Towards global-scale assessments of primary production in lakes and reservoirs from space

Lakes (and reservoirs) are now recognised to play an important role in regional and global carbon cycling through the production and consumption of carbon gases and the transport, transformation and storage of organic carbon (Battin et al. 2008; Tranvik et al., 2009).
The uptake, conversion and respiration of carbon by phytoplankton during photosynthesis is the main pathway for CO2 transfer across the air-water interface in lakes and a process that contributes significantly to ecosystem biogeochemistry and metabolism, trophic energy flows and the maintenance of biodiversity. It is currently estimated that global primary production in lakes consumes some 0.65 Pg C yr-1 (Pace & Prairie, 2005). However, measurements of primary production in lakes are very sparse in space and time and, as such, global estimates of carbon uptake currently necessitate crude extrapolations that inevitably lead to unacceptably large uncertainties.
Additionally, as phytoplankton primary production is influenced by a number of factors, including the availability of light and nutrients, water temperature and vertical mixing, it is tightly coupled to climate. There is an increasing body of research ascribing changes in primary production in lakes and reservoirs to climate variability. However, the precise nature of the response to climate change has been observed to vary considerably depending on factors such as the size, depth, latitude and type of lake. For instance, there is evidence of increased productivity in shallow polymictic lakes (Wilhelm and Adrian, 2008) but decreased productivity in large stratified lakes (Coats et al., 2006). However, our understanding of how primary production in lakes is responding to climate change at regional and global scales is very limited because of the scarcity of empirical data to support such studies.
There are some recent advancements that now make the challenge of obtaining reliable estimates of primary production in lakes at local, regional and global scales more feasible. Firstly, the launch of new Earth observation (EO) satellites, such as the European Space Agency’s Sentinels, now provides a means of obtaining data on key parameters such as the phytoplankton chlorophyll concertation, light availability and temperature (Fig. 1) required to model primary production in lakes and upscale these in space and time (Fig. 2).
Secondly, in parallel, new approaches have been developed based on fast repetition rate fluorometry (FRRF) and high-frequency dissolved oxygen time series data which now enable in situ measurements of primary production to be obtained more readily without the need for laborious, expensive and potentially-hazardous 14C-based methodologies. However, while satellite data have been used successfully to derive estimates of primary production in the oceans, shelf seas and coastal waters, there have been relatively few attempts to extend these models to lakes. Moreover, the use of FRRF- and oxygen-based measurements for the calibration and validation of satellite algorithms is not well established.
The overarching aim of this PhD project is to develop the science needed to improve current estimates of carbon fixation in lakes and reservoirs and to advance our understanding of how lake primary production is being influenced by climate and other drivers of environmental change. To this end, the student will develop improved algorithms for the estimation of phytoplankton primary production in lakes from satellite data. The models will be calibrated and validated using in situ measurements of primary production obtained from FRRF- and/or O2-based sensors with verification using more conventional 14C photosynthesis-irradiance (PE) curves. The derived algorithm(s) will then be used to produce time series of primary production data for a global population of lakes to enable the effect of climate and other pressures on lake production to be assessed.