Tides and currents affect and influence the natural world and human society. Tides and currents carry nutrients, moderate temperatures, and influence conditions in numerous ecosystems. The relationship between society and tides and currents is obvious as coastal cities built to withstand prescribed levels of tidal surge are now being impacted by record breaking tidal surges, and the building of solid coastal structures do not allow waves to dissipate energy on land but deflect their energy to other coastal areas along the shore.
There is no doubt that Earth's climate plays a vital role in determining which organisms thrive in different areas across the world. As environmental conditions change, then organisms will be affected. This is especially true for ecosystems along the land/ocean margin. As global temperature and sea levels rise, the location, strength, and extent of tides, tidal surges, and currents will be effected. As sea level rises, water will push across low-lying coastal areas and modify existing shorelines. One example of how climate affects and has affected currents is when thermohaline circulation was slowed or stalled during the last ice age. As a large volume of fresh water accumulated in the North Atlantic, major ocean currents were disrupted, diminishing the ocean's capacity to distribute heat around the entire planet. This disruption impacted world climate, which influenced glaciers' extent on land. The glacier's forming displaced the location of shorelines worldwide. Much of the coastal land area in the Northern Hemisphere went from being under water to above water and exposed the previously underwater continental shelf to the air. Mud on the continental shelf, then exposed to the atmosphere, was eroded by winds increasing the concentration of particulate mater in the atmosphere. This disrupted the amount of sunlight reaching Earth's surface further impacting climate.
Warm surface waters in the ocean also provide the energy source for tropical storms. As the surface waters of the ocean have warmed, so too has the energy source for storms. This may also impact the severity and frequency of future storm systems. Although more storms may provide increased regional mixing of ocean water and, an increased supply of nutrients for marine organisms, human populations and infrastructure will likely be more adversely impacted.
Ecosystems that span several tidal zones include anchored plants, and animals, and ephemeral species that move with the daily tides or during some portion of their life cycle. Intertidal ecosystems can be found in estuarine environments and shallow water coastal communities. Organisms that live in intertidal zones must be able to live both above and below water depending on the tide. Other organisms depend on currents for survival. Coral reefs are dependent on ocean currents to deliver their food to them (zooplankton) and disperse their larvae. Reefs located in shallow waters are most at risk from tidal emersions which can lead to bleaching and death. Without tides and currents these ecosystems would not exist.
Estuaries are bodies of water and their surrounding coastal habitats typically found where rivers meet the sea. Estuaries harbor unique plant and animal communities because their waters are brackish—a mixture of fresh water draining from the land and salty seawater.
Estuaries are some of the most productive ecosystems in the world. Estuaries are often protected from severe ocean conditions but depend on the currents created by tributary streams, long shore currents, and tidal fluctuations to cycle nutrients and provide a transportation mechanism for organisms such as shellfish, eels, and turtles during some stages of their life cycles.
Many animal species rely on estuaries for food and as places to nest and breed. One example is the blue crab. Blue crabs are native to the western edge of the Atlantic Ocean from Nova Scotia to Argentina. Their life cycle is dependent on estuarine habitats, and they have become one of the most recognizable icons of the Chesapeake Bay. Egg masses are produced in the warm salty water near the mouth of Chesapeake Bay where the newly hatched eggs can be swept out into the Atlantic by water currents. After a few months in the ocean they are able to swim vertically and can be transported back to the grass beds in the lower Bay by tides and wind-blown surface currents. From there they migrate north into the Bay where their mating cycle can start again in brackish waters.
Intertidal zones are regions between the highest water line and the mean low tide level. This area is exposed to the air at low tide and submerged at high tide and can include many different types of habitats, including steep rocky cliffs, sandy beaches or vast mudflats. Organisms in the intertidal zone are adapted to harsh extremes. Water can be high due to tides, rain and run off, and this water can be very salty at one time and very fresh another. These areas can also become very dry when tides are low for extended periods of time, very hot with full sun or freezing in colder climates. Some examples of organisms that live in the intertidal zone include: shore birds, marsh grasses, kelp, shrimp and fish (when water is present), snails, mussels and oysters, and burrowing worms.
Some shorelines have a rocky intertidal zone and harbor organisms that need a "rock solid" bottom for attachment. Examples of these are some seaweeds, mussels, barnacles, anemones, starfish, and snails. Intertidal shorelines that are comprised of mostly sand are constantly moving from wind and waves and may provide habitat for organisms including clams, horseshoe crabs, and microscopic invertebrates.
Coral reefs have been called rain forests of the sea. Coral reefs are typically mounds or ridges of living coral, coral skeletons, and calcium carbonate deposits from other organisms such as calcareous algae, mollusks, and protozoa. They often form in warm, shallow sea waters and rise to or near the surface. The organisms these reefs support are located only where the bottom is rocky and provides a solid surface to adhere to, and the tides, and currents are favorable. Coral do not grow in areas where the tides typically become low enough to expose them to the air. Coral are in symbiotic relationships with a type of algae called zooxanthellae that provide the coral with oxygen through photosynthesis, help the coral remove wastes, and supply glucose, glycerol, and amino acids, which the coral uses to make proteins, fats, carbohydrates, and produce calcium carbonate. If corals are exposed to air for long periods of time, it may prompt them to expel their zooxanthellae, losing a valuable source of energy.
Currents also carry coral eggs, larvae, and planulae to others areas expanding their colonies.
Tides and currents affect society directly and indirectly. Settlements in new lands were often established along the coast and in safe harbors created at the mouth of tidal rivers. These rivers provided freshwater, and access to fish populations that supported the growing coastal communities and their wealth. These ports also grew into transportation hubs that received manufactured goods and returned local bounty in the form of timber, fur, and fish.
From these port towns, goods traveled inland to remote parts of developing nations. In times of war, they also became strategic points of defense protecting these trading and economic hubs. Because of their position on or near major rivers these ports often had challenging currents and tides that are the site of numerous shipwrecks, providing an extra degree of difficulty to navigating.
Other cities near warm water also support society by harboring immense coral reef communities. The relatively low current, low energy, and low tidal water environments, foster the growth of coral and make some of the safest harbors and port towns. Unfortunately the two do not always coexist well. Increased tourism, development, and boat traffic can stress coral reefs and the biodiverse communities they support. Along the Florida coast is the largest coral reef system in the continental United States, and the third largest in the world. This coral reef system supports over 500 fish species and numerous flora species. It exists because of the local currents that transport food and disperse coral larva, and its ideal water clarity and productivity. In Looe Key, near the southern portion of the Florida Keys, exists one of the most biodiverse reefs in the system and home to 63 taxa of stony corals, 42 species of octocorals, and two species of fire coral. The local tides and currents also provided the right conditions to make shipping and transportation easy. The Floridian coast is also the home of one of 13 marine underwater havens that comprised the 18,000 square nautical miles of the United States National Marine Sanctuary Program. The Florida Keys National marine Sanctuary is a 2,800 square nautical mile long component of the larger program which serves to manage and protect some of the most unique species in the United States and the world.