Article: Hopkins, J., S. A. Henson, S. C. Painter, T. Tyrrell, and A. J. Poulton (2015), Phenological characteristics of global coccolithophore blooms, Global Biogeochem. Cycles, 29, 239–253, doi:10.1002/2014GB004919.
Coccolithophores: A worldly phytoplankton
While coccolithophore might seem like just another funny word, it actually denotes an elite member of the phytoplankton community, the microscopic photosynthesizers which make-up the base of the marine food web. Coccolithophores are a cosmopolitan phytoplankton adorned with outer plates made of calcium carbonate, a form of inorganic carbon that is the same material found in seashells and classroom chalk. Globally, coccolithophores are estimated to make up anywhere from 5 to 40% of the global primary production, or food creation using photosynthesis. And since calcium carbonate is a pretty heavy material for a phytoplankton exoskeleton, coccolithophores sink faster than many other phytoplankton species. In other words these guys transport a lot of inorganic carbon to the deep ocean.
The composition and phenology of phytoplankton blooms, or when phytoplankton grow at a very fast rate, has been a hot topic for decades. Phytoplankton phenology studies life cycle events, such as the peak time and concentration of a bloom, in reference to seasonal cycles, geographic position, and interannual variability. One of the old theories is that blooms would be dominated by a single phytoplankton group. However, this study wanted to test the hypothesis that more than one phytoplankton species could co-exist in a bloom!
Phytoplankton blooms favor succession, meaning that as environmental conditions and nutrients change, different phytoplankton species become better adept at thriving. So where do coccolithophores fit into the bloom hierarchy? and can they co-exist with other phytoplankton groups?
The approach: using satellites to see phytoplankton
Scientists now can estimate phytoplankton concentrations, or assess the time of a bloom, from the comfort of anywhere they have a computer and internet, using satellites to measure chlorophyll-a. For example, NASA’s Ocean Color product uses the AQUA MODIS satellite to measure the molecule chlorophyll-a throughout the global open ocean (Figure 2).
Coccolithophores are photosynthetic organisms, thus produce chlorophyll-a. However, they produce much less than other phytoplankton groups. Instead, the unique calcium carbonate shell can be used to remotely measure coccolithophores from space. While chlorophyll-a is measured by how much light is absorbed, particulate inorganic carbon (such as calcium carbonate) is measured by how much light is scattered.
Hopkins et al., used this backscattered light to estimate the concentration of particulate inorganic carbonate with the assumption that changes in these concentrations reflect changes in coccolithophore populations. Looking back at satellite data, they were able reconstruct a 10 year history of coccolithophore blooms for most of the global ocean. Figure 3 depicts how the researchers assessed when a bloom occurred, peaked, and ended (a.k.a. phenology).
What did they find?
For the Northern Hemisphere, coccolithophore blooms generally began in the low latitudes (~30°N) between February and March and would last for 6 to 7 months (Figure 4). Moving northward, coccolithophore blooms began later and got shorter. For example, closer to the pole, blooms would begin between April and May and last for 3 to 4 months. The peak bloom timing also followed a similar northward latitude.
Patterns were similar in the Southern Hemisphere, but followed the southern hemisphere seasons. (Winter in the northern hemisphere is summer in the southern hemisphere.) Start coccolithophore bloom times began between August and September and last for about 8 months.
One of the most exciting finds from this study was the co-occurrence of chlorophyll-a and particulate organic carbon in some regions. This indicates the coccolithophores co-existed in a bloom with another taxon of phytoplankton and were not just part of a seasonal succession. Co-existence was defined as a ± 16 day lag between peak chlorophyll-a and particulate organic carbon concentrations while species succession was defined as a greater than 24-day lag.
Of course, co-existence did not exist everywhere. Succession, or replacement, of phytoplankton group was observed in areas including upwelling zones, coastal shelves, and high latitudes. It was hypothesized that environmental conditions, such as iron availability, may help one taxon become ecologically dominant. An example would be when earlier diatom blooms occurred; these phytoplankton would use up most of the nutrients, leading to the smaller species like the coccolithophore to come in and replace them.
First and foremost, the study created global maps of coccolithophore phenology including bloom start and peak timing and bloom duration using satellite data. Additionally, in the open ocean, coccolithophores were shown to potentially co-exist with other phytoplankton groups, rather than just be replaced through species succession. This helps to change the way we think of ecological niches and theory by suggesting two taxa can get along!