Phytoplankton photophysiology in lakes and oceans

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Phytoplankton are photosynthetic microbes that are ubiquitous in natural waters; they are an essential part of aquatic ecosystems as they form the base of many food webs. They include an incredible diversity of shapes, sizes, and species. When ecosystems are altered by human influences, in particular by the addition of nutrients from agricultural runoff, phytoplankton species that were not previously abundant can proliferate. When these species are harmful (producing toxins), such as some bloom-forming cyanobacteria species, the quality of water for drinking or recreation can be compromised. In our research program, we are interested in understanding why some phytoplankton species are present at certain times and under which environmental conditions they are favored. To this end, we have deployed an autonomous research platform that allows us (using a sophisticated "submersible microscope" in a lake) to follow through time the species present as well as their grazers (zooplankton), in addition to other important factors such as nutrients and light. We also aim to understand how changes in the phytoplankton community affect other parts of the ecosystem. In the next few years, we will be focusing on the important role of the abundant aquatic bacteria. Because they are photosynthetic, just like plants on land, some are adapted to high light environments (growing near the surface) and others prefer dim light. Some phytoplankton can easily adjust to fast transitions from dim to high light, but others cannot. In our program, we will study in the laboratory how different species acclimate to these shifts in light levels, in particular how efficiently they can achieve this acclimation at different times of day. Such studies will allow us to create appropriate models to estimate future changes in phytoplankton abundance and productivity. Because phytoplankton absorb light, we can measure their abundance in surface waters using satellites (green waters contain high concentrations of phytoplankton, while blue waters contain low concentrations). This provides us with information about phytoplankton abundances over the whole planet every few days. Phytoplankton also emit a small amount of red light in the form of fluorescence. This fluorescence contains information about the physiology - or health - of phytoplankton communities. This can also be measured using satellites. In our program, we will try to further understand how to interpret and correct measurements of this fluorescence emission. Satellites measuring the color of the ocean also provide information about other absorbing and scattering (light bouncing off of particles) components and many algorithms and methods exist to obtain such measurements. Yet, there appears to be untapped information in this remotely measured signal that remains to be understood. We will be using a novel method to look at anomalies in the satellite signal to try to identify the sources of this signal and explore how it could be used to obtain more information about the ocean from space. To understand satellite measurements of light, one very important aspect is to understand which particles are backscattering light to the satellites. Indeed while we are able to measure the light, we do not yet know which particles are responsible for scattering the sunlight back to the satellite sensors. In our program, in collaboration with 4 groups of international collaborators, we are launching a large project to examine the backscattering efficiencies of particles in order to better understand the sources of this backscattering. This should allow us to better interpret the information we are measuring using satellites and gain new insights into the functioning of the oceans.