Historical records of coastal eutrophication-induced hypoxia (original) (raw)
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Coastal hypoxia and sediment biogeochemistry
2009
The intensity, duration and frequency of coastal hypoxia (oxygen concentration <63 µM) are increasing due to human alteration of coastal ecosystems and changes in oceanographic conditions due to global warming. Here we provide a concise review of the consequences of coastal hypoxia for sediment biogeochemistry. Changes in bottomwater oxygen levels have consequences for early diagenetic pathways (more anaerobic at expense of aerobic pathways), the efficiency of re-oxidation of reduced metabolites and the nature, direction and magnitude of sediment-water exchange fluxes. Hypoxia may also lead to more organic matter accumulation and burial and the organic matter eventually buried is also of higher quality, i.e. less degraded. Bottom-water oxygen levels also affect the organisms involved in organic matter processing with the contribution of metazoans decreasing as oxygen levels drop. Hypoxia has a significant effect on benthic animals with the consequences that ecosystem functions related to macrofauna such as bio-irrigation and bioturbation are significantly affected by hypoxia as well. Since many microbes and microbial-mediated biogeochemical processes depend on animal-induced transport processes (e.g. re-oxidation of particulate reduced sulphur and denitrification), there are indirect hypoxia effects on biogeochemistry via the benthos. Severe long-lasting hypoxia and anoxia may result in the accumulation of reduced compounds in sediments and elimination of macrobenthic communities with the consequences that biogeochemical properties during trajectories of decreasing and increasing oxygen may be different (hysteresis) with consequences for coastal ecosystem dynamics.
Biogeochemical and environmental drivers of coastal hypoxia
Journal of Marine Systems, 2014
Recent reports have demonstrated that hypoxia is widespread in the coastal zone of the Baltic Sea. Here we evaluate the long-term trends of dissolved oxygen in bottom waters and of the drivers of coastal hypoxia. Eleven of the 33 sites evaluated had increasing trends of bottom water dissolved oxygen, but only the Stockholm Archipelago presents a consistent positive increasing trend in time. The vast majority of sites continue to worsen, especially along the Danish and Finnish coasts, in spite of remediation efforts to reduce nutrients. Surface temperatures were relatively comparable across the entire coastal Baltic Sea, whereas bottom water temperatures varied more strongly among sites, most likely due to differences in mixing (or stratification) and water exchange with the open Baltic Sea. Nutrient concentrations varied by factors 2-3 with highest levels at sites with restricted water exchange and higher land based nutrient loading. None of the sites were permanently stratified during the summer seasonal window although most of the sites were stratified more than half of the time. The frequency of hypoxia was also quite variable with sites in Gulf of Bothnia almost never experiencing hypoxia to enclosed sites with more than 50% chance of hypoxia. There are many factors governing hypoxia and the complexity of interacting processes in the coastal zone makes it difficult to identify specific causes. Our results demonstrate that managing nutrients can create positive feedbacks for oxygen recovery to occur. In the absence of nutrient reductions, the recovery from hypoxia in coastal marine ecosystems is unlikely.
2009
Hypoxia has become a world-wide phenomenon in the global coastal ocean and causes a deterioration of the structure and function of ecosystems. Based on the collective contributions of members of SCOR Working Group #128, the present study provides an overview of the major aspects of coastal hypoxia in different biogeochemical provinces, including estuaries, coastal waters, upwelling areas, fjords and semi-enclosed basins, with various external forcings, ecosysCorrespondence to: J. Zhang (jzhang@sklec.ecnu.edu.cn) tem responses, feedbacks and potential impact on the sustainability of the fishery and economics. The obvious external forcings include freshwater runoff and other factors contributing to stratification, organic matter and nutrient loadings, as well as exchange between coastal and open ocean water masses. Their different interactions set up mechanisms that drive the system towards hypoxia. Coastal systems also vary in their relative susceptibility to hypoxia depending on the...
Eutrophication-Driven Deoxygenation in the Coastal Ocean
Oceanography, 2014
ABSTR ACT. Human activities, especially increased nutrient loads that set in motion a cascading chain of events related to eutrophication, accelerate development of hypoxia (lower oxygen concentration) in many areas of the world's coastal ocean. Climate changes and extreme weather events may modify hypoxia. Organismal and fisheries effects are at the heart of the coastal hypoxia issue, but more subtle regime shifts and trophic interactions are also cause for concern. The chemical milieu associated with declining dissolved oxygen concentrations affects the biogeochemical cycling of oxygen, carbon, nitrogen, phosphorus, silica, trace metals, and sulfide as observed in water column processes, shifts in sediment biogeochemistry, and increases in carbon, nitrogen, and sulfur, as well as shifts in their stable isotopes, in recently accumulated sediments.
Biogeosciences
The anthropogenically forced expansion of coastal the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.
Sediment Response to 25 Years of Persistent Hypoxia
Aquatic Geochemistry, 2012
We investigated the impact of persistent hypoxia on sediment chemistry by comparing total, reactive (extractible with 1 M hydroxylamine-hydrochloride in 25 % acetic acid), and dissolved forms of the redox-sensitive elements Mn, Fe, and As in cores recovered between 1982 and 2007 at two sites in the Lower St. Lawrence Estuary (LSLE) where the bottom water has been severely hypoxic since the early 1980s. The data reveal that the concentrations and the vertical distributions of total solid-phase and dissolved Mn as well as total solid-phase Fe and As were not significantly affected by persistent hypoxia. In contrast, the composition of solid-phase Fe and As changed significantly as did the porewater concentrations of both these elements. The relative amounts of solid-phase reactive Fe and As increased, and the abundance of pyrite and pyritic-As decreased in the sediment layer that accumulated since 1982. We propose that persistent hypoxic conditions restrict the supply of oxygen to the sediment and increase the relative contribution of alternate electron acceptors-Mn(IV), Fe(III), and sulfate-to microbial oxidation of organic matter. In marine iron-rich environments, such as the LSLE sediment, increased sulfate reduction may promote conversion of less reactive Fe phases to more reactive Fe phases which, in turn, interfere with pyrite formation. Consequently, a chalcophile element such as As, which would normally be sequestered with authigenic pyrite, remains available for recycling across the oxic-anoxic boundary in the sediment.
Paleoceanography, 2006
Sedimentary molybdenum, [Mo] s , has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic-rich marine facies deposited under oxygen-depleted conditions. A detailed analysis of [Mo] s-total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo] s , while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo] s-TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal. These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction. In the silled-basin environments, increasing restriction is correlated with a systematic decrease in [Mo] s /TOC ratios, from 45±5forSaanichInletto45 ± 5 for Saanich Inlet to 45±5forSaanichInletto4.5 ± 1 for the Black Sea. This variation reflects control of [Mo] s by [Mo] aq , which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the ''basin reservoir effect''). The temporal dynamics of this process are revealed by high-resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long-term trends toward lower [Mo] s /TOC ratios following development of water column stratification and deepwater anoxia. Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo] aq availability and enhanced Mo transport to the sediment-water interface via Fe-Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord. These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (i.e., more sulfidic conditions). These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (e.g., silled basins) and are not valid for low-oxygen facies in open marine settings such as continent-margin upwelling systems.
Persistent eutrophication and hypoxia in the coastal ocean
Cambridge Prisms: Coastal Futures
Impact Statements Nutrient loading (notably nitrogen and phosphorus) to coastal oceans from food production, fossil fuel burning, aquaculture operations, and wastewater from humans, livestock, and industry has accelerated during the past decades, causing over-enrichment of nutrients, or eutrophication. Eutrophication degrades coastal water quality with two most common symptoms, hypoxia and harmful algal blooms, creating profound ecological and societal consequences such as biodiversity decline, seagrass loss, coral bleaching, fish kills and marine mammal mortalities, and human health threats. Such marine pollution symptoms have persisted although billions of dollars have been invested in both research and management as well as efforts of restorations in many developed countries. Consequently, we are still witnessing trends in the expansion of coastal eutrophication and hypoxia from developed regions into developing regions. Though the limited efficacy of mitigation witnessed so far suggests the complexity of the issue, we contend that closing the knowledge gaps in the causality between eutrophication and hypoxia is essential and crucial towards making science-and evidence-based policies. We recognize that the non-linear cause-effect relationship in coastal marine ecosystem degradation caused by multi-stressors is complex. The strength and synergistic effect of multiple driving forces of coastal eutrophication is dependent on regional geographic feature, economic development, and societal management, while the long-term trends of eutrophication and hypoxia are subject to the control of the trends in nutrient loadings and physical dynamics under a changing climate. This review also examines lessons from past eutrophication management practices, and advocates for a better, more efficient indexing system of coastal eutrophication and an advanced regional earth system modeling framework to facilitate the development and evaluation of effective policy and restoration actions.