Many of us know that plankton form the basis of the aquatic food chain, but aside from that, these miniature creatures are probably a bit of a mystery, right? Well, if you breathe air, then you should really take a closer look at these tiny aquatic powerhouses that create much of the oxygen we take into our lungs.
So what exactly are plankton? Plankton are microscopic organisms that are found in marine and freshwater, drifting with the currents because they are unable to swim against the force of the water. The term “plankton” is a collective name for such organisms, including certain algae, bacteria, protozoans, and the juvenile life stages of crustaceans, mollusks and coelenterates (such as jellies and anemones).  These organisms really know how to go with the flow.
Above: Microscopic view of a copepod, a type of zooplankton. Photo by: Eric Schroeder.
Phytoplankton are the base of the food chain and contain chlorophyll like plants on land, so they are dependent upon light and nutrients to photosynthesize and produce their energy. As such, their productivity fluctuates throughout the year and usually takes place within 20 meters of the surface where the sunlight can penetrate the water.  Carbon dioxide, water, and light are necessary for photosynthesis. Nitrate, phosphate, silicate, and carbonate are all important in the production of plant matter. Silicon, phosphorus, and nitrogen also play a key role in their ability to grow.  Phytoplankton consume carbon dioxide and release oxygen; you can thank them for their contributions on your next visit to the ocean, as they provide between 50-70% of the oxygen we breathe. Some common types of phytoplankton include cyanobacteria, diatoms, green algae and dinoflagellates 
Springtime brings warmer temperatures and increased sunlight, creating a thermocline — the transition layer between warmer mixed water at the ocean’s surface and the cooler deep water below — that traps nutrients at the ocean surface. This allows phytoplankton to absorb energy and take in the nutrients they need to photosynthesize and multiply. The warming of the surface layer keeps this section of the water less dense, so it stays afloat. Phytoplankton respond very quickly when the right conditions occur, growing and reproducing as soon as a slight stratification of the water column occurs. 
Phytoplankton populations explode when sunlight and nutrients are plentiful, allowing them to grow and reproduce to a point where there are so many that the water seems to change to a different color — green, brown or red from the chlorophyll they contain. These plankton “blooms” can be quick events that begin and end within a few days or they may last several weeks; the length and intensity is dependent upon the concentration of nutrients in the water column and their residence time there. They can occur on a relatively small scale or cover hundreds of square kilometers of the ocean's surface. 
As the phytoplankton use up the available nutrients, many begin to die and drift to the bottom. As autumn begins, cooler days cause some vertical mixing of the water that may bring nutrients up from below resulting in a somewhat smaller fall bloom. Once winter begins, plummeting temperatures and frequent storms cause heavy mixing. Phytoplankton don’t stay at the water surface in this mixing, which means they don’t have access to as much sunlight—this is why blooms do not occur in the winter. 
But let’s take a step back to the springtime bloom we mentioned — this is one of the year’s most important biological events for the Gulf of Maine. The annual bloom in temperate and high latitude continental shelf waters may represent more than half the total input of organic matter to deep water and the organisms that live there in the “benthos,” or near the bottom (Parsons et al., 1984; Smetacek et al., 1978; Smetacek, 1980; Peinert et al., 1982).  Amazing, huh?
That leads to the other portion of the equation: animal plankton, or zooplankton. They feed on phytoplankton, and then the zooplankton become food for schooling fish like herring and progressively larger organisms. Zooplankton like copepods are very small in size—no more than ¼” long—but they are signaling a shift in the Gulf of Maine that has potentially massive impacts on marine life. In recent years, warming ocean temperatures have sped up the life cycle of one type of copepod, Calanus finmarchicus. The warmer waters have shifted peaks in C. finmarchicus abundance to earlier in the spring. Small fish depend on the timing of plankton development—the fish have a preference for certain sized copepods. Changes in the ocean temperature can lead to a mismatch in timing between the fish and their preferred size of zooplankton, which then leads to starvation for the young fish. 
Basic changes to phytoplankton communities could resonate up through the entire food web and impact creatures higher up in the food chain — including those of economic and ecological importance. Changes in the zooplankton composition may alter assemblages, forcing large marine predators to shift northward to areas where their prey species have survived, or to areas with alternative prey species. 
The take-home message from all this: Plankton are small but they have a big impact on life in the sea and our lives.
 britannica.com, accessed 21 February, 2016. Authored by the Editors of Encyclopedia Britannica.
 http://serc.carleton.edu/eet/phytoplankton/all_parts.html When is Dinner Served? - Predicting the Spring Phytoplankton Bloom in the Gulf of Maine
 website: gulfofmaine-census.org. Oceanography. Condensed from Townsend, D.W.,A.C. Thomas, L.M. Mayer, M. Thomas and J. Quinlan. 2006.
Oceanography of the Northwest Atlantic Continental Shelf. pp. 119-168. In: Robinson, A.R. and K.H. Brink (eds). The Sea, Volume 14, Harvard University Press.
 Biological Impacts of Climate Change in the Gulf of Maine, Scott D. Kraus, PhD, New England Aquarium, Boston, MA. Website: neaq.com. Changing Climate, Changing Coasts, Statement by Scott Kraus, PhD.
 http:// http://earthobservatory.nasa.gov/Features/Phytoplankton/ What are phytoplankton? Accessed 4/14/2016 Authored by Rebecca Lindsey and Michon Scott.