Aquatic Sciences 58/2, 1996 1015-1621/020120-19 $1.50 + 0.20/0 © 1996 Birkh~iuserVerlag, Basel Projective ordination by SIMCA: A dynamic strategy for cost-efficient environmental monitoring around offshore installations Bernard G. J. Massart 1, Olav M. Kvalheim 1, *, Fred Olav Libnau 1, Karl I. Ugland 2, Kjell Tjessem 3, Karl Bryne 4 1 Department of Chemistry, University of Bergen, N-5007 Bergen, Norway 2 Department of Biology, University of Oslo, N-0316 Oslo, Norway 3 Statoil Mongstad, N-5154 Mongstad 4 Statoil, Environmental department, N-4333 Forus Key words: offshore installations, multivariate analysis, SIMCA, benthic macro- fauna, ordination. ABSTRACT A method for environmental monitoring using benthic species profiles as input is developed in this work. The method, referred to as projective ordination, utilises local principal component model- ling (SIMCA) to obtain a cross-validated model which spans the natural variation in a region around offshore oil-producing installations. The borderline between regions with disturbed and non-disturbed species communities is subsequently decided from the residual distribution. This distribution is used to design an approximate F-test for assessing whether a community at a partic- ular sampling location is disturbed or not. If so, the nature of the disturbance is determined by pro- jecting the data on the PC model. Projective ordination utilises information from previous surveys to define the permissible variation in species communities, i.e. the limit of the natural variation. In addition, the method is dynamic, in a sense that the sampling locations may vary from survey to survey. Furthermore, our analysis shows that the customary number of replicate samples per station can be reduced. Modelling with only four randomly chosen replicates out of the complete set of five for each sampling location, does not affect the model significantly. On the other hand, taking only 3 replicates into account leads to significant divergences. A model based on the 1990 and 1993 surveys at the Staffjord field is presented as an example of the technique. Introduction In accordance with Norwegian legislation, regular chemical and biological surveys have been performed since 1979 around the oil fields in Norwegian sector of the North Sea. According to Kingston (1987), investigations should be focused on * To whom correspondence should be addressed.