Environmental Optics

What do we do?

The Environmental Optics theme is interested in problems of radiance transfer in seawater, light utilisation by phytoplankton, optical monitoring of ecological processes, and remote sensing in the marine environment. These problems all involve the application of physical principles in an interdisciplinary context.

Why do we do it?

This is an exciting field to work in because

• Optical oceanography has undergone a rapid expansion in recent years, with six ocean colour satellites currently in orbit and growing networks of moored optical sensors in the Atlantic and Pacific oceans.
• Optical techniques are applied on a wide range of spatial scales: they can be used to detect phytoplankton blooms from space (using satellite radiometers to measure patterns of ocean colour extending over hundreds of kilometres) and to measure the fluorescence of single cells under water (using laser-based flow cytometers with a resolution of the order of one micron).
• A theoretical framework is emerging that links the absorption and scattering characteristics of individual suspended particles to the colour of the sea measured by remote sensing reflectance. This theoretical framework is the key to deriving information on ecological processes in the marine environment from remotely sensed data.
• It is an area where physics research can make a direct contribution to our understanding of the way the earth’s life-support systems function, and help assess the vulnerability of these mechanisms to changes brought about by human activity.

Is it useful?

Practical applications of our research arise from the following:

• Remote sensing measurements of ocean colour can be used to identify water masses and to study their dispersal patterns.
• Changes in the intensity and spectral distribution of the underwater light climate profoundly affect the growth and behaviour of planktonic and pelagic organisms
• Optical measurements provide efficient methods of tracking sediment plumes (often loaded with contaminating substances), phytoplankton blooms (which can produce toxins) and pollutants (oil spills for example).
• Optical instruments can be used to measure biological processes in the ocean (such as phytoplankton and zooplankton population density changes) at the same temporal and spatial resolution as physical and chemical variables such as temperature and salinity

What are the big issues?

Phytoplankton photosynthesis.

Phytoplankton photosynthesis is the basis of nearly all marine food chains. Zooplankton, fish and sea mammal populations reach their highest densities where phytoplankton grow best. It also exerts a major influence on the exchange of gases between atmosphere and ocean. The quantity of atmospheric carbon fixed annually by phytoplankton is about the same as that fixed by the tropical rain forests. Unlike tropical forests, however, the growth of phytoplankton populations occurs in episodic bursts that are widely dispersed in space and time. Improved methods of quantifying this growth are required to provide inputs to mathematical models of ocean/atmosphere interactions and climate change, and to predict problems associated with the anthropogenic enrichment of coastal waters. Optical methods, either recording instruments on moorings of radiometers carried by aircraft and satellites, play a large part in this field.

Radiance transfer and remote sensing in shelf seas.

The relatively shallow seas around continental margins are highly productive regions. This is because seasonal cycles of stratification and mixing produce well illuminated nutrient-rich surface layers which support active growth by phytoplankton populations. These seas are the sites of most of the world’s sustainable fisheries and aquaculture, and gain additional economic value from amenity and recreational uses. Most countries devote considerable resources to monitoring the state of their coastal waters. Optical remote sensing can provide information pollutant dispersal, sediment transport, changes in water opacity and the occurrence of nuisance or toxic algal blooms that is otherwise unobtainable. However in order to retrieve this information it is necessary to develop algorithms which separate the optical signals arising from the sea surface from those generated by the atmosphere, and then to interpret the sea-leaving signal in terms of seawater composition. Shelf seas are an optically complex mixture of suspended mineral particles, phytoplankton and coloured organic material. In order to realise the full potential of remote sensing, a great deal of work on the physical optics of this mixture has yet to be done.

Optical instrument development.

Marine science suffers from a problem of chronic under-sampling: only a very small percentage of the sea floor has been visually surveyed, and the dynamic nature of the pelagic ecosystem defeats the surveying capacity of research vessels. There is therefore much current interest in ‘post-expeditionary oceanography’, a shorthand term for the idea that the oceans can be surveyed by a combination of autonomous vessels and a network of sea-bed installations. One of the problems with this concept is devising instrument packages capable of measuring the variables of interest. Good sensors already exist for most physical variables (conductivity, temperature, flow velocity). Chemical sensors (for nutrients, dissolved oxygen, trace metals) are undergoing rapid development. Populations of fish and zooplankton can be measured, once they reach sufficient biomass density, by acoustic techniques. Obvious gaps in this list include phytoplankton, low densities of zooplankton including eggs and larvae, and aggregations of organic material derived from faeces or senescent phytoplankton blooms. In theory, these ‘missing variables’ can all be measured by optical instruments including fluorometers, scattering and absorption meters, particle analysers and imaging systems. In practice, there are problems with stability, power consumption, ability to operate under varying pressures and temperatures, resistance to biofouling, and calibration which all have to be overcome before the data from these optical sensors can be reliably interpreted. This is a fruitful field for exercising ingenuity in opto-electronics and instrument engineering.

How do you do marine science in Glasgow?

All our work is collaborative in nature, and we have undertaken joint research projects with marine science laboratories throughout the UK and in Europe. Group members regularly work at sea, and have sailed on the James Clark Ross (Atlantic meridional transect), Prince Madog (Irish Sea), Challenger (north east Atlantic shelf), Terschelling (English Channel) and Calanus (Clyde Sea and Scottish west coast waters).

What facilities are available?

The group is well equipped with modern marine instrumentation, including two ac-9 absorption/attenuation meters, two free-fall profiling radiometers (Satlantic SPMR and Hyperpro), a LISST laser particle sizer, Hydroscat 2 backscattering meter, a floating platform carrying Trios hyperspectral radiometers, and a Seabird CTD system. Laboratory facilities include custom-built spectrometers for measuring phytoplankton absorption and fluorescence, inverted and epifluorescence microscopes, and a small-scale algal culture facility. We have computing facilities for in-house SeaDAS processing of SeaWiFS satellite images, and use Matlab and Curt Mobley’s Hydrolight for modelling studies.

Can I do a PhD in Environmental Optics?

Projects are available in all the areas where we currently have research projects. Typical thesis titles might include:

• Fluorescence lifetimes and photo-physiology: studies of chlorophyll fluorescence in intact phytoplankton cells.
• Colour and composition: interpreting satellite images of optically complex shelf seas.
• Phytoplankton optical properties from single cells to water columns.
• Optical monitoring of ecosystem function in the marine environment.

For more information on projects see the postgraduate section of the departmental website.

Selected publications

• Ship-borne measurements of ocean colour: development of a CCD-based reflectance radiometer and trials on a longitudinal transect of the Atlantic Ocean. Wood P and Cunningham A (2001) International Journal of Remote Sensing 22: 99-111
• Reflectance properties of hydrographically and optically stratified fjords (Scottish sea lochs) during the spring diatom bloom. Cunningham A, Wood P and Jones K J (2001) International Journal of Remote Sensing 22:2885-2897.
• Radiative transfer modelling of the relationship between seawater composition and remote sensing reflectance in sea lochs and fjords. Cunningham A, Wood P and Boyle J C (2002) International Journal of Remote Sensing 23:3713-3724
• Assessment of a microscopic photobleaching technique for measuring the spectral absorption efficiency of individual phytoplankton cells. Neumüller M, Cunningham A and McKee D (2002) Journal of Plankton Research 24:741-746
• Three-parameter active in-situ optical measurements: theory, instrumentation and results from coastal waters. McKee D, Cunningham A and Jones K (2002) Journal of Optics A: Pure and Applied Optics 4:S66-S70
• Brewster angle measurements of sea-surface reflectance using a high resolution spectroradiometer. Cunningham A, Wood P and McKee D (2002) Journal of Optics A: Pure and Applied Optics 4:S29-S33
• Two models for absorption by coloured dissolved organic matter (CDOM). Schwarz J N, Kowalczuk P, Kaczmarek S, Cota G F, Mitchell B G, Kahru M, Chavez F P, Cunningham A, McKee P, Gege P, Kishino M, Phinney D A, Raine R (2002) Oceanologia 44:209-241
• Optical and hydrographic consequences of freshwater run-off during spring phytoplankton growth in a Scottish fjord. McKee D, Cunningham A and Jones K J (2002) Journal of Plankton Research 24:1163-1171
• Inherent and apparent optical properties in coastal waters: a study of the Clyde Sea in early summer. McKee D, Cunningham A and Slater J (2002) Estuarine Coastal and Shelf Science (in press)
• Retrieval of inherent optical properties from in situ radiometric measurements in case II waters. McKee D, Cunningham A and Craig S (2002) Applied Optics (in press)

Research funding

• Quantitative studies of the inherent optical properties of marine particle suspensions and their influence on remote sensing reflectance in Case 2 waters. Natural Environment Research Council, January 2000: £195,470
• Scattering of natural light by organic and inorganic particles in shelf seas. Leverhulme Trust, July 2000: £59,500
• Factors determining the magnitude of solar-stimulated fluorescence peaks in water-leaving radiance spectra from shelf seas. Natural Environment Research Council, May 2001: £230,000
• Picosecond fluorescence decay measurements of competing deactivation mechanisms in the light harvesting antenna of intact phytoplankton cells. Natural Environment Research Council. January 2002: £29,909