Phytoplankton are abundant where nutrients are plentiful - coastlines, continental shelves, equatorial zones and high-latitude polar areas, whereas their numbers are far lower in oligotropic zones in the centre of the ocean where nutrients are scarce.
SPOTLIGHT ON PHYTOPLANKTON
➤ What are phytoplankton?
➤ Phytoplankton dynamics
➤ Phytoplankton and climate change
➤ Studying phytoplankton
Winds play a strong role in the phytoplankton distribution because they drive currents that cause deep water, loaded with nutrients, to be pulled up to the surface. These upwelling zones, including one along the equator supported by the convergence of the easterly trade winds, and others along the western coasts of continents, are among the most productive ocean ecosystems.
Given that phytoplankton are a food source for fish, it is not surprising that coastal upwelling habitats are some of the most economically productive areas in the world and support many of the world’s most important fisheries. Although coastal upwelling regions account for only one percent of the ocean surface, they contribute roughly 50 percent of the world’s fisheries landing.
Tropical waters hold the richest diversity of phytoplankton species throughout the year. Phytoplankton diversity is particularly high in the seas of the Indonesian-Australian archipelago, in parts of the Indian Ocean and in the equatorial Pacific Ocean. In the subtropics, biodiversity drops off markedly beyond 30 degrees latitude North and South, reaching its lowest values around a latitude of 55 degrees.
Diversity then picks up again slightly towards the poles. This is unusual: species diversity typically decreases continuously towards the poles, where it is normally at its lowest. Phytoplankton have shown that they do not behave with this norm. Phytoplankton diversity in the mid-latitudes, unlike in the tropics, varies greatly from season to season. The number of species in the mid-latitudes is constant over time, but the species composition changes over the course of the year. Species composition over time is a subject with a lot of potential for further research.
Phytoplankton photosynthetic physiology can be investigated through single turnover active fluorometry (STAF) approaches, which carries the unique potential to autonomously collect data at high spatial and temporal resolutions. Chelsea Technologies’ LabSTAF is our newest fluorometer that uses single turnover active fluorometry to measure very low levels of photosynthesis, ideal for oligotrophic zonesThe other end of the scale, where phytoplankton are least abundant, are oligotrophic zones. Phytoplankton are scarce in oligotrophic zones due to nutrient limitations and other reasons including ocean stratification, and this is also an area with a lot of scope for further research and for which the Chelsea Technologies’ STAF product range, with its ability to detect low concentrations, is uniquely suited. In the ocean, the subtropical gyres north and south of the equator are regions in which the nutrients required for phytoplankton growth (for instance, nitrate, phosphate and silicic acid) are in limited supply.
Oligotrophic zones host little excess aquatic vegetation and are uncontaminated by excess biological activity, while eutrophic zones tend to temporarily host large quantities of organisms, including algal blooms out of control due to excess nutrients, before oxygen is removed by the excessive growth and the organisms die. Each trophic level supports different types of fish and other organisms. If the algal biomass in a water body reaches too high a concentration, fish mortality events occur as decomposing biomass deoxygenates the water.
Seasonal phytoplankton dynamics
Just as land-based plant life has seasonality, phytoplankton growth also varies seasonally. In high latitudes, blooms peak in the spring and summer, when sunlight increases and the turbulence, and mixing of the water by winter storms, subsides. Research suggests that vigorous winter mixing sets the stage for explosive spring growth by bringing nutrients up from deeper waters into the sunlit layers at the surface and separating phytoplankton from their zooplankton predators. In the subtropical oceans, by contrast, phytoplankton populations drop off in summer. As surface waters warm up through the summer, they become very buoyant. With warm, buoyant water on top and cold, dense water below, the water column does not mix easily, which means fewer nutrients for phytoplankton growth.
Phytoplankton will use up the nutrients available, and growth falls off until winter storms kick-start mixing. In lower-latitude areas, including the Arabian Sea and the waters around Indonesia, seasonal blooms are often linked to monsoon-related changes in winds. As the winds reverse direction (offshore versus onshore), they alternately enhance or suppress upwelling, which changes nutrient concentrations. In the equatorial upwelling zone, there is little seasonal change in phytoplankton productivity.
El Niño events influence weather patterns beyond the Pacific, in the eastern Indian Ocean around Indonesia, for example, phytoplankton productivity increases during El Niño. Productivity in the Gulf of Mexico and the western sub-tropical Atlantic has increased during El Niño events in the past decade, probably because increased rainfall and runoff delivered more nutrients than usual. Compared to the El Niño-related changes in the productivity in the tropical Pacific, year-to-year differences in productivity in mid- and high latitudes are small.
La Niña, the opposite of El Niño, works in the opposite manner. ENSO (El Niño–Southern Oscillation) events such as La Niña and El Niño are one of the most important climate phenomena on Earth due to their ability to change the global atmospheric circulation, which in turn, influences phytoplankton dynamics across the globe.
Phytoplankton dynamics in the water column
Given that they are dependent on photosynthesis, phytoplankton can only exist at the top of the water column where there is light, as far down as the sunlight can penetrate – the photic zone. Only when they die, do they sink down to deeper layers in the ocean. Phytoplankton require various key ingredient to be active including nitrogen. Just as fertilisers or legume plants are necessary to grow crops on land, nitrogen supplies the nutrient value that phytoplankton need to grow in the ocean. Obtaining sufficient nitrogen in the ocean can be tricky.
Coasts receive Nitrogen through rivers or upwelling of deep waters rich in Nitrogen, but the middle of the ocean is remote from such sources. Upwellings involve wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface, replacing warmer and usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. The biomass of phytoplankton and the presence of cool water in those regions allow upwelling zones to be identified by cool sea surface temperatures and high concentrations of chlorophyll-a.
The importance of phytoplankton for global climate studies cannot be overstated. This process, phytoplankton at the top consuming carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments, is the biological component of the “marine carbon pump”, which will be further explored in the next article in this phytoplankton series. The oceanic carbon cycle is responsible for the cycling of organic matter and transfers about 11 gigatonnes of carbon every year into the ocean’s interior. An ocean without a biological pump may result in atmospheric carbon dioxide levels about 200-400 ppm higher than the present day.
Phytoplankton monitoring options from Chelsea Technologies
LabSTAF is the world’s leading portable instrumentation option for Phytoplankton primary productivity.
- Benchtop instrument to measure primary productivity using fluorescence, giving data for over 50 useful parameters within 15 minutes
- LabSTAF includes a peristaltic pump, solenoid unit and flow-through stirrer unit to provide for mixing, sample exchange and a periodic cleaning cycle.
- Data from LabSTAF is interpreted and analysed internally in the included Surface Go