Showing papers by "Woods Hole Oceanographic Institution published in 2002"

Harmful Algal Blooms and Eutrophication: Nutrient Sources, Composition, and Consequences

Abstract: Although algal blooms, including those considered toxic or harmful, can be natural phenomena, the nature of the global problem of harmful algal blooms (HABs) has expanded both in extent and its public perception over the last several decades. Of concern, especially for resource managers, is the potential relationship between HABs and the accelerated eutrophication of coastal waters from human activities. We address current insights into the relationships between HABs and eutrophication, focusing on sources of nutrients, known effects of nutrient loading and reduction, new understanding of pathways of nutrient acquisition among HAB species, and relationships between nutrients and toxic algae. Through specific, regional, and global examples of these various relationships, we offer both an assessment of the state of understanding, and the uncertainties that require future research efforts. The sources of nutrients poten- tially stimulating algal blooms include sewage, atmospheric deposition, groundwater flow, as well as agricultural and aquaculture runoff and discharge. On a global basis, strong correlations have been demonstrated between total phos- phorus inputs and phytoplankton production in freshwaters, and between total nitrogen input and phytoplankton pro- duction in estuarine and marine waters. There are also numerous examples in geographic regions ranging from the largest and second largest U.S. mainland estuaries (Chesapeake Bay and the Albemarle-Pamlico Estuarine System), to the Inland Sea of Japan, the Black Sea, and Chinese coastal waters, where increases in nutrient loading have been linked with the development of large biomass blooms, leading to anoxia and even toxic or harmful impacts on fisheries re- sources, ecosystems, and human health or recreation. Many of these regions have witnessed reductions in phytoplankton biomass (as chlorophyll a) or HAB incidence when nutrient controls were put in place. Shifts in species composition have often been attributed to changes in nutrient supply ratios, primarily N:P or N:Si. Recently this concept has been extended to include organic forms of nutrients, and an elevation in the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC:DON) has been observed during several recent blooms. The physiological strategies by which different groups of species acquire their nutrients have become better understood, and alternate modes of nutrition such as heterotrophy and mixotrophy are now recognized as common among HAB species. Despite our increased un- derstanding of the pathways by which nutrients are delivered to ecosystems and the pathways by which they are assimilated differentially by different groups of species, the relationships between nutrient delivery and the development of blooms and their potential toxicity or harmfulness remain poorly understood. Many factors such as algal species presence/ abundance, degree of flushing or water exchange, weather conditions, and presence and abundance of grazers contribute to the success of a given species at a given point in time. Similar nutrient loads do not have the same impact in different environments or in the same environment at different points in time. Eutrophication is one of several mechanisms by which harmful algae appear to be increasing in extent and duration in many locations. Although important, it is not the only explanation for blooms or toxic outbreaks. Nutrient enrichment has been strongly linked to stimulation of some harmful species, but for others it has not been an apparent contributing factor. The overall effect of nutrient over- enrichment on harmful algal species is clearly species specific.

Soil Warming and Carbon-Cycle Feedbacks to the Climate System

Abstract: In a decade-long soil warming experiment in a mid-latitude hardwood forest, we documented changes in soil carbon and nitrogen cycling in order to investigate the consequences of these changes for the climate system. Here we show that whereas soil warming accelerates soil organic matter decay and carbon dioxide fluxes to the atmosphere, this response is small and short-lived for a mid-latitude forest, because of the limited size of the labile soil carbon pool. We also show that warming increases the availability of mineral nitrogen to plants. Because plant growth in many mid-latitude forests is nitrogen-limited, warming has the potential to indirectly stimulate enough carbon storage in plants to at least compensate for the carbon losses from soils. Our results challenge assumptions made in some climate models that lead to projections of large long-term releases of soil carbon in response to warming of forest ecosystems.

Microbial diversity of hydrothermal sediments in the Guaymas Basin: evidence for anaerobic methanotrophic communities.

Abstract: Microbial communities in hydrothermally active sediments of the Guaymas Basin (Gulf of California, Mexico) were studied by using 16S rRNA sequencing and carbon isotopic analysis of archaeal and bacterial lipids The Guaymas sediments harbored uncultured euryarchaeota of two distinct phylogenetic lineages within the anaerobic methane oxidation 1 (ANME-1) group, ANME-1a and ANME-1b, and of the ANME-2c lineage within the Methanosarcinales, both previously assigned to the methanotrophic archaea The archaeal lipids in the Guaymas Basin sediments included archaeol, diagnostic for nonthermophilic euryarchaeota, and sn-2-hydroxyarchaeol, with the latter compound being particularly abundant in cultured members of the Methanosarcinales The concentrations of these compounds were among the highest observed so far in studies of methane seep environments The delta-(13)C values of these lipids (delta-(13)C = -89 to -58 per thousand) indicate an origin from anaerobic methanotrophic archaea This molecular-isotopic signature was found not only in samples that yielded predominantly ANME-2 clones but also in samples that yielded exclusively ANME-1 clones ANME-1 archaea therefore remain strong candidates for mediation of the anaerobic oxidation of methane Based on 16S rRNA data, the Guaymas sediments harbor phylogenetically diverse bacterial populations, which show considerable overlap with bacterial populations of geothermal habitats and natural or anthropogenic hydrocarbon-rich sites Consistent with earlier observations, our combined evidence from bacterial phylogeny and molecular-isotopic data indicates an important role of some novel deeply branching bacteria in anaerobic methanotrophy Anaerobic methane oxidation likely represents a significant and widely occurring process in the trophic ecology of methane-rich hydrothermal vents This study stresses a high diversity among communities capable of anaerobic oxidation of methane

Deep-sea paleotemperature record of extreme warmth during the Cretaceous

Abstract: Oxygen isotope analyses of well-preserved foraminifera from Blake Nose (30°N paleolatitude, North Atlantic) and globally distributed deep-sea sites provide a long-term paleotemperature record for the late Albian–Maastrichtian interval that is difficult to reconcile with the existence of significant Cretaceous ice sheets. Given reasonable assumptions about the isotopic composition of Cretaceous seawater, our results suggest that middle bathyal water temperatures at Blake Nose increased from ∼12 °C in the late Albian through middle Cenomanian to a maximum of 20 °C during the latest Cenomanian and earliest Turonian. Bottom waters were again ∼12 °C during the middle Campanian and cooled to a minimum of 9 °C during the Maastrichtian. Correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from Blake Nose. Comparison of global bottom-water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late Albian through late Cenomanian to a hot greenhouse phase during the latest Cenomanian through early Campanian, then to cool greenhouse conditions during the mid-Campanian through Maastrichtian.

Carbon emissions from tropical deforestation and regrowth based on satellite observations for the 1980s and 1990s.

Abstract: The increase of carbon dioxide in the atmosphere relative to emissions from fossil-fuel burning and land-use change indicates that terrestrial and marine environments are absorbing approximately one-half to three-quarters of the emitted carbon dioxide. Several lines of evidence indicate uptake of carbon dioxide in the terrestrial extratropical Northern Hemisphere including land-inventory data, atmospheric CO2 and O2 data, isotopic analyses, and ecosystem models (1–5). Regrowth on abandoned agricultural land, fire prevention, longer growing seasons, and fertilization by increased concentrations of carbon dioxide and nitrogen have been proposed as possible mechanisms responsible for the Northern Hemisphere uptake (6–8). Future atmospheric carbon-dioxide concentrations and consequent climate change depend to a large extent on the future course of the terrestrial uptake (9). If the underlying mechanisms are no longer able to sequester carbon at some point in the future, as for example would be the case once regrowing forests mature, a larger proportion of emitted carbon dioxide would remain in the atmosphere, and carbon-dioxide concentrations would increase at a greater rate for the same level of emissions. Atmospheric inversion studies, which calculate net sources and sinks of carbon dioxide from the spatial distribution of atmospheric concentrations, indicate a net land sink of 0.6–2.3 petagrams (Pg)⋅yr−1 in the extra tropics (6). In the tropics, inverse models are poorly constrained but indicate that the region, overall, is neutral or a small source of carbon to the atmosphere (10). Although inversion studies locate and quantify the net terrestrial sources or sinks, the attribution to mechanisms and their possible future trajectories depend on quantifying the gross sources and sinks. For a net sink, the mechanisms responsible for uptake of carbon dioxide must be powerful enough to offset the sources from fossil fuel and deforestation. The carbon dioxide emitted from fossil-fuel combustion is well quantified (11), but the emission from tropical land-use change is highly uncertain. Without more precise estimates of this source term, deciphering possible mechanisms sequestering the missing carbon remains problematic. The flux of carbon to the atmosphere from tropical land-use change is one of the largest uncertainties in the contemporary carbon budget (6, 12) because of the difficulties in quantifying deforestation and regrowth rates, initial biomass, and fate of carbon in areas where vegetation has been cleared. Estimates of carbon fluxes from tropical deforestation as reported by the Intergovernmental Panel on Climate Change (IPCC; ref. 12) from refs. 5 and 13 range from 0.6 to 2.5 Pg⋅yr−1 for the 1980s, based primarily on calculations using cropland statistics from the United Nations Food and Agriculture Organization (FAO) and deforestation rates from the FAO Forest Resource Assessment (FRA). The FRA information is obtained through national reporting supplemented by limited satellite analysis in the assessment for the 1990s (14–16). Participation of individual countries through national reporting is a strength from some perspectives, but it generates problems from varying definitions of forest cover among countries and time intervals (17). These problems are particularly acute in developing countries, where most tropical deforestation occurs. Comparisons of national statistics from the FRA with other country-level analyses suggest that the FRA overestimated changes in forest cover in some African countries (18), Bolivia (19), and other developing countries (20, 21). For the 1990–2000 interval, the FRA also conducted a remote-sensing survey, analyzing 10% of all tropical land area (15, 21). Forest area and deforestation rates from the FRA remote-sensing survey are generally lower than the FRA (15, 22) country reports for the 1990–2000 interval for Latin America and tropical Asia, although the differences are not statistically significant. For tropical Africa, the difference is very large (3 million ha/yr), suggesting exaggerated deforestation rates in the country data (15). For the 1980–1990 interval, on which the IPCC estimates of carbon fluxes from tropical deforestation are based, the country reports are the sole source of information for the FRA analysis. Satellite data offer the possibility of spatially and temporally consistent estimates of forest cover to complement national reports. Data acquired by the Landsat platform, with a pixel resolution of ≈30 m for the thematic mapper sensor and 60 m for the multispectral scanner sensor before the early 1980s, have provided estimates of deforestation rates for individual regions such as the Amazon basin (23). However, because of cloud coverage and limited acquisitions over the past several decades, it has not been possible to obtain comprehensive coverage for the entire tropics. Global data from the early 1980s to present acquired by the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer (AVHRR) provide daily coverage but at a coarse spatial resolution of 8 km (24). AVHRR data at the sensor resolution of ≈1 km are not available for the full time series with adequate spatial coverage. In this study we estimate changes in forest area by using an approach to estimate subpixel changes in tree cover within the coarse spatial resolution of the AVHRR data. This analysis thus provides a spatially explicit alternative to the FAO's nationally reported changes in forest area and an alternative estimate for carbon fluxes over the past two decades.

Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments

Abstract: No microorganism capable of anaerobic growth on methane as the sole carbon source has yet been cultivated. Consequently, information about these microbes has been inferred from geochemical and microbiological observations of field samples. Stable isotope analysis of lipid biomarkers and rRNA gene surveys have implicated specific microbes in the anaerobic oxidation of methane (AOM). Here we use combined fluorescent in situ hybridization and secondary ion mass spectrometry analyses, to identify anaerobic methanotrophs in marine methane-seep sediments. The results provide direct evidence for the involvement of at least two distinct archaeal groups (ANME-1 and ANME-2) in AOM at methane seeps. Although both archaeal groups often occurred in direct physical association with bacteria, they also were observed as monospecific aggregations and as single cells. The ANME-1 archaeal group more frequently existed in monospecific aggregations or as single filaments, apparently without a bacterial partner. Bacteria associated with both archaeal groups included, but were not limited to, close relatives of Desulfosarcina species. Isotopic analyses suggest that monospecific archaeal cells and cell aggregates were active in anaerobic methanotrophy, as were multispecies consortia. In total, the data indicate that the microbial species and biotic interactions mediating anaerobic methanotrophy are diverse and complex. The data also clearly show that highly structured ANME-2/Desulfosarcina consortia are not the sole entities responsible for AOM at marine methane seeps. Other microbial groups, including ANME-1 archaea, are capable of anaerobic methane consumption either as single cells, in monospecific aggregates, or in multispecies consortia.

Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean

Abstract: [1] Particle fluxes measured with time series sediment traps deployed below 2000 m at 68 sites in the world ocean are combined with satellite-derived estimates of export production from the overlying water to assess the factors affecting the transfer of particulate organic matter from surface to deep water. Multiple linear regression is used to derive an algorithm suggesting that the transfer efficiency of organic carbon, defined as the settling flux of organic carbon normalized to export production, increases with the flux of carbonate and decreases with water depth and seasonality. The algorithm predicts >80% of the organic carbon transfer efficiency variability in diverse oceanic regions. The influence of the carbonate flux suggests that the ballasting effect of this biogenic mineral may be an important factor promoting export of organic carbon to the deep sea by increasing the density of settling particles. However, the lack of a similar effect for biogenic opal suggests that factors other than particle density also play a role. The adverse effect of increasing seasonality on the transfer efficiency of carbon to the deep sea is tentatively attributed to greater biodegradability of organic matter exported during bloom events. In high latitude opal-dominated regions with high f-ratios and seasonality, while a higher fraction of net production is exported, a higher fraction of the exported organic matter is remineralized before reaching bathypelagic depths. On the other hand, in warm, low latitude, carbonate-dominated regions with low f-ratios and seasonality, a higher fraction of the exported organic matter sinks to the deep sea.

Resolution of Prochlorococcus and Synechococcus Ecotypes by Using 16S-23S Ribosomal DNA Internal Transcribed Spacer Sequences

Abstract: Cultured isolates of the marine cyanobacteria Prochlorococcus and Synechococcus vary widely in their pigment compositions and growth responses to light and nutrients, yet show greater than 96% identity in their 16S ribosomal DNA (rDNA) sequences. In order to better define the genetic variation that accompanies their physiological diversity, sequences for the 16S-23S rDNA internal transcribed spacer (ITS) region were determined in 32 Prochlorococcus isolates and 25 Synechococcus isolates from around the globe. Each strain examined yielded one ITS sequence that contained two tRNA genes. Dramatic variations in the length and G+C content of the spacer were observed among the strains, particularly among Prochlorococcus strains. Secondary-structure models of the ITS were predicted in order to facilitate alignment of the sequences for phylogenetic analyses. The previously observed division of Prochlorococcus into two ecotypes (called high and low-B/A after their differences in chlorophyll content) were supported, as was the subdivision of the high-B/A ecotype into four genetically distinct clades. ITS-based phylogenies partitioned marine cluster A Synechococcus into six clades, three of which can be associated with a particular phenotype (motility, chromatic adaptation, and lack of phycourobilin). The pattern of sequence divergence within and between clades is suggestive of a mode of evolution driven by adaptive sweeps and implies that each clade represents an ecologically distinct population. Furthermore, many of the clades consist of strains isolated from disparate regions of the world's oceans, implying that they are geographically widely distributed. These results provide further evidence that natural populations of Prochlorococcus and Synechococcus consist of multiple coexisting ecotypes, genetically closely related but physiologically distinct, which may vary in relative abundance with changing environmental conditions.

Heteroduplexes in mixed-template amplifications: formation, consequence and elimination by ‘reconditioning PCR’

Abstract: Although it has been recognized that PCR amplification of mixed templates may generate sequence artifacts, the mechanisms of their formation, frequency and potential elimination have not been fully elucidated. Here evidence is presented for heteroduplexes as a major source of artifacts in mixed-template PCR. Nearly equal proportions of homoduplexes and heteroduplexes were observed after co-amplifying 16S rDNA from three bacterial genomes and analyzing products by constant denaturing capillary electrophoresis (CDCE). Heteroduplexes became increasingly prevalent as primers became limiting and/or template diversity was increased. A model exploring the fate of cloned heteroduplexes during MutHLS-mediated mismatch repair in the Escherichia coli host demonstrates that the diversity of artifactual sequences increases exponentially with the number of both variable nucleotides and of original sequence variants. Our model illustrates how minimization of heteroduplex molecules before cloning may reduce artificial genetic diversity detected during sequence analysis by clone screening. Thus, we developed a method to eliminate heteroduplexes from mixed-template PCR products by subjecting them to 'reconditioning PCR', a low cycle number re-amplification of a 10-fold diluted mixed-template PCR product. This simple modification to the protocol may ensure that sequence richness encountered in clone libraries more closely reflects genetic diversity in the original sample.

The Current System in the Yellow and East China Seas

Abstract: During the 1990s, our knowledge and understanding of the current system in the Yellow and East China Seas have grown significantly due primarily to new technologies for measuring surface currents and making high-resolution three-dimensional numerical model calculations. One of the most important new findings in this decade is direct evidence of the northward current west of Kyushu provided by satellite-tracked surface drifters. In the East China Sea shelf region, these recent studies indicate that in winter the Tsushima Warm Current has a single source, the Kuroshio Branch Current in the west of Kyushu, which transports a mixture of Kuroshio Water and Changjiang River Diluted Water northward. In summer the surface Tsushima Warm Current has multiple sources, i.e., the Taiwan Warm Current, the Kuroshio Branch Current to the north of Taiwan, and the Kuroshio Branch Current west of Kyushu. The summer surface circulation pattern in the East China Sea shelf region changes year-to-year corresponding to interannual variations in Changjiang River discharge. Questions concerning the Yellow Sea Warm Current, the Chinese Coastal Current in the Yellow Sea, the current field southwest of Kyushu, and the deep circulation in the Okinawa Trough remain to be addressed in the next decade.

The economic effects of harmful algal blooms in the United States: Estimates, assessment issues, and information needs

Abstract: During the last several decades, harmful algal bloom (HAB) events have been observed in more locations than ever before throughout the United States. Scientists have identified a larger number of algal species involved in HABs, more toxins have been uncovered, and more fisheries resources have been affected. Whether this apparent increase in HAB events is a real phenomenon or is the result of increased sampling and monitoring is a topic of intense discussions within the scientific community. We also have an inchoate understanding of the reasons for the apparent increase, particularly concerning the role of anthropogenic nutrient loadings as a causal factor. Whatever the reasons, virtually all coastal regions of the U.S. are now regarded as potentially subject to a wide variety and increased frequency of HABs. It is important to begin to understand the scale of the economic costs to society of such natural hazards. It is a common, but not yet widespread, practice for resource managers and scientists in many localities to develop rough estimates of the economic effects of HAB events in terms of lost sales in the relevant product or factor markets, expenditures for medical treatments, environmental monitoring and management budgets, or other types of costs. These estimates may be invoked in policy debates, often without concern about how they were developed. Although such estimates are not necessarily good measures of the true costs of HABs to society, they may help to measure the scale of losses and be suggestive of their distribution across political jurisdictions or industry sectors. With adequate interpreta- tion, our thinking about appropriate policy responses may be guided by these estimates. Here we compile disparate estimates of the economic effects of HABs for events in the U.S. where such effects were measured during 1987-1992. We consider effects of four basic types: public health, commercial fisheries, recreation and tourism, and monitoring and management. We discuss many of the issues surrounding the nature of these estimates, their relevance as measures of the social costs of natural hazards, and their potential for comparability and aggregation into a national estimate.

Benthic eukaryotic diversity in the Guaymas Basin hydrothermal vent environment.

Abstract: Molecular microbial ecology studies have revealed remarkable prokaryotic diversity in extreme hydrothermal marine environments. There are no comparable reports of culture-independent surveys of eukaryotic life in warm, anoxic marine sediments. By using sequence comparisons of PCR-amplified small subunit ribosomal RNAs, we characterized eukaryotic diversity in hydrothermal vent environments of Guaymas Basin in the Gulf of California. Many sequences from these anoxic sediments and the overlaying seawater represent previously uncharacterized protists, including early branching eukaryotic lineages or extended diversity within described taxa. At least two mechanisms, with overlapping consequences, account for the eukaryotic community structure of this environment. The adaptation to anoxic environments is evidenced by specific affinity of environmental sequences to aerotolerant anaerobic species in molecular trees. This pattern is superimposed against a background of widely distributed aerophilic and aerotolerant protists, some of which may migrate into and survive in the sediment whereas others (e.g., phototrophs) are simply deposited by sedimentary processes. In contrast, bacterial populations in these sediments are primarily characteristic of anoxic, reduced, hydrocarbon-rich sedimentary habitats.

Evaluation of Wind Vectors Observed by QuikSCAT/SeaWinds Using Ocean Buoy Data

Abstract: Wind vectors observed by the QuikSCAT/SeaWinds satellite mission are validated by comparing with wind and wave data from ocean buoys. Effects of oceanographic and atmospheric environment on scatterometer measurements are also assessed using the buoy data. Three versions of QuikSCAT/SeaWinds wind data were collocated with buoy observations operated by the National Data Buoy Center (NDBC), Tropical Atmosphere Ocean (TAO), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) projects, and the Japan Meteorological Agency (JMA). Only buoys located offshore and in deep water were analyzed. The temporal and spatial differences between the QuikSCAT/SeaWinds and buoy observations were limited to less than 30 min and 25 km. The buoy wind speeds were converted to equivalent neutral winds at a height of 10 m above the sea surface. The comparisons show that the wind speeds and directions observed by QuikSCAT/SeaWinds agree well with the buoy data. The root-mean-squared differences of the wind speed and direction for the standard wind data products are 1.01 m s - 1 and 23°, respectively, while no significant dependencies on the wind speed or cross-track cell location are discernible. Inaddition, the dependencies of wind speed residuals on oceanographic and atmospheric parameters observed by buoys are examined using the collocated data. A weak positive correlation of the wind speed residuals with the significant wave height is found, while dependencies on the sea surface temperature or atmospheric stability are not physically significant.

The role of the Beaufort Gyre in Arctic climate variability: Seasonal to decadal climate scales

Abstract: [1] This paper presents a new hypothesis along with supporting evidence that the Beaufort Gyre (BG) plays a significant role in regulating the arctic climate variability. We propose and demonstrate that the BG accumulates a significant amount of fresh water (FW) during one climate regime (anticyclonic) and releases this water to the North Atlantic (NA) during another climate regime (cyclonic). This hypothesis can explain the origin of the salinity anomaly (SA) periodically found in the NA as well as its role in the decadal variability in the Arctic region.

Characterization of an Autotrophic Sulfide-Oxidizing Marine Arcobacter sp. That Produces Filamentous Sulfur

Abstract: A coastal marine sulfide-oxidizing autotrophic bacterium produces hydrophilic filamentous sulfur as a novel metabolic end product. Phylogenetic analysis placed the organism in the genus Arcobacter in the epsilon subdivision of the Proteobacteria. This motile vibrioid organism can be considered difficult to grow, preferring to grow under microaerophilic conditions in flowing systems in which a sulfide-oxygen gradient has been established. Purified cell cultures were maintained by using this approach. Essentially all 4′,6-diamidino-2-phenylindole dihydrochloride-stained cells in a flowing reactor system hybridized with Arcobacter-specific probes as well as with a probe specific for the sequence obtained from reactor-grown cells. The proposed provisional name for the coastal isolate is “Candidatus Arcobacter sulfidicus.” For cells cultured in a flowing reactor system, the sulfide optimum was higher than and the CO2 fixation activity was as high as or higher than those reported for other sulfur oxidizers, such as Thiomicrospira spp. Cells associated with filamentous sulfur material demonstrated nitrogen fixation capability. No ribulose 1,5-bisphosphate carboxylase/oxygenase could be detected on the basis of radioisotopic activity or by Western blotting techniques, suggesting an alternative pathway of CO2 fixation. The process of microbial filamentous sulfur formation has been documented in a number of marine environments where both sulfide and oxygen are available. Filamentous sulfur formation by “Candidatus Arcobacter sulfidicus” or similar strains may be an ecologically important process, contributing significantly to primary production in such environments.

The anaerobic oxidation of methane; new insights in microbial ecology and biogeochemistry

Abstract: As the major biological sink of methane in marine sediments, the microbially mediated anaerobic oxidation of methane (AOM) is crucial in its role of maintaining a sensitive balance of our atmosphere’s greenhouse gas content. Although there is now sufficient geochemical evidence to exactly locate the “hot spots” of AOM, and to crudely estimate its contribution to the methane cycle, a fundamental understanding of the associated biology is still lacking, consequently preventing a thorough biogeochemical understanding of an integral process in the global carbon cycle. Earlier microbiological work trying to resolve the enigma of AOM mostly failed because it was largely focussed on the simulation of AOM under laboratory conditions using cultivable candidate organisms. Now again, understanding the biological and biochemical details of AOM is the declared goal of several interational research groups, but this time in a combined effort of biogeochemists and microbiologists using novel analytical tools tailored for the study of unknown microbes and habitats. This review gives an overview on very recent progress in the study of AOM that dramatically advanced this ~ 30-yr-old field. New insights on the quantitative significance of AOM are combined to refine older estimates.

Estimating tidally driven mixing in the deep ocean

Abstract: [1] Using a parameterization for internal wave energy flux in a hydrodynamic model for the tides, we estimate the global distribution of tidal energy available for enhanced turbulent mixing. A relation for the diffusivity of vertical mixing is formulated for regions where internal tides dissipate their energy as turbulence. We assume that 30 ± 10% of the internal tide energy flux dissipates as turbulence near the site of generation, consistent with an estimate based on microstructure observations from a mid-ocean ridge site. Enhanced levels of mixing are modeled to decay away from topography, in a manner consistent with these observations. Parameterized diffusivities are shown to resemble observed abyssal mixing rates, with estimated uncertainties comparable to standard errors associated with budget and microstructure methods.

Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise

Abstract: Glassy Turonian foraminifera preserved in clay-rich sediments from the western tropical Atlantic yield the warmest equivalent ?18O sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic. We estimate Turonian SSTs that were at least as warm as (conservative mean ~30 °C) to significantly warmer (warm mean ~33 °C) than those in the region today. However, if independent evidence for high middle Cretaceous pCO2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea-surface pH, our conservative and warm SST estimates would be even higher (32 and 36 °C, respectively). Our new tropical SSTs help reconcile geologic data with the predictions of general circulation models that incorporate high Cretaceous pCO2 and lend support to the hypothesis of a Cretaceous greenhouse. Our data also strengthen the case for a Turonian age for the Cretaceous thermal maximum and highlight a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth and peak inferred tectonic CO2 production. We infer that this mismatch is either an artifact of a hidden Turonian pulse in global ocean-crust cycling or real evidence of the influence of some other factor on atmospheric CO2 and/or SSTs. A hidden pulse in crust cycling would explain the timing of peak Cretaceous-Cenozoic sea level (also Turonian), but other factors are needed to explain high-frequency (~10–100 k.y.) instability in middle Cretaceous SSTs reported elsewhere.

The Alongshore Transport of Freshwater in a Surface-Trapped River Plume

Abstract: The alongshore transport of a surface-trapped river plume is studied using a three-dimensional model. Model simulations exhibit the previously observed rightward veering (in the Northern Hemisphere) of the freshwater and establishment of a downstream geostrophically balanced coastal current. In the absence of any ambient current, the plume does not reach a steady state. The downstream coastal current only carries a fraction of the discharged freshwater; the remaining fraction recirculates in a continually growing “bulge” of freshwater in the vicinity of the river mouth. The river mouth conditions influence the amount of freshwater transported in the coastal current relative to the growing bulge. For high Rossby number [O(1)] discharge conditions, the bulge shape is circular and the coastal current transport is smaller than for the model runs of low Rossby number discharges. For all model runs conducted without an ambient current, the freshwater transport in the coastal current is less than the fr...

Does food web theory work for marine ecosystems

Abstract: More recent and extensive food web studies have questioned some of the prevailing paradigms of food web theory. Yet with few exceptions, most food webs and associated metrics are reported for freshwater or terrestrial systems. I analyzed the food web of the Northeast US Shelf ecosystem across a large spatial and temporal extent. This speciose food web exhibits a predator:prey ratio (0.95) and percentage of intermediate species (89%) similar to most other food webs. Other statistics, such as the percentage of omnivory (62%), percentage of cannibalistic species (31%), num- ber of cycles (5%), and the total number of links (L; 1562) and species (S; 81) are similar to more recent and extensively studied food webs. Finally, this food web exhibits a linkage density (L/S; 19.3), connectivity (C; 48.2%), and Lyapunov stability proxy (S × C; 39.1) that are an order of magnitude higher than other webs or are disproportionate to the number of species observed in this system. Although the exact S and C relationship is contentious, the connectivity of food webs with more than 40 species is approximately 10%, which is very different from the near 50% observed for this ecosys- tem. The openness of marine ecosystems, lack of specialists, long lifespans, and large size changes across the life histories of many marine species can collectively make marine food webs more highly connected than their terrestrial and freshwater counterparts, contrary to food web theory. Changes in connectivity also have ramifications for ecosystem functioning and Lyapunov stability. The high con- nectivity of this food web and the mathematical determinants for stability are consistent with the weak nature of species interactions that have been observed and that are required for system persis- tence. Yet the historically high exploitation rates of marine organisms obfuscate our understanding of marine food web stability. It is possible that marine food webs are inherently very different from their terrestrial or freshwater counterparts, implying the need for modified paradigms of food web theory.

Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay

Abstract: [1] Sediment cores from the intertidal zone of Waquoit Bay (Cape Cod, Massachusetts) yielded iron oxide-coated sands in the subterranean estuary, which underlies the head of the bay. The oxides were dark red, yellow and orange colors and are formed by the oxidation of ferrous iron-rich groundwater near the groundwater-seawater interface. Within these iron oxide-rich sediments, the concentration of the combined amorphous and crystalline forms of iron oxides ranged between 2500 and 4100 ppm of Fe. These concentrations were 4–6 times greater than the surface sands, and 10–15 times more Fe rich than sands collected from an off-site location. The precipitation of iron oxides in subterranean estuaries could act as a geochemical barrier by retaining and accumulating certain dissolved chemical species carried to the coast by groundwater. Indeed, phosphorus concentrations in the iron oxide-rich sands of Waquoit Bay were 5–7 times greater than the overlying surface sands.

Spatial and Temporal Offsets Between Proxy Records in a Sediment Drift

Abstract: Chronologies for Late Quaternary marine sediment records are usually based on radiocarbon ages of planktonic foraminifera. Signals carried by other sedimentary components measured in parallel can provide complementary paleoclimate information. A key premise is that microfossils and other indicators within a given sediment horizon are of equal age. We show here that haptophyte-derived alkenones isolated from Bermuda Rise drift sediments are up to 7000 years older than coexisting planktonic foraminifera. This temporal offset, which is apparently due to lateral transport of alkenones on fine-grained particles from the Nova Scotian margin, markedly influences molecular estimates of sea surface temperatures. More broadly, the observation raises questions about both the temporal and the geographic fidelity of paleoenvironmental records encoded by readily transported components of sediments.

Invasion theory and biological control

Abstract: Recent advances in the mathematical theory of invasion dynamics have much to offer to biological control. Here we synthesize several results concerning the spatiotemporal dynamics that occur when a biocontrol agent spreads into a population of an invading pest species. We outline conditions under which specialist and generalist predators can influence the density and rate of spatial spread of the pest, including the rather stringent conditions under which a specialist predator can successfully reverse a pest invasion. We next discuss the connections between long distance dispersal and invasive spread, emphasizing the different consequences of fast spreading pests and predators. Recent theory has considered the effects of population stage-structure on invasion dynamics, and we discuss how population demography affects the biological control of invading pests. Because low population densities generally characterize early stages of an invasion, we discuss the lessons invasion theory teaches concerning the detectability of invasions. Stochasticity and density-dependent dynamics are common features of many real invasions, influencing both the spatial character (e.g. patchiness) of pest invasions and the success of biocontrol agents. We conclude by outlining theoretical results delineating how stochastic effects and complex dynamics generated by density dependence can facilitate or impede biological pest control.

Reactive Transport in Porous Media: A Comparison of Model Prediction with Laboratory Visualization

Abstract: Groundwater transport models that accurately describe spreading of nonreactive solutes in an aquifer can poorly predict concentrations of reactive solutes. The dispersive term in the advection-dispersion equation can overpredict pore-scale mixing, and thereby overpredict homogeneous chemical reaction. We quantified this experimentally by imaging instantaneous colorimetric reactions between solutions of aqueous CuSO4 and EDTA4- within a 30-cm long translucent chamber packed with cryolite sand that closely matched the optical index of refraction of water. A charge-coupled device camera was used to quantify concentrations of blue CuEDTA2- within the chamber as it was produced by mixing of the two reactants at different flow rates. We compared these experimental results with a new analytic solution for instantaneous bimolecular reaction coupled with advection and dispersion of the product and reactants. For all flow rates, the concentrations of CuEDTA2- recorded in the experiments were about 20% less than predicted by the analytic solution, thereby demonstrating that models assuming complete mixing at the pore scale can overpredict reaction during transport.

Hydrography of the Labrador Sea during Active Convection

Abstract: The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February‐March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (; 200 Wm 22). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself—something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current ‘‘throughput’’ and offshore ‘‘recirculation.’’ Finally, the overall trends in stratification of the observed mixed layers are described.