Intro to Marine Biology | Florida International University | Chapter 2.3
Difficulty: Introductory to Intermediate | Prerequisites: Chapter 2.1 (Tectonic Processes) helpful for understanding seafloor topography's influence on currents. Chapter 2.2 (Weathering, Erosion and Sedimentation) helpful for context on how tides and waves shape shores.
Tides and ocean currents are the two main systems that move seawater around the planet. Tides are driven by the gravitational pull of the Moon and Sun; currents are driven by wind, temperature, salinity and the Earth's rotation. Together they control nutrient distribution, climate regulation, coastal habitats and the productivity of marine ecosystems. This section connects to nearly every later topic in the course, from intertidal ecology to global climate patterns. The El Nino/La Nina cycle in particular shows how disruptions to normal current patterns can cascade through entire ecosystems and economies.
Tides are the regular rise and fall of sea level caused by gravitational interactions between the Moon, Sun and Earth. Spring tides (greatest range) happen during full and new moons; neap tides (smallest range) happen during quarter moons. Ocean currents move seawater continuously, driven by wind, temperature, salinity and the Coriolis effect. Surface currents follow global wind patterns; deep-water currents (thermohaline circulation) are driven by density differences. Upwelling brings cold, nutrient-rich water to the surface and supports highly productive ecosystems. El Nino disrupts this by weakening trade winds, shutting down upwelling off South America and causing global weather shifts; La Nina is the recovery phase.
Tides
The regular rise and fall of bodies of water caused by the gravitational interactions of the Moon, Sun and Earth. Found in all coastal areas and large lakes. Think of it as the ocean breathing in and out, pulled by the Moon and Sun.
Tidal range
The vertical difference between high tide and low tide at a given location. Affected by the slope of the coast, the size of the body of water, and local weather. In simple terms, how far the water level changes between high and low tide.
Spring tide
Tides that occur during full and new moons, when the Earth, Sun and Moon are aligned in a straight line. They produce the greatest tidal range: the highest high tides and the lowest low tides. Think of it as the "extreme" tides (nothing to do with the season of spring).
Neap tide
Tides that occur during first and third quarter moons, when the Earth, Sun and Moon form a right angle. They produce the smallest tidal range: the lowest high tides and the highest low tides. In simple terms, the "mild" tides, when the Sun and Moon are pulling at right angles and partly cancelling each other.
Diurnal tides
A tidal pattern with one high tide and one low tide per day.
Semi-diurnal tides
A tidal pattern with two high tides and two low tides per day.
Tidal amplitude
The difference in height between the low-water mark and the high-water mark. Varies with gravitational effects and physical features of the coastline. Think of it as another way to express how big the tide swing is at a specific spot.
Fetch
The duration (and distance) of wind blowing over a body of water. Longer fetch produces larger waves. In simple terms, the more time and space wind has over open water, the bigger the waves it builds.
Ocean current
The continuous movement of seawater in a particular direction. Currents carry nutrients, dissolved gases and heat. Think of it as rivers within the ocean.
Surface current
A steady, predictable current driven primarily by wind. Global wind patterns arise from the uneven heating of the Earth's surface by the Sun and from convection currents in the atmosphere. In simple terms, the top layer of the ocean moving in response to prevailing winds.
Coriolis effect
A force resulting from the Earth's rotation that deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is the reason ocean currents spiral clockwise in the north and counter-clockwise in the south. Think of it as the Earth's spin bending the path of anything moving across its surface.
Thermohaline circulation
Deep-water circulation driven by differences in water density caused by temperature ("thermo") and salinity ("haline"). It moves slowly along the ocean floor and carries huge volumes of water. Often called the "global conveyor belt." In simple terms, the slow, deep-ocean circulation powered by cold, salty, dense water sinking and warm water rising.
Upwelling
The movement of cold, nutrient-rich water from the deep ocean to the surface. Caused by wind pushing surface water away from a coast (creating a low-pressure zone that deeper water fills) or by seabed topography deflecting deep currents upward. Think of it as the ocean's fertiliser delivery system: deep, nutrient-loaded water rises to feed surface life.
El Nino (El Nino Southern Oscillation, ENSO)
A climate event occurring every 3 to 5 years in which trade winds over the Pacific weaken or stop, allowing warm surface water to pool along the coast of South America instead of being pushed westward. This suppresses upwelling, cuts off nutrient supply, and causes widespread effects: heavy rainfall in the Americas, drought in Australia and Indonesia, and reduced marine productivity off South America. Defined by sea surface temperatures sustained 0.5 degrees C or more above the mean. In simple terms, the Pacific's normal wind-and-current pattern breaks down, warm water piles up in the wrong place, and weather patterns go haywire worldwide.
La Nina
The cooling phase that follows an El Nino event. Trade winds resume stronger than average, pushing warm surface water westward and allowing the cold, nutrient-rich Humboldt Current to rise to the surface off South America. Productivity improves greatly. Defined by sustained surface temperatures 3 to 5 degrees C below the average ocean temperature. Think of it as the system snapping back and overcorrecting: extra-strong trade winds, extra-cold upwelling, extra-productive waters.
Humboldt Current
A cold, nutrient-rich deep-ocean current that normally rises along the west coast of South America, supporting highly productive fisheries. Suppressed during El Nino events. In simple terms, the cold current off Peru and Chile that feeds the fishing industry when conditions are normal.
Tides are the regular rise and fall of water levels, driven by gravitational interactions between the Moon, Sun and Earth. They occur in all coastal areas and in large lakes.
Tidal range is affected by:
Slope of the coast: narrow channels funnel water, producing higher highs and lower lows; open beaches spread water evenly.
Size of the body of water.
Local weather, including wind (generates waves, causes fetch) and air pressure (low pressure allows greater tidal surge).
Types of tides by frequency:
Diurnal: one high tide and one low tide per day.
Semi-diurnal: two high tides and two low tides per day.
Types of tides by lunar alignment:
Spring tides: occur during full and new moons. The Earth, Sun and Moon are aligned. Greatest tidal range (highest highs, lowest lows).
Neap tides: occur during first and third quarter moons. The Earth, Sun and Moon form a right angle. Smallest tidal range (lowest highs, highest lows).
Tidal amplitude is the vertical difference between the high-water mark and the low-water mark. It varies with gravitational effects and the physical features of the coastline.
Currents are the continuous movement of seawater in a particular direction. They carry nutrients, dissolved gases and heat around the planet.
Currents are created by:
Wind
The Coriolis effect (Earth's rotation)
Temperature differences
Salinity differences
Density differences
Tides
The shape of the seabed
Surface currents are steady, predictable flows driven primarily by wind. Global wind patterns result from the uneven heating of the Earth's surface by the Sun, which creates convection currents in the atmosphere.
How surface currents work:
Water near the equator is heated by near-direct sunlight.
Warm air rises, cools, then sinks in areas with less radiation and cooler temperatures.
The Coriolis effect deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Result: currents spiral clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.
Thermohaline circulation:
Driven by differences in water density, which is determined by temperature and salinity.
Happens along the ocean floor.
Often called the "global conveyor belt."
Slow-moving but carries huge volumes of water.
Upwelling:
The rise of cold, nutrient-rich deep water to the ocean's surface. Caused by:
Wind: Wind pushes warm surface water away from the coastline, creating a low-pressure zone. Cold, nutrient-rich water rises to replace it.
Seabed topography: A mid-ocean ridge or seamount can deflect a deep current upward, pushing cooler water toward the surface.
Benefits of upwelling:
Cold, nutrient-rich water acts as a fertiliser for surface life.
Increases the productivity of primary producers (phytoplankton).
More producers support more consumers, creating highly productive ecosystems.
In areas where upwelling is the main nutrient source, this is a healthy, balanced process (not an algal bloom).
Normal conditions (Pacific Ocean):
Strong trade winds push warm surface water from South America westward toward Indonesia.
This allows cold, nutrient-rich water to rise (upwell) near South America.
Upwelling supports abundant producers and consumers, including major fisheries.
More rain falls over the warm western Pacific (near Indonesia).
El Nino conditions (every 3 to 5 years):
Trade winds weaken or stop.
Warm surface water pools along the coast of South America instead of being pushed west.
The Humboldt Current is blocked from bringing cold, nutrient-dense water to the surface.
Upwelling shuts down: nutrient supply collapses, marine productivity drops, fisheries suffer.
Warm water and air shift south toward Chile and north toward California.
Heavy rainfall hits the Americas; Australia and Indonesia suffer droughts.
Defined by sea surface temperature sustained 0.5 degrees C or more above the mean.
La Nina conditions (the recovery):
Follows an El Nino event. Considered the "cooling phase."
Trade winds resume, stronger than average.
Strong winds push surface water westward, allowing the cold, dense, nutrient-rich Humboldt Current to rise to the surface.
Marine productivity improves greatly.
Defined by sustained surface temperatures 3 to 5 degrees C below the average ocean temperature.
Fisheries off Peru and Chile depend on the Humboldt Current's upwelling to sustain fish stocks; El Nino events cause severe economic losses when that upwelling collapses. The Gulf Stream (a major surface current) carries warm water from the Gulf of Mexico across the Atlantic and is responsible for keeping northwestern Europe significantly warmer than it would otherwise be at that latitude. Tidal energy projects harness the predictable rise and fall of tides to generate electricity, particularly in areas with large tidal ranges such as the Bay of Fundy in Canada.
Students often think "spring tide" refers to the spring season. It does not. Spring tides happen every two weeks year-round, during full and new moons.
Students sometimes believe the Coriolis effect "creates" currents. It does not create them; it deflects their path. The driving forces are wind, temperature and density differences.
A common error is thinking El Nino makes everything warmer everywhere. El Nino redistributes heat: some regions get warmer and wetter, others (Australia, Indonesia) get drier and may be cooler.
Students sometimes confuse thermohaline circulation with surface currents. Thermohaline circulation is deep, slow, density-driven and follows the ocean floor. Surface currents are wind-driven and sit at the top of the water column.
⚠️ Know the difference between spring tides (full/new moon, aligned, greatest range) and neap tides (quarter moon, right angle, smallest range). This is a very common exam question.
⚠️ Be able to explain what the Coriolis effect does: deflects to the right in the Northern Hemisphere, to the left in the Southern Hemisphere. Causes clockwise spirals in the north, counter-clockwise in the south.
⚠️ Understand upwelling: what causes it (wind, seabed topography), what it brings to the surface (cold, nutrient-rich water), and why it matters (supports productive ecosystems).
⚠️ Be able to compare El Nino and La Nina: El Nino = weak trade winds, warm water pools off South America, no upwelling, rain in the Americas, drought in Australia/Indonesia. La Nina = strong trade winds, cold upwelling resumes, improved productivity.
⚠️ Know the difference between surface currents (wind-driven, top of water column) and thermohaline circulation (density-driven, deep, slow, "global conveyor belt").
⚠️ Understand diurnal (one high, one low per day) versus semi-diurnal (two highs, two lows per day) tidal patterns.
True or False: Spring tides have the smallest tidal range. Answer: False. Spring tides have the greatest tidal range.
Fill in the blank: The Coriolis effect deflects currents to the ______ in the Northern Hemisphere. Answer: Right.
True or False: Thermohaline circulation is driven mainly by wind. Answer: False. It is driven by differences in water density caused by temperature and salinity.
Fill in the blank: During El Nino, trade winds ______ and upwelling off South America ______. Answer: Weaken (or stop); shuts down (or is suppressed).
True or False: Upwelling brings warm, nutrient-poor water to the surface. Answer: False. Upwelling brings cold, nutrient-rich water to the surface.
Q: Compare spring tides and neap tides. When does each occur, and which produces the greater tidal range?
A: Spring tides occur during full and new moons when the Earth, Sun and Moon are aligned; they produce the greatest tidal range. Neap tides occur during first and third quarter moons when these bodies form a right angle; they produce the smallest tidal range.
Q: What is the Coriolis effect, and how does it influence ocean currents?
A: The Coriolis effect is a deflection caused by the Earth's rotation. It deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes surface currents to spiral clockwise in the north and counter-clockwise in the south.
Q: Explain what upwelling is, name two causes, and describe why it is ecologically important.
A: Upwelling is the rise of cold, nutrient-rich deep water to the ocean's surface. It can be caused by (1) wind pushing warm surface water away from a coast, and (2) seabed features (mid-ocean ridges, seamounts) deflecting deep currents upward. Upwelling is ecologically important because the nutrients it delivers act as a fertiliser, boosting primary producer productivity and supporting a larger biomass of consumers throughout the food web.
Q: Describe the sequence of events during an El Nino event and its effects on marine life off South America.
A: Trade winds over the Pacific weaken or stop. Warm surface water, normally pushed westward, pools along the coast of South America. The Humboldt Current is blocked from bringing cold, nutrient-rich water to the surface. Upwelling shuts down, nutrient supply collapses, primary productivity drops, and marine life (including commercially important fish stocks) declines sharply.
Q: How does La Nina differ from El Nino?
A: La Nina is the cooling phase that follows El Nino. Trade winds resume stronger than average, pushing warm water westward and allowing the cold, nutrient-rich Humboldt Current to rise again. Marine productivity off South America improves greatly. Sea surface temperatures drop 3 to 5 degrees C below the average.
Q: Name three factors that create ocean currents.
A: Wind, the Coriolis effect, and differences in temperature (or salinity, or density). Other valid answers include tides and the shape of the seabed.
Tides and currents connect directly back to Chapter 2.1 (Tectonic Processes), because the shape of the ocean floor (ridges, trenches, seamounts) created by tectonics influences current paths and upwelling zones. They also connect to Chapter 2.2, since wave action driven by tides and currents is a primary agent of coastal erosion and sedimentation. Later in the course, understanding upwelling and nutrient distribution will be essential for studying marine productivity, food webs and the biogeography of ocean life.
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