Internal Biogeochemical Processes Research Articles

Controls on distributions of aluminium, manganese and cobalt in the South Atlantic Ocean along GEOTRACES transect GA10

Trace metals (TMs) manganese (Mn), cobalt (Co), and aluminium (Al) have important geochemical and biological roles in the ocean. Here, we present full depth profiles of dissolved (d) and particulate Al, Mn, and Co along the latitude of 40 °S in the South Atlantic Ocean from the GEOTRACES GA10 cruises that operated in austral spring 2010 and summer 2011. The region is characterized by enhanced primary productivity and forms a key transition zone between the Southern Ocean and South Atlantic Subtropical Gyre. The mean concentrations of dAl, dCo, and dMn (±standard deviation) were 3.36 ± 2.65 nmol kg−1, 35.3 ± 17.6 pmol kg−1, and 0.624 ± 1.08 nmol kg−1, respectively. Their distributions in surface waters were determined by external sources and complex internal biogeochemical processes. Specifically, surface ocean dCo was controlled by the interplay between phytoplankton uptake, remineralization and external inputs; dMn was likely determined by the formation and photoreduction of Mn-oxides; and dAl was supplied by atmospheric deposition and removed by scavenging onto particles. Fluvial and sedimentary inputs near the Rio de La Plata estuary and benthic sources from the Agulhas Bank resulted in elevated dTM concentrations in near-shore surface waters. These externally sourced dTMs were effectively delivered to the open ocean by offshore diffusion and/or advection, and potentially facilitated enhanced primary productivity along the transect.The distributions of dTMs at depth were predominantly controlled by the mixing of North Atlantic Deep Water (NADW) and waters of Antarctic origin (e.g., Upper Circumpolar Water (UCDW) and Antarctic Bottom Water (AABW)). The calculated endmember concentrations of dAl and dCo in NADW showed minor decreases in the SASTG following north–south transport, suggesting removal rates of 0.064 nM/year and 0.035–0.075 pM/year, respectively. The endmember concentration of dCo in AABW was maintained at ∼30 pmol kg−1 without evidence for scavenging removal in the Southern Ocean and SASTG (time frame >400 years). The concentrations of dMn in NADW and AABW were between 0.1 and 0.16 nmol kg−1, and any elevated dMn concentrations were ascribed to local external inputs (e.g., from sediments in the Argentine Basin and hydrothermal activity near the Mid-Atlantic Ridge). Hence, four controlling factors (sources, internal cycling, water mass mixing and time) need to be considered when assessing TM distributions in the global ocean, even for TMs that are vulnerable to scavenging removal processes. Because the deep waters formed in high latitude oceans are crucial components of the global thermohaline overturning system, any processes (e.g., glacier melting, upwelling and sinking, and biological activity) that impact the preformed dTM concentrations in high latitude oceans will determine the downstream dTM distributions. Therefore, the sources and sinks of TMs and associated biological activity in high latitude oceans could engender basin to global scale impacts on seawater distributions of Al, Co, and Mn and their stoichiometric relationships with macronutrients, and the global biogeochemical cycles of these scavenged-type TMs.

Stable isotope patterns in lake food webs reflect productivity gradients

AbstractStable isotopes 13C and 15N are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, δ13C and δ15N are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled δ13C and δ15N in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature–oxygen profiles and light levels to assess factors influencing isotope patterns. Isotope samples were baseline‐corrected using epilimnetic mussels in each lake (sample—mussel) to control for watershed‐level differences in isotope values. Results showed δ13C in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in δ13C relative to epilimnetic mussels in low‐productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston δ13C stayed similar to mussel δ13C in all lakes. This pattern was likely due to hypolimnetic phytoplankton in clearwater lakes incorporating more respired CO2, which is depleted in δ13C, and subsequently passing depleted δ13C values up the food chain. Results also showed habitat differences in δ15N with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low‐productivity lakes with higher O2 levels in the hypolimnion, while epilimnetic seston δ15N remained similar to mussel values. The δ15N pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic δ15N seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.

Comprehensive and quantitative assessment of nitrate dynamics in two contrasting forested basins along the Sea of Japan using dual isotopes of nitrate

The recent deposition rates of atmospheric nitrate derived from east Asia to the Japanese forested watershed facing the Sea of Japan are of serious concern. However, export ratios and the seasonality of atmospheric nitrate versus microbial nitrate from forest soils to upstreams have not yet been quantified. Furthermore, the influence of local nitrogen sources and internal biogeochemical processes are still unclear. To determine the influence of watershed properties and atmospheric nitrogen deposition on nitrate dynamics in two adjacent basins (the Kita and Minami Rivers) located in central Japan, we conducted seasonal synoptic surveys using the dual isotopes of nitrate. It was found that nitrate regenerated through nitrification in the forest soil was likely the dominant nitrogen source in both basins from the upstream to downstream waters. However, nitrate concentrations and the direct leaching ratio of atmospheric nitrate were considerably higher in the Kita River Basin than in the Minami River Basin, possibly due to the difference in forest environments. In the Kita River Basin, geographic trait such as altitude may be one factor regulating the sensitivity of forest ecosystem to nitrogen deposition. Quantitative assessments of nitrate outflows from the sub-basins revealed that nitrogen leached from the forest soil was a major source (61–81%) of nitrate loading to the coastal sea.

Spatial and Temporal Dynamics of Dissolved Organic Carbon, Chlorophyll, Nutrients, and Trace Metals in Maritime Antarctic Snow and Snowmelt

Despite scientific interest in the investigation of biogeochemical changes in meltwaters of the Antarctic Peninsula, we still lack an understanding of the seasonal dynamics and release of dissolved and particulate carbon, nutrients, as well as trace metals from Antarctic snowpacks. Harsh conditions, lack of appreciation of the heterogeneity of the environment, as well as logistical constraints during fieldwork mean there is great demand for more detailed and comprehensive research. Therefore, a unique, comprehensive study of snowpack biogeochemistry was performed in the Ryder Bay area of the Antarctic Peninsula during the entire 2016/2017 melt season. Two-hundred snowpack and snowmelt samples were collected throughout the campaign, to quantify for the first time, seasonal dynamics and export of dissolved carbon, in-vivo chlorophyll, nutrient and trace metals from Antarctic snowpack in various locations. Our study uncovered the importance of environmental heterogeneity with respect to the export of solutes and carbon. A distinctive split in the temporal dynamics of solute export was found, suggesting that some solutes are rapidly delivered to coastal environments early in the summer whilst others are delivered more gradually throughout it. Coastal, low elevation snowpacks were identified as “power plants” of microbial activity, playing an important role in the regulation of land-ocean fluxes of labile carbon and bio-limiting macro- and micro-nutrients. We also found that multiannual snow residing deep below the surface can further contribute to biogeochemical enrichment of coastal ecosystems. Additionally, inland snowpack have been identified as a store for nutrients, dissolved organic carbon and chlorophyll. Lastly, we show that a number of factors (environmental characteristics, geochemical heterogeneity and internal biogeochemical processes in snow) make simple snowpack surveys insufficient for the prediction of biogeochemical fluxes carried by snowmelt runoff into the marine environment. A return to significant fieldwork-based research in Antarctica is therefore necessary to advance our knowledge of the complex biogeochemical processes occurring there. This study therefore provides crucial data and process insights for more accurate predictions of how changing climate will influence the Antarctic carbon cycle and the globally important Southern Ocean ecosystem.

Monitoring water quality in Toronto's urban stormwater ponds: Assessing participation rates and data quality of water sampling by citizen scientists in the FreshWater Watch

From 2013 to 2015, citizen scientist volunteers in Toronto, Canada were trained to collect and analyze water quality in urban stormwater ponds. This volunteer sampling was part of the research program, FreshWater Watch (FWW), which aimed to standardize urban water sampling efforts from around the globe. We held training sessions for new volunteers twice yearly and trained a total of 111 volunteers. Over the course of project, ~30% of volunteers participated by collecting water quality data after the training session with 124 individual sampling events at 29 unique locations in Toronto, Canada. A few highly engaged volunteers were most active, with 50% of the samples collected by 5% of trainees. Stormwater ponds generally have poor water quality demonstrated by elevated phosphate concentrations (~30μg/L), nitrate (~427μg/L), and turbidity relative to Canadian water quality standards. Compared to other urban waterbodies in the global program, nutrient concentrations in Toronto's urban stormwater ponds were lower, while turbidity was not markedly different. Toronto FWW (FWW-TO) data was comparable to that measured by standard lab analyses and matched results from previous studies of stormwater ponds in Toronto. Combining observational and chemical data acquired by citizen scientists, macrophyte dominated ponds had lower phosphate concentrations while phytoplankton dominated ponds had lower nitrate concentrations, which indicates a potentially important and unstudied role of internal biogeochemical processes on pond nutrient dynamics. This experience in the FWW demonstrates the capabilities and constraints of citizen science when applied to water quality sampling. While analytical limits on in-field analyses produce higher uncertainty in water quality measurements of individual sites, rapid data collection is possible but depends on the motivation and engagement of the group of volunteers. Ongoing efforts in citizen science will thus need to address sampling effort and analytical limits to fully realize the potential value of engaging citizen scientists in water quality sampling.

Natural ponds in an agricultural landscape: External drivers, internal processes, and the role of the terrestrial-aquatic interface

The pleistocenic landscape in North Europe, North Asia and North America is spotted with thousands of natural ponds called kettle holes. They are biological and biogeochemical hotspots. Due to small size, small perimeter and shallow depth biological and biogeochemical processes in kettle holes are closely linked to the dynamics and the emissions of the terrestrial environment. On the other hand, their intriguing high spatial and temporal variability makes a sound understanding of the terrestrial-aquatic link very difficult. It is presumed that intensive agricultural land use during the last decades has resulted in a ubiquitous high nutrient load. However, the water quality encountered at single sites highly depends on internal biogeochemical processes and thus can differ substantially even between adjacent sites.This study aimed at elucidating the interplay between external drivers and internal processes based on a thorough analysis of a comprehensive kettle hole water quality data set. To study the role of external drivers, effects of land use in the adjacent terrestrial environment, effects of vegetation at the interface between terrestrial and aquatic systems, and that of kettle hole morphology on water quality was investigated. None of these drivers was prone to strong with-in year variability. Thus temporal variability of spatial patterns could point to the role of internal biogeochemical processes. To that end, the temporal stability of the respective spatial patterns was studied as well for various solutes. All of these analyses were performed for a set of different variables. Different results for different solutes were then used as a source of information about the respective driving processes. In the Quillow catchment in the Uckermark region, about 100km north of Berlin, Germany, 62 kettle holes have been regularly sampled since 2013. Kettle hole catchments were determined based on a groundwater level map of the uppermost aquifer. The catchments were not clearly related to topography. Spatial patterns of kettle hole water concentration of (earth) alkaline metals and chloride were fairly stable, presumably reflecting solute concentration of the uppermost aquifer. In contrast, spatial patterns of nutrients and redox-sensitive solutes within the kettle holes were hardly correlated between different sampling campaigns. Correspondingly, effects of season, hydrogeomorphic kettle hole type, shore vegetation or land use in the respective catchments were significant but explained only a minor portion of the total variance.It is concluded that internal processes mask effects of the terrestrial environment. There is some evidence that denitrification and phosphorus release from the sediment during frequent periods of hypoxia might play a major role. The latter seems to boost primary production occasionally. These processes do not follow a clear seasonal pattern and are still not well understood.

Recent Advances in Understanding Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento–San Joaquin River Delta

This paper, part of the collection of research comprising the State of Bay–Delta Science 2016, describes advances during the past decade in understanding flow dynamics and how water-quality constituents move within California’s Sacramento–San Joaquin River Delta (Delta). Water-quality constituents include salinity, heat, oxygen, nutrients, contaminants, organic particles, and inorganic particles. These constituents are affected by water diversions and other human manipulations of flow, and they greatly affect the quantity and quality of benthic, pelagic, and intertidal habitat in the Delta. The Pacific Ocean, the Central Valley watershed, human intervention, the atmosphere, and internal biogeochemical processes are all drivers of flow and transport in the Delta. These drivers provide a conceptual framework for presenting recent findings. The tremendous expansion of acoustic and optical instruments deployed in the Delta over the past decade has greatly improved our understanding of how tidal variability affects flow and transport. Sediment is increasingly viewed as a diminishing resource needed to sustain pelagic habitat and tidal marsh, especially as sea level rises. Connections among the watershed, Delta, and San Francisco Bay that have been quantified recently highlight that a landscape view of this system is needed, rather than consideration of each region in isolation. We discuss interactions of multiple drivers and information gaps.

Recent Advances in Understanding Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento–San Joaquin River Delta

This paper, part of the collection of research comprising the State of Bay–Delta Science 2016, describes advances during the past decade in understanding flow dynamics and how water-quality constituents move within California’s Sacramento–San Joaquin River Delta (Delta). Water-quality constituents include salinity, heat, oxygen, nutrients, contaminants, organic particles, and inorganic particles. These constituents are affected by water diversions and other human manipulations of flow, and they greatly affect the quantity and quality of benthic, pelagic, and intertidal habitat in the Delta. The Pacific Ocean, the Central Valley watershed, human intervention, the atmosphere, and internal biogeochemical processes are all drivers of flow and transport in the Delta. These drivers provide a conceptual framework for presenting recent findings. The tremendous expansion of acoustic and optical instruments deployed in the Delta over the past decade has greatly improved our understanding of how tidal variability affects flow and transport. Sediment is increasingly viewed as a diminishing resource needed to sustain pelagic habitat and tidal marsh, especially as sea level rises. Connections among the watershed, Delta, and San Francisco Bay that have been quantified recently highlight that a landscape view of this system is needed, rather than consideration of each region in isolation. We discuss interactions of multiple drivers and information gaps.

"Diffusive Gradients in Thin Films" Techniques Provide Representative Time-Weighted Average Measurements of Inorganic Nutrients in Dynamic Freshwater Systems.

Nutrient concentrations in freshwater are highly variable over time, with changes driven by weather events, anthropogenic sources, modifications to catchment hydrology or habitats, and internal biogeochemical processes. Measuring infrequently collected grab samples is unlikely to adequately represent nutrient concentrations in such dynamic systems. In contrast, in situ passive sampling techniques, such as the "diffusive gradients in thin films" (DGT) technique, provide time-weighted average analyte concentrations over the entire deployment time. A pair of recently developed DGT techniques for nitrate (A520E-DGT) and ammonium (PrCH-DGT), as well as the Metsorb-DGT technique for phosphate, were used to monitor inorganic nutrients in different freshwater systems (i.e., streams and wetlands) with a range of environmental values and that were affected by different catchment types. Measurements of grab samples collected frequently (1-2 times daily, 8-10 a.m. and 2-4 p.m.) showed that concentrations of NH4-N and NO3-N changed dramatically in most of the studied freshwater systems over short time scales, while there were only relatively small fluctuations in PO4-P. The DGT measurements were highly representative in comparison with the average nutrient concentrations obtained from daily grab samples over short-term (24 h) and long-term (72 h) deployments. The ratios of DGT-labile concentrations to the average concentrations from grab samples were between 1.00 and 1.12 over the studied deployment periods. The results of this study confirmed that DGT measurements provided a reliable and robust method for monitoring NH4-N, NO3-N, and PO4-P in a diverse range of dynamic freshwater systems.

Stable isotopes reduce parameter uncertainty of an estuarine carbon cycling model

Quantifying estuarine carbon cycling is complex due to the highly-variable environmental conditions associated with the interaction between tides, riverine inflows, meteorological forcing and internal biogeochemical processes. A Markov-Chain Monte Carlo algorithm was utilized to perform unbiased calibration of parameters used by a 1-D isotope-enabled carbon model applied to stable isotope data collected in Caboolture River Estuary, Australia. The parameter posteriors were ported into a 3-D finite-volume isotope-enabled carbon model and run over a range of hydro-meteorological conditions that occurred during a 1.5-year simulation period. The model highlighted the spatio-temporal variations and uncertainties associated with carbon cycling within the estuary, including the shift from being strongly heterotrophic in the upper estuary with a higher water-atmosphere flux of CO2, to a more balanced trophic state in the lower estuary. The approach demonstrates the usefulness of isotope data to constrain model uncertainty and advances our ability to undertake carbon budgeting in coastal environments.

Key biogeochemical processes evaluated by the stable nitrogen isotopes of dissolved inorganic nitrogen in the Yodo River estuary, Japan: significance of estuarine nutrient recycling as a possible source for coastal production

Isotope analysis of dissolved inorganic nitrogen (DIN) and a mixing model along the salinity gradient were used to clarify the dominant biogeochemical processes controlling nutrient dynamics within a shallow eutrophic estuary in Japan. Although delivery of riverine DIN into the estuary is largely dominated by mixing with seawater during most months, internal biogeochemical processes of DIN assimilation by phytoplankton and DIN production by remineralization and subsequent nitrification within the estuary offer the most reasonable explanation for observed deviations in the isotopic compositions of $$ {\text{NO}}_{3}^{ - } $$ from mixing behavior. However, the balance of each process changed over time. During phytoplankton blooming in summer, co-occurrence of $$ {\text{NO}}_{3}^{ - } $$ assimilation and $$ {\text{NO}}_{3}^{ - } $$ regeneration inhibited the accumulation of $$ {\text{NO}}_{3}^{ - } $$ within the estuary. Moreover, assimilation of $$ {\text{NH}}_{4}^{ + } $$ as well as $$ {\text{NO}}_{3}^{ - } $$ by phytoplankton complicates the nutrient dynamics within the estuary. However, mostly conservative or productive behavior of DIN as well as $$ {\text{PO}}_{4}^{3 - } $$ showed that recycled nutrients are significant net source within the estuary. These results suggest recycled nutrients within the estuary could have a non-negligible impact on eutrophication in Osaka Bay.

Synergies between observational and modeling studies at the Takayama site: toward a better understanding of processes in terrestrial ecosystems

AbstractThis paper reviews past and ongoing ecological studies at the Takayama forest site, one of Japan's long‐term research sites with intensive studies; the emphasis here is on the synergies between observational and modeling studies. This synergism has been encouraged because global environmental change is a complicated and interdisciplinary issue that requires this type of collaboration. The increasing amount and quality of observational data available from this site helps researchers to better constrain their model simulations because substantial uncertainties remain in the behavior of models in their current form. In addition, modeling studies encourage observational research by integrating observational data within a comprehensive framework. They do this by demanding more long‐term and high‐quality data related to specific ecological processes, and by identifying high‐priority processes and parameters that should be studied. We describe the types of modeling studies that have been conducted and how observational data from the Takayama site have improved the accuracy of these models. Many terrestrial ecosystem models have been applied to data from the site, both to validate the present forms of the models and to refine the structure and parameterization of the models. The development of new or improved terrestrial ecosystem models will be further facilitated by the requirements to simulate atmosphere–ecosystem exchanges and internal biogeochemical processes, as well as simulating their responses to a changing environment. We conclude by discussing the remaining research gaps and opportunities for deepening our understanding of terrestrial ecosystems through future collaborative studies.

Temperature‐dependent remineralization in a warming ocean increases surface pCO2 through changes in marine ecosystem composition

AbstractTemperature‐dependent remineralization of organic matter is, in general, not included in marine biogeochemistry models currently used for Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projections. Associated feedbacks have therefore not been quantified. In this study we aim at investigating how temperature‐dependent remineralization rates (Q10 = 2) in a warming ocean impact on the marine carbon cycle, and if this may weaken the oceanic sink for anthropogenic CO2. We perturb an Earth system model used for CMIP5 with temperature‐dependent remineralization rates of organic matter using representative concentration pathway (RCP)8.5‐derived temperature anomalies for 2100. The result is a modest change of organic carbon export but also derived effects associated with feedback processes between changed nutrient concentrations and ecosystem structure. As more nutrients are recycled in the euphotic layer, increased primary production causes a depletion of silicate in the surface layer as opal is exported to depth more efficiently than particulate organic carbon. Shifts in the ecosystem occur as diatoms find less favorable conditions. Export production of calcite shells increases causing a decrease in alkalinity and higher surface pCO2. With regard to future climate projections, the results indicate a reduction of oceanic uptake of anthropogenic CO2 of about 0.2 PgC yr−1 toward the end of the 21st century in addition to reductions caused by already identified climate‐carbon cycle feedbacks. Similar shifts in the ecosystem as identified here, but driven by external forcing, have been proposed to drive glacial/interglacial changes in atmospheric pCO2. We propose a similar positive feedback between climate perturbations and the global carbon cycle but driven solely by internal biogeochemical processes.

Mercury contamination in aquatic ecosystems under a changing environment: Implications for the Three Gorges Reservoir

Mercury is one of the primary contaminants of global concern. As anthropogenic emissions of mercury are gradually placed under control, evidence is emerging that biotic mercury levels in many aquatic ecosystems are increasingly driven by internal biogeochemical processes, especially in ecosystems that have been undergoing dramatic environmental changes. Here we review the unique properties of mercury that are responsible for the exceptional sensitivity of its biogeochemical cycles to changes in climatic, geochemical, biological and ecological processes. We show that, due to rapid climate warming, a shift from sources-driven to processes-driven mercury bioaccumulation is already happening in the Arctic marine ecosystem. We further suggest that such a shift might also be operating in the Three Gorges Reservoir due to changes in these biogeochemical processes induced by the damming. As a result, the effectiveness of mercury emission control is expected to be followed by long delays before ensuing reduction is seen in food-web levels, making it all the more pressing to control and reduce mercury emissions to the reservoir. Long-term monitoring and targeted studies are urgently needed to understand how biotic mercury levels in the reservoir are responding to changes in mercury emissions and in biogeochemical processes.

Trade-off between conservation and exploitation of the transitional water ecosystems of the northern Adriatic Sea

Transitional waters (TWs) provide ecosystem goods and services that are essential for the well-being of human populations. These unpredictable aquatic systems, characterised by large environmental fluctuations, are under severe stress due to human activities. Increasing pressures (e.g. over-harvesting, eutrophication, habitat loss) inevitably lead to the degradation of these ecosystems. Analysis of the complexity of species distribution patterns within and among TW habitats is relevant to understanding the underlying processes and promoting appropriate management strategies. Assessment of the trophic status is one of the most critical aspects of TWs. Untangling the relevance of anthropogenic nutrient inputs from internal biogeochemical processes is of primary importance in defining appropriate restoration strategies. Biotic indices have been suggested as an operational tool to assess environmental quality in TWs. However, the application in TWs of indices developed for coastal waters can give distorted results (e.g. low species diversity and high abundance are natural features). The BITS approach provides a rapid assessment of ecological quality, although its sensitivity in reflecting field conditions remains to be assessed. The major challenge to TWs management is to couple long-term conservation with productive activities. This goal can be achieved using an integrated approach, forecasting conservation of TW ecosystem functioning together with sustainable economic development. North-western Adriatic TW habitats have been exploited for centuries and major shifts in ecological processes have occurred. In this study, knowledge of the ecological features of these habitats is summarised and analysed using recent ecological tools. Based on these findings, possible strategies for conservative management have been discussed.

Phytoplankton variation and its relation to nutrients and allochthonous organic matter in a coastal lagoon on the Gulf of Mexico

In tropical and subtropical zones, coastal lagoons are surrounded by mangrove communities which are a source of high quantity organic matter that enters the aquatic system through litter fall. This organic matter decomposes, becoming a source of nutrients and other substances such as tannins, fulvic acids and humic acids that may affect the composition and productivity of phytoplankton communities. Sontecomapan is a coastal lagoon located in the southern Gulf of Mexico, which receives abundant litter fall from mangrove. To study the phytoplankton composition and its variation in this lagoon from October 2002 to October 2003, we evaluated the concentrations of dissolved folin phenol active substances (FPAS) as a measure of plant organic matter, salinity, temperature, pH, O 2, N-NH 4 +, N-NO 3 −, P-PO 4 3−, Si-SiO 2, and phytoplanktonic cell density in different mangrove influence zones including the three main rivers that feed the lagoon. Nutrients concentrations depended on freshwater from rivers, however these varied seasonally. Concentrations of P-PO 4 3−, N-NH 4 + and FPAS were the highest in the dry season, when maximum mangrove litter fall is reported. Variation of these nutrients seemed to depend on the internal biogeochemical processes of the lagoon. Blooms of diatoms ( Skeletonema spp., Cyclotella spp. and Chaetoceros holsaticus) and dinoflagellates ( Peridinium aff. quinquecorne, Prorocentrum cordatum) occurred seasonally and in the different mangrove influence zones. The high cell densities in these zones and the occurrence of certain species and its ordination along gradient of FPAS in a canonical correspondence analysis, suggest that plant organic matter (i.e. mangrove influence) may contribute to phytoplankton dynamics in Sontecomapan lagoon.

A review of water column processes influencing hypoxia in the northern Gulf of Mexico

In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model.

Nutrient Dynamics in Jiaozhou Bay

Three cruises were carried out in Jiaozhou Bay (JZB) in the neap tide in October 2002 (fall) and in both neap and spring tides in May 2003 (spring) to understand the relative importance of external nutrient inputs versus physical transport and internal biogeochemical processes. Nutrients (\( NO_{3} ^{ - } \), \( NO_{2} ^{ - } \), \( NH_{4} ^{ + } \), \( PO_{4} ^{{3 - }} \), silicic acid, total dissolved nitrogen (TDN) and phosphorus (TDP), dissolved organic nitrogen (DON) and phosphorus (DOP)) were measured. The concentrations of nutrients were higher in the northern part than in the southern part. High concentrations of \( NH_{4} ^{ + } \) and DON in JZB demonstrated the anthropogenic input. Ambient nutrient ratios indicated that the potential limiting nutrients for phytoplankton growth were silicon, and then phosphorus. Nutrients showed an obvious tidal effect with low values at flood tide and high values at ebb tide. Nutrient elements were transported into JZB in the north and output in the south (i.e., into the Yellow Sea), which varied with season, tidal cycle and investigation sites. Water exchange between JZB and the Yellow Sea exports \( NO_{3} ^{ - } \), \( NH_{4} ^{ + } \) and DON out of JZB, while it inputs \( PO_{4} ^{{3 - }} \), silicic acid and DOP into JZB. Nutrient budgets demonstrate that riverine input and wastewater discharge are major sources of nutrients, while residual flow is of minor importance in JZB ecosystem. JZB is a sink for the nutrient elements we studied except for DON. Stoichiometric calculations demonstrate that JZB is a net autotrophic system.

Effect of Different Chloride Concentrations on Nutrient Release in Wetland Soils: A Phytometer Assessment in the Botshol Wetlands, The Netherlands

Turbid water, high phosphorus (P)-loading and disappearing Chara communities forced local water authorities to carry out restoration measures in the lakes and marshes of the Botshol The Netherlands. The reduction of the external-P input could be reached by chemical treatment of the brackish suppletion water and by separating the area from nearby agricultural areas. A side effect of these measures was an increase of chloride from 400 mg l−1 to 1000 mg l−1 in the surface water of Botshol. Internal biogeochemical processes were investigated with phytometers and direct measurements of soil nutrient availability in greenhouse experiments. The increased chloride levels were assumed to increase soil pore water P. The first experiment showed higher P in the peat-soils treated with the highest Cl-concentration and an increased leaching of PO4 from the lake-bottom peat-soils. No reaction of the phytometer Epilobium hirsutum was found. In the second experiment the 800 mg l−1 Cl-treatment resulted in significantly higher biomass of Carex acutiformis grown on treated bank soil. N-uptake by the phytometer Carex acutiformis was significantly higher. The available-P and total-P in the bank soil did not show a treatment effect. The two studies showed under similar ‘standardized’ conditions a treatment effect of chloride on the P-availability, resulting in higher PO4 leaching and increased plant nutrient concentrations and biomass. The field study showed higher available-P concentrations in the shore zone than in remote areas. The high chloride levels after restoration impact internal nutrient availability in the Botshol wetlands, on soil loaded with P in recent and historic times.

Hydrological controls on chemical export from an undisturbed old-growth Chilean forest

Stream water chemical concentrations from an undisturbed, temperate, old-growth Chilean forest were analyzed to gain insights into hydrological controls on watershed-scale losses of different nutrients and chemical elements. Our goal was to understand how hydrological and biogeochemical processes interact to control patterns of export of dissolved chemicals from these types of forested watersheds. The study is unique in that the monitored watershed lies in a part of the world that receives very little anthropogenic influence and this is the first study of its type in this part of the world. A small, densely forested, montane watershed was continuously monitored for stream discharge and precipitation and was sampled biweekly for stream-, precipitation-, and soil-water chemistry. Three different forms of hydrological controls were used to describe how concentrations of different elements changed as a function of increased stream water discharge: dilution of elements due to decreased relative contributions by baseflow sources, increased concentration due to enhanced hydrological access to elements in to shallow soil horizons, and no net effect, i.e. hydrologically constant. A hyperbolic relationship was used to describe element concentration as a function of stream discharge to quantify the variations for major ions, cations, and organic forms of nutrients. In general, this hydro-biogeochemical system showed the most distinct dilution and enhanced hydrological access for chemicals with large concentration differences between deep and shallow soil water. Deep soil water sources were most distinct for Ca2+ and Si, both of which demonstrated dilution. In contrast, variations in H+ concentrations resembled enhanced hydrological access indicating shallow soil sources, coupled with the consumption of acidity by weathering or cation exchange in deeper soil layers. Rainwater sometimes enhanced and sometimes tempered the observed trends. Potassium was the least hydrologically variable element (hydrologically constant) in this study, possibly due to strong biological mechanisms internal to the watershed. The approach used here is a simple method to identify the importance of hydrologic variations in controlling streamwater chemistry relative to internal biogeochemical sources and processes.