that putative cyanomyoviruses are ubiquitous in both summer and winter months in this large freshwater lake system and that they are an abundant component of the virioplankton group.