deep sea Ecology

Not only Popeye but phytoplankton need iron too.

Massive Southern Ocean phytoplankton bloom fed by iron of possible hydrothermal origin

There are vast areas in the oceans that are referred to as high nutrient low chlorophyll (HNLC) regions; these areas seemingly have all the nutrients required for phytoplankton growth but anomalously don’t have the expected phytoplankton numbers. Phytoplankton are tiny microscopic organisms dwelling close to the ocean surface; much like the plants on land, they form the base of the food chain in the oceans. Phytoplankton contain chlorophylls and other pigments to capture solar energy to produce food. Hence the quantity of chlorophyll acts as a primary indicator for phytoplankton numbers. Though we can’t see phytoplankton with naked eyes, under the right conditions, their numbers increase tremendously, appropriately called the phytoplankton bloom. Over the years, oceanographers have recognized that the HNLC regions, despite the required nutrients, lack one essential nutrient: iron. During photosynthesis, the phytoplankton take up CO2 from the atmosphere, and when they die, a part of this carbon sinks with them to the ocean floor, where it is stored for millions of years to come. This carbon transfer from the atmosphere to the ocean floor is called the carbon pump and helps regulate atmospheric CO2. Hence, HNLC regions can potentially increase CO2 drawdown from the atmosphere only if we can figure out a way to supply iron to these regions. This had inspired oceanographer John Martin to say, “Give me half a tanker of iron, and I will give you next ice age.” in a lecture at Woods Hole Oceanographic Institution.

Satellite image of a phytoplankton bloom in the southern ocean. (Source: Bigelow Laboratory for Ocean Sciences)

Southern Ocean houses one of the three largest HNLC regions in the world ocean. During certain parts of the year, the HNLC regions abandon their characteristic definition and support phytoplankton blooms so large that even satellites can spot them. These blooms suggest that there is a temporary supply of iron to these areas. A recent study by a team led by Dr. Casey Schine from Stanford University explores the possible sources of iron in one such bloom in the southern ocean. The team confirmed the presence of the bloom by both the satellite imagery and field observations. The bloom contained high chlorophyll and organic matter content compared to the surrounding non-bloom regions.

Interestingly the authors noted the concentrations of CO2 dissolved in waters was half in the bloom regions, indicating a rapid uptake. While comparing the iron concentrations in the bloom and non-bloom areas, the authors curiously found extremely low concentrations in both regions. But on probing further, they noticed the iron concentrations increased rapidly with water depth in the bloom regions, whereas this increase with depth was slower in the non-bloom regions. That suggests the supply of iron from deep waters was higher at the bloom region and was quickly utilized by the phytoplankton in the surface waters.

Iron in its usable form is mainly obtained by weathering of iron-rich rocks; the action of weathering and transportation of weathered material is enhanced by the action of wind, water, and glaciers, among other forces.  The possible sources of iron to the bloom region in the Southern Ocean can be wind-driven dust deposition, icebergs, sea-ice melt, sediments from the coast, and hydrothermal vents. The authors were quick to rule out the first two possibilities as the dust deposition is extremely low in the study area, and the paths of the icebergs lie too far from the bloom to have any significant impact. While sea-ice can contribute iron to the bloom, it can not be the primary source  in the bloom region because it is hypothesiszed that if sea-ice was a primary source, the bloom would track the sea-ice edges to maximize the iron uptake. The authors couldn’t justify the possibility of sediments from the Antarctic coast, given the absence of any strong water currents in the vicinity to carry a significant amount of sediments. The team further looked into the possibility of hydrothermal vents in supplying the iron.

Hydrothermal vents on the seabed spewing minerals. (Source: Schmidt Ocean Institute)

Hydrothermal vents are openings in the seafloor through which hot waters gush out. These hot waters are rich in minerals and, on coming in contact with cold waters of the deep oceans, rapidly precipitate the minerals forming chimney-shaped mineral deposits. The bloom area in question has two hydrothermal vent fields in its vicinity. The consistent shape and position of the bloom year after year makes the case of hydrothermal source stronger by suggesting a stationary source of elevated iron to the surface waters. Further evidence for the hydrothermal source comes from water mass properties—the iron-rich waters from the vents mix with the adjacent colder and well-oxygenated waters. Therefore a water mass generated by emissions from these vents will bear this colder and more oxygenated signal. This observation is consistent with the presence of anomalously cold and more oxygenated waters closer to the bloom but was absent from the non-bloom regions. This bloom is the first recurring bloom that is likely to be stimulated by hydrothermal iron to be identified in the Southern Ocean.

Though the HNLC regions generate a lot of interest in part due to their potential to act as CO2 sink,  the consistent location and size of this bloom have made it a hotspot amongst the krills and humpback whales region. Unfortunately, the majority of the bloom region is not protected under the Ross Sea Marine protected areas. Without understanding the mechanisms controlling production in the bloom, it is difficult to predict how it will be impacted by future changes anticipated for the Southern Ocean and how those will affect the role of the bloom region as a CO2 sink and foraging grounds for whales and krills in the region.

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