You might assume that New Hampshire's seacoast is limited to the 18-mile stretch of shoreline from Seabrook to Rye, which comes alive in summer with swarms of boaters, shell fishers, sunbathers, boardwalk strollers, arcade gamers, early bird special diners, and all manner of people watchers. You'd be partly correct. As much as it bustles, New Hampshire does have the smallest patch of oceanfront property of any coastal state.
But just past the tip of Rye’s Odiorne Point lies the mouth of the Piscataqua River. Here the ocean tide comes flooding in twice each day, driving salty currents upstream, through Little Bay and into Great Bay.
Portsmouth Harbor Lighthouse in New Castle, NH
Running straight here, then in swirls and crosscuts there, the tide drives notoriously strong currents in the Piscataqua. Upriver the seawater is gradually diluted by the fresh waters of seven tributaries, which drain 930 square miles of watershed.
This brackish mix creates the Great Bay Estuary ecosystem, with habitats such as underwater sea grass beds, intertidal rocky shores, tidal marshes and tidal flats. Reaching 15 miles inland to Great Bay, this arm of the sea is one of the most recessed estuaries in the country, adding 144 miles of additional tidal shoreline to the state.
So, the New Hampshire seacoast doesn't really end at the mouth of the Piscataqua River. It just takes a left turn.
Salinity
Brackish water defines an estuary, one of the most productive ecosystems in the world, with habitats hosting some marine life, some riverine life, and some uniquely estuarine life.
Seawater gets its saltiness from the many elements dissolved within it. The most common are chloride, sodium, magnesium, potassium, sulfate, calcium, silicon, iron and aluminum. Since the salts are the greater portion of these dissolved solids, it is called saltwater.
All of the elements are building blocks for life in the estuary. Plants and algae absorb them to create the organic compounds necessary for life. Some invertebrates use the calcium in combination with carbon dioxide to construct their shells. Some decomposer bacteria breathe the sulfate as they convert dead plants and animals into usable nutrients.
One kilogram of ocean water typically contains 35 grams of dissolved salts, so its salinity is expressed as 35 parts per thousand, or 35‰. Where seawater from the Gulf of Maine flows into the lower estuary, water salinities typically range from 32 to 35‰. This is where you will find blue mussels, sea stars, anemones, and other marine organisms.
Colorful flora and fauna in a tide pool on a slab
of bedrock next to Portsmouth Yacht Club.
As salinity is gradually diluted upstream by fresh water you’ll start to see oysters, mummichogs and other estuarine animals that can handle brackish water. Oysters, for example, tolerate salinities between 5 and 30‰.
In the upper estuary surging river flows during spring and fall rains alternate with dehydrating summer heat, causing the water of Great Bay to have five times the salinity range and two times the temperature range of the water at the mouth of the Piscataqua River. Spring rains can drop the salinity in Great Bay to as low as 5‰ or less. You will sometimes find freshwater species like pumpkin seed fish, which tolerate some salinity.
For most freshwater and saltwater species, though, these fluctuating conditions are too stressful. They prefer the more consistent riverine or marine environments at either end of the estuary. Though fewer kinds of species can handle estuarine conditions, they flourish with less competition for food and space, and fewer predators.
Marine animals visiting the estuary have ways of adapting to these physical and chemical changes. Lobsters, for example, cope by heading out to the ocean each spring when the rain dilutes the salinity of the upper estuary. As the water grows progressively saltier throughout summer and fall, they’ll swim back upstream.
Energy Flow
Vast plant and algae growth is cultivated by the sheltered, sunlit waters, which are fertilized by the elements in seawater, as well as the nutrients clinging to sediment particles suspended in the rivers and the tides. This is how estuaries are able to produce at least as much biomass per acre than a comparable span of farmland, and far more than an acre of ocean.
The
sun’s radiant energy is converted into enormous amounts of food
energy by all of this greenery: plants and algae in the water column;
underwater eelgrass meadows; microalgae and seaweeds on rocks and in the
water column; and grasses, reeds, sedges, and other flowering plants in
the marsh. It is no wonder then that estuaries are sometimes called breadbaskets
to the sea.
Some herbivores graze portions of the tough plant fibers. But most of the plants die off and become fragmented by things like water flow and chewing animals. Their pieces swirl together with other living and dead organic materials like zooplankton, phytoplankton, dead animals and feces, and become colonized by decomposer bacteria and fungi. These microbes free up inorganic elements like carbon, phosphates, and nitrates, for use by a new generation of plants and algae.
All of this decomposing organic material is called detritus, and it is an essential part of the estuary’s food web. Many of the small animals of the estuary are called detritivores, since they feed on this nutritious mixture.
These bent mysid shrimp, found swimming near seaweed
at Pierce Island in the lower part of the estuary, are one of the links
between detritus and larger animals in the food web.
Detritivores, which include tiny crustaceans, bivalves, worms and snails, are also called primary consumers because they are the start of wide web of consumption. Next come the crabs and schooling forage fish to prey on them. Big offshore visitors like striped bass and blue fish swim in to eat them. Some of these get hauled in by a fisher's line.
Birds, depending on their appetites, feed on producers (plants and algae) and/or consumers (like shrimp, crabs, forage fish and smaller game fish). Seabirds, shorebirds, waterfowl, wading birds, marsh birds and birds of prey are found in the greatest numbers during spring and fall migrations, as they spend time feeding in the estuary.
Formation
The landscape that resulted in this ecosystem actually got its start during the last ice age. A mile-high sheet of ice bore down on the land from the Arctic to Pennsylvania, forming the depression that is now the estuary. Eventually the climate warmed and the ice melted. From more than 300 feet below its current level, the ocean rose up to flood the landscape, creating the drowned river valley that is now Great Bay Estuary. Despite all the freshwater flowing from the rivers, the estuary remains an ocean-dominated ecosystem. It takes 18 days during high river flow for water to flush from one end of the estuary to the other.
A drowned river valley is just one kind of estuary. Others develop from shifts in the earth’s crust, or steep glacier-forged valleys, known as fjords. At the southeastern tip of the state, seawater also flows into Hampton Harbor through the protective barrier beaches at Hampton and Seabrook. It mixes with the fresh water of five rivers, creating the 475-acre Hampton-Seabrook estuary. This bar-built estuary adds 72 miles of tidal shoreline to the state.
Much of Great Bay Estuary’s shoreline is inaccessible due to private property boundaries. In 1989, a portion of the ecosystem came under the protection of the federal estuarine research reserve program. Great Bay National Estuarine Research Reserve (GBNERR) is now one of 26 state-owned and managed reserves around the country, established to conserve, manage and study estuarine ecosystems. More than 23,000 acres of the Great Bay reserve are managed by the New Hampshire Fish and Game Department.
Great Bay Discovery Center, located in Stratham on the southern shore of Great Bay, is the reserve’s education center. It is a good place to visit and explore some of the estuary’s history and habitats.