🌊Oceanography Unit 7 – Tides and the Coastal Ocean

What's the Deal with Tides?

• Tides are the regular rise and fall of sea level caused by the gravitational pull of the moon and sun on Earth's oceans
• Tidal range is the vertical difference between high tide and low tide
  • Varies from place to place depending on factors like coastline shape and ocean basin size
• Tidal period is the time interval between successive high or low tides, typically around 12 hours and 25 minutes
• Tidal currents are horizontal water movements associated with the rise and fall of the tides
  • Flood currents flow towards the shore during rising tide, while ebb currents flow away from the shore during falling tide
• Tidal bulges are the two regions of elevated sea level on opposite sides of Earth caused by the gravitational attraction of the moon and sun
• Tidal forces also affect Earth's crust, causing slight deformations known as Earth tides

Ocean Meets Land: Coastal Zones Explained

• Coastal zones are the areas where the land meets the ocean, characterized by unique physical, chemical, and biological processes
• Shoreline is the boundary between land and water, which shifts with changing tides and sea levels
• Intertidal zone is the area between the highest high tide and lowest low tide, exposed during low tide and submerged during high tide
• Coastal landforms result from the interaction of tides, waves, and sediment
  • Examples include beaches, cliffs, estuaries, and tidal flats
• Estuaries are semi-enclosed coastal bodies of water where freshwater from rivers mixes with saltwater from the ocean
  • Highly productive ecosystems that serve as nurseries for many marine species
• Deltas form where rivers deposit sediment as they enter the ocean, creating fan-shaped landforms (Nile Delta)
• Barrier islands are elongated, narrow islands that run parallel to the mainland coast, protecting it from waves and storms (Outer Banks, NC)

Tidal Forces: Moon, Sun, and Earth's Dance

• Tides are primarily caused by the gravitational attraction of the moon and sun on Earth's oceans
• Moon's gravitational force is the dominant factor in tide generation due to its proximity to Earth
  • Moon's tidal force is about twice as strong as the sun's
• Tidal forces cause the oceans to bulge towards and away from the moon, creating two tidal bulges on opposite sides of Earth
• Earth's rotation causes these tidal bulges to move around the planet, resulting in the daily rise and fall of tides
• Sun's gravitational pull also contributes to tides, enhancing or diminishing the moon's effect depending on the alignment of the sun, moon, and Earth
  • Spring tides occur during new and full moons when the sun and moon are aligned, causing higher high tides and lower low tides
  • Neap tides occur during the first and third quarter moons when the sun and moon are at right angles, resulting in lower high tides and higher low tides
• Tidal forces also affect Earth's crust and atmosphere, though to a lesser extent than the oceans

Types of Tides: Daily, Spring, and Neap

• Diurnal tides are characterized by one high tide and one low tide per lunar day (24 hours and 50 minutes)
  • Occur in areas with strong tidal resonance, such as the Gulf of Mexico
• Semidiurnal tides feature two high tides and two low tides of approximately equal height per lunar day
  • Most common tidal pattern worldwide, found along the U.S. Atlantic coast
• Mixed semidiurnal tides have two high tides and two low tides per lunar day, but with significant differences in the heights of successive high and low tides
  • Prevalent along the U.S. Pacific coast
• Spring tides occur during new and full moons when the sun, moon, and Earth are aligned
  • Result in higher high tides and lower low tides due to the combined gravitational pull of the moon and sun
• Neap tides happen during the first and third quarter moons when the sun and moon are at right angles to each other
  • Cause lower high tides and higher low tides as the sun's gravitational force partially cancels out the moon's
• Tidal range is the difference in height between high tide and low tide, which varies depending on the type of tide and location

Coastal Processes: Erosion, Deposition, and More

• Coastal erosion is the removal and transport of sediment from the shoreline by waves, currents, and tides
  • Can lead to the retreat of cliffs, beaches, and dunes
• Coastal deposition occurs when sediment is deposited along the shoreline, leading to the formation of beaches, spits, and other coastal landforms
• Longshore drift is the transport of sediment parallel to the shore due to the oblique approach of waves
  • Contributes to the formation of spits and barrier islands
• Rip currents are strong, narrow channels of water that flow seaward from the shore, posing a hazard to swimmers
• Coastal flooding can occur during high tides, storm surges, or heavy rainfall events
  • Low-lying areas and estuaries are particularly vulnerable
• Sea level rise, caused by factors such as climate change and land subsidence, can exacerbate coastal erosion and flooding
• Coastal management strategies, such as beach nourishment and the construction of seawalls, aim to mitigate the impacts of erosion and protect coastal infrastructure

Tidal Ecosystems: Life in the Intertidal Zone

• Intertidal zone is the area between the highest high tide and lowest low tide, characterized by unique adaptations of organisms to the changing environment
• Zonation patterns emerge in the intertidal zone, with distinct communities of organisms occupying different elevations based on their tolerance to exposure and submersion
• Rocky intertidal ecosystems are characterized by a diverse array of sessile and mobile organisms
  • Sessile organisms, such as barnacles and mussels, attach themselves to the rocks and withstand the force of waves
  • Mobile organisms, like crabs and snails, move around to feed and seek shelter during low tide
• Sandy intertidal ecosystems, such as beaches and tidal flats, are inhabited by burrowing organisms like clams and worms
• Salt marshes are coastal wetlands dominated by salt-tolerant vegetation, serving as important nurseries for fish and crustaceans
• Mangrove forests are tropical and subtropical intertidal ecosystems characterized by salt-tolerant trees with complex root systems that provide habitat for diverse species
• Intertidal organisms exhibit adaptations to cope with the challenges of the tidal environment, such as desiccation, temperature fluctuations, and salinity changes
  • Examples include the hard shells of mollusks, the air-breathing organs of crabs, and the salt-excreting glands of some plants

Human Impact: Coastal Development and Management

• Coastal development, including urbanization, industrialization, and tourism, can have significant impacts on coastal ecosystems and processes
• Coastal habitat loss and degradation result from activities such as land reclamation, dredging, and the construction of coastal infrastructure
  • Affects biodiversity and ecosystem services, such as natural flood protection and nursery grounds for commercially important species
• Coastal pollution, from sources like sewage, agricultural runoff, and marine debris, can harm marine life and human health
  • Eutrophication, caused by excess nutrients, can lead to harmful algal blooms and oxygen depletion in coastal waters
• Overfishing and destructive fishing practices, such as bottom trawling, can disrupt coastal food webs and damage seafloor habitats
• Climate change impacts, including sea level rise, ocean acidification, and increased storm intensity, pose significant challenges to coastal communities and ecosystems
• Coastal management strategies aim to balance human activities with the protection and sustainable use of coastal resources
  • Integrated Coastal Zone Management (ICZM) is an approach that involves stakeholders in the planning and decision-making process
  • Marine Protected Areas (MPAs) are designated to conserve biodiversity and manage human activities within their boundaries
• Coastal restoration projects, such as wetland rehabilitation and coral reef transplantation, seek to restore degraded habitats and enhance coastal resilience

Cool Stuff: Tidal Energy and Extreme Tides

• Tidal energy is a renewable energy source that harnesses the power of tides to generate electricity
  • Tidal barrages use the difference in water level between high and low tides to drive turbines
  • Tidal stream generators extract energy from the kinetic motion of tidal currents
• Tidal power plants are operational in several countries, such as France (La Rance), South Korea (Sihwa Lake), and Canada (Annapolis)
• Tidal energy has the advantage of being predictable and reliable, but it can also have environmental impacts on coastal ecosystems and sediment transport
• Some of the world's most extreme tidal ranges occur in the Bay of Fundy (Canada), Severn Estuary (UK), and Penzhinskaya Guba (Russia)
  • The Bay of Fundy has the highest tidal range, with a difference of up to 16 meters (53 feet) between high and low tide
• Tidal bores are rare tidal phenomena where incoming tides form a wave that travels upstream in narrow estuaries or rivers
  • Examples include the Qiantang River (China), Amazon River (Brazil), and Severn River (UK)
• Tidal whirlpools, like the Saltstraumen (Norway) and Old Sow (Canada/USA), form when strong tidal currents encounter underwater obstacles or constrictions
  • These whirlpools can be hazardous to navigation but also attract tourists and adventure-seekers