Difficulty: Introductory | Prerequisites: Basic biology (cell structure, photosynthesis, cellular respiration)
Big Picture: Nutrient cycles describe how essential elements move between living organisms and the non-living environment in the ocean. This chapter sits at the foundation of marine ecology: before you can understand food webs, productivity, or ocean chemistry, you need to know how nutrients get into the water, how organisms use them, and how they return. If you are coming in cold, make sure you are comfortable with the difference between abiotic (non-living) and biotic (living) components of an ecosystem, and that you recall what photosynthesis and cellular respiration do at a basic level.
Nutrients cycle between abiotic (non-living) and biotic (living) states in the ocean. The surface layer acts as the main reservoir where dissolved nutrients fuel primary productivity, and a handful of processes (atmospheric dissolution, decomposition, upwelling, run-off, tectonic activity) keep that reservoir topped up. Nutrients leave the reservoir through biological uptake, sinking to the seafloor, incorporation into reef skeletons, and human harvesting.
Nutrient cycle
The continuous movement of essential elements (carbon, nitrogen, phosphorus, etc.) between abiotic and biotic states in an ecosystem. Think of it as a loop: organisms absorb nutrients from the environment, use them, and eventually return them through death, waste, or decomposition.
Abiotic
The non-living component of an ecosystem, such as dissolved minerals in seawater, atmospheric gases, or sedimentary rock. In simple terms, this is everything in the environment that is not alive.
Biotic
The living component of an ecosystem, including all organisms from bacteria to whales. When a nutrient enters a cell, it has moved from abiotic to biotic.
Reservoir
The abiotic "storage pool" where nutrients accumulate in dissolved or solid form before organisms take them up. In marine biology, the ocean itself is the primary reservoir.
Residence time
The average time a particle or dissolved nutrient spends within a given system before being removed or transformed. In simple terms, it is how long a nutrient "sits" in the reservoir before something uses it or moves it.
Upwelling
The process by which cold, nutrient-rich deep water rises to the ocean surface, typically driven by wind pushing surface water away from a coastline. Think of it as natural fertilisation of the surface layer.
Marine snow (sinking)
The slow descent of dead organic matter, faeces, and other biological debris from the surface to the seafloor. In simple terms, it is the ocean's version of leaf litter falling to the forest floor.
Atmospheric dissolution
The dissolving of gases from the atmosphere directly into surface seawater. This is how nitrogen gas and carbon dioxide enter the ocean reservoir.
Run-off
Water flowing over the land surface into rivers and eventually the ocean, carrying dissolved nutrients, soil particles, fertilisers, and pollutants along the way.
Decomposers
Organisms (mainly bacteria and fungi) that break down dead organic matter and waste, releasing nutrients back into the abiotic environment. Think of them as the recycling crew of the ecosystem.
When an organism absorbs a nutrient from the environment, that nutrient moves from an abiotic state to a biotic state. Example: CO₂ being fixed into glucose by a photosynthetic organism.
Once inside the biotic phase, nutrients are transferred between organisms via feeding (food chains and food webs).
The reverse transition (biotic to abiotic) happens through decomposition. Decomposers, mainly bacteria and fungi, break down urine, faeces, and dead organisms, releasing nutrients back into the environment.
Example: bacteria decompose a dead fish and release phosphorus back into the water, where phytoplankton can use it to build DNA.
The ocean is the largest reservoir for dissolved nutrients on Earth. Reservoirs are strictly the abiotic side of the nutrient cycle.
Dissolved nutrients such as nitrogen, phosphorus, and carbon can be taken up quickly by organisms, or they can remain in the reservoir for thousands of years. The difference depends on the nutrient, the location, and the biological demand.
Residence time is the average duration a dissolved nutrient remains in the reservoir before being consumed or removed.
Short residence time: nutrients in the surface layer, where biological demand is high.
Long residence time: nutrients locked in deep-water sediments or incorporated into rock.
The surface layer of the ocean is the most nutrient-rich zone and the most important for primary productivity.
Nutrients are consumed quickly here because many organisms (phytoplankton, algae) are competing for them.
Core relationship: the more dissolved nutrients available for absorption in the surface layer, the higher the primary productivity of that region.
Gases such as nitrogen (N₂) and carbon dioxide (CO₂) dissolve directly from the atmosphere into seawater.
Three factors control the dissolution rate:
Temperature of surface waters (colder water dissolves more gas)
Concentration of the gas in the atmosphere (higher concentration, more dissolves)
Turbulence and mixing in the surface layer (more mixing, more contact with the atmosphere)
The system is generally in equilibrium: roughly the same amount of gas dissolves into the ocean as diffuses back out into the atmosphere.
Living organisms return nutrients to the reservoir through excretion and, after death, through decomposition.
Nitrogen return: organisms excrete urea (in urine) or ammonia directly into the water.
Dead organic matter and faeces:
Broken down by both fungi and bacteria.
Decomposers produce CO₂ through cellular respiration during this process.
Some bacteria convert amino acids from dead tissue into ammonia and nitrates.
Broken-down material sinks past the thermocline and settles on the seafloor, mixing with cool, dense deep water.
When upwelling later brings this deep water to the surface, it carries decomposition-derived nutrients back up.
Cold water from the deep ocean is brought to the surface, carrying high concentrations of dissolved nutrients.
Mechanism: wind blows surface water away from the coast; cold, dense, nutrient-rich water rises to replace it.
Upwelling zones are among the most biologically productive areas in the ocean. The nutrient input acts like a natural fertiliser.
Water flows over exposed land surfaces into rivers and streams, then into the ocean.
Along the way it leaches nutrients from soil and picks up fertilisers, waste, toxins, and other materials.
Run-off is a major pathway for nitrogen and phosphorus to enter the marine reservoir.
Disadvantage: excess nutrients from agricultural fertilisers can trigger harmful algal blooms.
Hydrothermal vents: minerals pour out from vents on the seafloor, dissolve, and join the reservoir. Hydrocarbons can also be created spontaneously at these sites.
Erosion and weathering: rocks are broken down on land, carried to the ocean, and their ions dissolve into the water.
Volcanoes: volcanic ash fertilises the ocean surface; underwater eruptions pour minerals directly into seawater.
Convergent boundaries: subduction and mountain-building expose new rock, which is then weathered and its ions dissolved into the reservoir.
Humans remove organisms (and therefore nutrients) from the ocean through fishing and other extraction.
Those nutrients leave the marine cycle entirely and enter terrestrial systems.
Producers (phytoplankton, algae) absorb dissolved nutrients from the reservoir to carry out photosynthesis and cellular respiration.
This is the main route by which nutrients move from abiotic to biotic.
Coral and their symbiotic zooxanthellae take in dissolved nutrients.
Coral use the nutrients to build calcium carbonate skeletons, effectively trapping carbon and calcium in solid form.
Those nutrients remain locked in the reef structure until the skeleton is physically or chemically broken down.
Dead organic matter, faecal pellets, and other debris sink from the surface to the seafloor.
This material becomes part of the ocean-floor sediment.
Three possible fates for the nutrients in marine snow:
They become incorporated into sedimentary rock.
They are released back into the reservoir through further decomposition on the seafloor.
They remain trapped in the sediment indefinitely.
Upwelling zones off the coasts of Peru, California, and West Africa support some of the world's largest commercial fisheries precisely because the nutrient influx drives enormous phytoplankton blooms at the base of the food web. Understanding run-off and nutrient loading is also central to managing agricultural pollution: the "dead zones" in the Gulf of Mexico and the Baltic Sea are direct consequences of excess nitrogen and phosphorus entering the ocean via run-off and triggering algal blooms that deplete oxygen.
Students often think "reservoir" means "deep ocean." It does not. The surface layer is the most important reservoir for primary productivity because that is where most biological uptake occurs.
Students sometimes assume upwelling is a rare event. In fact, upwelling is a continuous, wind-driven process in many coastal regions and is the reason those regions are so productive.
Run-off is frequently treated as purely beneficial ("it adds nutrients"). In reality, excess nutrients from run-off are a leading cause of harmful algal blooms and marine dead zones.
Students confuse the direction of the abiotic/biotic transition. Remember: absorption into a cell = abiotic to biotic. Death, decomposition, or excretion = biotic to abiotic.
⚠️ Be able to list and distinguish all five replenishment processes (atmospheric dissolution, excretion and decomposition, upwelling, run-off, tectonic activity) and all four removal processes (harvesting, uptake, reef incorporation, sinking).
⚠️ Know the direction of state change: nutrient entering a cell = abiotic to biotic; nutrient released by decomposition = biotic to abiotic.
⚠️ Understand why the surface layer is the most productive reservoir and how nutrient availability drives primary productivity.
⚠️ Be prepared to explain the mechanism of upwelling (wind displaces surface water, cold nutrient-rich deep water rises) and why upwelling zones are biologically productive.
⚠️ Expect a question linking run-off to harmful algal blooms (excess nitrogen and phosphorus from fertilisers).
True or false: When a nutrient enters a living cell, it transitions from biotic to abiotic. (False. It goes from abiotic to biotic.)
Fill in the blank: The average time a nutrient spends in a reservoir is called its __________. (Residence time.)
True or false: Upwelling brings warm, nutrient-poor water to the surface. (False. It brings cold, nutrient-rich water to the surface.)
Fill in the blank: Excess nitrogen and phosphorus from agricultural run-off can trigger __________. (Harmful algal blooms.)
True or false: Marine snow refers to debris sinking from the surface to the seafloor. (True.)
Q: Name three processes that replenish dissolved nutrients in the ocean's surface-layer reservoir.
A: Any three of: atmospheric dissolution, excretion and decomposition, upwelling, run-off, tectonic activity (hydrothermal vents, erosion/weathering, volcanoes, convergent boundaries).
Q: Explain how upwelling increases primary productivity in coastal regions.
A: Wind pushes surface water away from the coast. Cold, dense, nutrient-rich water from the deep ocean rises to replace it. The influx of dissolved nutrients (nitrogen, phosphorus, carbon) fuels phytoplankton growth, which drives primary productivity.
Q: What is the difference between the abiotic and biotic phases of a nutrient cycle? Give one example of each transition.
A: The abiotic phase is when a nutrient exists in a non-living form (dissolved in seawater, in rock, in the atmosphere). The biotic phase is when it has been incorporated into a living organism. Abiotic to biotic: CO₂ fixed into glucose by photosynthesis. Biotic to abiotic: bacteria decompose a dead fish and release phosphorus back into the water.
Q: Why is run-off both beneficial and harmful to marine ecosystems?
A: Beneficial because it delivers dissolved nutrients (nitrogen, phosphorus) from land to the ocean, supporting productivity. Harmful because excess nutrients, particularly from agricultural fertilisers, can trigger harmful algal blooms that deplete oxygen and create dead zones.
Q: Describe two ways nutrients are permanently or semi-permanently removed from the marine nutrient cycle.
A: (1) Incorporation into coral reef skeletons, which traps carbon and calcium in solid calcium carbonate until the skeleton is broken down. (2) Sinking (marine snow) that buries nutrients in seafloor sediment, where they may become part of sedimentary rock and remain locked away on geological timescales.
This material connects directly to the individual nutrient cycles (carbon, nitrogen, phosphorus, magnesium, calcium) covered in Part 2 of these notes, because each cycle is a specific instance of the general reservoir/replenish/remove framework described here. It also links to primary productivity and food webs: the availability of nutrients in the surface layer determines how much energy enters the marine food web at its base. Ocean chemistry topics (carbonic acid equilibrium, ocean acidification) build on the atmospheric dissolution and carbon-cycle concepts introduced in this chapter.
Nutrient cycling, biogeochemical cycles, marine nutrient cycles, abiotic vs biotic, ocean reservoir, residence time, upwelling, coastal upwelling, marine snow, sinking organic matter, atmospheric dissolution, gas dissolution seawater, run-off pollution, agricultural run-off algal bloom, eutrophication, dead zones, hydrothermal vents nutrients, tectonic nutrient input, reef incorporation, calcium carbonate skeleton, decomposition marine, excretion nitrogen ammonia, primary productivity nutrients, surface layer ocean, thermocline, FIU marine biology, intro to marine biology Chapter 3