Dynamics of oxygen depletion in the nearshore of a coastal embayment of the southern Benguela upwelling system (original) (raw)
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In January 2008 there was an intensive and extensive upwelling event in the southern Hum-boldt Current System. This event produced an intrusion of water with low dissolved oxygen into Coliumo Bay, which caused massive mortality and the beaching of pelagic and benthic organisms, including zooplankton. During this event, which lasted 3 to 5 days, we studied and evaluated the effect of the hypoxic water in the bay on the abundance of macrozoo-plankton, nanoplankton and microphytoplankton, the concentration of several nutrients and hydrographic conditions. At the beginning of the hypoxia event the water column had very low dissolved oxygen concentrations (<0.5 mL O 2 L-1), low temperatures and high salinity which are characteristics of the oxygen minimum zone from the Humboldt Current System. Redox, pH, nitrate, phosphate, silicate and chlorophyll-a values were the lowest, while nitrate and the phaeopigment values were the highest. The N:P ratio was below 16, and the abundance of nano-and microphytoplankton were at their lowest, the latter also with the lowest proportion of live organisms. Macrozooplankton had the greatest abundance during hypoxia, dominated mainly by crustacean, fish eggs and amphipods. The hypoxia event generated a strong short-term alteration of all biotic and abiotic components of the pelagic system in Coliumo Bay and the neighboring coastal zone. These negative effects associated with strong natural hypoxia events could have important consequences for the productivity and ecosystem functioning of the coastal zone of the Humboldt Current System if, as suggested by several models, winds favorable to upwelling should increase due to climate change. The effects of natural hypoxia in this coastal zone can be dramatic especially for pelagic and benthic species not adapted to endure conditions of low dissolved oxygen.
Potential and timescales for oxygen depletion in coastal upwelling systems: A box‐model analysis
Journal of Geophysical Research: Oceans, 2016
A simple box model is used to examine oxygen depletion in an idealized ocean-margin upwelling system. Near-bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom. Upwelling-supplied nutrients are consumed in the surface box, and some surface particles sink to the bottom where they respire, consuming oxygen. Steady states characterize the potential for hypoxic near-bottom oxygen depletion; this potential is greatest for faster sinking rates, and largely independent of production timescales except in that faster production allows faster sinking. Timescales for oxygen depletion depend on upwelling and productivity differently, however, as oxygen depletion can only be reached in meaningfully short times when productivity is rapid. Hypoxia thus requires fast production, to capture upwelled nutrients, and fast sinking, to deliver the respiration potential to model bottom waters. Combining timescales allows generalizations about tendencies toward hypoxia. If timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing, the steady state will generally be hypoxic, and results indicate optimal timescales and conditions exist to generate hypoxia. For example, the timescale for approach to hypoxia lengthens with stronger upwelling, since surface particle and nutrient are shunted off-shelf, in turn reducing subsurface respiration and oxygen depletion. This suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia, even as the oxygen levels in upwelled source waters decline.
Harmful Algae, 2011
Oxygen deficiency in the southern Benguela has a pronounced negative impact on living marine resources and within the greater St Helena Bay anoxia is the cause of large episodic mortalities of the rock lobster Jasus lalandii. These impacts have motivated further investigation, specifically of the role of high biomass dinoflagellate blooms, commonly known as red tides, in the development of anoxic conditions. A high resolution time series of dissolved oxygen concentrations obtained from a bottom mooring off Elands Bay, located within the greater St Helena Bay region, is examined in relation to the development of an exceptional bloom of the dinoflagellate Ceratium balechii and an anoxia-induced mass mortality. A clear seasonal trend is evident in bottom dissolved oxygen concentrations, initiated in spring by upwelling events that advect low oxygen waters across the shelf. Increased deposition of organic carbon derived from primary production maxima in summer and autumn, together with the development of an increasingly stratified environment exacerbate dissolved oxygen deficits leading to a progressive decline in dissolved oxygen concentrations in the cold bottom layer. Within this seasonal timeframe episodic anoxia may occur throughout the water column of shallow inshore regions following the decay of red tides accumulated within these environments under conditions of persistent downwelling. Anoxia within these shallow non-stratified nearshore regions is dependent on exceptional organic loading of the water column as afforded by the decay of red tide and to the absence of wind-induced mixing or wave action. These requirements contribute to the local and transient character of these events of anoxia. With the onset of winter strong mixing results in reduced primary production and increased ventilation of bottom waters causing an increase in dissolved oxygen concentrations.
Journal of Marine Systems, 2014
Long-term (1992-2010) water quality monitoring records reveal that the dissolved oxygen (DO) concentration in Mass Bay exhibits a well-defined seasonal cycle, highest in March-April and lowest in October. This pattern persists in all years with insignificant interannual variability. A multi-domain-nested coupled physicalbiogeochemical model was developed and applied to simulate the DO field over the 16-year period 1995-2010. The model-computed DO and nitrogen concentrations were in good agreement with observations. An EOF analysis of the modeled DO field indicates that DO in Mass Bay features both well-defined seasonal and spatial modes. The magnitude and phase of the DO seasonal cycle vary more significantly in the southern bay than in the northern bay. Horizontal advection, which is connected to the western Gulf of Maine coastal currents, plays a dominant role in the DO variability in the northern bay. The southern bay features a well-defined local retention mechanism with a longer residence time. In this region, the DO variation is controlled predominantly by local biogeochemical processes. Since the photosynthetic minus respiration production of DO is always balanced to a large degree by the oxidation of organic matters, reaeration becomes a major driver for the seasonal cycle of DO.