BENTHIC REPOPULATION OF THE RARITAN RIVER ESTUARY FOLLOWING POLLUTION ABATEMENT1

Abstract

A total of 69 samples of benthic animals was taken in the lower 20 km of the Raritan River estuary from 1957 to 1960. During 1957, under heavily polluted conditions, no freshwater species were discovered. Of the 17 marine species found, the barnacle Balanus improvisus extended 8.5 km above the river mouth; the remaining species were confined to the seaward 4.6 km of the river.In January 1958, a trunk sewer system began operation in the lower Raritan Valley, and pollution was abated in the river. Rapid repopulation of the estuary occurred. The sequence and numbers of freshwater and marine species invading the estuary and colonizing the bottom sediments were followed in the samples of 1958, 1959, and 1960.The most obvious change in 1958 was the distribution and density of Balanus improvisus. These barnacles coated all firm substrata in the previously uninhabited section, extending upriver to the limit of salt penetration. The 12 stations sampled in both 1958 and 1959 yielded 6 freshwater and 21 marine species in 1958 and 8 freshwater and 28 marine species in 1959. In 1960, freshwater species continued to increase, but there was a slight decrease in the number of marine species. Dominant components of the freshwater fauna were the oligochaetes Limnodrilus spp., the leech Erpobdella punctata, and the bivalve Sphaerium sp. A density of 7,102 organisms/m2 was found at one of the freshwater stations in 1960.Marine species that invaded the river following pollution abatement are placed in five groups—three of pioneers, one of secondary invaders, and one of progressive penetrators— on the basis of their year of arrival, penetration, and length of stay.By the end of the study, biotic recovery had so progressed that a plot of the quantitative distribution of species illustrated the classic V‐shaped curve for estuaries. A similarly shaped curve was obtained for the distribution of population densities.