Phytoplankton — tiny, marine plants that form the basis of our oceans’ food webs — absorb and sequester large amounts of CO2 from the atmosphere and generate half of the world’s oxygen supply. Given such important ecosystem services as these, one would hope that our oceans’ algae population numbers stay high…but, the results of a three-year data analysis are anything but encouraging.
According to a recent report in the journal Nature by researchers at Dalhousie University in Halifax, Nova Scotia, oceanic phytoplankton has been declining roughly 1% per year for at least the last one hundred years (1899-2008, the time period for which data on “ocean transparency” has been collected).
Declines in phytoplankton world-wide are thought to be the result of atmospheric warming, but exactly how warming is causing this impact is still a matter of debate.
It is speculated by some marine scientists that over-fishing (a major problem facing marine ecosystem integrity at present) may be contributing to the decline; the more big fish are caught, the less zooplankton (tiny animal forms of plankton) get eaten by these fish and the more their numbers increase. And, since zooplankton eat phytoplankton, more of these tiny plant organisms get eaten.
This type of impact is seen more and more often these days (consider the over-abundance of jellyfish invading many marine ecosystems due to over-fishing of their natural predators) as a key predator is eliminated or reduced in number, allowing its prey to over-populate.
Other scientists believe that warming oceans mean less mixing of ocean layers (i.e., the ocean is becoming more stratified). The normal mixing of water column strata churns up sediments which in turn provide nutrients that the phytoplankton need to grow. In this view, the plankton are being starved from lack of nutrient circulation in the water column.
There is also a possible influence from ocean acidification due to excess CO2 absorption and conversion into carbonic acid. This acidification may make certain nutrients less available, or, it may promote bacterial growth that is damaging or adversely impacting the natural plankton growth cycle (as is the case with over-growth of Vibrium bacteria on some species of coral).
Researchers analyzed over 400,000 data points –many of which were collected/measured using a simple white disc (known as a Secchi disc, invented by a Jesuit priest in the mid 19th Century) that is dropped into the water and allowed to sink until it is no longer visible (thus being a measure of the water’s transparency). Several factors can influence the transparency of marine water (such as the upwelling of sediments, pollution, etc.) but the presence of chlorophyll in algae is a primary one. Thus, water transparency is a proxy record or indicator for phytoplanktonic growth.
In addition to the Secchi disc data, the scientists used direct sampling/analysis and satellite data to augment their findings.
The pattern of decline is not, however, consistent everywhere. In some places, such as the South Indian Ocean, phytoplankton appears to be increasing. In certain coastal regions, such as eastern China, fertilizer-rich land run-off promotes algal blooms (which can have deleterious impacts on many marine organisms).
Also, natural phenomena like the El Niño Southern Oscillation (ENSO) and the Arctic Oscillation contribute to variation in phytoplankton population growth.
Most forms of phytoplankton (algae) are known as photoautotrophs which means that they make their own food from sunlight (photosynthesis), plus nutrients derived from sediments. Common types of phytoplankton are diatoms, cyanobacteria (blue-green algae), dinoflagellates, and coccolithophorids. These latter species produce dimethylsulfide (DMS), which enters the atmosphere as sulfate and forms the chemical nuclei around which water vapor condenses to form clouds.
Middle Image: satellite photo of algae circulation via NASA
Bottom Image: diatoms via NOAA