Biogeochemical Impacts


Emiliania huxleyi blooms may be very important in terms of global planetary temperature. When these blooms appear over large expanses of the ocean they have myriad effects on the water and on the atmosphere above. Although each cell is invisibly small, there can be as many as a thousand billion billion (10^21) of them in a large bloom, and the population as a whole has an enormous impact. Ongoing work is trying first of all to estimate whether the net effect of the blooms is to exacerbate or to ameliorate global warming; subsequently the magnitude of the combined forcing on global temperature will be estimated.


Ehux blooms are processed through the food web, with viruses, bacteria and zooplankton all contributing to the demise and decomposition of blooms. Some debris from the bloom survives to sink to the ocean floor, taking chemicals out of the water column. While they live and when they die, the phytoplankton cells leak chemicals into the water. A bloom can be thought of as a massive chemical factory, extracting dissolved carbon dioxide, nitrate, phosphate, etc from the water, and at the same time injecting other chemicals such as oxygen, ammonia, DMS and other dissolved organic compounds into the water. At the same time, the chemical factory pumps large volumes of organic matter and calcium carbonate into the deep ocean and to the ocean floor. Some of this calcium carbonate eventually ends up as chalk or limestone marine sedimentary rocks, perhaps to cycle through the Earth's crust and to reappear millions of years later as mountains, hills and cliffs. Picture courtesy of Glynn Gorick (copyright held).

Global abundance: a global satellite study by Brown & Yoder detected an annual area of blooms of 1.4 million km2. This is the total area each year that was classified as coccolithophore blooms, i.e. that was unobscured by clouds and that satisfied the imposed thresholds in terms of brightness, size of bloom area, etc. We know that there is also a lot of coccolithophore productivity in non-bloom areas, for instance in oligotrophic tropical oceans (Balch & Kirkpatrick, 1996), where there is a continual background turnover of coccolithophores even though their abundance never reaches bloom proportions.

Total calcite production: because calcite fluxes are not usually determined just for Ehux calcite, we will consider the biogeochemical impacts of global ocean calcite productivity, i.e. productivity of Ehux and other coccolithophores, and also other organisms. Although Ehux is frequently the most abundant coccolithophore (by number) in seawater samples, because of its smaller size it doesn't always dominate the calcite flux (by weight). Other coccolithophores with larger coccoliths, such as Calcidiscus leptoporus will usually be more important to the calcite export flux. From productivity estimates, sediment trap measurements, and from geochemical models, we can estimate how much calcium carbonate (calcite) is produced annually by coccolithophores and foraminifera and coral reefs (the main producers of calcite in the oceans). Several authors (Morse & Mackenzie, 1990; Wollast, 1994; Milliman, 1993; Shaffer, 1993; Archer & Maier-Reimer, 1994) have estimated total global calcite productivity as 0.63, 1.13, 0.64, 1-2 and 1.2 (average ~1.0) Gt calcite-C year-1. The relative contributions of coccolithophores, foraminifera and other organisms to the total remain relatively poorly known.

Total calcite burial: a significant percentage of the produced calcite (somewhere between 0 and 80%) probably gets dissolved before incorporation into sediments, which raises the question: how much calcite does get buried? This figure is more easily obtained, from averaging the amount of calcite in different marine sediment cores and dividing by the rate of accumulation of the material in the cores. Milliman (1993) estimates a calcium carbonate sedimentary accumulation rate of 0.46 Gt C year-1.

A lot of the dissolution of calcite (the difference between production and burial) probably occurs in surface waters, but also a lot occurs at very great depths. From comparison of the amounts of calcite in different marine sediment cores, it is apparent that no calcite at all reaches some parts of the sea bed, especially the sea bed underneath the very deepest waters. This because of what is known as the calcite lysocline (Broecker & Peng, 1982). Below a certain depth (4-5km or so) the great pressures cause a change in the seawater chemistry such that calcite is forced into solution. The water suddenly becomes much more corrosive to calcite, and all of the calcite fragments raining towards the sea floor suddenly get rapidly dissolved below the critical lysocline depth.

Types of climate forcing: there are several ways in which coccolithophores influence regional and global temperature:

1. Ocean Albedo: as described on the optical impacts page, coccoliths act somewhat like little mirrors, because of the special optical properties of calcite. This causes a typical bloom (containing 100 mg m-3 of calcite carbon) to increase the ocean albedo from ~7.5% to ~9.7%, as shown in the model-derived diagrams below. Another type of optical model (more accurate for this purpose) calculates an increase from ~6.2% to ~9.7%. If each bloom is assumed to persist for about a month, then an annual coverage of 1.4 x 10^6 km squared (see above) will increase the global annual average planetary albedo by
(9.7-6.2) x (1/12) x (1.4/510) = 0.001%
where 510 is the surface area of the Earth in 10^6 km squared. This is a lower bound on the total impact, because sub-bloom concentration coccolith light scattering will have an impact, over much larger areas (estimated maximum albedo impact = 0.21%), as will cloud-obscured blooms, which were not included in the global total of Brown & Yoder because they could not be seen by the satellite. A 0.001% albedo change corresponds to a 0.002 W m-2 reduction in incoming solar energy, whereas an albedo change of 0.21% causes a reduction of 0.35 W m-2. These two numbers can be compared to the forcing due to anthropogenic addition of CO2 since the 1700's, estimated to be about 2.5 W m-2. Coccolith light scattering is therefore a factor of only secondary importance in the radiative budget of the Earth.




Photon budgets for water without any coccoliths (top diagram), and for water with a concentration of 100 mg m-3 of calcite carbon in the form of coccoliths.

2. Ocean Heat Retention: the scattering caused by coccoliths causes more heat and light than usual to be pushed back into the atmosphere. It also causes more of the remaining heat to be trapped near to the ocean surface, and only allows a much smaller fraction of the total heat to penetrate to deeper in the water (see photon budgets above). Because it is the near-surface water which exchanges heat with the atmosphere, all three of the effects just described conspire to mean that coccolithophore blooms may tend to make the overall water column dramatically cooler over an extended period, even though this may initially be masked by a warming of the surface skin of the ocean (the top few metres). The importance of this effect, both regionally and globally, is currently being worked upon but has not yet been established.

3. Cloud Albedo: phytoplankton produce a sulphur compound called dimethyl sulphide (DMS), with coccolithophores and other species producing 100 times more of it than diatoms (Keller, 1989). After the cells die, and after several subsequent biological and chemical transformations in the ocean and in the atmosphere, some of this DMS eventually ends up as cloud condensation nuclei (CCN) in the atmosphere. These CCN help to stimulate cloud formation, especially in areas where other sources of CCN are in short supply, and the amount of CCN is limiting possible cloud growth. In this way, coccolithophore productivity helps to increase the reflectiveness (albedo) of the planet by allowing more clouds to form. Due to the complexities of the processes involved in the transformation from phytoplankton DMS to CCN, we do not yet have a good estimate of how important this forcing is.

4. CO2 Greenhouse: all phytoplankton growth removes carbon dioxide (CO2) into organic matter and reduces atm. CO2 (click here for explanation). However, coccolithophores are unique in that they also take up bicarbonate (HCO3), with which to form the calcium carbonate of their coccoliths. The chemical reaction for coccolith formation is:
Ca + 2HCO3 ---> CaCO3 + H2O + CO2
There are three forms of dissolved carbon in seawater: CO2, HCO3 and CO3; and carbon can shift very easily from being in one of these dissolved forms to being in another. How much of the total carbon is in each form is determined mainly by the alkalinity and by the water temperature. When the seawater carbon system is perturbed by coccolithophore cells removing HCO3 to form coccoliths, this causes a re-arrangement of how much carbon is in each dissolved form, and this rearrangement takes place more or less instantaneously. The removal of 2 HCO3 molecules and the addition of one CO2 molecule changes the alkalinity and this indirectly causes more of the dissolved carbon to be pushed into the CO2 form. Although the total dissolved carbon is obviously reduced by removal of dissolved carbon (bicarbonate ions) into solid calcium carbonate, yet the total effect, paradoxically, is to produce more dissolved CO2 in the water. In this way, coccolithophore blooms tend to exacerbate global warming (by causing increased atmospheric CO2), rather than to ameliorate it, as is the case when dissolved CO2 goes into new organic biomass.

However, recent work is showing that additional properties of coccoliths may make the situation yet more complicated. Coccolith calcite is rather dense (2.7 kg per litre compared to seawater density of 1.024 kg per litre), and the presence of coccoliths in zooplankton faecal pellets and `marine snow' (the two main forms in which biogenic matter sinks to the deep ocean) causes them to sink more rapidly. Slow-sinking organic matter may also adhere to the surfaces of coccoliths, hitching a fast ride out of the surface waters. If organic matter sinks faster then there is less time for it to be attacked by bacteria and so more of the locked-in carbon will be able to escape from the surface waters, depleting the surface CO2.

A recent paper (Buitenhuis et al, 1996) argues that this co-transport of organic matter with coccoliths offsets the atmospheric CO2 increase that would otherwise be caused, and makes coccolithophore blooms act to oppose global warming, rather than to intensify it.

References

Ehux home page

Toby Tyrrell : T.Tyrrell@noc.soton.ac.uk