VERTIMAR-2005 Symposium on Marine Accidental Oil Spills The effect of the Prestige oil spill on the phytoplankton in the N-NW Spanish coast during spring Manuel VARELA*1, Antonio BODE1, M Teresa ALVAREZ-OSSORIO1, Ricardo ANADÓN2, Marcos LLOPE2 and Luis VALDÉS3 1 Instituto Español de Oceanografía. Muelle de Animas s/n. A Coruña. Spain 2 Laboratorio de Ecología. Universidad de Oviedo. Spain 3 Instituto Español de Oceanografía. Paseo del Arbeyal s/n. Gijón. Spain ABSTRACT Changes phytoplankton variables were studied during spring 2003, after the Prestige shipwreck, in the N-NW Spanish coast using historical data available for the area. Some minor changes were occasionally observed, but they did not show any clear pattern and were related to natural variability of the ecosystem. The phytoplankton community structure did not change. The higher abundances of a few species were related to the high temperatures measured during 2003 that resulted in an advance of summer-like conditions to spring. The lack of effects is discussed in relation to the mechanisms removing fuel from surface waters to the sea floor. Zooplankton and diatoms play a key role in removing both oil particles and emulsions form surface waters. Presently, it is not possible to determine even some minor effects on plankton because the large variability of phytoplanktonic cycles. 1. INTRODUCTION Traditionally, there have been two main approaches to study the effects of oil on plankton: controlled laboratory conditions, and studies undertaken in natural conditions. The first approaches yield a good idea on the potential toxicity on planktonic species but the conclusions are often difficult to extrapolate to the natural environment. A more realistic approach is the in situ study. However, this approach is often difficult due to the appropriate temporal and spatial coverage, type of contaminants or characteristics of ecosystems. The response of plankton to fuel is species specific and dependent of the environmental conditions. Besides, trophic relationships among organisms can obscure the effects of contaminants (Johansson et al., 1980). The natural variability is the key factor to determine the effects of oil spills on plankton. The variability of phytoplankton is especially marked in the N-NW Iberian Peninsula (Bode et al., 2002) limiting our ability to differentiate anthropogenic from natural effects. A good knowledge of this variability is essential for a correct evaluation of oil impact. In 1989 the Spanish Institute of Oceanography (IEO) implemented the research core project Studies on Time Series of Oceanographic Data. (Valdés et al., 2002) The data set obtained during the last years, allowed for a quantification of the effects caused by the Prestige oil spill as relevant information on the ecosystem variability is available. Additional information from opportunistic cruises (Pelacus, core surveys from the IEO) and satellite data has been used likewise. The study has covered the spring bloom the first important production period occurring at annual scale (Varela et al., 2001) after the accident. This period coincides with the spawning of many species of high commercial interest, so that any catastrophic impact on the first links of the pelagic food web would result in failed or reduced recruitment. 2. RESULTS AND DISCUSSION No detectable effects on phytoplankton were observed during spring 2003, after the accident. The changes in chlorophyll were not noticeable as compared to previous years and fitted to range variation of previous data. ANOVA showed no statistically significant differences for both areas of study (Galicia and Asturias). Phytoplankton community structure showed minor changes with only four species showing higher abundances after the spill. However, these differences seem to be related to natural ecosystem variability as most of species of the spring bloom do not change after the accident. Large scale study in the whole NW-N Spanish coast carried out during Pelacus cruises in spring from 2000 to 2003 showed not significant differences in chlorophyll concentration for different areas of Galicia and Bay of Biscay after the accident as compared to previous years. VERTIMAR-2005 Symposium on Marine Accidental Oil Spills A Coruña shelf station chlorophyll 300 250 250 mg Chl a m-2 100 1993 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 0 0 2002 50 2001 2000 1999 1998 50 1997 150 1996 100 1995 150 200 2003 200 1994 mg Chl a m -2 Cudillero shelf station chlorophyll 300 Figure 1: Chlorophyll concentrations during spring before (white boxes) and after spill (2003, grey) in Galicia and Asturias. Dominant phytoplankton species were also similar. Sea-WiFS derived primary production for the area in front of Rias Bajas from Galicia showed also not significant differences for the period 1998-2003 Satellite Primary Production Integrated Chl a Pelacus Cruises 180 350 Bf: 00-02 160 She lf 250 120 100 80 gC m-2 y-1 -2 140 mg Chl a m Oce an 300 Af: 03 60 40 200 150 100 50 20 0 0 SG Bf SG Af NG Bf NG Af WB Bf WB Af EB Bf EB Af 1998-2002 2003 Figure 2: Chlorophyll for Pelacus Cruises before (Bf) and after the Prestige spill (Af) in the N-NW Spanish coast. Derived satellite primary production for the area of Rias Bajas. SG: south Galicia, NG: north Galicia, WB: Western Bay of Biscay, EB: Easter Bay of Biscay. The lack or reduced effects are related to physical, chemical and biological factors. The high dynamic of water masses favours the dispersion of oil and the transport of phytoplankton populations from unaffected areas. The crude of Prestige had a low solubility and a low concentration of aromatic hydrocarbons, the most toxic for plankton. On the other hand, bacteria can degrade up to 60-80% of crude oil. Zooplankton is able to feed oil particles (Johansson et al 1980), which are incorporated to faecal pellets and eventually sediment to sea floor. In addition, vertical transport of oil emulsions can be conducted after adsorption to detritus and living diatoms (Ohwada et al 2003), both abundant in the studied area. Therefore plankton acts as biological pump removing from pelagic system, both, particulate and dissolved fuel. The combination of these described effects favours the dispersion, degradation and sinking of fuel reducing the effects and the fast recovery of possible losses in the pelagic system. REFERENCES Bode A., Varela M., Casas, B. and Gonzalez, N. (2002) Intrusions of eastern North Atlantic Central Waters and phytoplankton in the northwestern Iberian shelf during spring. 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