Constrained enrichment contributes to hypoxia formation in Long Bay, South Carolina (USA), an open water urbanized coastline (original) (raw)
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Temporal responses of coastal hypoxia to nutrient loading and physical controls
Biogeosciences, 2009
The incidence and intensity of hypoxic waters in coastal aquatic ecosystems has been expanding in recent decades coincident with eutrophication of the coastal zone. Worldwide, there is strong interest in reducing the size and duration of hypoxia in coastal waters, because hypoxia causes negative effects for many organisms and ecosystem processes. Although strategies to reduce hypoxia by decreasing nutrient loading are predicated on the assumption that this action would reverse eutrophication, recent analyses of historical data from European and North American coastal systems suggest little evidence for simple linear response trajectories. We review published parallel time-series data on hypoxia and loading rates for inorganic nutrients and labile organic matter to analyze trajectories of oxygen (O 2) response to nutrient loading. We also assess existing knowledge of physical and ecological factors regulating O 2 in coastal marine waters to facilitate analysis of hypoxia responses to reductions in nutrient (and/or organic matter) inputs. Of the 24 systems identified where concurrent time series of loading and O 2 were available, half displayed relatively clear and direct recoveries following remediation. We explored in detail 5 well-studied systems that have exhibited complex, non-linear responses to variations in loading, including apparent "regime shifts". A summary of these analyses suggests that O 2 conditions improved rapidly and linearly in systems where remediation focused on organic inputs from sewage treatment plants, which were the primary drivers of hypoxia. In larger more open systems where diffuse nutrient loads are more important in fueling O 2 depletion and where
Journal of Geophysical Research: Oceans, 2014
Acquisition of high resolution time series of water column and bottom dissolved oxygen (DO) concentrations inform the dynamics of oxygen depletion in St Helena Bay in the southern Benguela upwelling system at several scales of variability. The bay is characterized by seasonally recurrent hypoxia (<1.42 ml l 21 ) associated with a deep pool of oxygen-depleted water and episodic anoxia (<0.02 ml l 21 ) driven by the nearshore (<20 m isobath) decay of red tide. Coastal wind forcing influences DO concentrations in the nearshore through its influence on bay productivity and the development of red tides; through shoreward advection of the bottom pool of oxygen-depleted water as determined by the upwelling-downwelling cycle; and through its control of water column stratification and mixing. A seasonal decline in bottom DO concentrations of 1.2 ml l 21 occurs with a concurrent expansion of the bottom pool of oxygen depleted water in St Helena Bay. Upwelling of this water into the nearshore causes severe drops in DO concentration (<0.2 ml l 21 ), particularly during end-of-season upwelling, resulting in a significant narrowing of the habitable zone. Episodic anoxia through the entire water column is caused by localized degradation of red tides within the confines of the shallow nearshore environment. Oxygenation of the nearshore is achieved by ventilation of the water column particularly with the onset of winter mixing. No notable changes are evident in comparing recent measures of bottom DO concentrations in St Helena Bay to data collected in the late 1950s and early 1960s.
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.
Impact of global change on coastal hypoxia
Biogeosciences Discussions, 2012
Coastal hypoxia, the depletion of oxygen concentration in coastal waters, is becoming more prominent on a global scale. Changes in climate and nutrient loadings are two aspects of global change that are expected to profoundly impact coastal hypoxia. We investigated the role of these drivers on the evolution of hypoxia in a stratified, temperate coastal system using a one-dimensional model. The model couples three submodels, describing the physical characteristics, the pelagic ecosystem and benthic diagenesis. The model is calibrated for the Central North Sea but the model approach is generic, and can be applied in stratified coastal ecosystems. Our results indicate that the projected changes in climatological conditions for the North Sea over the next 100 yr will increase the risk of hypoxia. On average the oxygen concentration is predicted to decrease by 17 µM, mostly due to a reduced solubility at higher water temperature (responsible for 65 % of the decrease). Increased stratification (22 %) and enhanced biological rates due to higher water temperature (13 %) also affect the future oxygen concentration. Nutrient loadings also have a strong effect on the occurrence of hypoxia. Decreasing nutrient concentrations strongly decreases the probability of hypoxic events, stressing the importance of continued extensive eutrophication management to mitigate the effect of increased temperature.
Biogeosciences, 2010
Hypoxia has become a worldwide 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, ecosys
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...