Frontiers in Ocean Optics and Ocean Colour Science – An IOCCG lecture series: Thoughts and South African perspectives.
For two weeks in July I was one of a very privileged few chosen to attend a summer lecture series organised by the International Ocean Colour Coordinating Group (IOCCG). The IOCCG operates as a voice for the ocean colour community, encouraging collaborative calibration and validation exercises, producing synthesis reports and providing training for those who work with ocean colour data. Our two week course was designed to expose early career scientists to the current frontline research in ocean optics and create an environment where students could discuss questions arising with the very best academics from the field.
I think the course will prove absolutely invaluable for my PhD research and for my future career, but I also came away with a number of thoughts and ideas for how frontiers in ocean optics and ocean colour research fit in a broad South African context.
Regional questions for a global data set
A lot of the major problems with using ocean colour data, from atmospheric corrections to chlorophyll and suspended sediment algorithms, result from differences in fundamental optical properties at a regional level, making it hard to find a “one size fits all” solution. The Case 1/Case 2 definition has been used to help separate waters to address this in some algorithm applications. However there are always exceptions and today many approaches are expanding on these old distinctions, classifying water types based on their optical properties. In South Africa we have a huge diversity of water types, allowing us the opportunity to really look in depth at the natural range of variability in optical properties and the consequences for ocean colour data. We have highly productive waters, waters with high sediment content, in land lakes carpeted in cyanobacteria and the deep, open ocean waters of the Southern Ocean, all within relatively easy access.
With the recent demise of ENVISAT it has become apparent how much we can come to rely on the specific characteristics of technology to help answer our scientific questions. For my research in the Benguela, the bands available on a satellite (how often the satellite measures the different colours over the light spectrum) are really important, particularly those in the red. The resolution of these measurements i.e. the number of and placement of the bands, can be crucial for resolving oceanic components of interest – high chlorophyll biomass, and the pigments indicative of different phytoplankton species. A fascinating lecture by Curtiss Davis (Oregon State University) awakened my curiosity through the use of HICO data – hyper spectral radiometry from the International Space Station. Although the data from this amazing instrument is not regularly available, the wavelength resolution is more comparable to our in water radiometers – unveiling massive potential to resolve things like pigment induced absorption peaks. Great for helping to identify potentially toxic phytoplankton species. A series of lectures from Richard Stumpf (NOAA, National Ocean Service) highlighted these specific requirements for Harmful Algal Bloom monitoring, a major focus of my research work. With the occurrence of high biomass, mono-specific blooms in South African waters, there’s real potential for us to evaluate and develop these methods.
Modelling is a key component in many modern oceanographic studies. Whether you are looking at currents, biological production or (as in our case) interactions between light (physics!) and phytoplankton (biology!), you can be sure there’ll be some modelling to be done. As we progress in asking more interdisciplinary questions, it is becoming more and more necessary to couple our models together to understand interactions between components. During the IOCCG course, I got to use Hydrolight for the first time. Hydrolight is a model which allows you to investigate the impact of different water properties on the light field that is observed by instruments like our satellite mounted radiometers. As we found out in a series of lectures from Curtis Mobley (Sequoia Scientific), Hydrolight now has the potential to be coupled to more complex physical and biological models – providing greater insight in to how incident light heats water and how much is available for biology at different depths. Crucially, this model coupling allows us to investigate the extent that biology can feed back in to these processes. With expertise in modelling already existing in South Africa using the Regional Ocean Modelling System (ROMS), the potential to investigate these coupling methods could be enormous.
Understanding and quantifying uncertainty in measurements is an essential background that must be in place to investigate trends in observations. How to “know what you don’t know” was a major component of a lot of the lectures during the IOCCG course. Errors in ocean colour data can come from many sources, including the instruments themselves, atmospheric corrections, extrapolations to ocean surface signals and algorithm assumptions. For our applications in South African waters, the diversity of sources of uncertainty are as numerous as the water types themselves.
In highly turbid waters such as those dominated by high biomass or sediment content, typical atmospheric correction methods fail due to the bright reflectances caused by these conditions. In high latitude waters, additional uncertainties are introduced. The viewing angle of the satellite and unique inherent optical properties have to be accounted for.
For long term applications such as the investigation of global climate change impacts, further uncertainties can be introduced. To detect trends and attribute causal mechanisms, long time series are needed. Current thinking suggests we may need between 18 and 60 years (location dependent) to detect trends from ocean colour data. Whilst ocean colour data collection has been happening since the 1970’s, the data is not continuous with respect to spatial and temporal sampling, or instrument used. As mentioned above, the specifications of particular instruments can make them more or less suitable for certain applications. Changing these specifications over time, as instruments change, can introduce error in the parameters generated. However the pay off for establishing usable, long time series with appropriate errors, is potentially massive, with something like just 1/8th of a satellite orbit providing more data than even our longest in situ biomass estimates.
So where is ocean colour science going from here? And where does South Africa in particular fit in to this? Well, we have several new satellites to work with. From an SA perspective, this particularly means OLCI aboard Sentinel 3 to replace our use of MERIS. The diverse range of water types in SA should provide a great chance to test the capabilities of this new instrument and hopefully in return it will provide us with continued time series of data for Harmful Algal Bloom and water quality monitoring. The global push towards understanding high latitude regions, thought to be so susceptible to climate change, shows no signs of abating. The Antarctic still remains poorly understood and undersampled, perhaps even more so than the Arctic. With the arrival of the new SA Agulhas II, South Africa has the potential to address this. Add in the new suit of optical instrumentation and developing expertise in this field and there is an exciting amount of work to be done in the near future! With great opportunities to gather in situ data, new satellites on the horizon, modelling expertise and some of the most interesting waters in the world, I think it’s safe to say that South Africa can definitely be at the frontiers of ocean optics and ocean colour science.
I would like to thank the IOCCG for this amazing opportunity and in particular David Antoine and Tatiana Donnay for their fantastic organisational skills and welcoming hospitality. Also many thanks to all the lecturers for their content which has inspired this post. Final thanks to the great group of students who I look forward to seeing again at conferences and on the seas in the future!