At present there are still considerable gaps in our knowledge as to what ultimately controls the vertical and horizontal distribution o f living coccolithophores. Latitude, ocean currents, water masses (Okada & Honjo, 1973a; McIntyre & Be, 1967), major nutrients, salinity, temperature and available light, vitamins and minerals, (such as iron, Martin et a l, 1989 & 1994; Kolber et a l, 1994; Watson et a l, 1994) are all thought to be important.
Most phytoplankton groups achieve their greatest diversity in equatorial and sub-tropical waters, whilst coccolithophores achieve their highest relative abundance in such conditions. The diversity o f coccolithophores is highest in warm, low productivity 'blue water' regions (Brand, 1994) such as sub-tropical oceanic gyre centres and areas o f restricted distribution (Hulburt, 1963, 1964). The Red Sea and the G ulf o f California are good examples o f areas o f restricted distribution and circulation. This is unlike most other phytoplankton groups (Winter et al., 1994) which prefer cold, high productivity regions. Under certain cold, nutrient rich conditions, however, such as the Norwegian Fjords, coccolithophorid blooms are known to occur. These bloom s are typically monospecific and in such situations concentrations o f 10^ - 10* coccolithophores per litre o f water have been recorded (Birkenes & Braarud, 1952; Berge, 1962). Such blooms are frequent, and were first recorded from space by the Coastal Zone Colour Scanner launched on the Nimbus-7 satellite in November 1978. Every year thereafter extensive patches o f water giving strong reflectance o f visible light were observed. These areas were recognized as being due to coccolithophorid bloom s, with the coccoliths reflecting light. The strong reflectance o f visible light was found (Sturm & Viollier, 1983) to be due to a combination o f back-scattering by the coccoliths, especially loose plates, and relatively low absorption by plant pigments and the other constituents in the water. The amount o f CaCO^ in the top 60m o f one o f these blooms, with a surface area o f approximately 7200km “, was conservatively calculated at 7.2 x lO'’ tonnes (Holligan et a l, 1983). Such high quantities o f calcite, produced by organisms reproducing by simple binary fission at rates as high as 2.6 divisions per day (Brand, 1981) must have an impact upon the global COg cycle and the chemistry o f the oceans.
In recent years coccolithophores along with other algal groups have received much attention as they have been recognized as important constituents in biogeochemical cycles and global climatic change. Coccolithophores are one o f the major producers o f calcareous sediments in the ocean. Honjo et al. (1982) estimated that 20-40% o f all the carbonate produced each year is formed by coccolithophores. Their production must, therefore have a great influence on the Earth's climate and global biogeochemical cycles.
Coccolithophores have an inordinate effect upon light scattering in the ocean because o f their coccoliths and high concentrations o f coccoliths are known to increase light reflectance back into space (albedo) (Balch et a i, 1991). Along with other prymnesiophytes and dinoflagellates they are also known to produce dimethylsulfide, a gas which is oxidised to sulphate aerosol nulceii in the atmosphere (Charlson et a l,
1987) and is believed to enhance cloud cover^increase the Earth's albedo. Both these features o f coccolithophores appear to be able to affect the Earth's climate.
Enhanced growth o f algae within the oceans has been proposed as a mechanism to soak up excess carbon dioxide from the atmosphere (more photosynthesis, less CO^) and thus reduce the 'greenhouse effect' (Martin et a/., 1994). Experiments are underway in the Pacific Ocean at present which involve adding iron and a tracer substance into areas o f sea water, and monitoring it's effect upon the phytoplankton community. Lack o f iron in the oceans is believed to restrict the growth o f algae as in large regions o f oceans where essential nutrients such as nitrate, phosphate and silicate are plentiful marine algae is scarce (Kolber et al., 1994). Preliminary results from such experiments have shown with certainty that the addition o f iron to the marine system stimulates plankton growth and increases levels o f chlorophyll. A single addition o f iron, however, does not prove a feasible approach to combating the greenhouse effect as only small changes have been noted in CO^ dissolution (Watson et a i, 1994).
It appears that coccolithophores have a significant effect on their environment, and the importance o f cocccolithophores and marine algae in the global climate system has been recognized worldwide.
3 .2 .2 M ethods o f collection o f recent and living coccolithophores
A wide range o f techniques are, and have been employed in the collection o f recent and living coccolithophores in oceanic waters due to the variety o f controls that influence their presence. Much o f the present knowledge on the distribution o f living coccolithophores comes from the collection o f surface water samples (0 - 10m) by a slow moving research vessel. A variety o f techniques collect surface water samples; these include neuston nets (dragged alongside the ship), filtration pump systems (to filter the coccolithophores out from the sea water) and CTD rigs. Many new species have been discovered and described in this way, but only limited information has been obtained on the vertical distribution o f species or on those which inhabit deeper waters (80 - 220m). There is also a lack o f quantitative studies o f surface water species and little has been done to note the relative abundances o f coccolithophores to other phytoplankton and the cause o f any variations within the assemblages collected.
Sediment traps and multicorers are among the other sampling techniques used to collect ocean bottom oozes and soft sediments for the study o f recent coccoliths. Sediment traps are devices that passively capture particles settling through the water column. They come in many varieties, shapes and forms, all adapted to their specific requirements and can be deployed either in anchored, vertical arrays or as free floating collective devices, for hours, days or even months at a time. No one trap is used by all scientists but all designs have been calibrated against one another in various sediment trap inter comparison experiments (Gardner, 1980a, 1980b; Spencer, 1981). Sediment samples can also be taken from the tops o f cores for recent studies.
It is important to remember that employing different collecting techniques can influence which species are recorded in the samples collected, because they may sample different parts o f the biocoenosis (through time/season or depth), or different stages o f the thanatocoenosis (sediment crop).
3 .2 .3 . Preseivation, prepai^tion and obseivation techniques o f living species
Over the years a variety o f different techniques have been used in the preservation, preparation and observations o f living coccolithophores collected from surface water samples due to advancements in technologies.
The first workers (Huxley, 1868; Wallich, 1877) used the Light Microscope under normal polarized light to identify species and these were described with delicate line drawings. The quality o f these drawings varied greatly. Polarizing microscopes were not introduced until much later, as they became available, in geological studies o f coccoliths in chalk. Great advances came in the 1950's and 60's due to greatly increased magnification with the use o f Transmitted Electron Microscopy (TEM) in which carbon replicas were made o f the specimens and photographs taken. This technique was then taken one step further in the 1970's with the development o f Scanning Electron Microscopy (SEM) and photographs o f the actual specimens rather than the replicas.
3.2 .4 . Biogeographic distiihution o f living coccolithophones in ocean waters
The distribution and ecology o f pelagic coccolithophores in surface waters have been studied by many different authors including Has le (1960), Smayda (1963), McIntyre & Be (1967), McIntyre el a l (1970), Marshall (1970), Okada (1970), Gaarder (1971), Okada & Honjo (1973a), Okada & McIntyre (1977) and more recently Jordan (1988), Kleijne et al. (1989) and Kleijne (1990 & 1991) amongst others.
Coccolithophores are seen to form 5 major latitudinal zones. These zones are - SubArctic, Temperate (Transitional), Subtropical (Central), Tropical (Equatorial) and Sub Antarctic (Figure 3.2) and were described in the Atlantic Ocean by McIntyre & Be (1967) and from the Pacific Ocean by Okada & Honjo (1973a). McIntyre & Be (1967) were able to distinguish five different assemblages in the Atlantic, whilst Okada & Honjo (1973a) identifiée^ four biogeographic provinces in the Pacific, very similar to those in the Atlantic. Figure 3.3 summarizes the information from both o f these studies. The rough estimations o f the zones were determined mainly by the distinct species assemblages which can be assigned to each o f the zones. These are, in most cases, similar to their counterparts in the opposing hemisphere. The zones are associated with major water masses and their movements, e.g. the Subtropical Zone
in the Pacific and Atlantic Oceans is synonymous with the position o f the mid-ocean gyres. The boundaries between these zones, however, are not simple but are instead highly convoluted and changeable due to the continual movement o f the ocean water masses forming a region o f transition between the two converging zones in which the properties are mixed to varying degrees.
McIntyre & Be (1967) collected surface water and sediment core top samples with the use o f 4 research vessels on 8 different cruises from 1964 - 1966. Each cruise lasted a few months. More than 70 species were identified and o f these McIntyre & Be (1967) described relative abundances o f 13 placolith and cyrtolith species due to their importance in the sediments collected. They erected 5 zones based upon the floral % abundances o f the species in the surface waters as well as in the sediment samples. These floral zones were defined on the distribution and associated o f certain species and are described below ;
I. Tm pical
Umbel/osphaera irregularis C yclolithella anuuJus Cyclococcolithus fra g ilis Umbellosphaera tenuis D iscosphaera tubifera Rhabdosphaera stylifera Helicosphaera carteri Gephyrocapsa oceanica Coccolithus huxleyi Calcidiscus leptoporus III. TiTUisitional Coccolithus huxleyi Calcidiscus leptoporus Gephyrocapsa ericsonii Rhabdosphaera stylifera Gephyrocapsa oceanica Umbellosphaera tenuis Coccolithus pelagicus II. Subtropical Umbellosphaera tenuis Rhabdosphaera stylifera Discosphaera tubifera C yclolithella annulus G ephyrocapsa oceanica Umbilicosphaera m irabilis H elicosphaera carteri Calcidiscus leptoporus C yclococcolithus fra g ilis Coccolithus h uxleyi