Estuaries are tidally-influenced ecological systems where rivers meet the sea and fresh water mixes with salt water.

The fundamental goal of ecology is to understand the
distribution and abundance of organisms.

Ecology explores relationships between organisms and biotic (living) factors or abiotic (nonliving) factors in the environment. Ecological interactions range from the impact of predation, a biotic factor, on clam  abundance, to the effect of salinity, an abiotic factor, on where a species of  marsh grass grow.

The sheer number, diversity, and complexity of abiotic and biotic  factors makes understanding natural systems extremely challenging. That is nevertheless, the goal of ecology.

From the largest landscape features to the smallest microscopic organisms, an estuary is a fascinating place. When viewing an estuary from the air, for example, one is awed by striking river bends as freshwater finds its  way back to the sea, the vast expanse of marsh grasses, mangroves, or mudflats, extending out into the calm waters, or perhaps the  elegant curve of an expansive barrier beach. Wherever there are estuaries, there is a unique beauty, as rivers meet the sea, and both ocean and land contribute to a unique ecosystem of  specialized plants and animals.

At high tide, seawater changes estuaries, submerging the lants and flooding creeks, marshes, pannes, mudflats, or mangroves, until what once was land is now water. Throughout the tides, the days, and the years, an estuary is cradled between outreaching headlands and is buttressed on its vulnerable  seaward side by fingers of sand or mud.

Estuaries metamorphosize with the tides, the incoming waters seemingly bringing back to life organisms that have sought shelter from their temporary exposure to the non-aquatic world. As the tides ebb, organisms return  to their protective postures, receding into sediments and adjusting to changing  temperatures and exposure to differing degrees of sunlight and different kinds of weather.

Flocks of shorebirds stilt through the shallows, lunging long bills  at their abundant prey of fish, worms, crabs or clams. Within the sediments, whether mud, silt, sand, or rocks, live billions of microscopic bacteria, a  lower level of the food web based largely on decaying plants.