The words ‘Clyde-built’ and ‘Clyde-navigation’ originated in the heyday of maritime engineering and commerce in Clydeside at the beginning of this century and had a double meaning; they respectively became the hallmarks of high-quality workmanship and astuteness in business. The words are symbolic of and have a rooting in the proud tradition of the lower Clyde with its flat carse-land suitable for human settlement, its sheltered navigable firth and estuary facing the New World and its handy reserves of coal. A new industrial society was founded from communities of poor, rural people in Scotland and Ireland ready to immigrate and settle in this land of opportunity. The sons of crofters became skilled tradesmen, engineers and businessmen; the Clyde was made navigable to the heart of Glasgow and for a time became an open sewer.

The relative resource value and use of the Estuary and Firth of Clyde have varied through time. Until the eighteenth century the estuary was difficult to navigate. The firth, with its rich fishing grounds and sheltered harbours, was the focus of early settlement. However, increasing trade with the Americas, larger ships and growing competition between Glasgow and the more accessible ports of Greenock and Dumbarton stimulated plans to improve the navigability of the estuary. This was achieved by deepening and widening the main channel, a process which continued as the size of ships increased and more sophisticated techniques of river improvement developed. Within a century and a half the bed of the estuary was transformed from an obstacle to navigation into a major water-route, from which the largest passenger liners in the world were launched.

Since the Second World War the relative resource values of the estuary and the firth have changed with the increase in air travel, the decline of ship building and heavy industry on the Clyde, and the increase in the size of cargo vessels. The economic focus shifted from the estuary to the natural, deepwater harbours of the firth.

At present redevelopment of derelict docklands and industrial sites along the estuary is taking place, while the firth is suffering from the effects of economic recession. Planning is now being increasingly directed toward the development of the amenity, recreational and tourist potential of both the estuary and the firth.

In late-Precambrian and early Palaeozoic times, the Clyde Sea Area lay at the junction of a major land mass and the Iapetus Ocean. Closure of the Ocean on its northwestern margin corresponded broadly with initiation of the Highland Boundary and Southern Uplands Faults and formation of southwestnortheast oriented basins of continental sedimentation. Northwest-southeast fractures of the Clyde Belt, along which the Northeast Arran and Southwest Arran troughs of the firth are emplaced, developed across the southwest-northeast Caledonoid grain in Carboniferous-Permian times. The firth and estuary began to assume their distinctive forms in Tertiary times. The area experienced several major glaciations during the Quaternary Period. Late Devensian effects are most distinct: overdeepening of the firth to depths exceeding — 160mOD; deposition of till and meltwater sediments up to 80 m in thickness. Clyde Beds, fossiliferous clays and silty sands up to 15 m in thickness, accumulated c. 13,150–10,000 BP in sub-arctic marine waters in the firth, estuary and Paisley embayment, and in the Lomond basin prior to the Loch Lomond Stadial. The Holocene marine transgression, which produced further modifications in shoreline position, e.g. in Ayrshire, the Paisley embayment and the Lomond basin, was followed by marine regression due to isostatic land rebound.

The climatic character of this particular shoreline environment is analysed with a major emphasis on the contrasts between the upper estuary and the Clyde coast. The relatively mild, dry and sunny conditions along much of the Ayrshire coast are described and explained, although attention is also drawn to the effects of smaller-scale variations in topography, especially on temperatures. Sea breeze mechanisms are identified and illustrated with reference to a specific event.

Rainfall and river flow data have been used to calculate the fresh water input to the Firth of Clyde at key locations. The importance of seasonal variation and the recurrence of period of low flow is illustrated. Long term flow frequency is also analysed and related to recent events.

A front on the Great Plateau separates tidally mixed water of the North Channel from quieter less saline surface layers of the Outer Firth. These water bodies exchange about 1·6 × 104m3 s−1, leading to a residence time of two months in the Outer Firth surface layers. The position of the front is determined by the balance of mixing and buoyancy supply. The wind modulates weak tidal currents in the Outer Firth by driving surface currents and setting up deeper compensation flows. Deep water in the Arran Deep and Kilbrannan Sound is irregularly renewed by inflow of dense water over the Eastern Plateau from autumn to spring of about 1 × 104m3 s−1 but stagnates in summer below a thermocline. A minor part of this renewal comes over the Davaar sill into Kilbrannan Sound.

Bottom water of the fjordic sea-lochs is isolated in summer and renewed during the winter. The renewals occur by density currents flowing in from the sills and these produce a characterising grading of the bottom sediments from coarse to fine away from the sills.

The basic mechanisms governing the general circulation of water in the Firth of Clyde are reviewed with particular reference to their changing relative importance throughout the system, this including the inner estuary. Attention is then focused on the effect of storm surge. A numerical model for the depth-integrated equation of motion is applied to the surges recorded in 1972 and 1979. The proportion of surge generated within the outer firth is thereby assessed.

A review is given of some of the hydrodynamic features of the Clyde Estuary and examples are used to emphasise their relevance to an understanding of the effect of pollution on water quality. Computer modelling of the estuary and its role in pollution management are briefly described.

Reference is made to the oceanographic survey methods regularly employed in the Firth of Clyde and examples provided of the results and their application to pollution studies. The potential for computer modelling of coastal waters is stressed.

Over the period 1973–75, detailed studies were made of the water density structure and circulation of Irvine Bay. Observations showed that offshore waters were influenced by water originating in the Clyde Estuary and the inshore waters were affected by brackish outflow from the Irvine-Garnock Estuary. Long period current records indicated that the movement of water in Irvine Bay and the eastern Clyde Sea Area was particularly responsive to meteorological forcing. Based on data for 1974, the estimated annual mean longshore drift was about 5·0cm/s and directed towards the northwest. Studies of a tracer dye patch over a 1·5 day period showed that horizontal dispersion was influenced by vertical shears in the mean current and that vertical mixing was inhibited by a density interface at less than 4 m depth. The average longitudinal and transverse dispersion coefficients were 2·17m2/s and 0·25 m2/s respectively.

Recent publications reporting dissolved trace metal data have highlighted the susceptibility of such determinations to contamination during sampling and analysis. Results from recent surveys using stringent anti-contamination procedures have suggested that much of the earlier data has been affected by contamination. New surveys are desirable, therefore, particularly in areas which are known to receive considerable burdens of trace metal pollution.

Macrobenthic bioturbation and microbial activity can have major ecological effects on present day marine sediments some of which are also seen in the fossil record. The coastal and estuarine sediments in the Clyde Estuary area are described. They contain very high densities of benthic macroorganisms and microorganisms. These organisms have dramatic effects on the physical and chemical properties of sediments, and field and laboratory studies have shown that many geotechnical and geochemical properties of sediments are dictated by them. The effects can be on a microscale of a few millimetres horizontally or vertically (micro-spatial variation), or can have large scale consequences over hundreds of metres (macro-spatial variation).

Sediment stability (shear strength and critical erosion velocity), permeability and particle sedimentation are all altered by macro-invertebrate bioturbation or microbial activity, and many chemical properties such as Eh and pH may be influenced in a similar manner.

Biological activity may therefore play a major role in modifying sediment geotechnics and early sediment diagenesis. It also has direct relevance to the siting of man-made structures on the sea-bed and to sediment stability and transport in estuaries. In situ seeding experiments may lead to biological control and manipulation of these most important field phenomena.

During the period of severe sewage pollution of the Clyde Estuary and severe industrial pollution of the Garnock Estuary, these two estuaries could be distinguished by unusual abundances of one particular alga in each estuary, respectively Melosira nummuloides and Rosenvingiella polyrhiza. However, in terms of species presence data alone these estuaries did not differ from others in the British Isles. The unusual abundances of these two algal species might be ascribed to their special characteristics, related to the polluted conditions in the estuaries.

Epilithic diatom communities of the Clyde Estuary demonstrated no spring or autumn bloom. Species diversity and dominance were stable at the ends of the estuary and fluctuated through the year in midestuary. Species succession differed from regions where one species dominated throughout the year, e.g. Opephora martyi Heribaud at Bowling (high tide), to those where the dominant species was constantly changing e.g. Erskine (low tide). In all 307 species were recorded, twenty-nine very rare, 172 rare, nineteen were abundant, some e.g. Opephora martyi, Navicula mutica var. cohnii (Hilse) Grun. having wide niche breadth (B>60). others being restricted in their distribution e.g. Nitzschia paleacea Kütz. (B=10.4), Navicula pseudocomoides Hendey (B = 14–8). N. mutica var. cohnii and var. mutica Kütz. were difficult to distinguish in terms of valve morphology, as intermediate forms existed. In the estuary, smaller valves occurred towards the freshwater end. Two clones, originating from a 12/μm and a 26 nm valve (length) corresponding to descriptions of var. cohnii and var. mutica respectively, behaved similarly in culture throughout a range of salinity, in terms of size expression, suggesting no taxonomic difference. Diatom community photosynthesis was affected by salinity, reflecting the site of origin.

Changes in the status of fish populations in the Clyde Estuary between Woodhall and Glasgow since 1978 are described and data presented on species composition and distribution, temporal and spatial fluctuations in abundance and biomass and length frequency data for certain species are analysed.

The seaward part of the estuary is colonised by marine and euryhaline marine species dominated by flounder, while the ‘city reaches’ have a fresh water component dominated by three-spined stickleback and eel in addition to several euryhaline species, e.g. sand-goby and saithe.

The extent of biological recovery of the estuary from severe organic pollution can be gauged from the presence now of thirty-four species, including nineteen in the city reaches which were virtually fishless in the mid nineteenth century. The upper estuary is now able to support resident and migratory fish for much of the year, but species composition and abundance are markedly affected by seasonally variable fresh water flows to the estuary affecting primarily dissolved oxygen and salinity, leading to mortalities among resident species and providing a barrier to migrants.

The Clyde Estuary is recovering from gross pollution. In the early 1970s high densities of a small number of species of intertidal invertebrates tolerant of organic enrichment and low oxygen levels provided rich feeding for large numbers of dunlin, redshank, lapwing, oystercatcher and curlew. Winter numbers of the first three species have now declined considerably. The declines cannot be attributed to national changes in wader numbers and are not due to changes in breeding habitats or lack of reproductive success in particular regions, since each species wintering on the Clyde Estuary breeds in a different area. The most likely explanation of the decline, which is most pronounced in the species highly dependent on Corophium volutator, is either that reduced organic pollution has reduced the densities of the main prey, or that higher oxygen levels over the mudflats have allowed fish, particularly flounders, to enter the estuary and compete with the waders for the available prey. Although there is evidence of some birds having moved to other wintering areas, the fate of most birds is unknown.

Relationships between feeding ecology, population dynamics and conservation of estuarine shorebirds are becoming better understood mainly as a result of detailed long-term studies of birds in the northwest European estuaries most vulnerable to industrial and agricultural developments. Until five years ago the tidal flats of the Clyde Estuary held internationally and nationally important populations of ducks (Anatinae) and waders (Charadrii). To understand the reasons for changes in bird distribution it is necessary to know more about the factors which determine distribution. In this paper we examine the pattern of association between the Clyde's birds and their prey by evaluating the extent to which the distribution of birds is related to that of benthic invertebrates, heterospecific birds, season and tidal state.

Stepwise multiple regression analysis indicated that heterospecifics had effects on bird distributions over-riding those of prey. In summer, the significant independent variables explained 41–61% variation in bird density (number km−2) and 33% variation in bird feeding hours (bird-hours km−2); in winter they explained 17–35% and 29–32% respectively. The amount of variation explained was greater during flow than ebb tides, and the number of species for which some of the variation was explained was greater in winter than in summer. Three associations, each consisting of two bird species, are suggested: redshank with lapwing, dunlin with mallard, and shelduck with gulls. We provide explanations for some of the mechanisms underlying the above patterns and indicate areas for more detailed observational and experimental work. The integrity of the avian component of the estuarine ecosystem appears to be dependent on associations and interactions between birds as well as between birds and their prey. The associations are labile, and in many species stem from the effect of tidal movement on prey availability.

Analysis of the factors influencing the seasonal changes in biomass of the ‘net’ phytoplankton in 1972– 73 snowed that the dynamics of the spring waxing of the diatom populations were controlled by narrow ‘windows’ of climatic events, and that subsequent fluctuations in cell numbers were linked with the interplay between zooplankton grazing and wind induced dispersion. Data for 1976–77, set against a similar background of events with the ‘net’ plankton, showed that the nanophytoplankton constituted a less variable biomass through the seasons and, on an annual basis, contributed some 50% of the total carbon fixed.

Lochs Fyne and Striven are deep-silled fjords opening into the northern Firth of Clyde. They receive relatively little freshwater from local runoff, have a relatively low rate of tidal exchange, and undergo periods of deep water stagnation. Both contain fish farms which have lost stock because of phytoplankton blooms. Loch Striven was investigated in detail in 1980. Observations are also reported for 1979 and 1981, and for Loch Fyne from 1980–1982. Most stratification in Striven is due to salinity layering, but most of the freshwater in the loch probably originates in the Clyde Estuary and is associated with high concentrations of nitrate. Typical phytoplankton biomasses in Striven are the same as those in the western seaboard Loch Creran, but maxima exceed those in Creran. In addition to a red tide of Gyrodinium aureolum in September, 1980, three periods of high biomass seem to be a regular feature of the phytoplankton calendar in Striven. The spring increase takes place in March or early April, is dominated by the diatom Skeletonema costalum, and probably depends on the stabilizing effects of reduced near-surface salinities. A summer flourishing of Leptocylindrus danicus and dinoflagellates is also associated with near-surface salinity layering. A late spring bloom of diatoms, dinoflagellates and small flagellates, some ichthyotoxic, appears to be related to the occurrence of thermohaline stratification and near-surface nutrient depletion; on some occasions lochhead upwelling may also have been involved. Insofar as data are available, the ecology of phytoplankton in Loch Fyne appears similar to that in Loch Striven.

Zooplankton investigations in the Firth of Clyde can be considered as starting in 1885, when the Fishery Board for Scotland built a laboratory at Tarbert and the Granton based Scottish Marine Station extended its studies to the west coast. The work of both groups, and their successors, waxed and waned, with a particularly active period starting in the early 1920s, when the Millport Station received government aid.

Although the euphausiids Meganyctiphanes norvegica and Thysanoessa raschii are important in the deep waters around Arran and in Loch Fyne, in general, copepods are the most important pelagic crustaceans. Of these Calaims occurs in enormous numbers in surface waters in May and June. Thereafter it is found mainly in deep waters such as those of Loch Fyne. Other abundant copepods are Pseudocalanus elongatus, Acarlia clausi and Oithona similis which are particularly characteristic of the area extending from Kilbrannan Sound via the east of Arran to the upper Firth of Clyde. In these latter areas the peak standing stock of the larger zooplankters (as caught by a 250μm mesh net) is about 5, occasionally 8, g dry weight per 100cum, occurring from April to July. The smaller zooplankters (as caught by a 68μm mesh net) add a further 1.3g dry weight per 100 cu m.