Research has gone for over a century to measure phytoplankton levels as widely as possible. The conclusion, is that global phytoplankton levels are not only declining, they have been declining for over a century.
Since 1899, a very simple tool, the Secchi Disk, has been used to measure the density of phytoplankton at various levels of the ocean's daylight zone. The Daylight Zone of the oceans is the point where enough light penetrates to support light dependent aquatic life. In particular, phytoplankton is a form of algae, which is a class of photosynthetic aquatic plant.
The resulting measurements, which total over half a million measurements over the years, indicate that global phytoplankton levels have been decreasing at about 1 percent per year for the past 110 years. This adds up, when the measurements are interpreted, to a 40 percent decrease since the 1950s.
NASA's Earth Observation System has capabilities for measuring phytoplankton blooms and for identifying the levels from space.
Phytoplankton is the base of the food chain and as such, it provides food for the entire chain of life, from fish to humans. It also is essential to the health of the oceans as a vast supply of plants that takes in carbon dioxide and produces oxygen for the air and the water.
Phytoplankton come in many forms. Some are bacteria that are photosynthetic. Some are protophyta, or mostly single celled algae. Most phytoplankton are simply single-celled plants.
The common forms of phytoplankton includes cyanobacteria, green algae and dinoflagellates. There are "armor plated" varieties, such as coccolithophores that can be chalk coated with microscopic limestone scales.
The Secchi Disk is a very simple apparatus. It is a round, flat disk, about the size of a dinner plate, with a black and white alternating and contrasting pattern. The disk is lowered until the pattern is no longer visible. When it reaches that point, it is at the Secchi Depth. What is being measured is water turbidity, or cloudiness and haziness.
This is not an exact measurement, because of such visual intereference as the glare of the sun and differing visual acuity of the observers. Because of these issues, there are standardized methods for taking the measurements. Researchers, officials and even citizens who live or work on waterways can take the measurements and report the results.
Climate seems to be the main cause of the declining phytoplankton levels. A general rise in the temperature of the waters, along with fluctuations in water temperature is causing problems for the microscopic life forms.
Melissa Hanniger, "The State of Global Phytoplankton Levels", Geology.com, July 10 2010.