SALINITY AND FAUNAL DISTRIBUTION IN THE POCASSET RIVER, MASSACHUSETTS

Abstract

During the summer of 1963 and the spring of 1965, a study was made comparing the subtidal sediment salinities with those of the immediately overlying water in a small estuary north of Woods Hole, the Pocasset River, and relating these findings to the distribution of the estuarine fauna. The estuary is a fluctuating type with a nearly constant river flow. At each location, there are pronounced though precise fluctuations of the water salinity with tidal periodicity. Bottom waters show periods of highly saline, essentially marine water and periods of nearly freshwater, separated by abrupt and brief transitional periods throughout the upper half of the estuary. The low salinity phase is longer at the upper end of the estuary and the high salinity phase gradually predominates as one proceeds seaward. In contrast to the marked fluctuation in bottom salinity, the salinities in the sediments are stable and constant, with each station having its own characteristic salinity that gradually increases from the uppermost reaches to the mouth. This salinity regime has a marked effect on the distribution of the benthic fauna. The epifauna, subjected to the extreme salinity fluctuation and rapid and extreme salinity changes, is poorly represented, particularly in the upper part of the estuary. The infauna, living under much more stable salinity conditions, make up the vast majority of the fauna. The periodic short‐term fluctuations in the water salinity stabilize the sediment salinities and subject the infauna to less physiological stress than that imposed on the epifauna. A brackish water fauna dominates the uppermost part of the estuary. The fauna is transitional in a zone where the sediment salinities vary from 19 to 22%. At higher sediment salinity values, lower in the estuary, the marine element predominates. Animals collected from the transitional zone were exposed in the laboratory to low and high salinity water obtained from the same locality at low and high tide, respectively, and to a control salinity duplicating the average sediment salinity of the zone. There were four patterns of response: 1) stress symptoms and death usually in less than 24 hr; 2) stress symptoms and recovery; 3) behavioral response; and 4) normal, identical activity in the low, high, and control salinity water. These responses were related to the depth distribution patterns of the species, sediment salinities, and redox potential values at the collecting site. © 1965, by the Association for the Sciences of Limnology and Oceanography, Inc.