MARINE ECOLOGY - PROGRESS SERIES Vol. 2: 207-212, 1980 Published April 30 Mar. Ecol. Prog. Ser. 1 Effects of the 'Amoco Cadiz' Oil Spill on the Seagrass Community at Roscoff with Special Reference to the Benthic Infauna R. P. W. M. Jacobs Laboratory of Aquatic Ecology, Catholic University, Toernooiveld, 6525 ED Nijrnegen, The Netherlands ABSTRACT: The benthic fauna of an eelgrass (Zostera marina L.) community has been investigated at Roscoff (France) from October 1977 to April 1979. The impact of the 'Amoco Cadiz' oil spill of March 1978 on the comnlunity was studied. Direct effects on the eelgrass itself were only local during the first weeks after the spill, when many plants had black, 'burnt' leaves. This was, however, a temporary phenomenon, for the production of new leaf tissue continued normally. Effects on the benthic fauna were observed directly after the arrival of the oil at Roscoff. A sharp decrease in numbers of both individuals and species occurred - mainly caused by an almost total disappearance of the smaller Crustacea and Echinodermata, and a serious numerical decrease in other groups. Recovery took place relatively rapidly. In the beginning of 1979 all numbers were at the same level as the year before, the filter feeding Amphipoda being the only exception: on 1 May 1979 they were still absent. INTRODUCTION STUDY AREA In the night of 16 March 1978 the tanker 'Amoco At Roscoff, the eelgrass Zostera marina constitutes Cadiz' went aground on the rocks near Portsall at the the basis of the seagrass community and forms dense northwestern point of the coast of Brittany (France). In beds, which are distributed from mean low water neap the course of the next 15 d the cargo of 216,000 t of level (MLWN) down to a depth of 4 m below mean low crude oil and 4,000 t of bunker fuel were released into water spring level (MLWS). In these meadows a rela- the ocean and caused severe pollution as far as Ile de tionship exists between the ab~Ge-~roundplant bio- Brehat (Bay of St. Brieuc). mass, length of leaves, number of shoots m-* and water After the spill, investigators of a variety of scientific coverage, i.e. the percentage of time each contour is disciplines participated in the study of the physical, covered with water. The below-ground plant parts chemical and biological effects of the pollution and the form a dense mat of interwoven rhizomes and roots, the first reports were published after some months (Conan thickness of which is determined by the age of the bed et al., 1978; Hess, 1978). and the sedimentation rate. For more details concern- The 'Amoco Cadiz' spill brought on radical changes ing distribution, biomass and production of Z. marina, in an ecological study of structural and functional see Jacobs (1979). aspects of seagrass communities by the Laboratory of Apart from the seagrass itself, a number of other Aquatic Ecology in Nijmegen (The Netherlands). This structural elements can be recognized in the commu- investigation started in 1976 at Roscoff with a study of nity (den Hartog, 1979),e.g. (1) epiphytic algae on the the production and biomass of eelgrass Zostera marina seagrass; (2) a mat of loose-lying algae, caught by the L. (Jacobs, 1979),and was followed in October 1977 by seagrass; (3) vagile fauna on the seagrass; (4) sessile an investigation of the con~positionof the fauna of the fauna on the bottom surface; (5) vagile fauna on the seagrass community. Since March 1978 the sampling bottom surface; (6) benthic infauna. program has been continued in order to elucidate the On 20 March 1978 the first oil reached Roscoff and impact of oil on the fauna. Although the collected data remained well visible during the following weeks. have not yet been completely analysed, a first survey of Only thickness and extent of the oil slick varied from the results is presented here, and effects of the spill on day to day. During low-water periods oil covered the the seagrass community are discussed. eelgrass beds but, at high tide, vertical transport pro- O by Inter-Research 208 Mar. Ecol. Prog. Ser. 2: 207-212, 1980 cesses always resulted in a loosening of the direct contact, because the beds are situated below MLWN. During April and May the impact on the eelgrass was distinct: especially at the boundaries of the beds in the higher littoral, some leaves were black and looked burnt, in others transparent parts were visible. Later these leaves were shed; however, the plants were not dead: even production continued normally. For the study of the fauna of the eelgrass community 2 homogeneous beds were chosen: one just below MLWN level and the other approximately 0.5 m lower. The eelgrass bed situated in the higher littoral (just below MLWN) was characterized by a large number of shoots (700-800 m-'), which were, however, short (< 30 cm). There was no mat of loose-lying algae. The rhizome mat was approximately 9 cm thick. This eel- Fig. 1. Characteristics of the fauna of the upper Zostera grass bed fell dry at almost each low water period. In marina bed at Roscoff (just below MLWN level), October 1977 the lower Zostera marina bed there were fewer shoots to April 1979. Total numbers 400 of individuals (N)and (500-600 m-'), but they were longer (up to 50 cm); the species (S); calculated diversity index (H) and evenness (J) given as broken lines. Arrow: moment of oil arrival at Roscoff thickness of the rhizome mat was approximately 6 cm. This bed was situated in an enormous tidal pool, that retained a water level of some centimetres at each low tide. Between the shoots, a mat of loose-lying algae was present; locally this mat was several centimetres thick. The mat was mainly composed of Corallina officinalisL., Cladostephus spongiosus (Huds.) C. Ag. and Sphacelaria species. From October 1977 to April 1979 in both eelgrass beds a bottom sample of 20 X 20cm was taken monthly in order to study the fauna. In the laboratory these samples were sieved (1 mm mesh size) and all individuals were collected and preserved. After identi- fication to species, each sample was characterized by 4 parameters: number of individuals (N),number of spe- cies (S), calculated species diversity (H) and evenness (J) (Pielou, 1969). Since most diversity measurements are affected by sample size, the latter was kept constant in order to be able to compare samples taken in different seasons. However, spatial distribution of the component species affects the diversity patterns. Comparison of 4 samples of 20 X 20 cm, taken simultaneously, showed a ran- dom or aggregated distribution of the individuals over Fig. 2. Characteristics of the fauna of the lower Zostera marina bed at Roscoff (approximately 0.5 m below MLWN the samples for 99 "/U of all species. The number of species per sample was approximately 65 of the total level), October 1977 to April 1979. Total numbers 400 cm-' of individuals (N) and species (S);calculated diversity index (H) number of species present. and evenness (J)given as broken lines. Arrow: moment of 011 arrival at Roscoff RESULTS at MLWN). At this level the bed is exposed to increased sedimentation, which results in the deposi- The fauna in the upper eelgrass bed showed a tion of a sand layer several centimetres thick between decrease in both total numbers of individuals and shoots. Eventually this leads to a thickening of the numbers of species at the end of 1977 (Fig. 1).This has rhizome mat and a raising of the bed. These changes to be regarded as a normal fluctuation of the fauna of are normal stages in the development of the commu- an eelgrass bed at the upper limit of its occurrence (i.e. nity (Blois et al., 1961; den Hartog, 1973). 1977 1978 1979 Group OND JFMAMJ JASOND JFMA Polychaeta Errantia l8(5) 38(5) 25(6) 13(7) 19(6) 21(7) 14(6) 23(4) 16(5) 18(5) 36(6) 21(5) 24(3) 22(3) 11(5) 15(5) 16(4) 14(4) 12(3) Polychaeta Sedentaria 85(10) 113(9) 219(8) 101(9) 114(8) 152(9) 41(4) 66(7) 51(5) 75(7) 95(8) 68(11) 36(6) 72(10) 111(12) 95(11) 61(10) 79(9) 109(9) Decapoda Reptantia 6(2) 5(2) 1(1) 1(1) 3(1) 4(1) 1(1) 3(1) lo(l) 9(2) 3(2) 1(1) 5(1) 6(2) 1(1) 1(1) Amphipoda 6(3) 1(1) 1(1) W21 1(1) 1(1) m Tanaidacea 218(1) 113(2) 22(1) 69(1) 52(1) 62(1) 2(1) l(1) 2(1) 2(1) 8(1) 30(1) 5(1) 11(1) 9(1) 19(1) 2(1) 4(1) 20(1) Gastropoda 3(2) 5(4) 2(2) 3(1) 1(1) 3(2) 2(2) 1(1) 4(3) 1(1) 3(2) 3(2) 6(2) 2(1) Blvalvia 49(2) 56(3) 26(4) 32(2) 36(3) 24(3) 44(2) 66(2) 41(2) 37(3) 56(3) 39(1) 14(2) 32(2) 41(2) 42(1) 47(3) 74(3) 72(2) Echinodermata 12(1) 17(1) 13(1) 1(1) 1977 1978 1979 Group 0 N D JFMAMJJASOND JFMA Polychaeta Errantia 22(5) 58(8) 22(4) 34(7) 19(4) 31(7) 20(4) 16(5) 96(7) 34(3) 29(5) 43(6) 42(3) 50(4) 22(3) 16(5) 37(7) 46(8) 18(7) Polychaetasedentaria 109(6) 206(10) 113(8) 105(10) 147(9) 90(10) 55(6) 31(8) 25(5) 35(6) 30(6) 47(6) 43(6) 104(7) 67(8) 30(9) 63(10) 54(9) 75(8) Decapoda Reptantia 20(6) 2(2) 9(3) 1(1) 6(3) 4(2) 1(1) 21(4) 7(2) 12(1) 9(3) 6(3) 4(2) 11(3) 3(2) 6(4) 1(1) Amphipoda 25(lO) 28(8) l6(9) 16(11)160(21) 120(20) 5(2) 3(3) 8(1) 5(l) 2(1) lO(2) 39(3) 31(6) 37(5) 12(2) Tanaidacea 231(2) 259(2) 121(3) 80(3) 416(3) 282(3) 62(3) 6(1) 12(2) 6(1) 5(1) 42(1) 68(1) 238(1) 214(1) 92(2) 122(2) 177(1) 337(1) Gastropoda 20(8) 53(5) lS(6) 156(5) 141(15) 69(7) 49(9) 17(4) 44(8) 21(6) 16(10) 13(7) l?(?) 7(3) 7(4) 12(2) 196(9) 79(13) 125(10) Bivalvia 24(3) 56(5) 12(5) 32(4) 26(5) 6(5) 13(4) 9(5) 6(4) 2(1) lO(3) 4(4) 3(3) 16(4) 8(4) 3(2) lO(5) 18(4) 18(6) Echinodermata 72(1) 210(1) 190(1) 120(1) 179(2) 134(1) 164(2) 20(1) 12(1) 7(1) 19(1) 37(1) 27(1) 20(1) 43(1) 41(1) 47(1) 82(1) 85(1) 210 Mar.