A model is given for multiple recapture studies on closed populations which allows capture probabilities to vary among individuals. The capture probability of each individual is assumed to be constant over time. Based on this model we give a nonparametric estimation procedure for population size. The estimator involves selecting one of a sequence of estimators which are each linear combinations of the capture frequencies. The individual estimators are derived from the generalized jackknife method. We also give a goodness of fit test for the model's assumption that individual capture probabilities do not change during the study. The robustness of this estimation procedure is investigated with a simulation study. By virtue of this study, and the theoretical nature of the estimator, it is judged to be robust to moderate variations in individual capture probabilities which may occur in commonly used short—term livetrapping studies.

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Despite hundreds of millions of dollars spent annually in the United States on environmental monitoring, policy and decision makers seldom have ready access to monitoring data to aid in prioritizing reasearch and assessment efforts or to assess the extent to which current policies are meeting the desired objectives. EPA is currently conducting research to evaluate options for establishing an integrated, cooperative monitoring program, with participation by federal, state, and private entities, that could result in annual statistical reports and interpretive summaries on the status and trends in indicators of adverse disturbance and corresponding 'health' of the nation's ecosystem on the regional and national scale.

Summer and winter distributional patterns of attached diatoms were investigated in Yaquina Bay and estuary, Oregon. Differences in species composition and diversity of diatom assemblages at selected stations from fresh water just below Elk City, Oregon, to the marine waters of lower Yaquina Bay were released to environmental gradients. A total of 16,475 diatoms from 30 samples was separated into 256 species and varieties, of which 97 were found in only one sample, and 72 were represented by a single individual. The most abundant diatoms in the August samples were Fragilaria striatual var. californica, Melosira moniliformis, M. nummuloides, Navicula mutica, and Synedra fasciculata, while in the February samples Achnanthes no.2 and no.4, Navicula diserta, N. mutica, and Nitzschia frustulum var. perpusilla were dominant. Of the most abundant taxa, Navicula no. 2, N. diserta, N. gregaria, Nitzschia frustulum var. perpusilla, Synedra fasciculata and Thalassionema nitzschioides were the most evely distributed among the stations. The mean species diversity for diatom assemblages sampled in February was slightly higher than for assemblages collected in August. In February the mean specific diversity within a genus was higher and the mean generic diversity slightly lower than in August. In general, differences in assemblages were closely related horizontally to the salinity gradient and vertically to the desiccation and insolation gradients. However, biological factors were more important in accounting for differences among assemblages in the summer than in the winter and these factors were primarily species interactions between diatoms and macro—algae.