The impact of the 1989 Exxon Valdez oil spill on phytoplankton as seen through the dinoflagellate cyst record




Genest, Maximilien

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Our knowledge of how oil spills affect coastal environments is severely limited by the shortage of research that addresses the impact of these events on phytoplankton, the single most important group of organisms in the marine ecosystem. This scarcity of knowledge is mainly attributed to the absence of baseline data, preventing the comparison of pre- and post-spill populations. This unique study aims to identify how dinoflagellates and diatoms, the two major groups of phytoplankton in coastal marine environments, have been affected by the 1989 Exxon Valdez oil spill in Prince William Sound (PWS), Alaska. To do this, sedimentary records of dinoflagellate cysts, produced during a dinoflagellate's life cycle and preserved in the sediment, and biogenic silica, a proxy for diatom abundance, were analyzed prior to, during and after the oil spill. The analysis of two well-dated cores in PWS reveals marked increases during the oil spill in the concentrations of total cysts of the species Operculodinium centrocarpum sensu Wall and Dale, (1966) and Dubridinium spp. Total cyst concentrations doubled in core P-10 from 362 to 749 per g, while in core P-12 the increase was from 1175 to 1771 cysts g-1. During this peak in cyst concentrations, total concentrations were 3 and 2 standard deviations greater than the mean in cores P-10 and P-12, respectively. Dubridinium spp. showed a five and sevenfold increase in concentrations in cores P-10 (4 to 20 cysts g-1) and P-12 (16 to 110 cysts g-1), respectively, while O. centrocarpum sensu Wall and Dale, (1966) doubled in concentrations in the two cores (P-10: 117 to 276 cysts g-1; P-12: 268 to 495 cysts g-1). Biogenic silica values did not show significant changes throughout the cores, with values varying between 8% and 9% in core P-10 and 9.0% to 10.9% in core P-12. These changes lie within or very close to the standard deviation of the analyzed standards, suggesting that much of the changes could be analytical noise. The dinoflagellate cyst signals seen in this study are comparable to those seen as a result of nutrient enrichment in estuarine systems, suggesting that the 1989 Exxon Valdez oil spill and its remediation had a stimulatory effect on some taxa of cyst- producing dinoflagellates. This impact appears to be short-lived, with cyst concentrations returning to pre-spill levels within two years of the event. The lack of change in diatom abundance, on the other hand, suggest that diatom abundance remained relatively constant during the entirety of the sample period.



Dinoflagellate cysts, Biogenic Silica, Sediments, Exxon Valdez, Oil spill, phytoplankton