Organisms and sea floor sediments respond to a number of physical and physico-chemical factors, the understanding of which permits identification of areas with typical and potentially aberrant habitats and deposits. After a resumé of the bathymetry of Scottish water tides, tidal current and residual currents stemming from the circulation in the North Atlantic are examined. The salinity and temperature interrelationships in many areas are summarised in open shelf waters, sea lochs and estuaries. Sea floor sediments commonly reflect reworking of glacial deposits coupled with varying indigenous populations of organisms which contribute skeletal debris to the particulate assemblages characterising the bed.

The lives of the plants, animals and bacteria of the plankton are ruled by the diffusive nature of their fluid medium, in which vertical turbulence is sometimes checked by density gradients caused by surface heating or inflow of freshwater. Seasonal and spatial changes in the production and species composition of phytoplankton and associated microheterotrophs result from interactions amongst light penetration, supply of mineral nutrients, and vertical mixing; many species are capable of rapid increase when multiplication exceeds losses by dispersion and predation. Water depth, currents, dispersion and food supplies control mesozooplankton.

Although Scottish coastal waters include a variety of pelagic environments, there is little evidence of site-specific flora or fauna. Planktonic assemblages are, however, locally more diverse than expected. If this excess diversity is a result of ever-changing conditions, the composition of microplanktonic associations may be largely decided by chance. Explanations based on niche theory seem to apply convincingly only to the larger, semi-nektonic, pelagic crustaceans. Thus, consideration of issues relating to conservation raises fundamental and difficult questions about the biology of plankton.

Conservation of species is impractical if accident plays a large part in recruitment and replacement. Conversely, the dispersive nature of the pelagic environment and the weakness of interspecific relations may lessen the sensitivity of plankton to anthropogenic perturbations. Nevertheless, eutrophication and climatic change can disturb pelagic communities, especially in enclosed waters.

This paper reviews the physical and biological characteristics of Scottish estuaries. A case is made for the conservation of a range of habitats, including sand dunes, saltmarshes and intertidal flats, although it is recognised that the large-scale protection of estuarine intertidal areas may depend more on their perceived importance as feeding areas for shorebirds and fish, rather than their very real intrinsic value. Contamination by metals and hydrocarbons, organic enrichment by sewage, agricultural run-off, land-claim and sea-level rise are identified as major threats posed to Scottish estuaries. Land-claim and sea level rise both lead to large-scale and permanent loss of intertidal area and they are considered the most pressing conservation problems.

Some of the more basic features of intertidal rocky shore communities are reviewed in order to point out the biological variability of this habitat type. This means that biological survey of them could be very detailed or could alternatively be based on physical features combined with basic community data. A previous conservation survey of Northern Ireland has shown that habitats and their communities can be classified without as much data as is customarily collected in transect surveys. In the Scottish context there is a particular variety of shores meriting conservation on the west coast sealochs. So far no intertidal effects of fish fanning have been demonstrated on them. Seaweed harvesting is localised although it may need control in the longer term. It is suggested that a change in the approach to protect areas might enable such habitats to be given protection without necessarily pursuing all the biological survey possible in such diverse habitats. East coast shores and those of the Solway area are less well-known but merit further study.

The species assemblages which occur on sublittoral hard substrata in Scotland reflect the following major environmental features: physiographic nature of the coastline, salinity, wave exposure, strength of tidal streams and depth modified by factors such as geographical location, geology, local topographical features, artificial substrata, scour, grazing, disturbance and pollution. Sublittoral hard substratum habitats and communities are affected by many of mans' activities including fishing and harvesting, damage by fishing equipment, organic input, nutrient input, chemical inputs, harbours and moorings, and construction work. Energy generation through wave and tidal power are potential threats. Some of mans' activities also create habitats of scientific interest. Conservation of sublittoral hard substratum habitats and communities requires: description of the character and distribution of habitats, communities and species; evaluation of survey information through application of nature conservation criteria; identification of areas/sites of nature conservation importance; establishment of appropriate consultative and statutory frameworks; obtaining information on biology of species, dynamics/longevity of natural communities and species, and mechanisms ‘driving’ communities; evaluation of damage and recovery; evaluation of effects of diffuse pollution; informing, consulting and educating.

A brief outline of the characteristics and habitat of marine sedimentary benthos is given together with a consideration of the factors influencing the structure and distribution of benthic communities and the role of larval strategies in influencing recruitment to such communities. The bearing these ecological factors have for the conservation of such communities in Scottish waters is discussed in the context of current and potential threats to their present stability and distributions. It is suggested that the rich communities of the fjordic and shelf areas of the western coast and island groups merit the greatest current concern for conservation, in the face of increasing pressures from a range of developmental actions.

Scottish coastal waters contain populations of both common and grey seals. In addition, the area is used by at least thirteen cetacean species. National and international agreements are in place which aim to protect these species. However, limited data on the ecology of marine mammals and on the threats which their populations face has prevented the implementation of an effective conservation strategy for marine mammals.

This paper reviews current information on the distribution, abundance and population dynamics of seals and cetaceans in Scottish waters. It then outlines the nature and likely extent of threats to these species from human impacts such as incidental killing, disturbance, pollution. Finally, conservation and management objectives for Scottish marine mammal populations are discussed, particularly in relation to international treaties which require the U.K. to provide adequate protection measures.

This paper reviews the status of marine fish populations in Scottish waters and considers current and potential threats. The inshore environments considered (shallow sandy bays, deeper and muddier areas, sea lochs, estuaries and firths) can be characterised by fish communities of 20-50 species, with some 20 species being present at any one locality. All of them functioned as nursery areas for commercial species and as habitats for smaller non-commercial residents. As few as 3-5 species were dominant, in terms of both numbers and biomass, in each habitat. More species occurred on the open shelf (over 200 ?) with up to 90 being taken regularly, but only 10 species accounted for 90% of both total biomass (in each of three North Sea areas) and commercial landings in Scotland by U.K. vessels. Mackerel (32.3%), herring (20.4%) and haddock (14.7%) dominated the landings. Both coastal fish communities and shelf stocks of commercial species have historically suffered from over-exploitation, a situation that has continued to date despite changes in management leading to their supposed protection. Other threats are identified, as is the need for conservation, while some legislative and research recommendations are made.

A comparison of macrofouling assemblages on offshore structures in the North Sea has revealed some similarity in their general characteristics, with a predominance in climax communities of relatively few species, such as the plumose anemone Metridium senile (L.) and the soft coral Alcyonium digitatum L., covering large proportions of the substratum. Specific geographical differences in community composition, diversity and successional development have been identified, however, and these are discussed in relation to environmental conditions and larval dispersal. In practical terms, the conservation of offshore fouling communities depends upon the fates of structures supporting these assemblages. Since the U.K.'s approach to platform decommissioning could ultimately involve the in situ toppling of structures in sea depths greater than 100 m, there would appear to be little threat to the long-term existence of offshore habitats for deep-water fouling assemblages. Moreover, an abundance of largely uninvestigated habitats for fouling organisms exists on submerged wrecks and other man-made structures. Thus, there appears to be no basis for a conservation requirement in relation to the fouling communities on fixed structures in the North Sea.

The papers presented in this volume clearly demonstrate the biological richness of the marine environment of Scotland. The system is controlled by complex interactions between the various physical, chemical and biological components which together make up the whole. This apparent equilibrium is the result of the natural regulation of what is in fact a highly dynamic and potentially unstable system. The forces involved in applying this overall regulation however are as yet only very poorly understood.