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takyon writes:

A multinational team of marine biologists have completed a 3.5-year-long study of plankton across the world's oceans:

The ocean is the largest ecosystem on Earth, and yet we know very little about it. This is particularly true for the plankton that inhabit the ocean. Although these organisms are at least as important for the Earth system as the rainforests and form the base of marine food webs, most plankton are invisible to the naked eye and thus are largely uncharacterized. To study this invisible world, the multinational Tara Oceans consortium, with use of the 110-foot research schooner Tara, sampled microscopic plankton at 210 sites and depths up to 2000 m in all the major oceanic regions during expeditions from 2009 through 2013.

Sampling, usually 60 hours per site, followed standardized protocols to capture the morphological and genetic diversity of the entire plankton community from viruses to small zooplankton, covering a size range from 0.02 µm to a few millimeters, in context with physical and chemical information. Besides the sampling, a lab on board contained a range of online instruments and microscopes to monitor the content of the samples as they were being collected. The main focus was on the organism-rich sunlit upper layer of the ocean (down to 200 m), but the twilight zone below was also sampled. Guided by satellite and in situ data, scientists sampled features such as mesoscale eddies, upwellings, acidic waters, and anaerobic zones, frequently in the open ocean. In addition to being used for genomics and oceanography, many samples were collected for other analyses, such as high-throughput microscopy imaging and flow cytometry. The samples and data collected on board were archived in a highly structured way to enable extensive data processing and integration on land. The five Research Articles in this issue of Science describe the samples, data, and analysis from Tara Oceans (based on a data freeze from 579 samples at 75 stations as of November 2013).

The team produced 5 research articles and a "perspective," all paywalled by Science. You can read this editorial about "Oceans and Earth's habitability" (PDF). Abstracts below:

Eukaryotic plankton diversity in the sunlit ocean
Structure and function of the global ocean microbiome
Patterns and ecological drivers of ocean viral communities
Determinants of community structure in the global plankton interactome
Environmental characteristics of Agulhas rings affect interocean plankton transport
Uncovering hidden worlds of ocean biodiversity

Images at BBC News.