Natural and human-induced hypoxia and consequences for coastal areas: synthesis and future development

Abstract

Abstract. 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 their physical and geographic settings. It is understood that coastal hypoxia has a profound impact on the sustainability of ecosystems, which can be seen, for example, by the change in the food-web structure and system function; other Published by Copernicus Publications on behalf of the European Geosciences Union. 1444 J. Zhang et al.: Natural and human-induced hypoxia and consequences for coastal areas influences include compression and loss of habitat, as well as changes in organism life cycles and reproduction. In most cases, the ecosystem responds to the low dissolved oxygen in non-linear ways with pronounced feedbacks to other compartments of the Earth System, including those that affect human society. Our knowledge and previous experiences illustrate that there is a need to develop new observational tools and models to support integrated research of biogeochemical dynamics and ecosystem behavior that will improve confidence in remediation management strategies for coastal hypoxia.

Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

“…If bottom-water oxygen drops significantly, ecosystems undergo successive deterioration, eventually turning into permanently anoxic environments where microorganisms replace all higher life. This collapse of aquatic communities leads to a dramatic decline in ecosystem functioning and services such as biodiversity, fisheries, aquaculture and tourism Zhang et al, 2010). Remediation measures in coastal ecosystems, however, do not always lead to ecosystem recovery; ecosystem responses tend rather to follow nonlinear trends that may include hysteresis and time lags (Kemp et al, 2009), as observed, for example, in the northwestern Black Sea (Mee et al, 2005, BSC, 2008.…”

“…Observations with an improved spatio-temporal coverage also facilitate the identification of hypoxia drivers and help constrain numerical models. The need to develop new observational tools and models to support integrated research of biogeochemical dynamics and ecosystem behavior that will improve confidence in remediation management strategies for coastal hypoxia has also been highlighted by SCOR working group 128 on natural and human-induced hypoxia and its consequences for coastal areas (Zhang et al, 2010).…”

“…When oxygen concentrations decrease, pelagic organisms may escape vertically, develop migration strategies to avoid hypoxic areas, or develop adaptation mechanisms . Benthic invertebrates may initially show aberrant behavior before they finally undergo mass mortality (Zhang et al, 2010). All these responses lead to changes in community structure and functioning, and typically to a reduction in species diversity accompanied by the dominance of a few oxygentolerant species (Levin, 2003).…”

“…The earliest systematic records of coastal hypoxia from Europe and North America date back to the 1910s (Zhang et al, 2010;Petersen, 1915;Sale and Skinner, 1917).…”

“…Rapidly changing oxygen concentrations and the occurrence of hypoxia impose stress on aquatic organisms, and the physiological responses and survival strategies of the biota are very complex (Vaquer-Sunyer and Duarte, 2008;Zhang et al, 2010;Levin, 2003;Ekau et al, 2010). When oxygen concentrations decrease, pelagic organisms may escape vertically, develop migration strategies to avoid hypoxic areas, or develop adaptation mechanisms .…”