A number of mechanisms which transport dissolved constituents and maintain the salt balance in estuaries are described. Although some of these have already been analysed, two which appear to be the most important in many real estuaries have received little previous attention. The Mersey Estuary is used as an example in which to estimate the amount of mass transported by each mechanism. The computations show in part how little is known and how much hypothesis is still required in spit?e of the number of previous estuarine studies; one may conclude, however, that in many real estuaries the most important mass transport mechanism is the net (non-tidal) transverse circulation, which is induced in part by the boundary geometry and in part by the longitudinal density gradient.

Mass transport mechanisms in partially stratified estuaries

Two rivers, the Mandovi and the Zuari, with their interconnecting canal, form an estuarine system in Goa on the west coast of India. Physical, chemical and biological features of this estuary are adapted to a seasonal rhythm induced by the annual cycle of the monsoon. Heavy precipitation and land runoff from June to September bring about large changes in temperature, salinity, flow pattern, dissolved oxygen and nutrients when the estuary becomes freshwater dominated. The monsoon season (July–September) is followed by a recovery period during the post-monsoon season (October–January) and thereafter a stable period of the pre-monsoon season (February–May) when the estuary becomes marine dominated. During the pre-monsoon (dry) season, the water in the estuarine system remains well mixed and the intrusion of salt water is felt as far as 65 km upstream in both the rivers; but during the monsoon season the rivers become stratified and a salt wedge is formed in each river which extends up to about 10 km upstream in the Mandovi and 12 km in the Zuari. The flow of the estuarine system is regulated by the entry of seawater with the incoming tide through Zuari which reaches Mandovi through the canal. The flow is reversed during the outgoing tide when the estuarine system is flushed. Dilution factors in both the estuaries are similar and vary from 1·2 to 8; highest values occur during the pre-monsoon season. Two shoals/sand bars occur permanently in Mandovi (Aguada Bay) close to a ramp-like inlet to the sea. This inlet poses no navigational problems for about 9 months during the dry season; but for a 3-month period during the monsoon, the waterway becomes hazardous and is closed to boat traffic. Heavy swell and intense wave activity lead to the transfer of sediments into the navigational inlet and the calm season brings the materials back to their original position with practically no overall change in the bathymetry of the bay. The oxygen cycle in the estuarine system is closely related to seasonal changes in temperature and bears an inverse relationship with salinity. In both the estuaries, the sulphate/chlorinity relationship remains uniform and similar to that of the sea except during the monsoon months when the relationship gets disturbed. Changes in phosphorus, nitrogen and silicon are largely regulated by rainfall and land runoff. There is no significant difference in the phytoplankton counts between Mandovi and Zuari which follow a rhythm similar to that of nitrate. Zooplankton biomass is higher in Zuari because of its greater marine influence. No seasonal variation was observed in the density of microflora in the two rivers. Bacterial counts were higher in the lower reaches of the estuarine system and decreased upstream. From its environmental features, the estuarine system can be classified as a tide-dominated coastal plain estuary.

Environmental characteristics of the Mandovi-Zuari estuarine system in Goa

The salt balance in stratified estuaries

Analysis of salinity data from Mobile Bay indicates that stratification-destratification events within this broad, shallow estuary are not uncommon. These events are related to the strength of the winds, through their influence on wave generation and subsequent bottom drag coefficient increases, and to the strength of river discharge. They do not appear to be due to the strength of tidal currents, as has been observed elsewhere. Furthermore, river flow appears to be the dominant control, the winds being important only in the absence of large freshwater discharges. The annual spring freshet can flush most of the salt from the bay. During other times of the year the relative strengths of river discharge and wind stress change the bay from highly stratified to nearly homogeneous and back on a variety of time scales ranging from daily to seasonal.

Salinity Stratification in a River-Dominated Estuary

The horizontal pattern of mesoscale (1–4 km) variability in salinity was a poor predictor of mesoscale patterns in chlorophyll a, suspended particulate matter, and daily primary productivity in the South San Francisco Bay estuary during spring 1987. The tidally-averaged salinity distribution varied over weekly time scales, reflecting inputs of freshwater as well as transport processes. Spatial distributions of the other quantities also varied weekly, but not in concert with the salt field. Spatial patterns of phytoplankton biomass (chlorophyll a) deviated from the salinity patterns, largely reflecting in situ production of phytoplankton biomass during the spring bloom. The tidally-averaged distribution of suspended particulate matter (SPM) was highly dynamic and responded to (1) the riverine input of suspended sediment during a freshet, (2) neap-spring variations in tidally-driven resuspension, and (3) resuspension in shallows following a period of wind mixing. Two-dimensional distributions of primary productivity P′, derived from maps of biomass and turbidity (SPM), also varied weekly, but the spatial variability of P′ was only about half that of SPM and chlorophyll. Since the magnitude and patterns of spatial variability differ among nonconservative quantities, at least in part because of local sources and sinks, we conclude that the spatial distributions of nonconservative quantities cannot be predicted from distributions of conservative tracers, such as salinity.

Spatial and temporal variability in South San Francisco Bay (USA). I. Horizontal distributions of salinity, suspended sediments, and phytoplankton biomass and productivity

This paper reports a series of measurements of salinity, temperature, and current velocity in the Vellar estuary. This typical bar-built estuary is shown, by comparison with dimensionless stratification parameters, to be a salt wedge at high river discharge and well stratified at low river discharge. During the tidal cycle, the salt wedge near the bottom became isolated in a series of basins during ebb tide; short period diffusion along the intense halocline seems to have been an important process in the salt balance of the water column. The water circulation at high river discharge was dominated by a meander system of alternating scour holes, with the high seaward flow in the deepest areas. Superimposed on this was a secondary lateral circulation system that broke down and reversed its sense at low river flows when the saline inflow developed.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1969.14.1.0004

K. Ramamoorthy

Papers

Salinity and water circulation in the vellar estuary