Oceanography-Chapter 5.pdf

TIDES Tides- the rise and fall of ocean water caused by the gravitational attraction among the earth, moon, and the sun. Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuarine...

TIDES Tides- the rise and fall of ocean water caused by the gravitational attraction among the earth, moon, and the sun. Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the bathymetry of oceans, seas and estuaries. Sea level measured by coastal tide gauges may also be strongly affected by wind. More generally, tidal phenomena can occur in other systems besides the ocean, whenever a gravitational field that varies in time and space is present A tide is a repeated cycle of sea level changes in the following stages: Over several hours the water rises or advances up a beach in the flood The water reaches its highest level and stops at high water. Because tidal currents cease this is also called slack water or slack tide. The tide reverses direction and is said to be turning. The sea level lowers or falls over several hours during the ebb tide. The level stops falling at low water. This point is also described as slack or turning. Tides vary on timescales ranging from hours to years, so to make accurate records tide gauges measure the water level over time at fixed stations which are screened from variations caused by waves shorter than minutes in period. These data are compared to the reference (or datum) level usually called mean sea level. The Moon Tide The gravitational and inertial forces are constant, always pulling water towards the moon and directly away from the moon. Range variation: springs and neaps The semidiurnal tidal range (the difference in height between high and low waters over about a half day) varies in a two-week or fortnightly cycle. Around new and full moon when the Sun, Moon and Earth form a line (a condition known as syzygy), the tidal forces due to the Sun reinforce those of the Moon. The tide's range is then maximum: this is called the spring tide, or just springs and is derived not from the season of spring but rather from the verb meaning "to jump" or "to leap up". When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth, and the forces induced by the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is minimum: this is called the neap tide, or neaps. Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven day interval between springs and neaps. Tides may be semidiurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). In most locations, tides are semidiurnal. Because of the diurnal contribution, there is a difference in height (the daily inequality) between the two high waters on a given day; these are differentiated as the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are referred to as the higher low water and the lower low water. The daily inequality changes with time and is generally small when the Moon is over the equator. The moon does not rotate around the earth’s equator, but follows an orbit that is inclined in relation to the earth’s axis. Because of this, northern and southern latitudes commonly face only one high tide and one low tide in a day, called diurnal tides. The inclination of the moon changes in relation to the earth on a 19 year cycle. The earth’s inclination in relation to the sun also effects the tides. The sun’s inclination follows a year-long cycle, and is in highest inclination in the summer and winter months. During these months the "bulges" in the ocean are offset the most from the equator, and it is most likely to encounter only one tide cycle per day, or diurnal tides. Semidiurnal tides As the earth turns upon its own axis in about 24 hours, a point on the earth moves through areas with these different forces acting on it. In one rotation (one day), a point on earth travels from an area of high tide (where there is a force pulling water outward), through an area of low tide, through an area of high tide again (the opposite pull), and through another area of low tide, before it returns to the point of origin at high tide. This results in two high tides and two low tides in a day (called semidiurnal tides). The Tidal Day The moon does not stay put, but rotates around the earth at a rate of about 12° a day, or one rotation a month. The rotation is in the same direction as the earth’s spin, so by the time the earth has done one rotation, the moon has shifted 12° further, and it takes an extra 50 minutes for the moon to be in the same position relative to a point on the earth. Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and low tides are about 50 minutes later every day. The Sun Tide The tides are caused mainly by the gravitational attraction of the moon and the earth, but there is also a gravitational attraction between the earth and the sun. The effect of the sun upon the tides is not as significant as the moon’s effects. Basically, the sun’s pull can heighten the moon’s effects or counteract them, depending on where the moon is in relation to the sun. The changing distance of the Moon from the Earth also affects tide heights. When the Moon is at perigee the range is increased, and when it is at apogee the range is reduced. Every 7½ lunations, perigee coincides with either a new or full moon causing perigean tides with the largest tidal range. If a storm happens to be moving onshore at this time, the consequences (in the form of property damage, etc.) can be especially severe. Orbit of the moon Apogee Intertidal ecosystem Intertidal ecology is the study of intertidal ecosystems, where organisms live between the low and high tide lines. At low tide, the intertidal is exposed (or ‘emersed’) whereas at high tide, the intertidal is underwater (or ‘immersed’). Interactions between intertidal organisms and their environment, as well as between different species of intertidal organisms within a particular intertidal community. The most important environmental and species interactions may vary based on the type of intertidal community Based on substrates - rocky shore and soft bottom communities. Organisms living in this zone have a highly variable and often hostile environment, and have evolved various adaptations to cope with and even exploit these conditions. One easily visible feature of intertidal communities is vertical zonation, where the community is divided into distinct vertical bands of specific species going up the shore. Species ability to cope with desiccation determines their upper limits, while competition with other species sets their lower limits. Intertidal habitats can be characterized as having either hard or soft bottoms substrates. Rocky intertidal communities occur on rocky shores, such as headlands, cobble beaches, or human-made jetties. Soft-sediment habitats include sandy beaches and mudflats differ in levels of ‘abiotic’, or non-living, environmental factors. Rocky shores tend to have higher wave action, requiring adaptations allowing the inhabitants to cling tightly to the rocks. Soft-bottom habitats are generally protected from large waves but tend to have more variable salinity levels. They also offer a third habitable dimension—depth—thus, many soft-sediment inhabitants are adapted for burrowing. Moisture Different areas of the intertidal zone may be wet or dry. Organisms must be able to adapt if they are left “high and dry” when the tide goes out. Sea snails such as periwinkles have a “trap door” called an operculum that they can close when they are out of water to keep moisture in. Waves In some areas, waves hit the intertidal zone with force, and marine animals and plants must be able to protect themselves. Kelp, a type of algae, has a root-like structure called a “holdfast” that it uses to attach to rocks or mussels, thus keeping it in place. Gastropods and bivalves have different attachment mechanisms Salinity Depending on rainfall, the water in the intertidal may be more or less salty, and tide pool organisms must adapt to increases or decreases in salt throughout the day. Temperature As the tide goes out, tide pools and shallow areas in the intertidal will become more vulnerable to temperature changes that could occur from increased sunlight or colder weather. Some tide pool animals hide under plants in the tide pool to find shelter from the sun. Marine Life in the Intertidal Zone Home to many species of animals and plants. Many of the animals are invertebrates (animals without a spine), which comprise a wide group of organisms. Some examples of invertebrates found in tide pools are crabs, urchins, sea stars, sea anemones, barnacles, snails, mussels and limpets. The intertidal is also home to marine vertebrates, some of whom prey on intertidal animals, such as fish, gulls and seals. ADVANTAGES TO LIVING IN INTERTIDAL ZONES Algae and other intertidal plants grow in the abundant sunlight and support an entire food chain of animals. Constant wave action supplies the tide pool with nutrients and oxygen. Food is abundant. Varied substrates provide hiding places and surfaces to cling to. ADAPTATIONS TO THE VARIABLE ENVIRONMENT 1. Small animals that live in the splash zone can avoid desiccation by closing their shells tightly to seal in moisture. 2. Some animals, like crabs and marine snails and bivalves, have thick, tough outer coverings to slow evaporation. Others, such as mussels and leaf barnacles, cluster together to reduce individual exposure. 3. With constant pounding of waves, animals have developed different adaptations to keep from being washed away. Some, like sea stars, cling fast to the rocky surfaces; others find shelter in crevices or hide under thick mats of seaweed when the tide is out. 4. Most intertidal life centers in the low intertidal level, which normally remains under water. Most of these inhabitants can only tolerate exposure to air for short periods. It is here and in the subtidal zone (below the intertidal) that marine plants provide fish and invertebrates with protective cover and food. ANIMAL ADAPTATIONS TO INTERTIDAL LIFE The ochre sea star can tolerate a longer time period exposed to air than many other sea stars. They regularly withstand up to eight hours of exposure during low tides. They are not found in high intertidal pools due to their lack of ability to withstand high water temperatures or low oxygen levels. Some abalones, limpets, and turban snails can smell approaching ochre stars and will move away to avoid being eaten. Sea cucumbers have few known predators, other than humans and sea stars. If disturbed, some species may eviscerate (expel their entrails), leaving the entrails to the predator while the sea cucumber escapes. Its organs will regenerate after several days. When the tide is out, periwinkle snails cluster in crevices, secrete a gluelike mucus to stick to the rock's surface, and withdraw into their shells to avoid drying out. Many fishes that inhabit tide pools, such as tide pool sculpin and young opaleyes, can breathe air at the surface—an adaptation that enables them to survive in oxygen poor water when the tide is out. These special adaptations are quite varied. They may consist of shells that close like trap doors when the tide goes out. Or clinging feet with suction cups, or muscles that grip the rocks. Some animals can fold themselves up or disappear down a long tube, to await the return of high tide. Others literally tie themselves to the rocks with secreted fibers, to resist the force of moving water. Rubber necks and flexible stalks allow some intertidal residents to bend with the waves. Many have a tough skin, or impregnable armor to keep them moist and cool while exposed to air. Intertidal regions are utilized by humans for food and recreation, but anthropogenic actions also have major impacts, with overexploitation, invasive species and climate change being among the problems faced by intertidal communities. In some places Marine Protected Areas have been established to protect these areas and aid in scientific research.