Biogeochemical Controls Research Articles

Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons: a brief overview

(1) The succession of primary producer communities in coastal lagoons is analysed in the light of the regime shift theory. Pristine coastal lagoons are considered to be dominated by extensive meadows of seagrass species, which are assumed to take advantage of nutrient supply from sediments. An increasing nutrient input is thought to favour phytoplankton and/or epiphytic micro-, macroalgae as well as opportunistic ephemeral macroalgae that coexist with seagrasses. In the latest stages of this succession, the imbalance of phosphorus to nitrogen ratio can favour macroalgal, cyanobacteria and/or picoplankton blooms, often causing dystrophy. (2) The primary causes of shifts and succession in the macrophyte community are nutrient loadings, mainly nitrogen, as well as changes in coastal hydrology or interactions between them. To some extent, in very shallow choked lagoons, benthic vegetation is mainly controlled by loading rates, while in open deep estuaries hydromorphological factors predominate. (3) External stressors/perturbations cause an amplification in benthic biogeochemical processes, e.g. wide variations in primary productivity and dark respiration, with large oscillations in oxygen and sulphide concentrations. Altered biogeochemical processes can determine positive feedbacks inducing a shift from pristine to altered macrophyte communities, which in turn amplify the perturbation until the shift becomes irreversible. (4) Macrophyte typology, organic matter composition and sedimentary geochemistry are primary factors in controlling feedbacks and shifts. For example, the sedimentary buffering capacity of iron controls sulphide and phosphates, while nitrogen cycling is mainly controlled by primary producers - microbial process interactions. (5) The alternative states which occur through the transition from pristine to modified primary producer communities can also be viewed as a sequence of stable states with different degrees of embedded information and with different ecological functions. Copyright © 2008 John Wiley & Sons, Ltd.

Hydrologic and biogeochemical controls on trace element export from northern Wisconsin wetlands

Wetlands play an important role in determining the water quality of streams and are generally considered to act as a sink for many reactive species. However, retention of chemical constituents varies seasonally and is affected by hydrologic and biogeochemical processes including water source, mineral weathering, DOC and SPM cycling, redox status, precipitation/dissolution/adsorption, and seasonal events. Relatively little is known about the influence of these factors on trace element cycling in wetland-influenced streams. To explore the role of wetlands with respect to the retention/release of trace elements to streams, we examined temporal and spatial patterns of concentrations of a large suite of trace elements (via ICP-MS) and geochemical drivers in five streams and wetland rivulets draining natural wetlands in a northern Wisconsin watershed as well as in their groundwater sources (terrestrial recharge, lake recharge, and older lake recharge). We performed principal components analyses of the concentrations of elements and their geochemical drivers in both the streams and rivulets to assist in the identification of factors regulating trace element concentrations. Variation in trace and major element concentrations among the streams was strongly related to the proportion of terrestrial recharge contributing to the stream. A dominant influence of water source on rivulet chemistry was supported by association of groundwater-sourced elements (Ba, Ca, Cs, Mg, Na, Si, Sr) with the primary statistical factor. DOC appeared in the first principal component factor for the streams and in the second factor for the rivulets. Strong correlations of Al, Cd, Ce, Cu, La, Pb, Ti, and Zn with DOC supported the important influence of DOC on trace metal cycling. A number of elements in the rivulets (Al, La, Pb, Ti) and streams (Al, Ce, Cr, Cu, La, Pb, Ti, Zn) had a significant particulate cycle. Redox cycling and precipitation/dissolution reactions involving Fe and Mn likely impacted Cu and Mo as evidenced by the low levels in the rivulets. Variance in Fe, Mn and the metal oxy-anions was associated with factors related to redox cycling and adsorption reactions in the wetland sediments. In streams, DOC and metals with a high affinity for DOC were associated with a factor which also included negative loadings for groundwater-sourced elements, reflecting the importance of seasonal hydrologic events which flush DOC and metals from wetland sediments and dilute groundwater sourced metals. Redox processes were of secondary importance in the streams but of primary significance in the rivulets, documenting the importance of anoxic conditions in wetland sediments on groundwater en route to the stream.

Kinetics and energetics of phosphate sorption in a multi-component Al(III)–Fe(III) hydr(oxide) sorbent system

Multi-component Al–Fe hydr(oxides) are ubiquituous in soil and aquatic environments, where they exhibit biogeochemical controls on nutrients and contaminants. Although, sorption on single-component Al and Fe hydr(oxides) have been extensively studied, limited studies have been done on their multi-component counterparts. In this study, effects of Al/Fe content on the kinetics and energetics of phosphate sorption in a poorly-crystalline co-precipitated mixed Al–Fe hydr(oxide) system were investigated using a combination of traditional batch techniques and flow adsorption calorimetry. Differences in Al/Fe content was found to influence the structural development and anion exchange capacity of the hydr(oxides) and subsequently their phosphate sorption characteristics. Higher structural development decreased phosphate sorption, while higher AEC was associated with increased phosphate sorption, initial sorption rate, and smaller losses in sorption with increasing pH. Results from flow adsorption calorimetry indicated that at pH 4.8 phosphate sorption: (i) occurred irreversibly on anion exchange sites, with a loss of 1.9 moles of AEC per mole of phosphate sorbed, and (ii) was exothermic, with molar heats of adsorption between −25 and − 39 kJ mol −1 . Molar heats of adsorption were ten times that for anion exchange and independent of hydr(oxide) composition with the amount of energy evolved being directly proportional to the quantity of phosphate sorbed.

Flushing of nitrate from a forested watershed: An insight into hydrological nitrate mobilization mechanisms through seasonal high-frequency stream nitrate dynamics

The variability of hydrologically induced mobilization of nitrate during sequences of hydrological events in different seasons was studied through high-frequency measurements of streamwater nitrate concentrations accompanied by a detailed hydrometeorological monitoring system. The study was carried out in 2006, within the 42 km2 forested Padež watershed in the southwestern part of Slovenia, which is characterized by distinctive flushing, an almost torrential hydrological regime influenced by impermeable flysch geological settings. More than 15 recorded hydrographs which, in the hydrological and biogeochemical sense, differed substantially, disclosed a highly variable but at the same time a strong linkage between hydrological and biogeochemical controls of nitrate exports from the spatial perspective of a watershed. During most of the hydrographs, with the exception of early spring rainfall events, a positive relationship between the nitrate concentration and discharge was observed with peak nitrate concentrations having a time delay in the order of a few hours after the hydrograph peaks. Peak nitrate concentrations in periods of rainfall events span from 3.5 mg/l-N in late spring to 14 mg/l-N in the case of the autumn hydrograph. However, the dilution effect as a consequence of high event water contributions was observed in certain events. The role of specific hydrological events on the nitrate mobilization proved to be important as the size of the accumulated nitrate pool available for mobilization was large throughout most of the hydrographs. The biogeochemical environment of the forest soils presumably drastically alters the size of the available nitrate pool at the studied watershed and, together with the specific hydrological conditions, shifts the watershed from the flush-limited state in the late spring, summer and autumn to the source-limited state in winter and early spring.

Mercury cycling in surface water, pore water and sediments of Mugu Lagoon, CA, USA

Mugu Lagoon is an estuary in southern California, listed as impaired for mercury. In 2005, we examined mercury cycling at ten sites within at most four habitats. In surface water (unfiltered and filtered) and pore water, the concentration of total mercury was correlated with methylmercury levels ( R 2 = 0.29, 0.26, 0.27, respectively, p < 0.05), in contrast to sediments, where organic matter and reduced iron levels were most correlated with methylmercury content ( R 2 = 0.37, 0.26, respectively, p < 0.05). Interestingly, levels for percent methylmercury of total mercury in sediments were higher than typical values for estuarine sediments (average 5.4%, range 0.024–38%, n = 59), while pore water methylmercury K d values were also high (average 3.1, range 2.0–4.2 l kg −1, n = 39), and the estimated methylmercury flux from sediments was low (average 1.7, range 0.14–5.3 ng m −2 day −1, n = 19). Mercury levels in predatory fish tissue at Mugu are >0.3 ppm, suggesting biogeochemical controls on methylmercury mobility do not completely mitigate methylmercury uptake through the food web.

Hydrologic and biogeochemical controls on the spatial and temporal patterns of nitrogen and phosphorus in the Kuparuk River, arctic Alaska

AbstractNitrogen (N) and phosphorus (P) dynamics in the Kuparuk River in arctic Alaska were characterized in a 3‐year study using routine samples near the mouth of the river at the Arctic Ocean, synoptic whole‐river surveys, and temporally intense sampling during storms in three headwater basins. The Lower Kuparuk River has low nitrate concentrations (mean [NO3]‐N] = 17 µg l−1 ± 1·6 SE) and dissolved inorganic N (DIN, mean [N] = 31 µg l−1 ± 1·2 SE) compared with rivers in more temperate environments. Organic forms constituted on average 90% of the N exported to the Arctic Ocean, and high ratios of dissolved organic N (DON) to total dissolved N (TDN) concentrations (mean 0·92) likely result from waterlogged soils formed by reduced infiltration due to permafrost and low hydrologic gradients. Annual export of TDN, DON, and particulate N averaged 52 kg km−2, 48 kg km−2, and 4·1 kg km−2 respectively. During snowmelt, the high volume of runoff typically results in the highest nutrient loads of the year, although high discharge during summer storms can result in substantial nutrient loading over short periods of time. Differences in seasonal flow regime (snowmelt versus rain) and storm‐driven variation in discharge appear to be more important for determining nutrient concentrations than is the spatial variation in processes along the transect from headwaters towards the ocean. Both the temporal variation in nitrate:DIN ratios of headwater streams and the spatial variation in nitrate:DIN between larger sub‐basins and smaller headwater catchments is likely controlled by shifts in nitrification and soil anoxia. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Biogeochemical controls on palaeoceanographic environmental proxies: an introduction

The current volume samples a selection of papers presented at the Geological Society of London meeting on ‘Biogeochemical Controls on Palaeoceanographic Proxies’, held at Burlington House, London, UK on 3–4 October 2005. The aim of the meeting was to bring together palaeontologists, geochemists and palaeoceanographers who could contribute evidence that, when considered together, would better constrain the proxies that are used for palaeoclimate reconstruction. An improved understanding and quantification of past climate change, and the processes that force climate to change, has a fundamental role to play in constraining model projections of future climate (e.g. Hegerl et al. 2006) but it remains a huge challenge. This is because key climate variables, such as temperature and ocean salinity, cannot be observed in a world which no longer exists, but must instead be teased from proxies in the geological and ice records. There are numerous proxy archives, but one of the most important, currently lying at the forefront of palaeoceanographic research, is the biogeochemical composition of sediment records. This publication consists of 11 papers which deal with various aspects of biogeochemical proxies and their interpretation in terms of past climate. Seven of these specifically focus on the Foraminifera. What are proxies? Primarily, these are biogenic components which have a close relationship to environmental parameters and may be identified as so-called ‘proxy variables’ (Wefer et al. 1999), providing measurable descriptors of key climatic and environmental variables. The methods commonly employed in palaeoceanography have their origins in the biological, chemical and physical sciences; palaeoceanography therefore represents a relatively young and truly crossdisciplinary field of research. At the time of writing, an excellent new book entitled Proxies in Late Cenozoic Paleoceanography has been published (Hillaire-Marcel & De Vernal 2007), providing a comprehensive review of the subject. This volume begins with an overview by James & Austin, highlighting some of the most important biological and geochemical proxies, and outlining their contribution to our understanding of the ocean–climate system. We anticipate that this review article will provide an accessible introduction to the topic as well as an insight into a wide range of analytical methods. The second paper by Williams highlights some of the fundamental features of the biomineralization process, initially contrasting these with inorganic mineral formation, before selecting examples from corals, foraminifera and coccoliths. The paper by Zeebe et al. develops a modellingbased approach to improve our understanding of stable carbon, oxygen, and boron isotopes as well as magnesium incorporation into foraminiferal calcite shells. The approach adopted by these authors is based on an integrated understanding of the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine palaeo-tracer signals in organisms used in climate reconstructions. They argue very convincingly that the integration of laboratory experiments, field and culture studies, theoretical considerations, and numerical modelling holds the key to the method’s success. These authors demonstrate that a mechanism-based understanding is often required before primary climate signals can be extracted from the geological record, because these signals can be heavily overprinted by secondary, nonclimate related phenomena. One such phenomenon is foraminifer test diagenesis, which Pearson & Burgess highlight in their study of high-latitude Eocene sites. These authors illustrate the textures using a combination of reflected light microscopy and high-resolution scanning electron microscopy, highlighting the fact that foraminifer tests are prone to diagenetic recrystallization on a micron scale, which can affect their geochemical composition. For example, diagenetic calcite added on the sea floor or in shallow burial at low temperature, may result in the severe underestimation of the apparent tropical sea surface temperatures obtained from calcite foraminifera (e.g. Pearson et al. 2001). Next, two papers consider the growth rates and growth patterns in biogenic calcite. The first of these, by Schmidt et al. provides an overview of the effect of growth rate on the trace element

Biogeochemical controls on photic-zone euxinia during the end-Permian mass extinction

Geochemical, biomarker, and isotopic evidence suggests that the end-Permian was characterized by extreme oceanic anoxia that may have led to hydrogen sulfide buildup and mass extinction. We use an earth system model to quantify the biogeochemical and physical conditions necessary for widespread oceanic euxinia and hydrogen sulfide release to the atmosphere. Greater than threefold increases in ocean nutrient content combined with nutrient-trapping ocean circulation cause surface-water H 2 S accumulation in the paleo–Tethys Ocean and in areas of strong upwelling. Accounting for the presence of sulfide-oxidizing phototrophs in the model suppresses but does not prevent widespread release of H 2 S to the atmosphere. Evidence from the geologic record is consistent with modeled geochemical distributions of widespread nutrient-induced euxinia during the end-Permian, suggesting H 2 S toxicity and hypercapnia may have provided the kill mechanism for extinction.

A global inventory of the soil CH4 sink

Methane uptake by soils is a small but important flux in the global budget of atmospheric methane, and could be susceptible to changes in land use and climate. Estimates of this sink vary between 20 and 45 Tg yr−1. We propose to develop a better constrained estimate using a mechanistic understanding of the biogeochemical controls of soil CH4 uptake. We reviewed over 120 published papers reporting field measurements of CH4 uptake and made over 318 annual estimates of CH4 uptake for various types of ecosystems. We collected data from these papers for a number of parameters that are known to influence the magnitude of the sink including climatic zone, ecosystem, latitude, annual mean rainfall, annual mean temperature, and the soil texture. Regression analyses with the continuous variables (latitude, rainfall, and temperature) yielded results with poor predictive ability and no significant relationship. Stratification by class variables such as climatic zone, ecosystem type and soil texture provided better predictive ability (R2 = 0.29, P &lt; 0.0001). The mean largest uptake rates were observed in temperate forests with coarse soil texture, but the variance within this stratum was also large. Without any stratification, we estimate that the global soil CH4 sink is 36 ± 23 Tg yr−1. With stratification, the best current estimate of the global soil uptake of CH4 is 22 ± 12 Tg yr−1. The ecosystem type accounted for the largest part of the variation in the global data set. This inventory showed that ecosystem type, geographic zone, and soil texture strongly control CH4 uptake. Inventory methods that take into account underlying factors that control the process provide better estimates of sink strength.

Development and test application of a screening-level mercury fate model and tool for evaluating wildlife exposure risk for surface waters with mercury-contaminated sediments (SERAFM)

Complex chemical cycling of mercury in aquatic ecosystems means that tracing the linkage between anthropogenic and natural loadings of mercury to watersheds and water bodies and associated concentrations in the environment are difficult to establish without the assistance of numerical models that describe biogeochemical controls on mercury distribution and availability to organisms. This paper presents an overview of a process-based, steady-state model developed for state and water quality managers and scientists to assist in ecological risk assessments for mercury in aquatic environments. SERAFM (Spreadsheet-based Ecological Risk Assessment for the Fate of Mercury) incorporates the chemical, physical, and biological processes governing mercury transport and fate in a surface water including atmospheric deposition, watershed transport and transformation, solid transport and cycling within the water body, and water body mercury processes. This modelling framework was designed to assist risk assessors in evaluating wildlife risk at the screening-level for an aquatic ecosystem with mercury-contaminated sediments. An example application of the model that is used to inform a regional risk assessment is presented in this manuscript. In the example provided, hazard quotients for exposed wildlife and humans are calculated by the model for three scenarios: historical case of mercury-contaminated sediments, required clean-up levels to protect the most sensitive species, and background conditions. The spreadsheet structure of SERAFM permits dismantling and reassembling of specific sub-modules, while maintaining transparency to permit flexibility in use and application.

Biogeochemical controls on distributions and fluxes of dissolved and particulate organic carbon in the Northern South China Sea

Abstract Dissolved organic carbon (DOC) represented the major portion of total organic carbon (80–95% TOC) in the Northern South China Sea (NSCS) that was influenced significantly by the Kuroshio intrusion and continental fluxes. The concentration of DOC ranged from 70 to 85 μM in the mixed layer of studied areas excluding the coastal zone. The concentration of particulate organic carbon (POC) was relatively low, ranging from 1.6 to 4 μM in the mixed layer. Distributions of DOC (43±3 μM) and POC (1.1±02 μM) were uniform in deep and bottom layers (>1000 m) of the deep basin areas. However, elevated concentrations of DOC and POC up to 132 and 13 μM, respectively, were found in the Zhujiang (Pearl River) plume during summer with the highest river discharge. Both DOC and POC were generally higher in shelf zones than in deep basins, and also higher in summer than in other seasons particularly for coastal zones. Terrestrial inputs and mixing processes may primarily control the horizontal and vertical distributions of DOC and POC in the NSCS, as both correlate significantly with water density and temperature but less significantly with salinity. Biological effect on DOC and POC distributions also may be significant in the central basin, as both are correlated well with chlorophyll a . A portion of surface DOM with poor nitrogen and phosphorus contents appears to be released during C org syntheses inferred from stoichiometric patterns of DOM and POM in the euphotic zone. The degradation rate of DOC in the mixed layer is generally higher in the Zhujiang plume (decay coefficient: −0.045 d −1 ) than in the deep basin (−0.011 d −1 ). The labile and semi-labile fractions of DOC account for roughly 42–50% of total DOC in the mixed layer. The increase of DOM and POM stoichiometry (C/N or C/P ratio) with depth in deep-basin waters implies a preferential decay of phosphorus (nitrogen) over carbon. Contribution of DOC oxidation to apparent oxygen utilization (AOU) below the mixed layer is fairly low, ranging from 11% and 30% with an average of ∼19%, implying that DOC may be largely recycled in the upper layer. Although the downward flux of DOC is smaller than the sinking flux of C org , its magnitude (0.27–4.4 mmol C m −2 d −1 ) constitutes a significant flux of total organic carbon transported through the depth of 100 or 150 m in the NSCS basin. With the exception of fluxes through 100 m in summer, the downward fluxes of total C org are nearly balanced by new productivity derived from upward fluxes of nutrients by considering the estimate uncertainty. DOC appears to play an important role in organic carbon cycling and budgets in the upper layer of NSCS.

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ 18O and δ 34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O 2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III) aq as an oxidant with varying δ 18O H 2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ 34S SO 4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O 2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O 2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water ( ε 18 O SO 4 – H 2 O ) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured ε 18 O SO 4 2 - – H 2 O value was then used to estimate the oxygen isotope fractionation effects ( ε 18 O SO 4 2 - – O 2 ) between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ 18O SO 4 values in the biological and abiotic experiments, it is suggested that δ 18O SO 4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III) aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water–oxygen under both aerobic and anaerobic conditions.

Simulation of DOM fluxes in a seasonal floodplain of the Okavango Delta, Botswana

In order to examine dissolved organic matter (DOM) fluxes in seasonal wetland systems that expand and contract seasonally, a time-variable model of dissolved organic carbon (DOC) was developed for a seasonal floodplain in the Okavango Delta of Botswana. The model simulates DOC concentrations from March 2001 to November 2002, during which time DOC concentrations varied between 8 and 31 mg C L −1. The model uses a continuously stirred tank reactor (CSTR) approach to describe the hydrologic and biogeochemical controls on DOC leached from litter within the floodplain and transported into the floodplain from upstream. In 2002, a fire burned the floodplain and less litter was available for leaching than in 2001. The model was driven by observations of discharge, water temperature, upstream DOC concentrations, and DOC leaching rates from leaching experiments. Leaching experiments with sedges and grasses indicated that on average 23 mg DOC g −1 were leached during the first day of wetting and 0.6 mg DOC g −1 d −1 were continuously leached afterwards. Leaching experiments also showed a decreased amount of DOC released from burned litter and soils than from unburned litter and soils. A two-pool first-order decay model that represents both rapidly (0.14 d −1 (at 22 °C)) and slowly (0.045 d −1) decaying pools of DOC provided the best representation of observed patterns in DOC concentration in 2001. The decay rate of the first pool decreased by nearly half in 2002, when an estimated 78% of litter was removed by fire. Upstream DOC transport into the floodplain was the dominant source of DOC (representing approximately 70% and 75% of the DOC input in 2001 and 2002, respectively), followed by DOC leaching from litter and DOC originating from microbial sources. In 2001, decomposition (representing approximately 36% of the DOC loss), outflow to an adjacent floodplain (36%) and infiltration (28%) were the major removal mechanisms for DOM from the study floodplain. The large amount of DOC transported by infiltration implies storage of DOC in the subsurface, which may influence subsurface heterotrophic activity. In light of future climate change anticipated for the region, a scenario using a 2 °C increase in average water temperature and 10% reduction in upstream DOC mass was performed and resulted in significant (11%) reduction in annual DOC mass within the study floodplain.

Selenium Geochemistry and Health

Selenium (Se) is a naturally occurring metalloid element, which is essential to human and other animal health in trace amounts but is harmful in excess. Of all the elements, Se has one of the narrowest ranges between dietary deficiency ( 400 µg day-1) (1) making it necessary to carefully control intakes by humans and other animals hence the importance of understanding the relationships between environmental exposure and health. Geology exerts a fundamental control on the concentrations of Se in the soils on which we grow the crops and animals that form the human food chain. The Se status of populations, animals and crops vary markedly around the world as a result of different geological conditions. Since diet is the most important source of Se in humans, understanding the biogeochemical controls on the distribution and mobility of environmental Se is key to the assessment of Se-related health risks.

Hydrological versus biogeochemical controls on catchment nitrate export: a test of the flushing mechanism

AbstractDeciphering the connection between streamflows and nitrate (NO−3) discharge requires identification of the various water flow pathways within a catchment, and the different time‐scales at which hydrological and biogeochemical processes occur. Despite the complexity of the processes involved, many catchments around the world present a characteristic flushing response of NO−3 export. Yet the controls on the flushing response, and how they vary across space and time, are still not clearly understood. In this paper, the ‘flushing response’ of NO−3 export from a rural catchment in Western Australia was investigated using isotopic (deuterium), chemical (chloride, NO−3), and hydrometric data across different antecedent conditions and time‐scales. The catchment streamflow was at all time‐scales dominated by a pre‐event water source, and the NO−3 discharge was correlated with the magnitude of areas contributing to saturation overland flow. The NO−3 discharge also appeared related to the shallow groundwater dynamics. Thus, the antecedent moisture condition of the catchment at seasonal and interannual time‐scales had a major impact on the NO−3 flushing response. In particular, the dynamics of the shallow ephemeral perched aquifer drove a shift from hydrological controls on NO−3 discharge during the ‘early flushing’ stage to an apparent biogeochemical control on NO−3 discharge during the ‘steady decline’ stage of the flushing response. This temporally variable control hypothesis provides a new and alternative description of the mechanisms behind the commonly seen flushing response. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Biogeochemical controls on the trace element composition of wine

Biogeochemical controls on the trace element composition of wine

Modeling physical and biogeochemical controls over carbon accumulation in a boreal forest soil

Boreal soils are important to the global C cycle owing to large C stocks, repeated disturbance from fire, and the potential for permafrost thaw to expose previously stable, buried C. To evaluate the primary mechanisms responsible for both short‐ and long‐term C accumulation in boreal soils, we developed a multi‐isotope (12, 14C) soil C model with dynamic soil layers that develop through time as soil organic matter burns and reaccumulates. We then evaluated the mechanisms that control organic matter turnover in boreal regions including carbon input rates, substrate recalcitrance, soil moisture and temperature, and the presence of historical permafrost to assess the importance of these factors in boreal C accumulation. Results indicate that total C accumulation is controlled by the rate of carbon input, decomposition rates, and the presence of historical permafrost. However, unlike more temperate ecosystems, one of the key mechanisms involved in C preservation in boreal soils examined here is the cooling of subsurface soil layers as soil depth increases rather than increasing recalcitrance in subsurface soils. The propagation of the 14C bomb spike into soils also illustrates the importance of historical permafrost and twentieth century warming in contemporary boreal soil respiration fluxes. Both 14C and total C simulation data also strongly suggest that boreal SOM need not be recalcitrant to accumulate; the strong role of soil temperature controls on boreal C accumulation at our modeling test site in Manitoba, Canada, indicates that carbon in the deep organic soil horizons is probably relatively labile and thus subject to perturbations that result from changing climatic conditions in the future.

Spatial and temporal variability in new production in the equatorial Pacific during 1980–2003: Physical and biogeochemical controls

Abstract The total new production in the equatorial Pacific ocean is still subject to debate. Early estimates range from 0.8 to 1.9 Pg C year−1 for the Wyrtki box (5°N–5°S, 180°W–90°W). More recent studies estimate a rate of less than 0.14 Pg C year−1 for the 1°N–1°S equatorial divergence, which is significantly lower than the previous estimates even when the difference in area is taken into account. To address this issue, we employ a coupled three-dimensional physical-biogeochemical model to simulate ecosystem dynamics and to estimate new production over the period of 1980–2003. The biogeochemical model consists of nine components: two size classes (large and small) each of phytoplankton, zooplankton and detritus, and three nutrients (ammonium, nitrate, and iron). The model is able to reasonably reproduce observed surface chlorophyll, ecosystem structure, primary productivity, and new production. The modeled primary productivity and new production are in the ranges of 20–100 and 5–40 mmol C m−2 d−1, respectively. The modeled primary productivity shows relatively weak spatial and temporal variations whereas the modeled new production shows considerable variability over space and on seasonal-to-interannual time scales. The model simulates two peaks in new production (e.g., in boreal spring and boreal fall) in the high-nitrate low-chlorophyll (HNLC) region. The high rate of new production in spring is associated with the seasonal warming and a shallow mixed layer whereas the high rate of new production during fall is related to the high iron concentrations due to the seasonal upwelling and entrainment in the eastern equatorial Pacific. The new production reveals clear spatial and interannual variability in association with the El Nino/Southern Oscillation (ENSO). The rate of new production is highly related to the mixed-layer concentrations of nitrate and iron, and the 20 °C isotherm depth (Z20) in the HNLC region. However, there is no relationship between the rate of new production and either the concentrations of nitrate or iron, or Z20 in the warm pool. Integrated new production from the model is 0.61±0.14 Pg C year−1 for the Wyrtki box, which is significantly lower than some previous estimates.

Physical and biogeochemical controls of microaggregate dynamics in a tidally affected coastal ecosystem

Tidal flat ecosystems exhibit pronounced tidal currents that cause high loads of suspended matter (SPM) and intense sedimentation. To identify systematic patterns of tidal SPM dynamics and the significance of physical forcing versus microbial processes in aggregation processes, we conducted a comprehensive study from January 2002 to October 2004 in a backbarrier tidal flat area of the German Wadden Sea. Further, various aggregate fractions were separated by their settling properties in June and October 2004, applying a new sampling device. Tidal dynamics of SPM, particulate organic carbon (POC), aggregate abundance and size, chlorophyll a (Chl a), the carbon to nitrogen ratio (C : N), numbers of bacteria, and DOC often exceeded seasonal dynamics of the tidal means of these properties. SPM, POC, Chl a, and aggregate abundance were positively correlated and aggregate size negatively correlated to the current. DOC concentrations and total bacterial numbers exhibited minima at high tide and maxima at low tide. Aggregate quality—i.e., POC : SPM, Chl a, size, amino acid content, and bacterial colonization—varied tidally among the fractions, relative to bulk SPM and was different in June and October. In June, tidal dynamics of these properties and bacterial biomass production were higher than in October. Aggregate abundance was substantially lower during the growing season and aggregate size larger than in fall and winter. Microbial processes were important during the growing season in affecting tidal dynamics of aggregation and sedimentation, whereas in fall and winter, physical forcing was the main factor controlling aggregate dynamics.

Biogeochemical controls on the oxygen, nitrogen and sulfur distributions in the water column of Golfo Dulce: an anoxic basin on the Pacific coast of Costa Rica revisited

Chemical distributions, respiration rates, and bacterial distributions were measured in 1994 in the water column and sediments of a small, tropical, anoxic basin (Golfo Dulce, Pacific coast of Costa Rica) to examine the biogeochemical controls on anoxia, sulfide, dissolved inorganic nitrogen, and organic carbon consumption. As reported previously, the deepest 100 m of the water column were anoxic, and sulfide concentrations in the bottom waters were less than 7 μM and then only transiently. Both free-swimming sulfide-oxidizing bacteria and Beggiatoa sp. (containing large vacuoles) were observed in the anoxic bottom waters or at the sediment-water interface. Aerobic respiration dominated the decomposition of organic matter in the surface waters and pycnocline, whereas sulfate reduction was principally restricted to the sediments. Bacteria were distributed in discrete zones and exhibited the highest densities where oxygen decreased below 1 μM around 100 m depth, and near the sediment-water interface. The sub-oxic, sub-pycnocline water column was characterized by a dissolved inorganic nitrogen (DIN) deficit of 2.9 mole m-2. With a water residence time of 35 – 57 d, estimated from a salt balance, this deficit corresponded to a DIN loss of 51 – 85 mmol m-2 d-1, comparable to the sub-pycnocline oxygen consumption. Sulfide in the water column was maintained at low concentrations by frequent inputs of oxygenated water from the Pacific Ocean. Sulfide production in the sediments due to bacterial sulfate reduction was scavenged by frequent deposition of iron-rich turbidites. Based on 210Pb distributions, the most recent emplacement of a turbidite in the basin sediments was determined to have occurred between 1989 and 1992. Rev. Biol. Trop. 54 (Suppl. 1): 171-191. Epub 2006 Sept. 30.