Macrobenthos of the Pamlico River Estuary, North Carolina

A study of the macrobenthos and its environment in the Pamlico River estuary, North Carolina, included measurements of salinity, temperature, and dissolved oxygen, as well as the particle size and concentrations of organic matter of the sediments. Salinity ranged from 1% to 20%, and temperature from 5° to 31° C. Anoxic conditions in the bottom water over—lying the deeper portions of the estuary occurred in the late summer. Two distinct sediment types were found: a sand substrate in the shallow area and a silty clay substrate in the deep area of the estuary. A preliminary sampling program in July 1968 showed that the macrobenthos was characterized by both low diversity and low density because of unfavorable environmental conditions of salinity and sediment. The statistical sampling design for this study was stratification of the estuary and equal random allocation of samples. Only 18 species were found, and the Shannon—Weaver information diversity index (H) showed low values of 0.66—1.77. In addition, low density values (maximum number of organisms at any station was 629/m²) further indicated that stressing salinity conditions and unstable sediments are factors that make this estuary unfavorable for the benthos. The absence of benthic life in much of the deeper portions of the estuary correlated directly with anoxic conditions in these areas. On the basis of trellis diagrams, the estuary was divided into three areas: (1) and oligohaline zone dominated by Rangia cuneata and Nereis succinea; (2) a mesohaline zone with a Macoma balthica—Heteromastus filiformis—Nereis succinea association; and (3) a polyhaline zone at the mouth of the estuary characterized by a Macoma phenax—Mulinia lateralis—Glycera dibranchiata association. A seasonal sampling program, with transects located in these communities, showed vast seasonal changes in species composition, range of distribution, density, and community delimitation caused by larva set in the estuary. A total of 36 species was found during this 1968—69 sampling program. Although a few species, such as Rangia cuneata and Nereis sucinea, showed relatively little change in densities and range of distribution, most of the benthos underwent great fluctuations due to recruitment from new set. Settling periods in the fall and spring added large numbers of juveniles to the benthos, not only in the deeper portions where re—establishment of life was found after the late fall mixing destroyed the anoxic conditions, but also in the shallow areas. Due to this recruitment, the distribution of communities in the estuary changed so drastically during the year that a great homogeneity was found throughout the estuary during the winter and spring sampling periods. Ho ever, the communities noted in the preliminary sampling reappeared in the summer. Thus the macrobenthos undergoes significant seasonal cycles in the Pamlico River estuary.