Phytoplankton of the Northeast U.S. Shelf Ecosystem
Microscopic algae known as phytoplankton are the base of most marine food webs. Phytoplankton contain chlorophyll which is used to capture sunlight and convert carbon dioxide and water into oxygen and carbohydrates.
What Are Phytoplankton?
Almost all life on Earth relies on photosynthetic organisms such as plants and algae. In the Northeast U.S. continental shelf ecosystem, microscopic single-celled algae known as phytoplankton are responsible for nearly all primary production. Globally, primary producers generate almost half of Earth’s oxygen and are the base of most marine food webs. They are also important components of carbon cycling and sequestration—the process of capturing and storing atmospheric carbon dioxide. Ultimately, how many organisms that are able to live and grow in a given area depends on the amount of primary production generated by the phytoplankton.
There are more than 20,000 species of phytoplankton distributed among eight major taxonomic groups. They range in size from less than 1 µm to greater than 100 µm. The distinct phytoplankton groups have different primary functions in the ecosystem. They also have different effects on food availability, biogeochemical cycling, and export of carbon to the deep ocean. The Northeast Fisheries Science Center’s Milford Laboratory actively cultures more than 230 species of phytoplankton. These cultures serve as a resource for research and a part of outreach and “extension” activities with the commercial aquaculture community.
What Is Primary Production?
Photosynthesis involves the capture of light energy by plant pigments such as chlorophyll. This energy converts water and carbon dioxide into carbohydrates (sugars) and oxygen. Primary productivity is simply the rate of photosynthesis and uptake of dissolved nutrients such as nitrate and phosphate to produce more plant matter or biomass. High primary productivity regions are often “hot spots” for fish, turtles, birds, and marine mammals, and also for benthic shellfish in shallower regions.
Phytoplankton and the Food Web
Phytoplankton blooms are a major component of the food web and a primary food source for zooplankton and filter feeders such as shellfish. At the Northeast Fisheries Science Center we are researching how different phytoplankton species influence shellfish growth of both natural and aquacultured populations. We are also using models to determine how changes in phytoplankton composition and productivity influence the food web and fisheries productivity.
Where Are Phytoplankton?
A number of environmental and oceanographic factors interact to influence the abundance, composition, spatial distribution, and productivity of phytoplankton. The principal factors are:
- Amount of sunlight.
- Availability of major nutrients.
- Water temperature.
- Physical oceanographic processes such as vertical mixing, upwelling, currents, and tides.
- Grazers such as zooplankton and shellfish.
When the growing conditions are ideal for a specific species, the population can grow exponentially and create a “bloom.”
The unique physical characteristics of the Northeast U.S. continental shelf help make it among the most productive continental shelf systems in the world. However, there are large seasonal and regional differences in phytoplankton. The most obvious spatial pattern, which can be observed using satellite remote sensing data, is the decrease in phytoplankton abundance from the coast to the shelf break. Georges Bank and Nantucket Shoals are shallow regions that are well mixed by tides. This mixing supplies sufficient nutrients to support phytoplankton growth throughout the year. In other regions, blooms of large diatom species occur on a seasonal cycle when growing conditions are ideal.
Phytoplankton in estuaries and coastal regions in the Northeast have similar seasonal cycles. However, excess nutrients from septic systems and non-point source pollution such as urban and agricultural runoff can lead to eutrophic conditions and large phytoplankton blooms in these nearshore regions. These dense blooms can lead to low-oxygen “dead” zones and fish kills when the phytoplankton decay. Eutrophication can also favor the development of harmful algal blooms (HABs), some of which produce toxins that are harmful to humans, fish, shellfish, marine mammals, and birds. HABs may also be called a “red tide,” as some harmful species contain red pigments that turn the water red at high populations. There are several HAB species in the Northeast. The most well known HAB is the toxic dinoflagellate Alexandrium catenella, which produces saxitoxins that accumulates in shellfish. This species can cause paralytic shellfish poisoning in humans and other consumers. In the Chesapeake and Delaware Bays, there are a variety of HABs that can kill fish and shellfish and have severe impacts on fisheries and aquaculture.