The Upper Palaeozoic bedrock, which is of sedimentary and volcanic origin, is briefly described. The origin of the Forth as a series of depressions in the bedrock surface probably owes much to erosion of a pre-existing Tertiary landscape during phases of Quaternary glaciation. The late Quaternary history of the area is described, relating the distribution of the sediments deposited in the Forth to climatic events and changes in relative sea-level. Since the acme of the last main glaciation about 20,000 years ago, late Devensian marine and estuarine sediments have been deposited on the underlying glacial till sheet at altitudes ranging from more than 120 m below O.D. to at least 46 m above O.D. Similarly, raised and buried beaches and their deposits occur at altitudes from 40 m above O.D. down to around 10 m below O.D. in the estuary. During the Flandrian, sea-level has fluctuated, reaching its maximum (about 11 to 15 m above O.D.) about 6500 years ago. The typical deposit of this period is the carse clay which forms a series of extensive, fertile raised mudflats around the estuary. The calcareous marine faunas of the carse clay and older deposits are outlined.

The pattern of climatic variation over estuaries and their environs is, to a considerable extent, controlled by aspect, slope and elevation in addition to orientation of the major relief features with respect to the movement of principal weather systems. Data from the few climatological stations within the immediate vicinity of the estuary and Firth of Forth demonstrate the essentially maritime nature of climate, and indicate also that there is a dominantly west–east gradient of change in most climatological variables. Below the high water mark the movement of water in tidal ebb and flow and river inflow modify atmosphere–surface interactions over relatively short spatial and temporal scales. Observations from a fixed instrument tower on Skinflats in the middle estuary and from Inchkeith Lighthouse indicate that tidal water movements exert varying degrees of control over sub-surface heat fluxes, air movement in the atmospheric boundary layer and the inland penetration of coastal weather systems such as the typical east coast haar.

The hydrography of the Forth estuary is described, and various modelling studies that have been carried out are briefly summarised. The description of the hydrography includes information on the currents, salinity, temperature, tidal elevations, and turbidity, as well as physical dimensions and freshwater inputs. The vertical salinity structure of the estuary varies from the well-mixed state to the partially-mixed state, depending on the freshwater input and the tidal range (which determines the amount of turbulent kinetic energy available for mixing). The tides in the estuary exhibit double high and low waters because of shallow-water effects, and there are observations of a small tidal bore. The resuspended solids are found to account for a large fraction of the biochemical oxygen demand in the water column. The modelling studies range from an early physical model, to an analytical model of water-quality, to more recent numerical models produced by the Forth River Purification Board and Imperial Chemical Industries, which were developed in order to assess the impact of proposed discharges on water-quality. Finally, currently active projects are described and suggestions for future work given.

The Firth of Forth, in terms of physical oceanography, is part of the North Sea. The general circulation pattern in the firth must be regarded as largely speculative. There have been insufficient measurements of insufficient quality, and what evidence exists leads to the view that what circulation there is, is sluggish and varies from season to season and from year to year.

A description is given of the three principal mechanisms that contribute to circulation. Tides, due initially to astronomical forces, manifest themselves in the Firth of Forth through the rise and fall of the adjacent open sea. This rise and fall, periodic in mid ocean, is no longer strictly so in the firth and neither are the tidal currents. The wind-driven currents in the sea are influenced by the earth's rotation. In the Firth of Forth, some of this influence is retained. Naturally, wind-driven currents are larger near the surface. Finally, when water of different densities meets, overturning causes convection currents. All of these effects are present to some extent in the Firth of Forth. In addition, specific account has to be taken of complicated coastal and bottom topography and river outflow. Some attempt to bring together these effects and available measurements is made in this paper. Lastly, several theoretical models are proposed which account for the magnitudes and directions of the observed steady circulation. Mathematical details are given in appendices.

The waters and coastal margins of the estuary and Firth of Forth are important and very valuable biological and recreational resources. They also receive, directly or indirectly, the aqueous domestic and industrial wastes of a quarter of the population of Scotland. It is one task of the Forth River Purification Board to minimise and eventually eliminate any conflict between the effects of these discharges and the various legitimate uses of the waters. Some of the main causes of environmental stress are briefly outlined and the basic strategy by which the problems will be overcome is described.

The natural background concentrations of radioactivity in the waters of the Forth are shown to be only marginally increased by anthropogenic inputs. The nature of the background levels and anthropogenic influences is briefly described.

The major sources of domestic and industrial wastes being discharged into the estuary and firth are described and quantified. The effects of these inputs on the quality of the receiving waters, in terms of their dissolved oxygen and micronutrient levels, are discussed.

In the estuary, nitrate is shown to behave in an essentially conservative manner in the winter months; the ammonia distribution exhibits summer maxima as a direct result of the effluent waste inputs; the orthophosphate distribution is characterised by considerable losses in the upper reaches. The data on lateral distributions of micronutrients show marked gradients.

In the firth the significant improvements in waters off the Edinburgh coastline are described. The influence of the existing major discharges on water quality is shown occasionally to extend to the eastern limit of the firth for ammonia and orthophosphate.

The long history of discharge of industrial and domestic waste to the Forth makes the area of particular interest to environmental scientists. The area has been studied both in its own right for the monitoring and control of contamination by trace metals and organohalogen compounds, and in the study of processes which influence the dispersion and effects of these substances. This paper discusses the inputs of contaminants to the area, their occurrence in the water, sediments and biota, and places the Forth in context with other coastal areas of Scotland. The paucity of information concerning organohalogen compounds is contrasted with the extensive work carried out on trace metals.

Since 1924 the Forth has received effluent from Scotland's major petrochemical industry and refinery in addition to hydrocarbon inputs from many other sources. The firth shows no major effects of this, but the history of inputs, together with the estuary's physical characteristics, have created a large residue of hydrocarbon contaminants especially in its sediments. This paper gives the degree and gradients of contamination in several components and describes the effect of hydrocarbon contaminants on the ecosystem structure.

The major inputs have produced localised lethal effects, in that productive estuarine intertidal habitat has been lost or its functioning altered. The changes are primarily due to organic enrichment with only very near-field toxic effects. Some of the effects result from historical contamination, but existing discharges continue to have a deleterious effect. New petrochemical discharges in both firth and estuary appear to have a minimal impact, although the historical contamination makes it difficult to assess their effects.

A current intensive mapping scheme for Forth seaweeds shows the normal attenuation gradient of species going upstream in the estuary with the existence of two areas of species depletion in the firth, around Kirkcaldy and Edinburgh. Ordination of species presence data suggests the continuation of estuarine influence into the firth and the possibility of slight differences in composition of the flora between the north and south banks in the mid and inner firth. Impoverishment of the flora around Edinburgh since the 19th century can be ascribed to increased pollution, but possibly also, to a lesser degree, to shore reclamation. Following abatement of sewage pollution, differences in the recovery of the algal flora on different shores may be related to the nature of the pollution-induced animal community. Recent work on the upper estuarine flora of the many small, highly-stratified sub-estuaries of the Forth system has revealed a marked shore zonation pattern of smaller mat-forming species.

The Forth saltmarshes occur as a more or less continuous strip on the south bank of the middle Forth and as scattered fragments of varying sizes elsewhere on the banks of the estuary and the Firth of Forth. The most complex and important from a conservation viewpoint are those at Aberlady and Tyninghame. Floristically the Forth saltmarshes are poor and lack several plant species which are imporant in England, notably Halimione portulacoides and Limonium vulgare (the latter recently extinct at Aberlady). Most of the Forth saltmarsh vegetation apparently falls into the two most common and widespread Associations of British saltmarshes, Puccinellietum maritimae and Juncetum gerardii, although several other community types occur. The most distinctive features of the Forth saltmarshes are: (a) the presence of the physiographically distinct and nationally rare beach-head saltmarshes; (b) the occurrence locally of community types which have a predominantly northern or southern distribution in Britain; and (c) communities which are subtly different from more widespread examples nationally.

The history of Edinburgh's sewage disposal to the sea from the open ditch system of the 17th century to the present day, together with the changes in legislation affecting sewage discharges to the firth, are reviewed. During the last thirty years sewage discharges have been monitored monthly for levels of faecal coliform bacteria at approximately thirty-four shore sites and twenty offshore sites. These coliform counts are used as bacteriological indicators of sewage pollution, in order to assess the risk to humans of infections that might result from the consumption of contaminated shellfish or from bathing in sewage-polluted waters. Most of the areas sampled from the inner firth were shown to be grossly polluted, particularly in late summer, and regularly to exceed guideline levels throughout the year. Since the commissioning of the new Seafield sewage works in 1979, a general improvement in bacterial and viral counts has been recorded on the south shore of the inner firth. In the last decade two European Community directives relating to the quality of seawater for bathing and shellfish growing have been promulgated, which set guideline and mandatory levels for the number of faecal coli in seawater. The Scottish Development Department has recently designated five beaches as bathing beaches and two areas on the north and south shores of the firth as shellfish growing areas. These areas have been regularly monitored by the Forth River Purification Board and the published results of both surveys are analysed. The virological studies carried out by the Forth Pollution Group of Napier College before and after the major change in the sewage discharge pattern into the Firth of Forth are reviewed.

A description of the community structure of the Forth estuary's zooplankton was obtained by sampling monthly at nine stations ranging from fresh water to fully marine conditions for an eighteen-month period. The variation in species abundance and distribution is related to environmental variables. The apparent spatial and temporal successions of members of the Acartia species complex are described, as are the episodes of predation spreading from the firth and the domination of the upper estuary by Eurytemora. It is observed that the estuary of the Forth is rare among British estuaries in its ability to support a truly pelagic self-maintaining assemblage of species.

The intertidal habitats of the Forth estuary are dominated by fine-grained muds, which experience a salinity range of 0–32%, j over the length of the estuary from Stirling to Queensferry. The current total area of the intertidal zone is 23.3 km2, the majority of which is found in the three central bays of Skinflats, Kinneil and Torry Bay. Over the centuries, agricultural and industrial reclamation has reduced by almost 50% the size of the Forth estuary intertidal habitat. The upper estuary has narrow intertidal shores, inhabited almost exclusively by oligochaete worms, which are very abundant due to organic enrichment of the area. The middle estuary is responsible for the greatest biomass and production of the intertidal benthic macrofauna. The lower estuary has a greater diversity of species than the upper and middle estuary, but has a lower biomass and abundance of macrofauna. It is shown that the production/biomass (P/B) ratio of several species in the Forth estuary is lower than that of the same species in other estuaries. This depression cannot be explained by natural environmental variables, and is suggested as being due to the levels of pollutants throughout the area. Despite this depressed productivity, and the threats from further reclamation, the Forth estuary retains abundant populations of intertidal animals, which are an important food source for birds and fish.

The paper summarises and briefly discusses published and unpublished data relating to the physicochemical characteristics of the sediments and the occurrence and distribution of the intertidal macrobenthos of paniculate shores in the Firth of Forth. In general, the shores described are poor faunistically and in some areas this results partly from the influence of waste water discharges. Detailed information is available for only a limited number of the numerous beaches in the Firth of Forth. In consequence, this paper focusses attention on the results of a long term study of four beaches along the south shore of the firth which was undertaken as part of an environmental impact assessment in relation to the introduction of improved sewage treatment and disposal for the City of Edinburgh. Edinburgh Sewage Scheme and the firth into which it discharges are briefly described and data pertaining to changes in the intertidal sediments and the intertidal macrobenthic communities, which occurred in the Seafield Bay area of the firth following pollution abatement, are presented and discussed.

Knowledge concerning the meiofauna of the Forth is reviewed and extended by presentation of the results of new surveys of the intertidal and subtidal provinces of the estuary. Most of the published information concerns the influence of sewage pollution on shores of the firth. Before 1979, the meiofaunal communities of sandy beaches along the Edinburgh coastline exhibited reduced species richness, although certain taxa displayed enhanced densities. Improvements in the sewage treatment process have initiated a phase of meiofaunal recovery.

In the estuary the salinity regime plays a key role in the determination of species composition, although the influence of local changes in sediment type and stability is recognised. Meiofaunal communities of lower shore mudflats in different salinity zones of the estuary are described. Pollution plays a major modifying role. In the upper reaches of the estuary, which suffer oxygen depletion in summer, only nematodes and oligochaetes were recorded in the oligohaline zone and impoverished nematode and copepod faunas in the mesohaline zone. In the polyhaline zone discharges from the industrialised region around Grangemouth modify meiofaunal community structure over an extensive area of mudflat. The subtidal meiofauna of the lower estuary is briefly described, particularly with respect to the copepods. Impoverishment again occurs in the vicinity of Grangemouth.

Molluscan species diversity decreased westward from the North Sea into the Forth with decreasing diversity of habitats, decreasing algae and increasing turbidity of water. Species diversity varied little from year to year or seasonally at individual sites. Adult Littorina littorea in particular were larger at lower than at higher shore levels, and larger in the outer than the inner firth or estuary, while Nucella lapillus adults were of more uniform size throughout the area.

Population parameters and reproductive output of L. littorea and L. saxatilis in the estuary varied without strict relation to mere position between open firth and up-estuary sites. On the other hand, shells of L. saxatilis became progressively wider, breeding seasons of L. littorea were longer and less defined, breeding in L. saxatilis became more sharply seasonal and eggs of L. saxatilis had greater weight, calorific value and inorganic content at up-estuary sites. L. littoreashowed two-weekly peaks in spawning on spring tides while L. saxatilis showed monthly peaks in release of young near times of full moon.

Comparison of fresh material and stranded shells with material from beach deposits up to 5500′ years old indicates changes in populations with changes in climate about 550 BC. Size of Littorina spp. continued to decrease while numbers of Mytilus edulis increased greatly near the onset of the industrial revolution.

The structure of the sublittoral benthic macro-invertebrate populations of the Forth estuary and firth, Scotland, is described, giving the species richness, abundance and biomass for the area from the freshwater tidal limit to the North Sea boundary. Eight faunal associations have been defined, which include classical Petersen communities in the marine area and transition associations within the estuary. The spatial distributions of the associations are predominantly the result of the physical environment, but superimposed on the effects of salinity, sediment type and bathymetry are the effects of urbanisation, industrialisation, dredging and spoil disposal and thermal discharges. The benthos of the major part of the firth has been little affected, although that of the peripheral and estuarine areas does show anthropogenic effects.

The water column-benthos and fisheries-benthos interactions are also discussed. Elevated levels of nutrients in the water column may be the cause of enriched benthic populations in the firth and an assessment of the fish-benthos coupling indicates an estuarine functioning similar to other northwestern European areas.

Until the late 1970s Edinburgh and the surrounding areas employed a series of relatively short sea outfalls to dispose of sewage to tidal waters. A centralised sewage treatment facility was provided in 1977, with disposal of liquid effluent by long sea outfall. The residual wet solids are dumped at two sites in the North Sea off the Firth of Forth. The sites, one near Bell Rock (56 25N 02 10W) and one ten miles north of St Abb's Head (56 04·5N 02 07·25W) are used on a rotational basis. The macrofauna at both sites shows a normal polychaete-dominated community exhibiting the normal low number/high diversity characteristics of many unimpacted communities in the North Sea. In general, seasonal differences in mean concentrations of lead, copper and zinc in sediments were small and within the limits of statistical variation. The concentrations of mercury and cadmium were mainly low and close to the detection limits of the analytical methods employed. The monitoring data from Bell Rock and St Abb's Head demonstrate that the dumping operation has had little effect on the benthic environment and that the macrobenthos showed no signs of stress due to the addition of sewage sludge.