Seasons in the Sea
By Ann Michelle Morrison, SD

The Gulf of Maine is the relatively shallow water located off the coasts of Massachusetts, New Hampshire, Maine, and Nova Scotia, Canada.  The region is home to some of the most productive fisheries in the world and hosts at least 18 species of marine mammals throughout the year.  Beneath the blue, wavy water of the Gulf of Maine, the sea, like the land, changes with the seasons.

Winter
The Gulf of Maine cycle begins in the dark and stormy months of winter.  Nor’easters and other large storms cross the Gulf, churning the water with the forces of wind and tide.  During Winter the deep and shallow waters of the Gulf of Maine are roughly the same temperature, which allows mixing of nutrients (particularly nitrogen) from the deep waters to the surface waters.  The short and frequently cloudy days of Winter also reduce light levels.  Under these conditions the tiny, floating plants, called phytoplankton, are in low abundance.  Zooplankton, the small animals that feed on phytoplankton, are also relatively few in number in the Winter.  Some species, such as the copepod Calanus finmarchicus, hide out in the deep basins of the Gulf of Maine until conditions are favorable for growth and reproduction.  Young fish, including cod and redfish, live along the bottom of the sea where food is more plentiful during the Winter months. 

Spring
As the inclination of the Earth changes and the northern hemisphere shifts towards the sun, sea surface temperatures increase, and the days get longer.  Storm frequency decreases, and there is less mixing in the ocean due to wind.  Life in the Gulf of Maine awakens in the Spring.  Warming of surface waters creates an invisible barrier called a thermocline, which reduces mixing between the warm waters above the barrier and the cold waters below.  Phytoplankton that need light and nutrients to thrive multiply rapidly in the warm surface waters above the thermocline.  The surface waters are rich in nutrients in the Spring thanks to Winter mixing from the deep.  These conditions initiate the annual “Spring Bloom.”  The Spring Bloom usually occurs in April in the Gulf of Maine, but the timing of the Spring Bloom can vary by as much as several weeks and has implications for other species further up the food chain.

In early Spring, populations of the copepod C. finmarchicus, which have been sheltered in the deep basins of the Gulf of Maine, rise to the surface waters.  With the advent of the Spring Bloom, C. finmarchicus numbers rapidly increase alongside the growing phytoplankton population.  Other species of zooplankton follow soon after.  By the end of the Spring phytoplankton bloom, zooplankton concentrations have increased substantially since the availability of food, or phytoplankton, allows zooplankton to multiply.  

Many species of fish in the Gulf of Maine, including cod, time spawning (egg laying) to capitalize on the Spring Bloom that provides a rich source of food for the growing fish larvae.  Female cod spawn in the spring by releasing millions of eggs that float on the surface of the ocean where they are fertilized.  The fertilized eggs develop into larvae that feed on zooplankton in the surface waters.  One hypothesis suggests that an early phytoplankton Spring Bloom results in an earlier zooplankton population, which provides a large source of food for the hungry young fish.  These conditions are thought to produce a strong fish population from that year’s larvae.  Conversely, a late Spring Bloom could mean that there is little food available for the young fish.  Fewer will survive, producing a weak fish population from that year’s larvae.  Not all fish time their spawning to coincide with the Spring Bloom, but, for those species that do, changes like early Spring warming or frequent Spring storms may have a large positive or negative effect on the fisheries.

Summer
In Summer the sea surface temperatures continue to rise and storm frequency continues to decline. The thermocline between the warm and cold waters of the Gulf of Maine intensifies and further restricts flow of nutrients from the nutrient-rich deep water to the, now, nutrient-depleted surface waters.  The principal nutrient that limits growth of phytoplankton in the ocean is nitrogen, and during the summer, the primary source of nitrogen to the surface waters is ammonium excreted by zooplankton.  The ammonium provides a reliable, though smaller, source of nitrogen for phytoplankton.  Therefore, despite the increased light intensity of Summer, which is beneficial to phytoplankton productivity, the reduced nitrogen supply limits phytoplankton growth. 

Zooplankton continue to increase in numbers through the summer as they consume phytoplankton.  C. finmarchicus populations, which account for the largest biomass of zooplankton in the Gulf of Maine, typically peak in late July and early August.  Other zooplankton species peak later in the summer and into the fall.  

Spring spawned larval fish continue to feed on zooplankton throughout the Summer.  Other fish species are preparing to spawn.  Herring adults fatten up on the large zooplankton population in the Summer before they release their eggs on the ocean floor at the end of summer and into early fall.  Fertilized herring eggs develop into larvae that rise to the surface waters to feed.  The herring larvae then drift with the plankton for about 6 months before they metamorphose into juvenile herring.

Fall
The advent of Fall brings reduced temperatures, reduced light intensity and duration, and an increase in storms that promote mixing.  Nutrients again rise up to the surface waters where there is still enough light for phytoplankton to multiply.  A “Fall Bloom” is common in the Gulf of Maine.  The Fall Bloom is typically smaller than the Spring Bloom because the mixing from the deep waters is not enough to add a large amount of nutrients to the surface waters.  The conditions allow just enough nitrogen to rise to the surface to stimulate phytoplankton growth once more.  The Fall Bloom may be more protracted than the Spring Bloom because mixing from storms incrementally increases the nitrogen in the surface waters, which remain relatively warm well into the Fall.

The Fall Bloom is beneficial to zooplankton and fish alike.  Zooplankton species that have been less abundant in the Spring and Summer often peak in the Fall.  This is particularly true for the inshore waters of the Gulf of Maine where the copepod Centropages typicus peaks and dominates the zooplankton assemblage.

The larval cod that have grown throughout the summer are now considered juvenile fish.  In Fall the juvenile cod swim out to deeper waters and sink to the bottom of the sea where they will feed through the Winter.  Some adult cod actually spawn again in the Fall.  These cod larvae feed on the abundant food from the Fall Bloom and subsequent zooplankton growth.  Larval herring that were spawned in the Summer continue to drift and feed off the plankton in the surface waters throughout the Fall and into Winter.  

Back to the beginning . . . 
Fall leads into Winter, and the seasonal cycle of the Gulf of Maine begins again.  Bacteria that live on the ocean floor have decomposed zooplankton and other animals that died throughout the year as well as fecal material that dropped through the thermocline and settled on the bottom.  Decomposition released the basic nutrients into the water column again.  Some of these nutrients will be mixed into the surface waters during the Winter as the waters of the Gulf of Maine become uniform in temperature again, dissolving the thermocline barrier.

Life is not easy in the Gulf of Maine.  Of the millions of fish eggs spawned each year, perhaps 1% will become adult fish.  Predation from fish, other animals, and humans affects fish stocks, but the seasonal dynamics that drive phytoplankton and zooplankton abundance may have a significant influence on early fish survival.  The Spring Bloom does not occur each year on the same day or even the same week.  It is said that “timing is everything,” and that adage may be fitting for life in the Gulf of Maine.  Scientists are studying how and why the Spring Bloom happens when it does and how the rest of life in the Gulf of Maine is impacted.  If scientists can begin to unravel the interconnectedness of weather, tide, current, sunlight, nutrients, phytoplankton, zooplankton, and other animals further up the food chain, we may begin to understand how to best manage and sustain fisheries, shellfisheries, and marine mammals within the Gulf of Maine. 

This information has been adapted from:
Cushing, D.H., 1990. Plankton Production and Year-class Strength in Fish Populations: an Update of the Match/Mismatch Hypothesis. Advances in Marine Biology 26, 249-293.

GMA, 2005. Herring Biology: Life-Cycle. Gulf of Maine Aquarium.  http://www.gma.org/herring/biology/life_cycle/default.asp.

GoMOOS, 2002. About the Gulf of Maine.  http://www.gomoos.org/aboutgulfme/.

HBOI, 2005. Gulf of Maine Copepod Primer. Harbor Branch Oceanographic Institution.  http://www.at-sea.org/missions/maineevent4/docs/copprimer.html.
Manning, C.A., Bucklin, A., 2005. Multivariate analysis of the copepod community of near-shore waters in the western Gulf of Maine. Marine Ecology Progress Series 292, 233-249.

Minister of Supply and Services, Canada. Redfish.  http://www.mi.mun.ca/mi-net/fishdeve/redfish.htm.

Platt, T., Fuentes-Yaco, C., Frank, K.T., 2003. Marine ecology: Spring algal bloom and larval fish survival. Nature 423, 398-399.

Runge, J., 1988. Should we expect a relationship between primary production and fisheries?  The role of copepod dynamics as a filter of trophic variability. Hydrobiologia 167/168, 61-71.

Ryan, S., Department of Fisheries and Oceans, Canada., The Evening Telegram, NFFAUW, Human Resources Development Canada, DataDisk, Inc., 1996. Northern Cod- A matter of survival.  http://www.cdli.ca/cod/history4.htm.

Townsend, D.W., Thomas, A.C., Mayer, L.M., Thomas, M.A., Quinlan, J.A., 2004. Oceanography of the northwest Atlantic continental shelf (1,W). In: Robinson, A.R. and K.H. Brink (Eds.), The Sea: The Global Coastal Ocean: Interdisciplinary Regional Studies and Syntheses. Harvard University Press, Boston.