Plankton are any organisms, plant or animal, that float, as opposed to swim, in the water. The word plankton comes from the Greek word, planktos, meaning wandering or drifting (size has nothing to do with the definition of plankton). As very weak swimmers, these plants and animals flow at the whim of currents and tides. Phytoplankton, or plant plankton, have chloroplasts (complex organelles found in plant cells, responsible for the green color of almost all plants) and use sunlight and nutrients for photosynthesis. Zooplankton are the drifting animals that feed on the phytoplankton.
The phytoplankton, like trees or grass on land, are primary producers – the first step in a complicated food web. They are the main source of food for zooplankton which are the main diet of other larger zooplankton, some sea birds, fish and even the North Atlantic right whale. Even though these organisms are barely visible, they are essential to life on Stellwagen Bank.
Sunlight and nutrients are essential for a phytoplankters growth and reproduction. Nutrients are found throughout the water column, but sunlight is only available in the top part of the water column, the area known as the photic zone. This makes survival for phytoplankton rather complicated: these plants need to find a way to stay up near the surface so they can take advantage of both the sunlight and the nutrients – a sink or swim situation. The many different species of phytoplankters are separated into four categories: the diatoms, the dinoflagellates, the flagellates and the coccolithoporids. Each species has its own special and unique adaptation that enables it to remain at or near the water’s surface.
Starting in mid-March, when the water column is stratified and a thermocline (boundary between warm top layer and cool bottom layer) is present, diatoms are generally the more dominant phytoplankton. Diatoms are relatively heavy organisms because their cell walls are made primarily of silicon and would quickly sink away from the surface (and light) without adaptations to keep them afloat. Certain diatoms like Chaetoceros debilis have tiny bristles that extend from the cell body which slow down their sinking. Chaetoceros debilis is one of the distinctive phytoplankters of the Gulf of Maine. From the four corners of each cell, long thin setae, or bristles, extend outwards. These then fuse with the neighboring cell and this arrangement can create a long sprial, up to 2 mm long. Although it may seem like one long snakelike organism, each cell is actually distinct and separate from the neighboring cell. By joining together they increase their surface area. This increases drag and slows down sinking. Joining into this long chain, ensures that the cells will remain at the surface for a longer period of time, having more of an opportunity to capture energy from the sun.
Thalassiosira uses the same technique as Chaetoceros to remain suspended in the water column. Little bristles extend out from the top and bottom of the cell, circling the perimeter. In the center, there is one, long organic thread that connects with the body of another Thalassiosira cell.
Skeletonema and Leptocylindrus are other examples of chain-forming diatoms commonly found in these waters. They don’t have the bristles found in Chaetoceros and Leptocylindrus, but they do chain together in an to increase their surface area in an attempt to extend their time at the surface.
There are other diatoms, like Coscinodiscus, however, that do not have the long bristles and that do not form these long chains. Termed centric diatoms, these are single cells that resemble pill boxes. The silicate frustule, or cell wall, is made up of two halves: the epitheca (top half) and the hypotheca (bottom half). Coscinodiscus do not have adaptations that help keep them afloat. They rely on the stratification of the water column to keep them at the surface: they grow in the warm water layer, at the top of the thermocline. This water can be moved about by wind and tide, but cannot easily mix with the cool, dark water below. In essence, Coscinodiscus is trapped in a world to its liking.
The dinoflagellates, another type of phytoplankton, have an advantage over the diatoms. They have two flagella, or threads, that extend from the body. In most cases, one flagellum circles the body horizontally at the center while the other extends vertically from the lower half of the cell body. The center flagellum causes the organisms to rotate around their axis while the lower flagellum pushes water away from the cell, catapulting the plant forward. These organisms travel in a forward corkscrew manner. Dinoflagellates therefore don’t need to worry about sinking. They can retrieve nutrients from way down at depth and then spiral up to the surface to capture energy from the sun. This advantage makes them one of the most common phytoplankters from September to mid-March.
A beautiful example of a dinoflagellate commonly found in these waters is Noctiluca scintillans. Despite its truly microscopic size, many people have witnessed these organisms. Their name comes from the Latin words nocti (moon) and lucere (to shine) which describes the greenish glow of their bodies, massed together on summer night tides. By combining luciferine, luciferase, oxygen and energy, Noctiluca can create a sudden burst of light to confuse would-be predators with eyes adapted to the dark.
Maybe one of the most famous dinoflagellates is Alexandrium tamarense. It is responsible for the so-called “red-tide” that can be found on the shores throughout the Gulf of Maine in August. Massive blooms of this plant occur offshore and then are carried to the beaches by currents. Even though these dinoflagellates can “swim”, their movement is still primarily governed by the tides and currents. Alexandrium is also responsible for paralytic shellfish poisoning (PSP) toxins that shut down some of the local shellfish fisheries.
The dinoflagellate Ceratium does not have the characteristic shape of other dinoflagellates. Shaped like an anchor (up to 2 mm long), it can have two or three horns emerging from the cell body – always with two from the top and possibly with one from the bottom. But true to form, there is always one exception to every rule. In addition to being able to perform photosynthesis, Ceratium and certain other dinoflagellates, can also ingest particles through a process called phagocytosis. They store particles in food vacuoles, little compartments within the cell. These particles are eventually broken down by different enzymes. Scientists therefore categorize dinoflagellates in the phylum of protozoa (as opposed to plants), but are generally regarded as phytoplankton.
An example of another type of phytoplankton is the marine flagellate Phaeocystis pouchetii. The life cycle of this plant is quite complicated for it has two different stages. The first is a motile flagellated cell that is only .003 to .008 mm in diameter. Populations grow very rapidly and quickly out compete the other phytoplankton for available nutrients. Once nutrients start to become too scarce, the single cells aggregate into a colony, surrounded by a sticky mucous membrane. These colonies can be up to 1 mm in size (about the size of the period at the end of this sentence). Phaeocystis has an advantage over the other phytoplankton for it can store nutrients in this outer membrane. When nutrients are no longer present in the surrounding water, it can draw upon the stored nutrients and continue to grow and reproduce. Once nutrients are no longer available either in the water column or in the colony membrane, the colonies start to lyse, or dissolve. This leaves behind a white foam which washes up on the nearby beaches. The odor and texture of this foam is not too pleasant! More critical to the Stellwagen environment though is that zooplankton cannot eat the colonial form of Phaeocystis. This limits zooplankton populations and has a spiral effect: less food is available for the fish and other animals that feed on zooplankton. Studying Phaeocystis is an integral component in the North Atlantic right whale habitat research (see here) that is conducted at the Center for Coastal Studies.