Dynamics of oxygen depletion in the nearshore of a coastal embayment of the southern Benguela upwelling system

TitleDynamics of oxygen depletion in the nearshore of a coastal embayment of the southern Benguela upwelling system
Publication TypeJournal Article
Year of Publication2014
AuthorsPitcher G.C, Probyn T.A, A. Randt du, Lucas A.J, Bernard S., Evers-King H., Lamont T., Hutchings L.
JournalJournal of Geophysical Research-Oceans
Volume119
Pagination2183-2200
Date Published2014/04
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000336213200004
Keywordsafrica; anchor station; california current; climate-change; ecosystem; episodic anoxia; hypoxia; jasus-lalandii; oceanography; physical; phytoplankton; Red tide; seasonal hypoxia; short-term variability; southern Benguela
Abstract

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(-1)) associated with a deep pool of oxygen-depleted water and episodic anoxia (<0.02 ml l(-1)) 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 approximate to 1.2 ml l(-1) 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(-1)), 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. Key Points Phenology of coastal upwelling influences bay productivity and red tides Bay is subject to seasonally recurrent hypoxia and episodic anoxia No change in deep pool of seasonally hypoxic water over past 50 years

DOI10.1002/2013jc009443
Short TitleJ Geophys Res-Oceans
Integrated Research Themes: 
Student Publication: 
No