118
Baumann et al.
and biochemical (dimethyl sulfide - which act as a
source molecule for cloud nucleation) effects they
likely produce an additional feedback to climate
change (Westbroek et al. 1993). Some species,
such as Emiliania huxleyi, contain alkenones, that
are long-chained (C 37 -C 39 ) di- and tri-unsaturated
ketones, first observed by Boon et al. (1978) in
Miocene to Pleistocene sediments from the Walvis
Ridge. The alkenone ratios are sensitive to the
coccolithophores' growth temperatures and, thus,
these compounds are potentially usefull in the reconstruction of past surface-water temperatures
(e.g. Brassell et al. 1986; Prahl et al. 1989). Hence,
variation in saturation ratios ofthese alkenones form
an important palaeothermometer (Uk'37 - index), but
can also be used to determine temperature affinities of the present species (Jordan et al. 1996).
Many studies on recent coccolithophores are
predominantly taxonomic (Halldal and Markali
1954; Gaarder and Markali 1956; Gaarder and
HeimdaI1977; Heimdal and Gaarder 1980; Heimdal
and Gaarder 1981; Kleijne 1991; Knappertsbusch
1993), although a number of investigations concerning the dynamics and ecology of calcareous
nannoplankton have been published. Individual species are mostly cosmopolitan but with more or less
limited latitudinal distributions. Although much information is available on the oceanic-scale distribution of coccolithophores (e.g. McIntyre and Be
1967; Okada and Honjo 1973; Honjo and Okada
1974; Kleijne 1993) and from smaller areas (e.g.
Winter et al. 1979; Reid 1980; Mitchell-Ines and
Winter 1987; Samtleben and Schroder 1992;
Samtleben et al. 1995; Ziveri et al. 1995), the environmental parameters that control their distribution
are still poorly understood. A recent summary of the
most important contributions on the distribution of
modem coccolithophores is given by Winter and
Siesser (1994). Using the available data, Young
(1994) separated three groups generally defined on
ecological and morphological criteria. (1) Species
characteristic for eutrophic environments. These
environments in equatorial waters, high latitudes,
as well as in upwelling areas, are dominated by
placolith-bearing species. All species within this
group are so-called r-strategists, i.e. they respond
to nutrient enrichment with enhanced growth
rates or productivity. Predominantly bloom-forming
coccolithophores (such as Emiliania huxleyi,
Gephyrocapsa
oceanica,
G.
ericsonii,
Umbilicosphaera
sibogae,
Coccolithus
pelagicus) are included in this group. (2) Upper
water communities in subtropical latitudes from all
oceans are dominated by the species
Umbellosphaera tenuis, U. irregularis, and
Discosphaera tubifera. These umbelliform species are so-called K-strategists, which are adapted
to low nutrient contents, especially of the
oligotrophic mid-ocean gyres. (3) The deep
euphotic-zone assemblages (150-200m) in low- to
mid-latitude are dominated by a third group of
species, such as Florisphaera profunda and
Gladiolithus flabellatus. The absence of these
floriform species in surface-waters suggest that
they live below the mixed layer were the environment is characterized not only by low light intensity, but also by relatively high nutrient levels. In
addition, miscellaneous species do not have an obvious distribution pattern, although some of them
occasionally dominate the assemblages.
The described distribution patterns of living
communities are generally reflected in bottom
sediments (e.g. McIntyre and Be 1967), although
relatively little is known about both the transformation from a living coccolithophore community in the
plankton into a coccolith assemblage of the underlying deep-sea sediment and ofthe coccolithophore
(carbonate) fluxes to the surface sediments. It is
obvious that coccolith assemblages in sediments
are reduced by selective destruction and/or dissolution and form a distorted image ofthe living communities (e.g. Samtleben and SchrOder 1992).
Fragile coccolith specimens, especially of the
umbelliform assemblages and many species ofthe
miscellaneous groups, are heavily dissolved and,
thus, less prevalent in sediment assemblages. Alteration processes during descent through the water column have been studied in sediment traps
employed in various water depths (e.g. Andruleit
1997). Also, sediment trap studies have been performed in order to examine the seasonal patterns
of coccolithophore communities and to estimate the
contribution of coccolithophores to the total carbonate flux (Samtleben and Bickert 1990; Steinmetz
1991; Knappertsbusch and Brummer 1995; Ziveri
et al. 1995; Andruleit 1997). However, most of
Baumann et al.
and biochemical (dimethyl sulfide - which act as a
source molecule for cloud nucleation) effects they
likely produce an additional feedback to climate
change (Westbroek et al. 1993). Some species,
such as Emiliania huxleyi, contain alkenones, that
are long-chained (C 37 -C 39 ) di- and tri-unsaturated
ketones, first observed by Boon et al. (1978) in
Miocene to Pleistocene sediments from the Walvis
Ridge. The alkenone ratios are sensitive to the
coccolithophores' growth temperatures and, thus,
these compounds are potentially usefull in the reconstruction of past surface-water temperatures
(e.g. Brassell et al. 1986; Prahl et al. 1989). Hence,
variation in saturation ratios ofthese alkenones form
an important palaeothermometer (Uk'37 - index), but
can also be used to determine temperature affinities of the present species (Jordan et al. 1996).
Many studies on recent coccolithophores are
predominantly taxonomic (Halldal and Markali
1954; Gaarder and Markali 1956; Gaarder and
HeimdaI1977; Heimdal and Gaarder 1980; Heimdal
and Gaarder 1981; Kleijne 1991; Knappertsbusch
1993), although a number of investigations concerning the dynamics and ecology of calcareous
nannoplankton have been published. Individual species are mostly cosmopolitan but with more or less
limited latitudinal distributions. Although much information is available on the oceanic-scale distribution of coccolithophores (e.g. McIntyre and Be
1967; Okada and Honjo 1973; Honjo and Okada
1974; Kleijne 1993) and from smaller areas (e.g.
Winter et al. 1979; Reid 1980; Mitchell-Ines and
Winter 1987; Samtleben and Schroder 1992;
Samtleben et al. 1995; Ziveri et al. 1995), the environmental parameters that control their distribution
are still poorly understood. A recent summary of the
most important contributions on the distribution of
modem coccolithophores is given by Winter and
Siesser (1994). Using the available data, Young
(1994) separated three groups generally defined on
ecological and morphological criteria. (1) Species
characteristic for eutrophic environments. These
environments in equatorial waters, high latitudes,
as well as in upwelling areas, are dominated by
placolith-bearing species. All species within this
group are so-called r-strategists, i.e. they respond
to nutrient enrichment with enhanced growth
rates or productivity. Predominantly bloom-forming
coccolithophores (such as Emiliania huxleyi,
Gephyrocapsa
oceanica,
G.
ericsonii,
Umbilicosphaera
sibogae,
Coccolithus
pelagicus) are included in this group. (2) Upper
water communities in subtropical latitudes from all
oceans are dominated by the species
Umbellosphaera tenuis, U. irregularis, and
Discosphaera tubifera. These umbelliform species are so-called K-strategists, which are adapted
to low nutrient contents, especially of the
oligotrophic mid-ocean gyres. (3) The deep
euphotic-zone assemblages (150-200m) in low- to
mid-latitude are dominated by a third group of
species, such as Florisphaera profunda and
Gladiolithus flabellatus. The absence of these
floriform species in surface-waters suggest that
they live below the mixed layer were the environment is characterized not only by low light intensity, but also by relatively high nutrient levels. In
addition, miscellaneous species do not have an obvious distribution pattern, although some of them
occasionally dominate the assemblages.
The described distribution patterns of living
communities are generally reflected in bottom
sediments (e.g. McIntyre and Be 1967), although
relatively little is known about both the transformation from a living coccolithophore community in the
plankton into a coccolith assemblage of the underlying deep-sea sediment and ofthe coccolithophore
(carbonate) fluxes to the surface sediments. It is
obvious that coccolith assemblages in sediments
are reduced by selective destruction and/or dissolution and form a distorted image ofthe living communities (e.g. Samtleben and SchrOder 1992).
Fragile coccolith specimens, especially of the
umbelliform assemblages and many species ofthe
miscellaneous groups, are heavily dissolved and,
thus, less prevalent in sediment assemblages. Alteration processes during descent through the water column have been studied in sediment traps
employed in various water depths (e.g. Andruleit
1997). Also, sediment trap studies have been performed in order to examine the seasonal patterns
of coccolithophore communities and to estimate the
contribution of coccolithophores to the total carbonate flux (Samtleben and Bickert 1990; Steinmetz
1991; Knappertsbusch and Brummer 1995; Ziveri
et al. 1995; Andruleit 1997). However, most of