Planktonic bacteria in estuaries and coastal waters of northern Massachusetts: spatial and temporal distribution

The Essex, Parker and Ipswich River estuaries are salt marsh estuaries that empty into Ipswich Bay in northern Massachusetts, USA. The Essex estuary was the subject of a year-long study of planktonic bacteria numbers and heterotrophic activity, with monthly sampling of the entire range of water within the estuary. Both numbers and activity were fractionated into 4 sizes using Nuclepore filters. The Ipswich and Parker River estuaries were sampled along with the Essex on 5 occasions for comparison purposes. In winter, counts and activity were uniformly low throughout the Essex estuary. During spring, there gradually developed a peak of bacteria in mid-estuarine waters which was sustained through the summer and subsided in fall. The seasonal range in these waters was 10-fold (0.7 to 7.0 X lo6 m l l ) , while the range was lower in coastal waters (4-fold). This peak also occurred prominently in the Parker, but was modified in the Ipswich because of physical factors. Maximum numbers and activity were consistently found in the 0.4 to 0.6 pm fraction. Bacteria counts were strongly related to temperature, with the greatest dependence in mid-estuarine waters. Heterotrophic activity was logarithmically related to temperature and followed the same pattern as counts. The increment of bacteria with linear distance upriver to where the maximum is found was very similar in the 3 estuaries. It is suggested that this is a function of the availability of substrate as influenced by tidal mixing, and involves considering the bacteria as dynamic constituents of estuarine and coastal waters rather than conservative constituents reflecting only the mixing of water masses.