While huge dinosaurs roamed the earth, tiny single-celled algae called coccolithophores lived in the surface waters of the seas. For millions of years, coccoliths, the calcite plates that surround each microscopic cell, accumulated on the seafloor where they formed layers of calcareous sediment tens to hundreds of meters thick.
When the dinosaurs disappeared, so did most coccolithophores, except for a few inconspicuous species that had been barely noticeable before. These few species survived when others, seemingly more successful, became extinct.
In the 65 million years since that time, coccolithophore species evolved new forms in response to changes in environmental conditions. In today's oceans, there are more than 200 known coccolithophore species. Although microscopists have studied coccolithophores for more than a century, along with other phytoplankton, very little is known about their life cycles and environmental requirements. (See "Scanning electron microscopy" for more information.)
Understanding how different coccolithophore species interact with the environment is becoming more important as we face the consequences of increasing global change. Coccolithophores play a role in the exchange of carbon dioxide between the ocean and atmosphere and may affect climate in other ways as well.
Coccolithophores in the Gulf of Mexico
My research focuses on the distribution and abundance of coccolithophore species in the Gulf of Mexico. The diverse environments of the gulf make it an excellent location for marine studies. Eddies that spin off the warm current that enters the gulf through the Yucatan Straits are particularly useful. These eddies are rings of water from the Caribbean Sea that move through the gulf. Sampling both inside and outside an eddy at different times of year enables us to find out whether or how phytoplankton in water transported from the Caribbean Sea differ from phytoplankton in local waters.(See "Warm-core eddies in the Gulf of Mexico" for more information.)
Previous research in the Gulf of Mexico identified about 35 species in October surface waters. One study noted greater change in proportions of different species present in samples taken over 200 meters of depth at one place than in samples taken in surface water over hundreds of kilometers across the gulf. Still, no comprehensive quantitative data were available on coccolithophore species. Which species are most common? Where do they live? What environments are associated with which species?
To find out more about coccolithophore species that live in the gulf I examined material filtered from samples that had been collected during several gulf cruises aboard Texas A&M's R/V Gyre. At first, to become familiar with the material and develop a research methodology, I briefly examined representative filters using scanning electron microscopy and I photographed examples of the species encountered. Eventually, I documented about 100 species, including at least two not yet described in scientific journals.
To obtain detailed quantitative data on coccolithophore species concentrations and distributions, I used samples from cruises in October 1990 and March 1991. Both cruises included sampling stations inside and outside Quiet Eddy, a warm-core eddy that moved west across the gulf between the two cruises.
How many coccolithophores live in a liter?
Coccolithophore concentrations ranged from about 100 to 100,000 cells per liter. The highest concentrations occurred in March, while in October the maximum was only about 30,000 cells per liter in one sample.
These concentrations are typical of the open ocean and are low compared to blooms, population explosions that produce millions of cells per liter. During a bloom, coccolithophores and the coccoliths they shed can cover thousands of square kilometers of ocean surface and are observed with remote sensing instruments carried on satellites.
Some analyses of satellite data suggest that such blooms may occur in the Gulf of Mexico over the outer continental shelf and slope, but coccolithophore concentrations of that magnitude have not been detected in water samples. The coccolithophore most commonly involved in such blooms is Emiliania huxleyi, which is also the species that accounted for the difference in coccolithophore abundance between October 1990 and March 1991 in the gulf. Emiliania huxleyi dominated 90% of the March samples and only a few of the October samples, although it was always present. Between October and March overall concentrations of other species did not vary much, but proportions of some species did change.
Some species live in the attic, others in the basement
A major goal of my research is to understand how coccolithophores respond to their environments. Coccolithophores live in the photic zone-the upper part of the ocean where there is enough light to support photosynthesis. (See "Light and nutrients" for more information.)
Think of the photic zone as a house. First, imagine a glass-roofed, three-story house with an attic, a main-floor kitchen, and a basement. The brightly lit attic is vented to the atmosphere and stirred by breezes; the temperature is uniform and warmer than the rest of the house. Between the attic and kitchen temperature changes dramatically. The kitchen is much cooler than the attic but somewhat warmer at the ceiling than at the floor. Temperature changes again between the kitchen and the basement, which is cooler and dimly lit. It contains the pantry full of food.
In October the photic zone in the Gulf of Mexico has three layers that correspond to the three stories of the house. Coccolithophores in the "basement," the deepest layer, have direct access to nutrients but are slow to use them for lack of sufficient light. Coccolithophores in the "kitchen," the middle layer, have moderate light and immediately use any nutrients that come in from the basement as well as leftovers, recycled nutrients released by the decay of dead plankton and other organisms. Most coccolithophores live in this layer. In the "attic," the uppermost layer, coccolithophores have plenty of light but a lack of new nutrients from below forces them to rely on recycled ones.
My census of coccolithophores living in the Gulf of Mexico in October found that different species live in different layers of this photic-zone "house." Most residents of the shallow layer belong to the Umbellosphaera irregularis species. The dominant species in the middle layer was Umbellosphaera tenuis and in the deepest, nutrient-rich layer it was Florisphaera profunda. These three species, as well as some others, were almost completely separated into the three layers. Other species lived mostly in one level but also in the next level, and a single species, Emiliania huxleyi, was common at all three levels.
Now imagine a two-story house where the kitchen and attic are combined into a single upper level and a basement lies beneath it. This corresponds to the structure of the photic zone in March. The high-ceilinged kitchen is uniform in temperature, but not as warm as the attic was in October. The basement is about the same as before but there is less difference in temperature between the basement and kitchen.
The census of coccolithophores revealed that in March, some of the kitchen species had become more numerous while the species formerly exclusive to the attic were now few in number. The basement species were still there but some of them had ventured upward and seemed to prosper by it. No species was very numerous compared to Emiliania huxleyi which dominated everywhere except a few areas of the basement.
The reason for the change in abundance and distribution of coccolithophore species is the change that occurs with the seasons. During the winter, the gulf surface receives less energy from the sun and more from the wind. Stronger northerly winds stir the ocean ever deeper while cold air cools the surface water. This mixing brings both nutrients and coccolithophores into the upper layers from deeper water. Those species that need more light and more nutrients are able to grow faster. Species that require higher temperatures suffer. Species that require higher concentrations of nutrients and are adapted to lower light conditions remain in the basement. When light is sufficient Emiliania huxleyi grows faster in response to increased inputs of nutrients.
Effects of Quiet Eddy
The three-level and two-level house analogy applied both inside and outside the eddy but the sizes of the different levels varied. Within the eddy in October the shallow, brightly lit layer was slightly larger, the middle layer was considerably expanded, and the deep layer was contracted.
Coccolithophores outside the eddy originated in the gulf while those inside the eddy originated in the Caribbean. If the species in the gulf and in the Caribbean were different initially, we would expect to see greater differences between species inside and outside the eddy in October than in March, after species had five months to adjust to environmental conditions in the gulf. As it turned out, proportions of species changed beween October and March, but the same changes occurred inside and outside the eddy.
In the Gulf of Mexico, it is clear that transport of coccolithophores from the Caribbean does not change the species composition. Instead, seasonal change is much more important in determining the species distribution.
During both cruises the inventory of species was similar inside and outside the eddy, but concentrations of coccolithophores were higher inside the eddy. Also, high concentrations of coccolithophores extended deeper into the photic zone inside the eddy. This was particularly evident in March when coccolithophores were abundant throughout the mixed, upper layer (the high-ceilinged kitchen), which reached depths of 100 to 130 meters. It remained true even though at a depth of only 70 meters, light was already reduced to the minimum amount thought to be necessary to support photosynthesis (1% of surface intensity).
Nutrients were undetectable in water as deep as 130 meters which implies that phytoplankton were actively growing 30 to 60 meters below the 1% light threshold. It could be argued that phytoplankton were not growing in the dark but had been mixed to those depths from shallower water. In October, however, nutrients were depleted in waters as deep as 120 meters even though the mixed, upper layer extended only to 55 meters at most. Thus, wind mixing does not account for the depth of nutrient depletion in October.
Elsewhere in the world ocean
Differences in species present at different depths and seasonal changes in abundance and proportions of species are not limited to the Gulf of Mexico. For example, one study found a similar three-layer structure of coccolithophore species in the Pacific in late summer, at 155°W between about 10°N and 35°N. Vertical segregation of various groups of coccolithophore species was the pattern in samples taken between 15°S to 35°N. In a study covering several years, phytoplankton species at about 28°N, 155°W varied both seasonally and vertically.
Warm-core rings that break from the Gulf Stream in the Atlantic, like eddies from the Loop Current in the gulf, also show seasonal influences. Texas A&M scientists found that phytoplankton species assemblages, including coccolithophores, were more alike in the same season in different rings in different years than in one ring sampled during different seasons of a single year.
Groups of coccolithophore species are associated with certain environments in the world's oceans. The broader outlines of the relationships between phytoplankton species and environment are beginning to emerge. Although phytoplankton occupy three times the earth's surface that land plants do, we know far less about these important residents of the oceans. We cannot yet anticipate how environmental change might affect species living in the oceans and ultimately world climates and food stocks. Investigation is likely to hold as many surprises in the future as it has in the past.
Emiliania huxleyi [~26K] (Photo by Vita Pariente)
Umbellosphaera irregularis [~31K] (Photo by Vita Pariente)
Umbellosphaera tenuis [~20K] (Photo by Vita Pariente)
Florisphaera profunda [~44K] (Photo by Vita Pariente)
[~33K] Map of the Gulf of Mexico showing sample locations and the
movement of Quiet Eddy.
[~24K] Distribution of coccolithophore species in the water column in
October.
[~24K] Distribution of coccolithophore species in the water column in
March.
