318
degradation of DMSP and the consumption of DMS (Kiene and Bates, 1990); Another
striking feature of the data in Fig. 9 is that the maximum DMS concentrations were about an
order of magnitude greater than average values for surface ocean waters (Andreae, 1986) and
represented as much as 20% of the algal DMSP. This high proportion suggest that the
breakdown of DMSP to DMS is relatively rapid in coccolithophore blooms.
However, it is not possible to estimate global values for DMSP or DMS production by
coccolithophores until there is comparable data on the productivity of this group of
phytoplankton. Measurements of sulphur gases or CCN abundance in the atmosphere above
coccolithophore populations have not been made, even though the large scale of the blooms
(Fig. 2) and the high levels of DMS in the water (Fig. 9) represent ideal conditions for such
an experiment. In the context of global budgets, a further point of interest is that the DMSP
content of coccolithophores (estimated from the data in Fig. 9 assuming a carbon-tochlorophyll ration of 40) represents about 15 % of cellular organic carbon. This compound
therefore appears also to have an important role in the marine carbon cycle given the
abundance of coccolithophores and other DMSP-producing phytoplankton (Keller et al., 1989)
in the oceans.
DISCUSSION
The information summarised in this paper suggests that the coccolithophores have a significant
effect on the radiation budget of the surface layers of the ocean and on the ocean-atmosphere
cycles of carbon and sulphur. Also their distributions change with variations in climate,
becoming more abundant and widespread in warmer periods. Such changes are most
pronounced at mid-latitudes following displacements of the oceanic polar fronts (Ruddiman
et al., 1980; Ruddiman and McIntyre, 1981), as shown by data on coccolith abundance in
sediment cores (Fig. 10) and on the distribution of CaC03 in the oceans (Bishop, 1989). Shifts
of the polar fronts towards the poles during climatic amelioration lead to a marked increase
in the areal extent of temperate and sub-polar waters and it is in this hydrographic regime that
coccolithophores are most abundant today. At the same time the continental shelves have
become flooded, creating shallow water environments in which E. huxleyi is abundant today
and which are comparable to the major sites of coccolithophore deposition during the Jurassic
and Cretaceous periods (Gallois, 1976).
The well-known seasonal succession in marine phytoplankton communities (Margalef, 1978)
from diatoms to coccolithophores and then flagellates is observed consistently in a wide range
of habitats. The appearance of coccolithophores is usually associated with the surface depletion
degradation of DMSP and the consumption of DMS (Kiene and Bates, 1990); Another
striking feature of the data in Fig. 9 is that the maximum DMS concentrations were about an
order of magnitude greater than average values for surface ocean waters (Andreae, 1986) and
represented as much as 20% of the algal DMSP. This high proportion suggest that the
breakdown of DMSP to DMS is relatively rapid in coccolithophore blooms.
However, it is not possible to estimate global values for DMSP or DMS production by
coccolithophores until there is comparable data on the productivity of this group of
phytoplankton. Measurements of sulphur gases or CCN abundance in the atmosphere above
coccolithophore populations have not been made, even though the large scale of the blooms
(Fig. 2) and the high levels of DMS in the water (Fig. 9) represent ideal conditions for such
an experiment. In the context of global budgets, a further point of interest is that the DMSP
content of coccolithophores (estimated from the data in Fig. 9 assuming a carbon-tochlorophyll ration of 40) represents about 15 % of cellular organic carbon. This compound
therefore appears also to have an important role in the marine carbon cycle given the
abundance of coccolithophores and other DMSP-producing phytoplankton (Keller et al., 1989)
in the oceans.
DISCUSSION
The information summarised in this paper suggests that the coccolithophores have a significant
effect on the radiation budget of the surface layers of the ocean and on the ocean-atmosphere
cycles of carbon and sulphur. Also their distributions change with variations in climate,
becoming more abundant and widespread in warmer periods. Such changes are most
pronounced at mid-latitudes following displacements of the oceanic polar fronts (Ruddiman
et al., 1980; Ruddiman and McIntyre, 1981), as shown by data on coccolith abundance in
sediment cores (Fig. 10) and on the distribution of CaC03 in the oceans (Bishop, 1989). Shifts
of the polar fronts towards the poles during climatic amelioration lead to a marked increase
in the areal extent of temperate and sub-polar waters and it is in this hydrographic regime that
coccolithophores are most abundant today. At the same time the continental shelves have
become flooded, creating shallow water environments in which E. huxleyi is abundant today
and which are comparable to the major sites of coccolithophore deposition during the Jurassic
and Cretaceous periods (Gallois, 1976).
The well-known seasonal succession in marine phytoplankton communities (Margalef, 1978)
from diatoms to coccolithophores and then flagellates is observed consistently in a wide range
of habitats. The appearance of coccolithophores is usually associated with the surface depletion