Physical Oceanography

Water may be the most obvious component of Stellwagen Bank. Currents, tides and storms work with shape of the Bank, seasonal changes in light levels and nutrients to form the habitat used by all the local inhabitants. But this richness does not stand alone. The nutrient rich currents that feed this habitat come from somewhere else.

Stellwagen lies at the southern end of the Gulf of Maine, a semi-protected body of water open to the Atlantic by the Great South Channel. Currents, flowing off the Atlantic, sweep counter clockwise through the Gulf gathering nutrients (and pollutants) from the rivers running to the coasts of Canada and New England. Minerals from rivers and the decaying bodies of marine plants and animals drift down, with gravity, to the cool currents below. As these deep currents run into obstructions, like Cape Cod and Stellwagen Bank, they are forced upward.

This process, called upwelling, fertilizes the sun-drenched surface waters creating the perfect habitat for drifting marine plants or, phytoplankton. This growth, in turn, will add to the richness of the Gulf’s currents, creating a cycle of nutrients: each local habitat within the Gulf gives energy to the currents and each habitat receives energy from the currents.

A cross section of water on Stellwagen, from the surface to the bottom, shows that the properties of the water are not the same throughout. These properties change with the seasons. Layers of water, separated by temperature (with the warmer, less dense waters on top), salinity and nutrient content form at different times. In a simplified model, the long days of summer create a surface layer of warm water, separated from the cooler, deep waters by the thermocline. This stratification breaks down through the storms of Fall: strong winds churn the seas creating a well-mixed water column where temperature, salinity and nutrients are similar from the surface to the bottom.

Spring

Well mixed water, from winter storms, begins to warm near the surface; strengthening sun and nutrients create the strongest bloom of phytoplankton for the year.

Summer

The sun-drenched upper layer has separated from the cooler depths (thermocline); nutrients within the upper layer are used up by the explosion of phytoplankton.

Fall

With less sun and more wind, the thermocline of summer breaks down. Unused nutrients from the bottom layer circulate to the surface creating a new, brief bloom of phytoplankton.

Winter

With few hours of sun and intense storms nutrients, temperature and salinity are the same throughout the water column. Phytoplankton numbers are at their lowest.

Changes in the composition of water, caused by weather, seasons and bathymetry (shape of the sea floor) are the basis of marine environments. These changes are key to understanding the lives of marine organisms, from right whales to copepods, clams to cod.