The general biology and pathology of Lepeophtheirus salmonis and Caligus elongatus and the prevention and treatment of such ‘sea-lice’ infestations on farmed salmonids are described from the literature and original observations. The life-cycle of L. salmonis and probably also that of C. elongatus comprises the egg and 10 stages separated by moults, namely, two nauplius, one infective copepodite, four chalimus, two pre-adult and the adult (male and female) stages. Water temperature greatly affects the rate of development, especially for early larval stages. Heavy infestations of wild fish seem rare, and lice are lost fairly rapidly in freshwater. In Scotland at least L. salmonis shows a succession of generations on farmed salmonids; generation time is about six weeks at 9–12 C. Post-chalimus stages of C. elongatus may exchange between farmed salmonids and wild fish (especially gadoids). Epizootics (particularly with L. salmonis) cause great damage to salmonids in Norwegian and Scottish farms largely through feeding on host skin. The dermis is oedematous and haemorrhaged where lice feed, and blood seeps between scales; deaths probably result from osmoregulatory failure. Whilst prevention of infestation is difficult, a bath treatment for 1 h with 1 ppm of the organophosphorus compound Dichlorvos is effective against post-chalimus stages of L. salmonis on caged salmonids. Side effects are minimal and clearance rates from fish tissues satisfactory, but treatment may be required every 3–4 weeks.

This chapter sets out to give a historical overview of the African rain forest from its origins, towards the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms which were then becoming established. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa which in turn led to the extension of tropical forest to various sites on the shores of the Tethys Sea. There were probably at this time common taxa and similar vegetation patterns stretching from the Gulf of Guinea to the Tethys Sea.

Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo–Zaïre basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, with a periodicity of about 100 000 years determined by the orbital variations of the earth around the sun, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. The most arid periods resulted in the fragmentation of the forest cover, and the forest biotopes and their biodiversity were preserved in a series of refugia. The lowering of temperatures also resulted in the extension of montane flora to low altitudes, with migration of montane flora and fauna between main mountain ranges. These compounded phenomena of isolation and migration, probably involving genie exchange, must have resulted in numerous speciation phenomena. Subsequently, such montane flora or fauna became isolated on mountain areas during periods of maximum warming, in the last instance in the course of the Holocene, when a vast forest cover became re-established from Guinea westwards, and to the East as far as the Lake Victoria area. The phases of maximum fragmentation, which appear to have been connected with only the coldest periods – in the last instance during the second part of isotopic stages 6 (from c. 160 to 130 000 years) and 2 (from c. 24 to 12000 years BP) – relate to less than 10% of the last 800 000 years, and the phases of maximum forest extension would likewise appear to be less than 10% of the period. The remaining 80–90% of the time relates to ‘intermediate situations’ which varied from period to period, and these intermediate extension situations seem to have been the norm over the larger part of the Quaternary, rather than the present situation which is closer to a situation of maximum extension.

The evolution, ecological processes, aerodynamics, and morphology of incipient or new foredunes on the upper beach is reviewed. Four types of incipient foredunes may be distinguished, namely: type 1, those initiated by shadow dune formation within zones of discrete individual pioneer annuals (e.g. Cakile spp.), and perennials (e.g. Spinifex spp.; Ammophila spp.); type 2, those initiated by dunelet or hummock formation within discrete colonies of perennial grasses and herbs; type 3, those initiated by sand deposition within laterally extensive colonies of pioneer seedlings; and type 4, those initiated by sand deposition within a laterally extensive plant rhizome cover.

Type 1 dunes are dominated by high, local flow deceleration and 3-D flow separation. Shadow dunes form within and downwind of the discrete plants. If colonised by annual plants, invasion by perennial plants is necessary for survival. Later lateral plant spread and accretion produces hummocky terraces and ridges. Type 2 dunes (dunelets) are characterised by local landward flow deceleration and marginal high velocity side flows. Sand deposition results in the formation of low, discrete, semi-circular convex mounds (seedlings), or more elongate hummocks and mounds (rhizomes). Types 3 and 4 dunes are characterised by laterally extensive, often homogeneous vegetation canopies. High canopies display more rapid landward flow deceleration than low canopies, thus narrow, asymmetric ridges and longer convex ridges result respectively. High plant densities result in rapid down-canopy flow deceleration, maximum traction load retardation and the formation of narrow asymmetric ridges. Dune height decreases and dune length increases as plant densities decrease. Maximum deposition zones occur for each wind velocity range. Whether ramp, terrace or ridge morphologies are formed depends on mode of beach colonisation, plant density and distribution, sand volume, wind speed, and plant species type and morphology.

A range of variables which affect foredune morphology and evolution, including plant canopy density, height and distribution, wind velocity, and various ecological environmental processes are examined.

A taxonomic review is provided of imagines of the forty-one known British species of Culicoides Latreille (Diptera, Ceratopogonidæ).

Four species new to science, C. cameroni, C. reconditus, C. segnis and C. machardyi are described from the British Isles.

The female of C. poperinghensis Goetghebuer is described for the first time. C. poperinghensis Goetghebuer and C. subfasciipennis Kieffer are recorded as new to the British Isles. C. lupicaris Downes and Kettle is defined anew. Distinguishing taxonomic characters are given for the females of C. achrayi Kettle and Lawson and C. pallidicornis Kieffer.

A number of female structural characters of taxonomic importance are discussed and described for all the British species: the antennal sensilla are found to be important as a group character and antennal ratio as a specific or subspecific character.

Descriptive notes with relevant taxonomic discussion are given for each species. The paper includes a key to species based on superficial characters and separate keys for each sex based on structural characters.

Green plants, within certain limitations, can adapt to a wide variety of unfavourable conditions such as drought, temperature changes, light variations, infectious attacks, air pollution and soil contamination. Depending on the strength of the individual impact(s), fluent or abrupt changes in visible or measurable stress symptoms indicate the deviation from normal metabolic conditions. Most of the visible or measurable symptoms are connected with altered oxygen metabolism principally concerning the transition from mostly heterolytic (two-electron transition) to increased homolytic (one-electron transition) processes. Homolytic reactions within metabolic sequences create, however, free radicals and have to be counteracted by the increase in radical-scavenging processes or compounds, thus warranting reaction sequences under metabolic control. At later states of stress episodes, the above control is gradually lost and more or less chaotic radical processes take over. Finally, cellular decompartmentalisations induce lytic and necrotic processes which are visible as the collapse of darkening cells or tissues. Every episode during this process is governed by a more or less denned balance between pro- and antioxidative capacities, including photosynthetic (strongly under metabolic and oxygen-detoxifying control) and photodynamic (only controlled by scavenger- and/or quencher-availability) reactions. This (theoretical) sequence of events in most cases can only be characterised punctually by strongly defined (analytical) indicator reactions (ESR) and is certainly species- and organ-specific.

Four phytogeographical regions of Pakistan are recognised in an analysis of the phanerogams of Pakistan. Among the uniregionals, the most common element is Irano-Turanian (45.6%) followed by Sino-Japancse (10.6%), Saharo-Sindian (9.1%) and Indian (4.5%). Though in terms of the area, the Saharo-Sindian region occupies by far the biggest territory, the density of uniregional elements is lowest in this region. Overall there are only 6 endemic genera and an estimated 372 endemic species in Pakistan out of 4882 species. Highest numbers of uniregional endemics per unit area are met with in the Sino-Japanese region, followed by the Irano-Turanian and Saharo-Sindian region. Most of the endemics (78.22%) are confined to mountainous regions (c. 1200 m or above). Four areas, i.e. Sino-Japanese region of Kashmir (10.21% endemics), N Baluchistan (Irano-Turanian; 9.4% endemics) and Chitral (9.1% endemics), may be recognised as centres of radiation in Pakistan.

Cyclitols are low molecular weight substances which accumulate in plant cells in response to various environmental stress situations, for example drought (Ford 1984), salinity (Gorham et al. 1984), low temperature (Richter et al. 1990).

Apart from their more general role in osmotic adjustment, only in the case of salt stress is their mode of function well understood. Cyclitols (e.g. pinitol) accumulate when plants are exposed to increasing salt concentration (Paul & Cockburn 1989) and act as compatible solutes (Sommer et al. 1990) as defined by Brown & Simpson (1972).

The significance of cyclitol accumulation in stress adaptation of plants to drought and cold still remains uncertain. However, it is generally accepted that drought and cold as well as several other stress situations lead to an enhanced generation of oxygen free radicals (Elstner 1990; Smirnoff & Colombe 1988), including the hydroxyl radical as the most harmful one. The report by Smirnoff & Cumbes (1989) that myo-inositol is an effective hydroxyl radical scavenger prompted us to test other naturally-occuring cyclitols like pinitol, quebrachitol, 1-D-1-O-methyl-muco-inositol, ononitol and quercitol for their ability to scavenge hydroxyl radicals.

The water masses, major circulation features and long-term variability of Rockall Channel waters are reviewed in relation to work in recent years. Data collected with modern instruments disclose more detail in the water mass structure than found previously, and the range of water masses present in the Channel is examined.

The use of recording current meter moorings has amplified our knowledge of the processes occurring within the Rockall Channel circulation, although net flows remain difficult to evaluate. Eddy motions suggested by current meter records and geostrophic data are confirmed by the trajectories of satellitetracked drogues.

Changes in the temperature and salinity of the upper waters of the Rockall Channel from the early years of this century to the present are clearly detectable, and a brief account is given of these.

The current state of Ghana's forest is summarised. Considerable changes have occurred in the last decade, since Hall & Swaine's account and classification, due mainly to fire and logging. The requirements and potential for sustainable forest use are explored through a summary of patterns of regeneration, and of local and national distribution of individual species.

Incisive indices of forest quality and condition are vital to good forest management. Various forest quality indices, summarising different properties of the plant community, are examined. These indices gloss over the statistically noisy behaviour of single species in small forest areas. The indices are: Forest Type – Hall & Swaine's forest ordination and classification; a Pioneer Index (PI) revealing the balance of ‘regeneration guilds’; a Genetic Heat Index (GHI), based mainly on the rarity value (Star rating) of all forest species, highlighting ‘hotspots’; and an Economic Index (EI) based on the concentration of common species (‘reddish Stars’) threatened by exploitation. Guild and Star are defined for all species and encapsulate trends of local and of global distribution and ecology. The national and local patterns and response to disturbance of the indices derived from the representation of these various guilds and stars are discussed.

Scale is crucial to all discussions. A strictly hierarchical model of forest ecology/biogeography is less suitable than a continuum-of-significant-scale, and non-hierarchical model. For instance, refugia are usually perceived as discrete biogeographical units. However, major biological ‘hotspots’, which are often described as refugia and attributed to Pleistocene climatic variation, differ only in position along a continuum of scale from mini-refugia as small as individual plants. The biogeographic Dahomey gap has much in common with a canopy gap, with scale as the main distinction.

There are conspicuous trends across Ghana's forests in the abundance of pioneer, rare or economic species. These differ in detail, but ‘hysteresis’ – the forest memory – and other factors related to the concept of refugia apply to all these aspects of forest quality. Major hotspot refugia are crucial to the national framework of biodiversity, but local refugia, between the size of individual plants and single forest blocks, are crucial to local regeneration and sustainable use, as they shape the probability cloud which defines the anatomy of and processes within each species' range. Short-term sustainable use depends on local refugia; longer-term sustainability requires maintenance of refugia on a wider range of scale.

The implications of these phenomena to forest management are discussed in conclusion. Forest health is a multi-scale, but particularly a broad-scale, phenomenon. Local processes like the regeneration of forest under canopy gaps, are subordinate to larger-scale patterns and not determined simply by a match between species physiology and gap dynamics or patterns in the physical environment. Success of a species in a certain landscape does not automatically imply the species can be successful in similar conditions in a different landscape elsewhere: the context of the landscape in terms of the broader mosaic is also important. Managers, whether of plantations or natural forest, need to monitor, plan, and protect indigenous species on all scales. Forest managers need also to be aware of and work with the ‘forest memory’ factor. Protective measures for rare or economically threatened species should be based on current refugia and, like them, be arranged on all scales from single trees to large forest blocks.

Researchers need to pay more attention to processes between the ecological and biogeographical, if they are to provide information for managers which has a useful synergy with existing types of data. Exploration is needed of the anatomy of the ‘probability clouds’ defining the statistics of dispersal and regeneration of rare or threatened species with respect to parent populations. What are the chances of a mahogany establishing at a point 500 metres from a mother tree? How is this statistic influenced by soil type? There is much to be learnt on scales between the canopy and the Dahomey Gap.

Evidence has accumulated over the past twenty years to suggest that the deep-sea environment is not as constant as was at one time thought, but exhibits temporal variations related to the seasonally in the overlying surface waters. Recent results from deep-moored sediment traps suggest that this coupling is mediated through the sedimentation of organic material, while observations in the Porcupine Seabight indicate that in this region, at least, there is a major and rapid seasonal deposition of aggregated phytodetritus to the sea-floor at slope and abyssal depths.

This paper summarises the results of the Porcupine Seabight studies over the past five years or so, using time-lapse sea-bed photography and microscopic, microbiological and chemical analyses of samples of phytodetritus and of the underlying sediment. The data are to some extent equivocal, but they suggest that the seasonal deposition is a regular and dramatic phenomenon and that the material undergoes relatively little degradation during its passage through the water column. The mechanisms leading to the aggregation of the phytodetritus have not been identified, and it is not yet known whether the phenomenon is geographically widespread nor whether it is of significance to the deep-living mid-water and benthic communities.

In experiments lakes 223 (L223) and 302 South (L302S) in the Experimental Lakes Area in north-western Ontario, and Little Rock Lake (LRL) in northern Wisconsin, were progressively acidified with sulphuric acid from original pH values of 6.1–6.8 to 4.7–5.1. Although the lakes were at different locations with different physical settings and assemblages of plants and animals including fish, there were remarkable similarities in their responses, particularly in regard to biogeochemical processes and effects on biota at lower trophic levels.

All three lakes generated an important part of their buffering capacity internally b\ the reduction of sulphate, and to a lesser extent by the reduction of nitrate. Alkalinity production increased as concentrations of biologically-active strong acid anions increased. Models relating the residence times of sulphate and nitrate to water renewal, or first-order kinetics, effectively predicted events.

Acidification disrupted nitrogen cycling in all three lakes. Nitrification was inhibited in L223 and L302S, while in LRL, nitrogen fixation was greatly decreased at low pH.

The phytoplankton communities of all three lakes were originally dominated by chrysophyceans and cryptophyceans. However acidification changed the dominant species and decreased diversity. Acidification tended to increase phytoplankton production and standing crop slightly, probably because light penetration was increased.

Littoral zones of all three lakes became increasingly dominated by a few species of filamentous green algae, which created nuisance blooms by pH 5.6. Mats or clouds of algae changed the entire character of the littoral zone.

Acidification of L223 and L302S caused the loss of several species of large benthic crustaceans as pH changed from 6 to 5.6. Large, acid-sensitive littoral crustaceans were absent from LRL before acidification, probably because the lake was already too acidic.

As acidity increased, the dominance of cladocerans within zooplankton communities increased. Daphnia catawba appeared at pH values near 5.6 and became more abundant at lower pHs as the lakes were acidified. Its appearance coincided with a decline in other Daphnia species: another cladoceran, Bosmina longirostris, increased in the experimentally-acidified lakes as did Keratella taurocephala: they became the dominant rotifers. Several sensitive zooplankton species declined or disappeared as the lakes were acidified, most notably Daphnia galeata mendotae, Epischura lacustris, Diaptomus sicilis and Keratella cochlearis.

The responses of different fish varied; they appeared to depend on the sensitivity of key organisms in the food chain. The ability of key fish species to reproduce was impaired as early as pH 5.8; their reproduction, except for yellow perch in LRL, had ceased at pH 5.0 in all the three lakes.

Acidification consistently reduced the diversity and richness of species in taxonomic groups studied, these effects resulting from losses of species and the increased dominance of a few acidophilic taxa.

Responses of experimentally-acidified lakes in north-western Ontario and atmospherically-acidified lakes in eastern Ontario were similar in most respects where records allowed comparisons to be made, notably in relation to biogeochemical processes and the disappearance of acid-sensitive biota.

When the acidification of L223 was reversed, several biotic components recovered quickly. Fish resumed reproduction at pHs similar to those at which it ceased when the lake was being acidified. The condition of lake trout improved as a result of greatly increased populations of small fish, their prey. Many species of insects and crustaceans that had been extirpated by acidification returned. Assemblages of phytoplankton and chironomids have retained an acidophilic character, although their diversity during recovery is similar to that at comparable pHs during progressive acidification. As their chemistry recovered, atmospherically-acidified lakes in the Sudbury area were able to sustain recruitment by species offish, including lake trout and white sucker, with rapid increases in the diversity of invertebrate taxa. Results from both L223 and lakes near Sudbury suggest a rapid partial recovery of lacustrine communities when acidification is reversed.

It is concluded that the experimental lakes responded similarly to acidification, and that experimental acidification can reliably indicate the effects of acidification attributable to acidic precipitation.

An analysis of the 8 published volumes of the Flora of Turkey was made in order to determine if there was a factual basis for the concept of an Anatolian Diagonal affecting plant distribution in Turkey. The data accumulated indicated that a substantial number of species (c. 2600) have distributions associated with the Diagonal this total accounts for c. 32% of the number of species in the 8 volumes: 5% are ± restricted to the Diagonal; 14% are absent from the W of it and 13% from the E. Some suggestions are made for the reasons for this floral break.

Current evidence for early fossil Salicaceae is summarised. Most data come from Europe and North America where revisionary studies are in progress in several laboratories. The earliest records of the modern genera are from North America with Populus section Abaso in the Late Palaeocene and Salix subgenus Salix in the early Eocene. This evidence is based mainly on leaves but the presence of Populus is confirmed by a leafy shoot with attached fruiting raceme. The two genera first occur later in Europe with Populus in the uppermost Eocene and Salix in the Middle Oligocene. Members of both genera in both continents apparently occupied riparian habitats early in their history.

Acidification in geologically-sensitive areas subject to appreciable acidic deposition has affected the activities of microbes, plants and animals in poorly-buffered freshwater ecosystems in Europe and N. America. Biota can be influenced directly by changes in water quality during both short acidic episodes and longer-term sustained periods of acidification: they are affected indirectly by alterations to the balance of acid-sensitive and acid-tolerant species at different trophic levels sometimes leading to a lessening of grazing pressures. Together the chemical and biological alterations result in adverse effects on some biogeochemical processes including the increased mobilisation and leaching of biologically active elements such as mercury, copper and zinc. Many field observations have now been corroborated by field (whole-lake) and laboratory experiments.

Decomposition of organic matter, attributable to microbes, has been found to be affected adversely at some locations. Conspicuously, acidification favours the prolific growth of filamentous algae particularly species of Mougeolia: it also shifts the balance of diatoms, minimising the occurrence of circumneutral species and favouring that of acidophilous and acidobiontic types. Because of their ‘resistant’ siliceous skeletons, it has been possible to trace the historical progression of acidification by examining the remains of diatoms in sediment accumulations. While their species diversity is significantly decreased, the biomass and productivity of dinoflagellates and chrysophytes are only slightly affected.

Like that of phytoplankton (dinoflagellates, chrysophytes and some diatoms), the species diversity of zooplankton is significantly decreased by acidification which has variable effects on the biomass and productivity of the group as a whole: the loss of daphnids is particularly conspicuous. Of the benthic macroinvertebrates, snails, crayfish, clams and freshwater shrimps (amphipods) have been found to be acid-sensitive whereas dragonflies and water boatmen (corixids) increase, particularly where populations of predetory fish have decreased.

Acidification has been responsible for the loss of fish from significant parts of Norway and Sweden: losses, partial or complete, have also been documented in geologically-sensitive areas of the U.K. and other parts of Europe, also Canada and the U.S.A. Eels seem to be the least sensitive to acidic conditions, followed by pike, trout, minnow and roach — the latter being the most sensitive. Prior to being eliminated, populations of fish become unbalanced because of recruitment failure and the mean age, and sometimes size, of the survivors increases. Accumulations of manganese and mercury have been detected in fish taken from acidified freshwaters. Short-term events (episodes) of extreme acidity and/or large concentrations of water-soluble aluminium have led to major fish kills.

The development of embryonic and larval stages of amphibians, including the common frog, toad and natterjack toad, is acid-sensitive. The distribution of fish-eating birds may be influenced by the effects of acidification on the performance of their prey. That of the dipper, a riverine bird, as well as its breeding success, has been restricted where acidification has decreased the availability of its macroinvertebrate prey. Deleterious effects on fish-eating mammals of reduced supplies of prey, sometimes with accumulations of mercury and cadmium and attributable directly and indirectly to acidification, have not, as yet, been confirmed.

In 1616 William Baffin, coasting up the icebound west coast of Greenland, reported: ‘The first of July we were come into an open sea, in the latitude of 75 degrees 40 minutes, which a new revived our hope of a passage…’ (Purchas 1625). From this point, at an unspecified longitude in the north part of Melville Bay, he cruised for 12 days in open water, up the Greenland coast to 77°30'N and down the west side of Baffin Bay to Bylot Island, seeing and naming on the way Smith, Jones, and Lancaster sounds. From Bylot Island south he found ‘a ledge of ice between the shoare and us’ as he continued past Pond Inlet and down the coast of Baffin Island. This is the first mention in written records, and the first known navigation, of an area that became well known two centuries later as the ‘North Water’.

Recent knowledge of physical-oceanographic processes is reviewed for the eastern shelf-edge boundary of the Rockall Channel.

A year-round northward current flows along the steep continental slope. Mean currents vary from 3 to 30cm/s, generally increasing northwards, but estimates of transport inshore of the 2000 m depth contour are much more consistent, averaging about 1·5 × 106m3/s. The current is thought to be forced largely by a long-shelf pressure gradient, associated with large-scale N-S density variations in the upper ocean.

Although cross-slope changes of temperature and salinity are much less than occur, for example, east of the U.S.A., cross-slope exchange velocities appear to be only around 2cm/s (1/5 of long-slope fluctuations). There is a sharp change between winter-cooled water on the shelf and adjacent slope water. Upwelling against the upper continental slope may occur following northerly long-slope winds. Tidal currents and surges depend strongly on continental shelf wave properties. At near-diurnal frequencies, and in response to winds of short scale (< 100 km), clockwise-rotating currents near the shelf break are expected, especially where the shelf is broadest. Tidal currents show this character.

Internal tides are significant; any non-linearity is most likely when stratification is present but weak. Bottom stirring has been observed, and may be important at some depth (where the bottom slope becomes as steep as the semi-diurnal internal characteristics) almost everywhere around the Rockall Channel. The energetic internal waves should contribute significant internal mixing as they approach the shelf break, intensify and dissipate.

The statistical material of genetics usually consists of frequency distributions—of genes, zygotes and mating couples—from which new distributions referring to their progeny arise. Combination of distributions by random mating is usually symbolised by the mathematical sign for multiplication; but this sign is not taken literally for the simple reason that the genetical laws connecting the distributions of progenitors and progeny are inconsistent with the laws governing multiplication in ordinary algebra. This is explained more fully in § 2.

This paper describes the environment, structure and internal physical conditions of the loch. The first section deals with structure, land use and climate of the catchment area. The structure of the loch itself is then considered together with a brief description of the sediments. The bulk of the paper is devoted to the effect of sun, rain and wind on the loch, e.g., radiation and water temperature, water balance and hydraulic conditions. It ends with a summary of the influence of environmental factors on phytoplankton production and higher trophic levels.

Photosynthetic productivity of phytoplankton in Loch Leven was studied over a 4-year period (1968–71), using the oxygen light and dark bottle technique. Marked seasonal changes in hourly and daily rates of gross photosynthetic productivity are described within the range 0·02 to 1·59 g O2/m2.h and 0·4 to 21·0 g O2/m2.day respectively. Hourly rates are shown to be relatively insensitive to variations in surface light intensity, whereas daily rates are influenced to a considerable extent by the duration of incident radiation (daylength).

The phytoplankton itself exerts a dominant influence on underwater light penetration, accounting for ca 75 per cent of light extinction at highest crop densities. This self-shading effect contributes to the poor correlation observed between crop density and areal gross productivity. The chlorophyll a content per unit area in the euphotic zone often approached its estimated theoretical limit of 430 mg/m2.

In general, increase in photosynthetic capacity (per unit content of chlorophyll a) accompanied increase in water temperature. During certain periods an inverse relationship between photosynthetic capacity and population density was evident. Reduction in photosynthetic capacity is attributed, in part, to the high pH values (> 9·5) with concomitant CO2-depIetion associated with dense phytoplankton crops.

Estimates of net photosynthetic productivity were frequently zero or negative, even over periods when algal populations were increasing and dissolved oxygen and pH values were above their respective air-equilibrium values. Underestimation of gross photosynthesis due to photochemical oxidation, photorespiration or the use of stationary bottles could not account for this apparent anomaly. The most probable sources of error in the estimates of net photosynthetic productivity are discussed.

The evolutionary origins and relationships of deer are briefly reviewed, with special reference to the species currently living wild or feral in Britain. Two species (red and roe) are native to this country; a third (fallow) was previously native but has been re-introduced from continental Europe; and three taxa (sika, muntjac and Chinese water-deer) have been introduced from eastern Asia. The Pleistocene history of red, roe and fallow deer in Europe is traced in detail, with emphasis on changing patterns of distribution and the climatic and vegetational conditions associated with fossil occurrences. Comparison of Pleistocene habitats with those occupied at the present day helps in an understanding of the ecological flexibility and rangelimiting factors of each species. The taxonomic status and habitats of the Asian species in Britain, as compared to their countries of origin, are briefly reviewed, and the dates of introduction into Britain of the Asian species and of fallow deer are discussed.