319
of inorganic nutrients particularly nitrate, but possibly also trace nutrients such as iron
(Martin, 1990) in the sub-polar regions, including the NE Atlantic, where nitrate levels remain
high through the summer. Experimental work with cultures has demonstrated that
coccolithophores continue to grow at very low iron concentrations (Kramer and Ryther, 1960;
Brand et al., 1983). Thus, as an extension to the hypothesis that the deposition of atmospheric
dust (iron) over the oceans during the glacial period maintained higher rates of diatom
productivity (Martin, 1990), it is suggested that low dust input during the interglacials favours
the growth of coccolithophores, and the pelagic deposition of CaC03•
The overall effect on the global carbon cycle of such a change in ocean ecology remains
uncertain, especially as the sedimentation of coccolithophores almost certainly enhances the
downward flux of organic carbon. In general, it appears that an increase in coccolithophore
productivity during interglacial periods would reinforce a lowering of surface alkalinity due
to changes in ocean circulation which has been postulated by Broecker and Peng (1989) as a
possible cause of higher atmospheric levels of CO2• Simple models have demonstrated the
potential effects of variation in carbon productivity due to corals (Berger, 1982) and
coccolithophores (Dymond and Lyle 1985; Taylor et al., 1991) on atmospheric CO 2 , but such
processes have yet to be incorporated into models which take account of changes in ocean
circulation and in the lysocline depth for calcite.
The sulphur cycle presents a special problem in that ice core records show greater deposition
biogenic sulphur during the glacial period (Legrand et al., 1988) when diatoms (which are not
an important source of DMS (Keller et al., 1989)) are thought to have made a proportionately
greater contribution to ocean productivity. However, interpretation of the ice core record must
take account of the short life time (days) of biogenic sulphur in the atmosphere and of the
possible scavenging role of the abundant dust (Petit et al., 1990) in the glacial atmosphere.
Thus, the observed changes in sulphur may reflect local variations in processes affecting
sulphur deposition rather than global variations in phytoplankton ecology and productivity.
In considering the climatic implications of possible future changes in oceanic phytoplankton
popUlations, in particular the effects of a potential increase in coccolithophore abundance at
high latitudes associated with global warming due to anthropogenic CO 2 , there are two major
uncertainties. The first concerns the nature of interactions between the biogeochemical cycles
of two or more critical elements and associated environmental impacts. For example, how
might variations in the light climate of surface ocean caused by the effects DMS on cloud
formation (Charlson et al., 1987) affect the relative rates of photosynthesis and calcification
and the exchange of CO2 between atmosphere and ocean? The second concerns ecological
of inorganic nutrients particularly nitrate, but possibly also trace nutrients such as iron
(Martin, 1990) in the sub-polar regions, including the NE Atlantic, where nitrate levels remain
high through the summer. Experimental work with cultures has demonstrated that
coccolithophores continue to grow at very low iron concentrations (Kramer and Ryther, 1960;
Brand et al., 1983). Thus, as an extension to the hypothesis that the deposition of atmospheric
dust (iron) over the oceans during the glacial period maintained higher rates of diatom
productivity (Martin, 1990), it is suggested that low dust input during the interglacials favours
the growth of coccolithophores, and the pelagic deposition of CaC03•
The overall effect on the global carbon cycle of such a change in ocean ecology remains
uncertain, especially as the sedimentation of coccolithophores almost certainly enhances the
downward flux of organic carbon. In general, it appears that an increase in coccolithophore
productivity during interglacial periods would reinforce a lowering of surface alkalinity due
to changes in ocean circulation which has been postulated by Broecker and Peng (1989) as a
possible cause of higher atmospheric levels of CO2• Simple models have demonstrated the
potential effects of variation in carbon productivity due to corals (Berger, 1982) and
coccolithophores (Dymond and Lyle 1985; Taylor et al., 1991) on atmospheric CO 2 , but such
processes have yet to be incorporated into models which take account of changes in ocean
circulation and in the lysocline depth for calcite.
The sulphur cycle presents a special problem in that ice core records show greater deposition
biogenic sulphur during the glacial period (Legrand et al., 1988) when diatoms (which are not
an important source of DMS (Keller et al., 1989)) are thought to have made a proportionately
greater contribution to ocean productivity. However, interpretation of the ice core record must
take account of the short life time (days) of biogenic sulphur in the atmosphere and of the
possible scavenging role of the abundant dust (Petit et al., 1990) in the glacial atmosphere.
Thus, the observed changes in sulphur may reflect local variations in processes affecting
sulphur deposition rather than global variations in phytoplankton ecology and productivity.
In considering the climatic implications of possible future changes in oceanic phytoplankton
popUlations, in particular the effects of a potential increase in coccolithophore abundance at
high latitudes associated with global warming due to anthropogenic CO 2 , there are two major
uncertainties. The first concerns the nature of interactions between the biogeochemical cycles
of two or more critical elements and associated environmental impacts. For example, how
might variations in the light climate of surface ocean caused by the effects DMS on cloud
formation (Charlson et al., 1987) affect the relative rates of photosynthesis and calcification
and the exchange of CO2 between atmosphere and ocean? The second concerns ecological