Biological Argo floats
Phytoplankton under the microscope
Arctic foodweb MORE
Some Argo floats collect information related to the biology and chemistry of the ocean. The data contributes to a better understanding of how phytoplankton productivity is linked to the biogeochemical cycles of oxygen, carbon and nutrients, and how these relate to variability and change in Earth's climate system.
Why study phytoplankton?
There are several reasons. Some of the most important are:
- Phytoplankton are the 'grass of the sea'- the bottom of the marine food chain. Where they bloom marine animals soon turn up to feed. Information about phytoplankton is used, for example, in marine ecology, fisheries, and marine conservation.
- Oceanic phytoplankton produce about half the oxygen in Earth's atmosphere, so they are as important for life on Earth as the forests on land are.
- They also play an important role in the ocean's uptake of atmospheric carbon dioxide. They 'drive' the biological carbon pump, which exports carbon from the surface to the deep ocean, where it is stored for centuries before returning to the atmosphere.
What biological Argo floats measure
The Bio-Argo floats are still experimental, so the biological data is not on line at Argo data centres, but our float selection includes a few floats with data provided by the scientists that operate them.
In addition to the usual temperature and salinity, these floats measure:
- chlorophyll-a: a pigment found in all plants, which harvests light for photosynthesis;
- photosynthetically available radiation (PAR): how much light-energy is available for photosynthesis;
- nitrate: an important plant nutrient, often in short supply;
- oxygen: essential for animal life;
- downwelling irradiance: the intensity of daylight penetrating to different depths in the water;
- backscatter: a measure of how light is scattered in a backward direction by particles in the water;
- turbidity or beam attenuation: a measure how quickly light is lost when travelling through the water;
- CDOM (Coloured Dissolved Organic Matter): also known as 'yellow substance' because of its colour.
With exception of nitrate and oxygen these are known as 'bio-optical parameters', because they tell us how the water interacts with visible light
(the optical part of the
electromagnetic spectrum) to give the water its colour.
Bio-optical float data are used, for example, with ocean colour data from satellites to study phytoplankton.
(See ocean colour: light in the water).
Bio-Argo and ocean colour satellites
Global chlorophyll from satellites MORE
Ocean colour is an important tool in marine ecology. Since 1997 satellites have provided daily measurements of chlorophyll in the global ocean.
Satellites 'see' only the surface, however. Argo floats complement the satellite data by providing the missing 3-D information. Just as the Argo temperature and salinity profiles complement satellite measurements of SST and ocean currents, so . biological Argo floats will complement ocean colour satellites as they become more numerous.
What the measurements tell us
All plants contain chlorophyll-a, the main pigment needed for photosynthesis. Chlorophyll captures energy from sunlight, and uses this to convert carbon dioxide and water into glucose. To grow and multiply phytoplankton also need nitrogen, phosphorus, sulphur, silicate, iron, and manganese. Without these the glucose is converted to oils and stored or just released into the water.
Many plant nutrients are plentiful in seawater, but a shortage of some can severely limit plankton growth. There is often a close, inverse link between chlorophyll and nitrate concentrations, as shown in the figure (right).
Chlorophyll is a proxy for phytoplankton biomass. The relationship between the weight of organic carbon in a water sample and the chlorophyll it contains varies with light levels and plankton types, but is close enough for us to chlorophyll to estimate the weight of organic carbon in phytoplankton populations. In turn this tells us how much food is avaialble for zooplankton and their predators (fish, squid, etc.)
Photosynthetically available radiation (PAR)
Like all plants, phytoplankton need light to grow. The light available for photosynthesis is known as phytosynthetically available radiation (PAR). Ocean colour satellites also provide estimates of PAR at the sea surface. Light diminishes with depth; how fast depends on the turbidity of the water (see below). The Argo profiles give PAR at different depths; combined with chlorophyll data this makes it possible to calculate actual phytoplankton production of new organic carbon and relate this to the ocean's uptake of carbon dioxide.
Coastal upwelling MORE
Nitrogen (N) is needed to build proteins. 79% of the Earth's atmosphere is nitrogen gas (N2), but most plants cannot use nitrogen in this form. Instead they need nitrate (NO3) or another form of nitrogen. These are often in short supply in surface water, and this limits plankton growth. In contrast deep water is usually rich in nitrate, which is released when organic carbon decomposes.
When phytoplankton grow, they remove plant nutrients (including nitrate) from the sunlit surface water. As the plants die and decompose these nutrients are released and become available for new plant growth. Most of this 'remineralisation' of carbon and nutrients occurs in the surface ocean, but the a 'snowfall' of marine detritus transports some organic matter into deeper water.
Nutrients in deep water cannot be directly re-used for plant growth because of the lack of light, so concentrations of nitrate build up. The nutrient-rich water can only be brought back to the surface through vertical mixing by waves or tides, or through upwelling of deep water.
All animals breathe oxygen, those that use gills rather than lungs are no exception. But oxygen can be in short supply in the deep ocean.
Oxygen can only be replenished in two ways:
- oxygen gas from the atmosphere is dissolved in surface water (the warmer and saltier sea water is, the less oxygen gas it holds);
- during photosynthesis marine plants release oxygen into the water.
This means that new oxygen is only available in the sunlit surface ocean. At depth there is no light for photosynthesis and no direct contact with the atmosphere. New oxygen can only enter the deep ocean when oxygen-rich surface water is mixed down or sinks towards the sea floor in the Southern Ocean or the North Atlantic. Oxygen depletion can be a problem in deeper water, sometimes with severe consequences for marine life.
Downwelling irradiance is a measure of the amount of light reaching to a particular depth in the ocean. It is usually measured at different wavelengths; bio-Argo floats measure in the blue violet and blue part of the optical spectrum. Phytoplankton chlorophyll and coloured dissolved organic matter both absorb blue light. Measuring downwelling irradiance in the blue, can therefore help tell to what extent these substances are present in the water.
Backscattering is a measure of how much of the downwelling light is reflected back up towards the surface. Along with light absorption by the water, backscattering can be used relate ocean colour measurements to concentrations of chlorophyll, suspended particles and CDOM (see below).
Turbidity and 'beam attenuation'
Suspended particles such as phytoplankton cells or sediment scatter light making the water appear murky and cloudy. Such turbid water has low visibility and light cannot penetrate very far.
Turbidity is measured by shining a beam of light through the water and recording the proportion that is lost. From this we can calculate the 'beam attenuation coefficient'. When this is high, light-penetration is low, visibility is poor, and photosynthesis may be limited to a surface layer of only a few metres.
Coloured dissolved organic matter (CDOM)
CDOM (also known as yellow substance) is another optical measurement useful for linking Argo measurements to ocean colour data from satellites. Like chlorophyll CDOM absorbs blue light, so where CDOM from rivers are present in the water, it can make it difficult to calculate chlorophyll concentrations accurately.