A Study of the Benthos of an Anoxic Marine Basin and Factors Affecting Its Distribution

Abstract

In a small, intermittently anaerobic basin located near the head of a shallow narrow estuary on the east coast of Canada, the species diversity of the benthos decreased with increasing depth and varied seasonally according to the available dissolved oxygen. Evenness also changed with the oxygen regime, but increased towards the deepest part of the basin (26m). Species composition, diversity, and evenness were asymmetrically distributed in the basin, although the distribution of dissolved oxygen was symmetrical. The sediments of the basin graded from fine silt/clay, high in organic carbon, on the inshore slope, to coarser fractions of silt and sand, low in organic content, on the seaward sill. The resulting benthic population of the landward slope consisted mainly of infaunal detritus feeders, characteristically refer red to as a Nephtys incisa-Yoldia limatuloides assemblage, while the sill was inhabited mainly by suspension feeders such as Pectinaria hyperborea and Mya arenaria. Species which were common to both habitats gave the two locations a moderately high faunal affinity. Below 5m a zone of transition was found, with several species more tolerant of deoxygenation occurring on both sides of the basin. Deeper than 15m a few polychaetes, harpacticoid copepods, and nematodes were found, and these only when there was sufficient oxygen to support life.