The shoreline changes dramatically when the tide goes out. Acres of sand or mud are exposed for hours until the next flood. Tidal flats composed mostly of sandy sediments are created by swift currents, like those of the Piscataqua River. Only the larger, sand-size particles are heavy enough to drop out of such fast-flowing water. By the time the tide reaches all the way up to Great Bay, it has slowed enough to allow the finer sediments to settle out. These combine with a far greater contribution of silt and clay from river flows to create the mudflats of Great Bay.
Shorebirds
Exposed sediments and shallow water provide forage areas for shorebirds. Their kinds vary, depending in part on the timing of their migrations, and even the level of the tide. Lesser yellowlegs, for example, pass through in April and again in mid-July, on their way to and from nesting in the far north. The greater yellow legs start their northern migration sooner, then lag behind into November on their way south. It’s a good bet you’ll be able to watch some of them toward the end of ebbing tide on the west side of Adams Point. They like to catch the fish swimming out from Crommet Creek.
Greater Yellowlegs feeding off the shore of Adams
Point, in Durham as the tide goes out from Crommet Creek on Labor Day
weekend. The disturbance of the water surface around them, looking like
drops of rain, is caused by the movement of so many schooling mummichogs
and shrimp in the water column.
Each kind of shorebird seeks out the zone that suits its feeding style
and appetite. 
Up
from the tide line, semipalmated plovers dash along and then stop, using
their large eyes to spot prey, which they quickly peck with their stout
little beaks. Plovers typically forage in the same general area, but each
hunts alone for its mollusks, crustaceans, worms and insects. Another
kind of plover, the killdeer, eats mostly insects, which live in the grassy
areas beyond the upper intertidal zone.
Many sandpipers have nerve receptors at the tips of their bills to help them taste and touch their prey as they probe the sediments. Flocks of semipalmated sandpipers dash toward receding waves, their legs a blur. They stop for quick probes before resuming their dash back ashore ahead of incoming waves. The ruddy turnstone, also in the sandpiper family, goes about overturning rocks and shells in search of worms, invertebrates and insects. The greater yellow legs, another large sandpiper, wades out to sweep its bill through the water for fish.
These are just a few of the shorebirds that have been seen in the estuary. It’s also common to see great blue herons at low tide. These wading birds stand stock-still just off the flat in shallow water, patiently waiting for the right moment to jab a fish or invertebrate.
When the birds take off, tidal flats seem so lifeless and barren. There is still plenty of activity, though. It doesn’t all fly away or swim out with the tide. On and within the sediments are thriving tidal flat communities. They have fewer species than other habitats, yet their populations flourish where others cannot.
The Surface
The
yellowish green film coating the top few millimeters of a mudflat is created
by millions of cells called diatoms. An electron microscope reveals each
diatom as a minute glass box. There are round ones that float along in
the water column, and elongated ones that move along sediment surfaces.
They all have beautifully intricate geometric designs on their cell walls.
Diatom colonies produce lots of food and oxygen through photosynthesis.
For the countless thousands of mud snails dotting the flats, these are
a big menu item. When the snails fall prey to other creatures, some of
the food energy produced by the diatoms starts to make its way through
the food web.
Snails and amphipods eat the soft sea lettuce strewn on the flats. Some of the rockweeds and knotweeds are also grazed, but most animals consume them only after bacteria and fungi have had a chance to decompose their rubbery blades and bladders into particles of detritus.
Within
Few other animals linger when the flats are exposed and dehydration threatens. Below, however, a whole community thrives. In a three-dimensional habitat of sediments, myriad organisms take refuge from the swimming and crawling predators mobilized by the tides. They are also shielded from the harsh extremes of estuarine water, with its ever-fluctuating temperatures and salinities. Such animals are known as the infauna.
Some prefer the sands of the lower estuary. Sandy sediments have their pros and cons. They are coarse enough to allow water to wash through, providing oxygen beneath the surface. At the same time the water is washing away much of the organic material that could be somebody’s next meal. If you dig into a sand flat you will find animals like clams and lugworms closer to the water’s edge, in the lower intertidal zone. Little hopping amphipods, or beach fleas, are found in the upper intertidal zone, where the tide is less frequent.
But in mud, animals don't seek out particular zones in the intertidal area, as they do in sandy sandy sediments, tidal marshes, or rocky shorelines. The mudflat community arranges itself from the surface down.
Compact, muddy sediments trap more organic material to eat, but they also prevent oxygen from percolating below the top few millimeters. The result is an anoxic (without oxygen) environment. This inhospitable situation is actually necessary for a lot processes, which the estuarine ecosystem depends on to function. The creatures responsible for these processes? Mere cells.
There are many kinds of photosynthetic cyanobacteria in the world, and some love living in salty environments. They are found on surface sediments in the tidal flats, releasing enormous amounts of oxygen as they produce carbohydrates through photosynthesis. They serve as a food source for other types of cells, as well as tiny animals living between the grains of sediment, and larger animals like worms.
In the sediments below, decomposer bacteria make their living disintegrating organic materials trapped in the flats. While breaking down sulfur-containing proteins they release lots of hydrogen sulfide, the source of the low tide aroma, reminiscent of rotten egg. You can see evidence of their work in the black stains of subsurface sediments. These are the result of hydrogen sulfide reacting with iron particles.
Other bacteria break the energy bonds holding hydrogen sulfide molecules together to power their food production. This is a kind of chemosynthesis, similar to using the sun’s radiant energy for photosynthetic food production. The hydrogen sulfide created during decomposition is eventually converted to sulfate, a form used by plants and algae.
These bacteria are responsible for just a few of the many complex chemical conversions taking place. Other bacteria are at work freeing nutrients like carbon, phosphorous, hydrogen and nitrogen from organic materials, so they can be used again by other organisms.
Invertebrates
Though necessary for decomposition, anoxic muds and hydrogen sulfide are
a deadly combination for infauna like worms and clams.
These invertebrates survive because they have ways of supplying themselves with oxygen. Certain worms and clams dig little burrows with surface access and survive by filter feeding, that is, pumping water from the surface over their gills. Unwanted debris is sorted, oxygen is absorbed, and food particles like diatoms and detritus are consumed.
The bamboo worm, which actually does resemble a piece of bamboo, lies head down in a sandy tube eating sediments, and breathing through water drawn into the burrow. This kind of food consumption is called deposit feeding.
You’ve
probably seen the signs of another deposit feeder, the northern lugworm.
It lives in a u-shaped burrow, eating sediments at one end, and eliminating
fecal pellets in the form of long, coil-shaped sediments at the surface
of the other end.
The oxygen these organisms supply themselves with often spreads into nearby sediments, converting the poisonous hydrogen sulfide to harmless sulfate. This oxygen spillover also benefits the large numbers of meiofauna (microscopic invertebrates) living between the grains of sediment. These include either juveniles of larger animals, or microscopic species of invertebrates like worms and crustaceans. Meiofauna thrive close to the surface, though some can tolerate the anoxia below. They feed on bacteria and microscopic protozoa.
The versatile Macoma balthica clam has two feeding methods: deposit feeding and filter feeding. This small clam lives in a burrow with two siphons extended to the surface. At low tide, it sweeps its incurrent siphon over the surface sediments for diatoms and bacteria. When high tide covers the flats, the Macoma balthica switches to filter feeding particles from the water. It sends waste out of its excurrent siphon.
Flood Tide
When the tide returns to cover the flats, it brings a flood of oxygen and organic particles to sustain the members of the infaunal community while carrying away their metabolic wastes. Moving along in the flows are larger animals hunting for their next meal. Juvenile flounder nip the siphons of the Macoma balthica poking out of the mud. Shrimp graze detritus and microalgae on the sediments. Crustaceans and fish dig for gemma gemma, a tiny, filter feeder clam common in Great Bay.
Horseshoe
crabs use their legs to pull worms and mollusks from the mud and carry
them into their mouths. Dozens of tiny hermit crabs also come scrambling
in, housed in their periwinkle and dogwhelk shells.