The ecosystem on Jeffreys Ledge centers around phytoplankton (small marine plants), the base of all marine food chains. Phytoplankton, produced by upwelling, attracts zooplankton (small marine animals). There are many kinds of zooplankton, but the most prominent one is called the copepod. This tiny animal is the prey of other forms of zooplankton, crustaceans, many fish species, and even the 50-plus ton northern right whale!
Productivity of plankton is not uniform through the year. Because the plants need nutrients and sunlight, the longer days which start in spring cause seasonal plankton blooms. Plankton productivity decreases in the latter part of the summer. However, in the fall much of the Gulf of Maine stock of herring comes into the area to spawn. Both the aggregated spawning herring and, later their developing eggs and larvae (also considered a form of plankton) provide a seasonal food source for many predators.
Because the fish and mammals which use the Ledge are powerful swimmers, many animals which spend some time on the Ledge also use other marine habitats as well. For example, in years when herring are abundant and sand lance are not, humpback whales are abundant on the Ledge. However, if sand lance are abundant and herring stocks are low, the same animals will generally use Stellwagen Bank and the Great South Channel. Because of this, it is often inaccurate to speak of a true "Jeffreys Ledge Ecosystem." However, since localized productivity is caused by localized upwelling, the Ledge and its surrounding waters are thought of as a management area.
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