If you'd ask people around you which hand tool characterizes a professional microbiologist, they would probably come up with the microscope. Much like hammer and saw would be named as typical carpenter tools, or the plough as typical for farmers ('hammer and sickle' is a similar case, yet not the point here). The light microscope is in fact so much at the heart of microbiology – and biology in general – that one central theory, the cell theory, could not have been developed without it. But when the small things gather en masse in lakes and in the seas their assemblies, called "blooms", can be so huge that microbiologists have to turn to "macroscopes", that is, satellite imagery, to see them in their full glory.
The NASA Earth Observatory published these days a stunning true-color satellite picture of a swirling green phytoplankton bloom in the Gulf of Finland, a section of the Baltic Sea (Figure 1). Though it's impossible to know the genus and species without sampling the phytoplankton from the water, earlier studies by satellite observations combined with in situ sampling suggest that this particular "swirly" green bloom is likely to be cyanobacteria, probably Nodularia. Some of the green could come from diatoms, unicellular eukaryotes that are also rich in chlorophyll. The Finnish Environment Institute (SYKE) has observed the recent bloom from the water and found it to be mostly cyanobacteria. This was also evident from the large amounts of cyanobacteria that were found at the Finnish beaches at the time of the bloom. (Figure 2).
If you look more closely at the superb vortex in Figure 1 you may wonder why it is not completely like the vortex you see when you pull the plug in your bathtub. First, and unlike the situation in your bathtub, the direction of rotation, clockwise or counter-clockwise, is dictated by the Coriolis force for vortices of this size. Second, there was nobody covertly pulling a plug at the bottom of the Baltic Sea near the Finnish coast. How do we know? What you do not see is the typical 'bathtub vortex' with an indent in the center where air is pulled down together with water. This indicates that the vortex in Figure 1 is oriented upwards (and technically called an "eddy" or "gyre"), that is, it moves water and dissolved nutrients (NO3, nitrate, PO4, phosphate, and trace metals) from the bottom of the Baltic Sea to the upper layers of the water column. This enables explosive phytoplankton growth in the upper ~200 m, close to the surface. But, alas, vortex physics are highly complicated, as are most of fluid dynamics. Not the least because physicists still struggle to fully understand the transition between laminar flow and turbulent flow, for which vortices are a special case. (Fun aspect: it's unlikely that you have estimated how long it would take to drain the world's oceans by pulling the plug from an imaginary 10 m wide "hole" in the Mariana Trench. I think you will enjoy the calculation here.)
Without diving any deeper into vortex physics (pun intended), it is safe to state that the rotary movement of the eddy is triggered by underwater currents at different depths that slowly mix water bodies of different temperatures and salinities, that is, different relative densities. The eddy is given its final shape by the Coriolis effect and the winds above sea surface. And yes, in contrast to what I said a few lines before, there is in fact an "upwelling" in the center of such eddies which can be measured by careful examination of satellite pictures, and was found to range between 0 and 3 m for eddies with a diameter of >50 km (not discernible in Figure 1 due to the limited resolution and the vertical imaging). The existence of such eddies, which can last for weeks or months, is now well documented for the Carribean Sea and can help to explain why phytoplankton blooms are also observed in the notoriously nutrient-poor Sargasso Sea.