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Primary Production in Oligotrophic waters

Primary productivity

Phytoplankton form the basis of the aquatic food web. By utilising pigments such as chlorophyll and phycoerythrin, they are able to absorb solar energy to convert carbon dioxide and water into high-energy organic carbon compounds. This fuels growth by synthesising vital components such as amino acids, lipids, proteins, polysaccharides and nucleic acids. Phytoplankton are in fact responsible for around half the photosynthesis on Earth and hence are a fundamental component of the global carbon cycle.

Vertical distribution of phytoplankton

Sunlight is, directly or indirectly, the ultimate energy source for almost all life on Earth, with phytoplankton harvesting it to convert inorganic carbon to an organic form. However, sunlight is absorbed and scattered by the upper layers of the ocean, such that very little light generally penetrates below a depth of around 80 m.

Additionally, the absorption of sunlight increases the temperature of the surface water, causing it to become warmer and more buoyant. This allows it to float on top of the cooler, denser deep ocean water below, that has previously sunk from the surface in high latitudes. This gradient of increasing pressure and decreasing temperature is known as the thermocline (vertical temperature gradient) and drives stratification across the majority of the ocean. Significant amounts of energy, whether that be from wind or other sources, are required to drive mixing across the thermocline. Therefore, the exchange of water between the sunlit surface of the ocean (euphotic zone) and the dark interior is limited. The dual effect of density stratification, that prevents phytoplankton from being mixed down below, and their dependency on sunlight, restricts primary production to the upper layers of the ocean surface.

Oligotrophic zones

Phytoplankton require nutrients and take them up at a ratio that varies between different algal species and environmental conditions. Nitrogen and phosphorus make up the major constituents of cellular biomass, while iron is required for a large number of enzymes and electron transfer proteins essential for photosynthesis. When phytoplankton die, they are exported as organic matter to depth, causing nutrients to accumulate in deep waters. The density gradient between the surface and deep waters mean that ocean circulation can only very slowly reintroduce dissolved nutrients into the euphotic zone. This process therefore depletes the surface ocean of inorganic forms of nitrogen, phosphorus, iron and silica, causing primary production to become limited by the availability of nutrients.

These environments, characterised by low concentrations of nutrients and low amounts of primary production, are known as oligotrophic. These vast “ocean deserts”, such as the North Pacific Subtropical Gyre, may account for over 30% of total marine primary production and cover more than 60% of the ocean’s surface area, making them the largest ecosystem in the surface ocean. Increased temperatures associated with global warming are predicted to strengthen water column stratification and therefore reduce nutrient fluxes and primary productivity, resulting in the expansion of the ocean’s oligotrophic regions. Considering the global importance of subtropical oligotrophic ocean ecosystems, it is vital that we understand the fundamental processes occurring within them, including primary productivity.

Measuring primary productivity

The 14C method is traditionally the most frequently used method for measuring primary productivity. However, considerable uncertainty surrounds what parts of primary production this method measures, its spatiotemporal limitations, and its accuracy for measurements in oligotrophic waters. Single Turnover Active Fluorometry (STAF) can probe phytoplankton primary productivity on much wider spatiotemporal scales than is possible with more direct methods such as 14C fixation.

Over the last ten years, Chelsea Technologies has worked closely with the research community to develop hardware and data processing algorithms that will allow for more reliable estimates of primary production. We have combined principles from our highly successful FastOcean/Act2 laboratory system and FastBallast portable monitoring system to create our new portable active fluorometer, LabSTAF. LabSTAF combines an unparalleled sensitivity with a wide dynamic range, allowing for measurements in extremely oligotrophic waters, open oceans, coastal waters and lakes, with greater precision, all in one instrument.

It provides a highly automated platform for running continuous Fluorescence Light Curves (FLCs) and incorporates new features to greatly improve the accuracy of STAF-based primary production assessment, including the correction of spectral errors, baseline fluorescence and the package effect. The introduction of the world-leading LabSTAF has the potential to revolutionise primary productivity measurement and our understanding of oceanic ecosystems.

For more information on LabSTAF and the use of active fluorescence in primary productivity monitoring, please contact Ben Goymer at bgoymer@chelsea.co.uk.