Influence of sewage pollution on a rocky intertidal community dominated by the mytilid Brachidontes rodriguezi

The characteristic Brachidontes rodriguezi community was absent from the highly polluted intertidal area surrounding a sewage outfall at Quequen (Argentina). It began to recover in lower tidal levels with better water renewal and shorter dessication periods. The chlorophyte Ulva lactuca was the most abundant organism at intermediate pollution levels, but reached its highest abundance in association with B. rodriguezi. The pulmonate limpet Siphonaria lessoni, the chlorophytes Cladophora sp. and Enteromorpha compressa, and blue-green algae increased in the vicinity of the outfall. A decrease in epilithic community biomass in the highly polluted area could be inferred from changes in percent cover, stratification and bare substratum proportion. Highest diversity was reached at intermediate pollution levels. Diversity decreased at higher levels due to low specific r~chness and also in relatively unpolluted areas due to space monopolization by B. rodriguezi. Therefore, sewage pollution could be regarded as a disturbance factor preventing space monopolization by the most successful competitor Sewage effects on the benthic community were similar to the decrease in abundance observed towards upper intertidal levels. Community structure proved to be a better pollution indicator than seawater variables, since it can be considered as reflecting an average of varying environmental conditions. INTRODUCTION The effects of domestic wastes on marine communities have been studied mainly in the northern hemisphere. In developed countries, sewage usually receives primary or secondary treatment in order to decrease its nutrient and organic matter content, and is discharged to the sea through pipeline systems opening at some distance from shore (Gerlach 1981). The influence of sewage pollution on intertidal communities has been assessed in relatively few cases (Borowitzka 1972, Munda 1974, 1980a, b, Littler & Murray 1977). A study similar to the present one was carried out in California by Littler & Murray (1975), who made a detailed analysis of the impact of very low sewage volumes on an intertidal community dominated by macrophytes. The use of benthic communities in marine pollution assessment is based on the concept that they reflect not only conditions at the time of sampling but also conditions to which the community was previously exposed (Reish 1987). Therefore, benthic organisms can be good indicators of the pollution degree in a given area (Anger 1977, Leppakoski 1979, Young & Young 1982, Reish 1986). The present study was carried out at Quequen (Buenos Aires Province, Argentina). At this locality, untreated sewage is discharged directly into a rocky intertidal community dominated by the bivalve Brachidontes rodriguezi. No previous reports exist on intertidal community structure at Quequen. However, several studies deal with foullng communities at Necochea Power Station in Quequen Harbor (Bastida & Brankevich 1980, 1981, 1982, Brankevich et al. 1984, 1988) and information about intertidal communities and populations is available for Mar del Plata, located ca 120 km NE of Quequkn (Olivier et al. 1966, Olivier & Penchaszadeh 1968, Bastida et al. 1971, Penchaszadeh 1973a). The main aim of this study was to evaluate community changes caused by different pollution levels, and the tolerance of benthic organisms to pollution stress. We suggest that the pollution gradient observed along the shoreline could also be interpreted as a disturbance gradient affecting the benthic community. Q Inter-Research/Printed in F. R. Germany Mar. Ecol. Prog. Ser. 63: 163-175, 1990 STUDY AREA According to the 1980 National Census, the current population of Necochea and Quequen (Buenos Aires Province) is about ' 65 000 inhabitants. Information obtained at the Department of Public Works and Services of Necochea Municipality indicates that ca 65 % of houses and buildings are connected to the sewage network, which represents ca 42000 inhabitants. Domestic effluents do not receive any kind of treatment. There is only one pumping station within the urban area, which propels part of the effluent towards the end of the pipeline system. Four km eastwards of Quequen Harbor, between Quequen and the summer resort of Costa Bonita (38'34' S, 58'38'30" W) (Fig. l), raw sewage is discharged onto the shore through a pipeline measuring 60 cm in diameter at its opening. No information on discharged volume or chemical and biological characteristics of the effluent was available. Based on flux measured at the pipeline opening, we obtained a rough estimate of ca 14 million 1 of untreated sewage per day. The outfall terminus is located in the lower intertidal area. However, there is a leak through a hole of irregular shape (ca 40 X 25 cm), on the upper side and 13.5 m before the pipeline end, in the mid-intertidal area. The intertidal area consists of a rocky substrate composed of loess platforms, separated from a coastal cliff 7 to 8 m high by sand strips. At low tide, 2 irregular parallel breaks of 40 to 60 cm height can be seen along the coast. The first divides the intertidal into 2 levels, and the second roughly indicates the beginning of the subtidal zone (Fig. 2). Grooves lying perpendicular to the shoreline are common in the lower intertidal area, as well as holes produced by the boring bivalve Fig. 1. Location of study area Lithophaga patagonica. There are 2 unequal daily tides, reaching a maximum average amplitude of 1.28 m. The horizontal intertidal distance ranges from 40 to 70 m in the study area. Changes in the benthic community caused by sewage outfall can be clearly seen from the top of the cliff. The affected area becomes wider to the east, owing to the influence of the west-east coastal current. The total affected area extends over ca 400 m of shoreline, comprising an area in which the Brachidontes rodriguezi community is absent (0 to 75 m west and 0 to 150 m east of the outfall), and also areas which show a gradual recovery of this community (75 to 150 m west and 150 to 250 m east of the outfall). METHODS A preliminary survey of the study area and collection of samples for taxonomic identification of species were carried out during October 1987 and March 1988. During spring (23 to 29 October 1988), the benthic community was sampled along 6 transect lines perpendicular to the shore, at 50 m intervals from 0 to 250 m to the west of the outfall. Nine to 14 quadrats were placed along each transect, at 5 m intervals (Fig. 2). A total of 65 quadrats were taken, all in the intertidal except for one quadrat at the beginning of the subtidal, 250 m from the outfall. Percent cover was chosen as the method for quantifying relative abundance of each species, due to difficulties in estimating algal density, and since substratum is regarded as the maln limiting factor in intertidal benthic ecology (Dayton 1971). A 45 X 45 cm square grid made up of nylon lines intersecting every 5 cm and Lopez Gappa et al.: Sewage pollution in a rocky intertidal community 165 Fig 2. Schematic map of study area show~ng outfall and transect locations. Symbols represent quadrat groups obtained from multivanate analyses. 1, quadrats with high proportion of bare substratun? ; 2 , crusts of blue-green algae; 3, Brachidontes rodriguezl community; 4 , tide-pools dominated by Ulva lactuca; 5, subtidal quadrat dominated by Corallina officinalis. Transect profiles with relative quadrat heights are shown on the right l . 0