The Salinity Characteristics of the Middle and Upper Reaches of the River Blyth Estuary

Abstract

The distribution of free-living nematodes inhabiting the intertidal muds of the Blyth estuary has been studied in relation to salinity distribution. This paper presents the results of the investigations into the salinity variations in both the tidal water of the estuary and the interstitial water of its muds, while the distribution of the free-living nematodes will form the subject of a later paper. In this country the salinity conditions in the estuary of the River Tees (Alexander, Southgate & Bassindale 1935), the estuary of the River Severn and the Bristol Channel (Bassindale 1943a, b), and the River Tamar (Cooper & Milne 1938, Milne 1938, 1940, Percival 1931, Spooner & Moore 1940) have been studied in detail. The River Blyth estuary is much smaller than any of these, and only the general principles found in the larger estuaries can be applied to it. Day (1951) and Yonge (1949) give reviews of these general estuarine conditions. The distribution of free-living nematodes in water of varying salinity has been the subject of much research in the Baltic Sea and in certain areas of the Zuiderzee, while in this country Rees (1940) has dealt with free-living nematodes as part of a larger study of the ecology of a mud flat in the Bristol Channel. Bassindale (1943b, p. 3) has pointed out that 'it has long been known that if the salt content of the water in which they live is changed sufficiently slowly, many animals can acclimatize themselves to large changes. Thus although the magnitude of the change is important, the rate of change is also significant'. In the Zuiderzee and Baltic, rates of salinity change are much less rapid than those found in estuaries, and the distribution of nematodes under conditions of high rates of change has not previously been studied. Nematodes live in the interstitial water of the mud, and Rees (1940) has shown that the greater part of this constituent of the microfauna is concentrated in the surface 2 cm of intertidal mud. It was therefore necessary to investigate salinity conditions in the interstitial water near the surface of the mud. Reid (1930, 1932), Alexander et al. (1932) and Smith (1955) have worked on the salinity of interstitial water in mud and sand, and all have found that salinity conditions tend to be more stable with increasing depth, and that salinities could be maintained at a high level-varying with depth-below nearly fresh overlying water at low tide. Reid states that '. . . marine burrowing animals living in estuaries are in no way