Effects of hypoxia on benthic macrofauna and bioturbation in the Estuary and Gulf of St. Lawrence, Canada

The bottom water in the 4300 m deep Lower St. Lawrence Estuary (LSLE) is persistently hypoxic in contrast to the normoxic bottom waters in the Gulf of St. Lawrence (GSL). We photographed the seabed at 11 stations in the Estuary and Gulf of St. Lawrence (EGSL) during the summers 2006 and 2007 and analysed the images to identify bioturbation traces (lebensspuren) and benthic macrofauna. The objective was to identify the environmental variables that influence the density and diversity of benthic macrofauna and bioturbation traces, and the differences that exist among regions with high, medium and low oxygen levels in the bottom water. The bottom water oxygen concentration is the variable that best explains the densities of total-traces as well as surface-traces. However, the density of these traces was higher in hypoxic regions than in well-oxygenated regions. The higher density of traces in the hypoxic region of the LSLE is mainly due to the activities of the surface deposit feeder Ophiura sp., which occurs in large numbers in this region. Possible explanations explored are stress behaviour of the organisms in response to hypoxia and different benthic macrofauna community structures between the hypoxic regions of the LSLE and the normoxic regions of the GSL. In the former, surface deposit feeders and low-oxygen tolerant species dominate over suspension feeders and low-oxygen intolerant species

Comparative diagenesis at three sites on the Canadian continental margin

Diagenesis of carbon, oxygen, nitrogen, and manganese at three sites on the Canadian continental margin is quantitatively compared and contrasted using results from a computer code (CANDI) published by Boudreau (1996a). The data at Station 3 (Cabot Strait) are well explained by the steady state output from CANDI, assuming a porewater balance created by diffusion and reaction only, whereas the data from Stations 4 (Emerald Basin-Scotia Shelf) and 5 (Scotia Slope) are not consistent, in one way or another, with this simple model. The deviations between model and data at Station 4 are best explained by nonsteady-state diagenesis. Model fits to the Station 5 ΣCO2 observations are improved dramatically by adding some irrigation at this site, but the ΣNH3 distribution appears to be subject to an additional anomalous transport to the O2 zone and subsequent oxidation to NO-3. The mechanism for this latter phenomena is unknown and in need of future research. In addition, the O2 and ΣCO2 profiles at all sites require the existence of at least two reactive organic matter types; furthermore, the initial amounts of these OM types at each station is strongly dependent on the intensity of particle bioturbation. Ammonia is preferentially regenerated at Station 3 at a high ratio of about 25 N to 106 C. The net kinetics of the deeper removal of Mn2+ appear to be fractional-order with respect to the concentration of this species, suggesting multiple removal processes. Finally, an oxidant balance, assuming steady state, indicates a considerable difference in the use of oxidants at each station even though the O2 fluxes are similar

Mixing and its effects on biogeochemistry in the persistently stratified, deep, tropical Lake Matano, Indonesia

This is the publisher's version, also available electronically from http://www.aslo.orgIn the > 590-m deep, tropical Lake Matano (Indonesia), stratification is characterized by weak thermal gradients (< 2°C per 500 m) and weak salinity gradients (< 0.14% per 500 m). These gradients persist over seasons, decades, and possibly centuries. Under these nearly steady-state conditions, vertical eddy diffusion coefficients (Kz) cannot be estimated by conventional methods that rely on time derivatives of temperature distributions. We use and compare several alternative methods: one-dimensional k-ε modeling, three-dimensional hydrodynamic modeling, correlation with the size of Thorpe instabilities, and correlation with the stability frequency. In the thermocline region, at 100-m depth, the Kz is ~ 5 × 10-6 m2 s-1, but, below 300 m, the small density gradient results in large (20 m) vertical eddies and high mixing rates (Kz ~ 10-2 m2 s-1). The estimated timescale of water renewal in the monimolimnion is several hundred years. Intense evaporation depletes the surface mixed layer of 16O and 1H isotopes, making it isotopically heavier. The lake waters become progressively isotopically lighter with depth, and the isotopic composition in the deep waters is close to those of the ground and tributary waters. The vertical distribution of Kz is used in a biogeochemical reaction-transport model. We show that, outside of a narrow thermocline region, the vertical distributions of dissolved oxygen, iron, methane, and phosphorus are shaped by vertical variations in transport rates, rather than by sources or sinks

The biogeochemistry of tropical lakes: A case study from Lake Matano, Indonesia

This is the publisher's version, also available electronically from http://onlinelibrary.wiley.comWe examined the chemical composition of the water column of Lake Matano, Sulawesi Island, Indonesia, to document how the high abundances of Fe (hydr)oxides in tropical soils and minimal seasonal temperature variability affect biogeochemical cycling in lakes. Lake Matano exhibits weak thermal stratification, yet a persistent pycnocline separates an oxic epilimnion from anoxic meta- and hypolimnions. The concentration of soluble P in the epilimnetic waters is very low and can be attributed to scavenging by Fe (hydr)oxides. Chromium concentrations in the epilimnion are high (up to 180 nmol L−1), but below U.S. Environmental Protection Agency guidelines for aquatic ecosystems. The concentration of chromium decreases sharply across the oxic-anoxic boundary, revealing that the hypolimnion is a sink for Cr. Flux calculations using a one-dimensional transportreaction model for the water column fail to satisfy mass balance requirements and indicate that sediment transport and diagenesis play an important role in the exchange of Fe, Mn, P, and Cr between the epilimnion and hypolimnion. Exchange of water between the epilimnion and hypolimnion is slow and on a time scale similar to temperate meromictic lakes. This limits recycling of P and N to the epilimnion and removal of Cr to the hypolimnion, both of which likely restrict primary production in the epilimnion. Owing to the slow exchange, steep concentration gradients in Fe and Mn species develop in the metalimnion. These concentration gradients are conducive to the proliferation of chemoautotrophic and anoxygenic phototrophic microbial communities, which may contribute a significant fraction to the total primary production in the lake

Pathways of manganese in an open estuarine system

International audienc

Ideas and perspectives: Sea-level change, anaerobic methane oxidation, and the glacial-interglacial phosphorus cycle

International audienceThe oceanic phosphorus cycle describes how phosphorus moves through the ocean, accumulates with the sediments on the seafloor, and participates in biogeochemical reactions. We propose a new two-reservoir scenario of the glacial-interglacial phosphorus cycle. It relies on diagenesis in methane hydrate-bearing sediments to mobilize sedimentary phosphorus and transfer it to the oceanic reservoir during times when falling sea level lowers the hydrostatic pressure on the seafloor and destabilizes methane hydrates. The stock of solid phase phosphorus mobilizable by this process is of the same order of magnitude as the dissolved phosphate inventory of the current oceanic reservoir. The potential additional flux of phosphate during the glacial period is of the same order of magnitude as pre-agricultural, riverine dissolved phosphate fluxes to the ocean. Throughout the cycle, primary production assimilates phosphorus and inorganic carbon into biomass, which, upon settling and burial, returns phosphorus to the sedimentary reservoir. Primary production also lowers the partial pressure of CO2 in the surface ocean, potentially drawing down CO2 from the atmosphere. Concurrent with this slow "biological pump", but operating in the opposite direction, a "physical pump" brings metabolic CO2-enriched waters from deep-ocean basins to the upper ocean. The two pumps compete, but the direction of the CO2 flux at the air-sea interface depends on the nutrient content of the deep waters. Because of the transfer of reactive phosphorus to the sedimentary reservoir throughout a glaciation cycle, low-phosphorus and high-CO2 deep waters reign at the beginning of a deglaciation, resulting in rapid transfer of CO2 to the atmosphere. The new scenario provides another element to the suite of processes that may have contributed to the rapid glacial-interglacial climate transitions documented in paleo-records

Comparative diagenesis at three site on the Canadian continental margin

Diagenesis of carbon, oxygen, nitrogen, and manganese at three sites on the Canadian continental margin is quantitatively compared and contrasted using results from a computer code (CANDI) published by Boudreau (1996a).The data at Station 3 (Cabot Strait) are well explained by the steady state output from CANDI, assuming a porewater balance created by diffusion and reaction only, whereas the data from Stations 4 (Emerald Basin-Scotia Shelf) and 5 (Scotia Slope) are not consistent, in one way or another, with this simple model. The deviations between model and data at Station 4 are best explained by nonsteady-state diagenesis. Model. fits to the Station 5 S CO2 observations are improved dramatically by adding some irrigation at this site, but the S NH3 distributionappears to be subject to an additional anomalous transport to the O2 zone and subsequent oxidation to NO3 2 . The mechanism for this latter phenomena is unknown and in need of future research. In addition, the O2 and ΣCO2 profiles at all sites require the existence of at least two reactive organic matter types; furthermore, the initial amounts of these OM types at each station is strongly dependent on the intensity of particle bioturbation.Ammonia is preferentially regenerated at Station 3 at a high ratio of about 25 N to 106 C. The net kinetics of the deeper removal of Mn21 appear to be fractional-order with respect to the concentration of this species, suggesting multiple renoval processes. Finally, an oxidant balance, assuming steady state, indicates a considerable difference in the use of oxidants at each station even though the O2 fuxes are similar

Comparative diagenesis at three site on the Canadian continental margin

Diagenesis of carbon, oxygen, nitrogen, and manganese at three sites on the Canadian continental margin is quantitatively compared and contrasted using results from a computer code (CANDI) published by Boudreau (1996a).The data at Station 3 (Cabot Strait) are well explained by the steady state output from CANDI, assuming a porewater balance created by diffusion and reaction only, whereas the data from Stations 4 (Emerald Basin-Scotia Shelf) and 5 (Scotia Slope) are not consistent, in one way or another, with this simple model. The deviations between model and data at Station 4 are best explained by nonsteady-state diagenesis. Model. fits to the Station 5 S CO2 observations are improved dramatically by adding some irrigation at this site, but the S NH3 distributionappears to be subject to an additional anomalous transport to the O2 zone and subsequent oxidation to NO3 2 . The mechanism for this latter phenomena is unknown and in need of future research. In addition, the O2 and ΣCO2 profiles at all sites require the existence of at least two reactive organic matter types; furthermore, the initial amounts of these OM types at each station is strongly dependent on the intensity of particle bioturbation.Ammonia is preferentially regenerated at Station 3 at a high ratio of about 25 N to 106 C. The net kinetics of the deeper removal of Mn21 appear to be fractional-order with respect to the concentration of this species, suggesting multiple renoval processes. Finally, an oxidant balance, assuming steady state, indicates a considerable difference in the use of oxidants at each station even though the O2 fuxes are similar

The functional group approach to bioturbation: the effects of biodiffusers and gallery-diffusers of the Macona balthica community on sediment oxygen uptake

International audienc

The functional group approach to bioturbation : II. The effects of the Macoma balthica community on fluxes of nutrients and dissolved organic carbon across the sediment-water interface.

International audienc