utheast U.S. coupled with one of the highest tide ranges along the east coast of North America. Each day, two high and two low tides averaging 6 to 8 feet in range inundate a vast area of the coastal zone, maintaining a system of creeks, channels, and rivers. Since tides are principally the result of the gravitational pull of the moon and the sun on the earth, the timing and range of the tides depend upon their relative positions. Tidal range is greater than average when the sun and moon are aligned with the earth during new and full moons and are called spring tides. Tidal range is smaller than average when they are not aligned during the other moon phases and are called neap tides.
Besides an average annual rainfall of over 50 inches per year, an enormous volume of freshwater enters the coastal zone from rivers. The fresh water that drains into the tidal zone is mixed with sea water and creating brackish conditions and one of the earth's most productive ecosystem. Energy captured by plants from sunlight flows from prey to predator throughout a complex estuarine food web in which nutrients are absorbed, redistributed, and recycled. Due to the abundance of early life stages of both coastal marine and oceanic species in the system, estuaries are called the "nurseries of the sea." The eggs and larvae of both estuarine and marine species are carried into a system where they find both resources and refuge. Estuarine habitats are subdivided into salt marsh shrub zone, salt marsh flats, needle rush marsh, smooth cord grass marsh, marsh edge zone, intertidal creek banks, bars and flats, and tidal pools, creeks, and rivers.
The saline influence of the ocean in coastal environments is significant. Sea water is salty because it contains 7 major ions (elements forming salts when not separated by water in a solution), 5 minor elements, and 11 trace elements. The salinity or saltiness of oceanic sea water is about 36 parts per thousand (3.6%) dissolved inorganic ions and elements. The salinity of the brackish water in the tidal creek and adjacent coastal waters can range from less than 5 to over 30 parts per thousand during the year. The plants and animals that inhabit this changing environment are generally well-adapted to it.
The major elements of the salt marsh ecosystem can be seen from the University dock adjacent to the Marine Biology building. Smooth cord grass is the dominant tall green grass found in the lower part of the marsh where the ground stays wet and very muddy as a result of the tides flooding the area twice daily. In late spring through late fall when most of the salt marsh is lush green, darker dead-looking sections of marsh grass can be seen. This dead-looking marsh grass has long, tubular stems and sharp brown- pointed tips and is called the needle rush. These two grasses are replaced toward higher ground by the sea oxeye, which has a succulent leaf with a yellow aster flower in June and the similar, but taller, marsh elder, a shrub that encroaches on part of the wooden walkway leading to the dock. None of these salt tolerant plants, called halophytes, is found beyond the influence of the brackish tidal water.
The tidal creek supports much more marine life during the summer than during the winter. In summer, the warmer temperatures and greater sunlight increases the overall productivity of the salt marsh while in winter the colder temperatures and less sunlight decreases the productivity. While the water in the tidal creek is never crystal clear because the tides constantly stir up the mud, it is significantly clearer in the winter because there are fewer microscopic organisms such as algal cells suspended in it.
At low tide, the surface of the mud banks along the campus' tidal creek is highlighted by the golden hue of millions of algal cells called diatoms. Mud snails aggregate by the thousands on the mud zone. Mullet, Atlantic menhaden, killifishes, blue crabs, mud crabs, white shrimp, and brown shrimp are most responsible for activity at the surface of the water in the creek. Because of the significant tidal 