1National Oceanography Centre, Southampton, SO14 3ZH, UK
2Laboratory of Recent and Fossil Bio-Indicators (UPRES EA 2644 BIAF), 2 Boulevard Lavoisier, 49045 Angers Cedex, France, and LEBIM, Ile d'Yeu, France
3Integrative Oceanography Division, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0218, USA
4NIOO-KNAW, Centre for Estuarine and Marine Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands
5Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands
6National Institute of Oceanography, Dona Paula, Goa 403 004, India
7Louisiana Universities Marine Consortium, Chauvin, Louisiana 70344, USA
8The School of Marine and Atmospheric Sciences (SoMAS), Stony Brook University, Stony Brook, NY 11794-5000, USA
9State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, Putuo District, Shanghai 200062, China
Received: 23 Dec 2008 – Published in Biogeosciences Discuss.: 27 Feb 2009
Abstract. Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical indicators (proxies) derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Those based on (1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), (2) sedimentary features (e.g. laminations) and (3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on (4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), (5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and (6) organic C, N and their stable isotope ratios reflect conditions in the water column. However, the interpretation of these indicators is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment caused by eutrophication, making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxia-specific proxies, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, together with multi-proxy approaches, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of human-induced hypoxia, and associated eutrophication, in vulnerable coastal environments.
Revised: 21 Jun 2009 – Accepted: 07 Jul 2009 – Published: 21 Aug 2009
Gooday, A. J., Jorissen, F., Levin, L. A., Middelburg, J. J., Naqvi, S. W. A., Rabalais, N. N., Scranton, M., and Zhang, J.: Historical records of coastal eutrophication-induced hypoxia, Biogeosciences, 6, 1707-1745, doi:10.5194/bg-6-1707-2009, 2009.