j, , EFPECTS OF THE AMOCO CADIZ OIL SPILL ON THE SEAGRASS COMMUNITY AT ROSCOFF (FRANCE) WITR SPECIAL REFERENCE TO THE BENTHIC INFAUNA. R.P.W.M. Jacobs Laboratory of Aquatic Eeology, Catholie University, Toernooiveld, 6525 ED N~egen, The Netherlands. ABSTRACT The benthic fauna of an eelgrass (Zostera marina L.) community has been invest1gated at Roscoff from October1977 to April 1979. The impact of the Amooo Cadiz oi1 spill of March 1978 on thc community could be studied. Direct effects on the eelgrasB itself were only local1y observed during the first weeks after the spill, when many p1ants had black, 'burnt', leaves. This was, however, a temporary phenomenon, for the production of new 1eaf tissue continued norma1ly. Effects on the benthic fauna were observed direct1y after the arrival of the oil at Roscoff. A sharpdecrease of the numbers • of both individua1s and species occurred, mainly caused by the almost total disappearance of the sma11er Crustacea and the . Echinodermata and the serious decrease in numbers in other groups. The recovery took place re1ative1y rapid. In the beginning of . 1979 tbe numbers were at the same level as the year before, the filter feedfng Amphipoda being the on1y exoeption: at 1 May 1979 they were still absent•. J .,...-------- ,. Jacobs - 2 INTRODUCTION In thc night of 16 March 1918 the tanker Amoco Cadiz went aground on the rocks near Portsall at the northwestern point of the coast of Brittany (France). In the course of the next 15 days the cargo of 216,000 tons öf crude oil and 4.000 tons of bunker fuel were released into the ocean and caused a severe pollution as far as Ile de Brehat (Bay of st. Brieuc). After the spill investigators of a variety of scientific disciplines participated in the study of the physical, chemical • and biological effects of the pollution and the first reports were published after some months (see Conan et al•• 1918; Hess. 1918). - The Amoco Cadiz spill brought on radical changes in an ecologicalstudy of structural and functional aspects of seagrass communities by the Laboratory of Aquatic Ecology in N~egen (The Netherlands). This investigation started in 1916 at Roscoff with a study of the production and biomass of eelgrass (Zostera marina L.) (see Jacobs, 1919), followed in October 1911 by an investigation of the composition of the fauna of the seagrass community. After March 1978 the sampling program has been continued in order to elucidate the impact of oil on the fauna. Although the collected data have not;yet been completely analysed. a first survey of the results c8.ii·;-·: be presented.here 'and effects of the spill on the seagrass community will be discussed. ... Jacobs-- :3 THE STUDY AREA At Roscoff the eelgrass, Zostera marina, 1s the frame of thc seagrass community and forms dense beds, which are distributed from meanlow water neap level (MLWN) down to a depth of 4 m below ~ean low water spring level (MLWS). In these meadows a relation exists bctween the aboveground plant biomass, length of leaves, number of shoots m-2 and water coverage, i.e. the percentage of time each contour is covered with water. The belowground plant parts form a dense mat of interwoven rhizomes • and roots, the thiokness of wh1ch is determined by the age of the bed and the sedimentation rate. For more det~ils concerning the distribution and aspects of production and biomass of !. marina can be referred to Jacobs (1979). Apart from the seagrass itself, a number of other structural elements can be reeognized in the community (see den Hartog, 19r9), e.s. 1~ .epiphytic algae on the seagrass 2. a mat of loose-lying algae, caught by the eeagrass 3. vagile fauna on the seagrass 4. sessile fauna on the bottom surface 5. vagile fauna on the bottom surface 6. benthic infauna On 20 March 1978 the first oil reached Roseoff and remained weIl visible during the following weeks. Only the thickness and the extent of the oil slick varied from day to day. During the low water periode oil oovered the eelgrass beds, but at high tide vertieal transport proeesses always resulted in a loosening of the direct contact, because the beds are situated below MLWN. During the months of April and May the impact on the eelgrass Jacobs - 4 , ' I wasdistinct:' especially at the boundaries of the beds in the higher littoral leaves were black and looked aa if they were burnt, whereas in other leaves transparent parts r.ere visible. Later on these leaves were shed. Notwithstanding this these plants were apparently not dead: even the produotion continued'normally. Por the study of the fauna of the eelgrass community two homogeneous beds were chosen: one just below MLWN level and the other one approximately 0.5 m lower. The eelgrass bed situated in the higher littoral' (just below MLWN) was charaoterized by • relatively many shoots (700 - 800 m-2), whioh were, however, short (no longer than 30 cm). There was no mat of loose-Iying algae present. The rhizome mat was approximately 9 cm thick. ' This eelgrass bad fell dry at each low water periode In the lower situated Zostera bed thera were less shoots ' (500 - 600 m-2), but they were longer (up to 50 cm). the thiokness of the rhizome mat was approximately 6 cm. This bed was situated in an enormous tidal pool, that retained some centimetres of water at each low tide period. Between the shoots a mat of loose-lying algae was present; locally this mat was several oentimetres thick. The frame of the mat was composed of Corallina otficinalis L., Cladostephus spongioßus (Huds.) C. Ag. and Sphncelnria species. From October 1977 to April 1979 in both eelgrass beds a bottom sampIe of 20 x 200m was taken monthly in order to study the fauna. In tne laboratory these sampIes were sieved (1 mm' mesh size) end all individuals were collected and preserved. After idontification to species level,.each samplewas oharacterized by four parameters: number of individuals (N), Jacobs - 5 number of species (S) and the 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 concerning species affects the diversity patterns. Comparison of 4 samples of 20 x 20 cm, taken simultaneously, showed for 99% of all species a random or aggregated distribution of the individuals over the samples. The number of species per sample was approximately 65% of the total number of species pr~sent• • Jaoobs - 6 RESULTS The fauna in the higher situated eelgrass bed showed a decrease of both total numbers of individuals and numbers of species at the end of 1977 (Fig. 1). This has to be regarded as anormal fluctuation of .the.fauna of an eelgrass bcd at the upper limit of its occurrence.(i.e. MLW!{). Atthis level the bed is exposed to increased sedimentation, which results in th~ deposition of a sand layer several centimetres thick between the shoots. Eventually this leads to a thickening of the rhizome • mat and a raise of the bed. These changes are normal s~ages in the development of the community (Blois et al., 1961; den Hartog, 1973). The first samples after thc Amoco Cadiz oil spill showed a decrease of the numbers of species. Further, only half of th~ numbers of individuals present befor~ the spill were found. This resulted in a decrease of the diversity index trom 2.26 in March 1978 to 1.66 in May 1978 (Fig. 1). After May the divcrsity recovered within 3 months due to an increasingly proportional • distribution of the individuals over the species faund in the sampIes. The numbers of individuals and species increased somewhat but remained fluctuating slightly during the further observation period in 1978 and 1979. The values from the end of 1977 were.' never reached. again due to the raise of this bed as described above. However, the relatively high and rather stable values of diversity during the second half of the investigation period (see Fig. 1) indicate a stable community with a high d1versity. After the spill the polychaetes comprised the most abundant -.... Jacobs - 7 group of the fauna. The best represented were. in order of relative abundance. the Maldanidae (2 species). the Capitellidae (3) and the Spionidae (4). The bivalve Loripes lacteus (L.) was also represented by numerous specimena. Representatives of the three polychaete families were also abundant before the spill. but the Spionidae reoovered apparently less quickly than the Capitellidae and Maldanidae (Table 3). In the lower situated eelgrass bed more individuala as weIl as species of the macrofauna were found during the whole • investigation period than in high littoral bed. This difference was most distinct during the time before the oil spill and again from November 1978 onwards (Fig. 2). It was only in May 1978 that the impact of the oil spill on the benthic fauna was perceptible in a prominent deorease of the numbers of individuals and apecies. This was mainly cauaed by the virtual disappearance of Amphipoda. Tanaidacea and Echinodermata. whereas the numbers ofGastropoda. Polyohaeta Sedentaria and Bivalvia also decreased (Table 2) • • The sampIe taken in June 1978 contained more apecies and individuals, mainly herbivorous apeoies, due to the effects of an algal bloom. At that time the littoral was covered with enormous quantities of Ulva lactuca L. t Enteromorpha spp.'and Ceramium rubruc (Huds.) C. Ag •• while the eelgrass was covered with the epiphyte Ectocarpus sp. Various stimulating factors can be pointed out as causes for this exceptional growth, which were probably effective in combination: 1.