Although less visible to us than the freezing and thawing that transform our local lakes from swimming holes to ice-fishing haunts, the waters off the coast of Maine undergo similarly dramatic seasonal cycles. And the physical transformations in the water trigger changes that create a sea of plenty-phytoplankton feeding zooplankton feeding tiny fish, larval lobsters, giant whales, and the thousands of other animals that live in or migrate to the Gulf of Maine.
As the Gulf’s deep waters are mixed up to the surface by winter winds and currents, they carry with them a wealth of nutrients-phosphate, nitrate, nitrite, iron-the basic fertilizers required to fuel life. Once the surface waters receive this enrichment, all that is required is a few days of calm weather and phytoplankton begin to bloom. Blooms can occur as early as mid-winter in the offshore waters of the Gulf, as long as winds stay at bay for long enough for a few generations of dinoflagellates or diatoms (two of our most important and common phytoplankton) to thrive in the sunlight of near-surface waters.
As the days lengthen in April and May, the surface waters begin to warm, floating above the heavier, colder water below. This stratification buoys phytoplankton in the sunlit waters near the surface, and the plankton truly bloom. The rivers and bays take on a green or brown tint. The NERACOOS ocean observing buoys record the blooms by measuring the light-reflecting characteristics of particular species, allowing researchers to trace the sequential blooms of various diatoms and dinoflagellates throughout the spring.
Rapidly following the growth of phytoplankton come the blooms of the zooplankton, the insects of the ocean. Copepods comprise many of the zooplankton in the Gulf, including the shrimp-like Calanus finmarchicus, considered a critical food source for many commercial fish species and whales. The life cycles of Calanus and other zooplankton are tuned to the blooms of the phytoplankton on which they feed. Similarly, the lobster, haddock, flounder and herring are tuned to release their eggs to the waters of the Gulf so that they become larvae at the right time and place to encounter the plankton food source they need to grow.
In addition to being timed with the phytoplankton and zooplankton blooms, the eggs and larvae of marine species are subject to the movements of the currents, tides, and waves, as they are carried by tides and nearshore currents, or the broad offshore circulation of the central Gulf.
Large spring rainstorms and associated rapid snowmelt during March and April can wreak particular havoc on carefully tuned marine ecosystems, as they do on coastal beachfronts. Given the tremendous volume of saltwater in the Gulf of Maine, it is somewhat surprising that large spring rains, snowmelt, and associated runoff can have any effect on the coastal waters. But they do. One reason for this is that freshwater is less dense than seawater, and so it forms a “lens” on the sea surface. Along the coast of Maine, the Penobscot, Kennebec and Saco rivers contribute to a coastal “plume” of relatively fresh water which flows west and south along the coast. During major runoff events, this plume is particularly fresh, and flows faster, bringing nutrients from land and carrying eggs and larvae more rapidly downstream.
Just as crocuses poking up through last year’s leaves, apple trees bearing first buds, and gardeners transferring from hot-house to garden are all gambling on the timing of the last frost and whims of spring rains, the success of each spring’s fish and shellfish depends on many factors. A warm or cold winter, heavy snowfall and an early melt, little snowfall with spring northeasters-each year creates a novel situation. Boon years and lean years among all of the Gulf’s species reflect their varying success at making it through another spring.
This article is made possible in part by funds from Maine Sea Grant.
Heather Deese holds a doctorate in oceanography and is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.