10.2 Nutrient cycling and energy flow in the oceans

Oceans depend on the continuous movement of nutrients and energy to sustain life at every scale, from microscopic phytoplankton to whales. Carbon, nitrogen, and phosphorus cycle through seawater, sediments, and living organisms, while energy captured by primary producers flows upward through food webs. Understanding these processes explains why some ocean regions teem with life while others are relatively barren.

Nutrient Cycles in the Ocean

Nutrient cycles in oceans

Three major nutrient cycles drive marine ecosystems. Each one involves biological, chemical, and physical processes that move elements between the water, the atmosphere, organisms, and the seafloor.

Carbon cycle

Carbon dioxide from the atmosphere dissolves in surface waters, forming carbonic acid ($H_2CO_3$). This dissolved carbon is then available to phytoplankton (diatoms, dinoflagellates), which incorporate it into organic compounds like glucose and amino acids through photosynthesis. When organisms respire, die, and decompose, bacteria and other decomposers release $CO_2$ back into the water column and eventually the atmosphere.

Not all carbon returns quickly. Some sinks to the deep ocean as dead organisms and fecal pellets, a process called the biological pump. This carbon can remain locked in deep waters and seafloor sediments for centuries to millennia, making the ocean a massive long-term carbon reservoir.

Nitrogen cycle

Nitrogen gas ($N_2$) makes up about 78% of the atmosphere, but most marine organisms can't use it in that form. The cycle depends on specialized bacteria at each step:

1. Nitrogen fixation — Cyanobacteria like Trichodesmium convert $N_2$ into ammonium ($NH_4^+$), a biologically usable form.
2. Nitrification — Bacteria such as Nitrosomonas oxidize ammonium to nitrite ($NO_2^-$), and then Nitrobacter converts nitrite to nitrate ($NO_3^-$). Nitrate is the form most phytoplankton prefer.
3. Assimilation — Phytoplankton take up ammonium or nitrate and build it into proteins and nucleic acids.
4. Denitrification — In low-oxygen (anoxic) zones, other bacteria convert nitrate back into $N_2$ gas, returning it to the atmosphere and completing the cycle.

Nitrogen is often a limiting nutrient in the ocean, meaning its scarcity controls how much phytoplankton can grow in a given area.

Phosphorus cycle

Unlike carbon and nitrogen, phosphorus has no significant atmospheric phase. It enters the ocean primarily through the weathering of rocks and minerals like apatite, carried by rivers to the coast. Phytoplankton incorporate dissolved phosphate ($PO_4^{3-}$) into essential molecules such as ATP and DNA. When organisms die, bacterial decomposition releases phosphate back into the water.

Phosphorus can be permanently lost from the active cycle when it settles into seafloor sediments and becomes buried. Because there's no atmospheric shortcut to replenish it, phosphorus cycling in the ocean is slow, and its availability can limit productivity in some regions.

Energy Flow in Marine Ecosystems

Primary producers in marine ecosystems

Primary producers are the organisms that capture solar energy and convert it into chemical energy through photosynthesis. In the ocean, the major groups are phytoplankton (which account for roughly half of all photosynthesis on Earth), along with seaweeds and seagrasses in shallower waters. They convert inorganic carbon ($CO_2$) into organic compounds like carbohydrates and lipids, storing energy in chemical bonds.

These producers form the base of marine food webs. Primary consumers such as zooplankton (copepods, krill), small fish, and filter-feeding mollusks graze on them. Energy then transfers upward to secondary and tertiary consumers: larger fish, seabirds, and marine mammals.

At each trophic level, roughly 90% of the energy is lost as metabolic heat. This means only about 10% of the energy at one level is available to the next, which is why top predators are far less abundant than the plankton that ultimately support them.

Factors influencing ocean productivity

Light availability

Photosynthesis requires light in the visible spectrum (wavelengths of 400–700 nm, called photosynthetically active radiation). Phytoplankton are most productive in the euphotic zone, the upper ~200 m of the ocean where enough light penetrates. Water clarity, season, and latitude all affect how deep light reaches. At high latitudes, for example, productivity spikes in spring and summer when daylight hours increase.

Nutrient concentrations

Primary producers need nitrogen, phosphorus, iron, and silica (for diatom shells). When any of these runs low, productivity drops. In much of the open ocean, nitrogen and iron are the most common limiting nutrients. Upwelling, where deep, nutrient-rich water rises to the surface along coastlines or at the equator, is one of the most important processes fueling high productivity. This is why upwelling zones off Peru, California, and West Africa support some of the world's richest fisheries.

Water temperature

Temperature controls metabolic rates and growth. Polar phytoplankton are adapted to cold water, while tropical species thrive in warmer conditions. Temperature also affects stratification: warm surface water sits on top of cold, dense deep water, creating a barrier that prevents nutrient-rich deep water from mixing upward. Strong stratification can starve the surface of nutrients, while storms or seasonal cooling can break it down and boost productivity.

Importance of nutrient and energy flow

Nutrient cycling

1. Ensures essential elements remain available for primary production rather than being permanently locked away
2. Maintains the overall balance and productivity of marine ecosystems
3. When disrupted, can cause problems like eutrophication (excess nutrients, often from agricultural runoff, triggering algal blooms and oxygen-depleted dead zones) or severe nutrient limitation that suppresses growth

Energy flow

1. Transfers energy captured by primary producers to consumers at every trophic level
2. Supports the growth, reproduction, and survival of marine organisms throughout the food web
3. Shapes food web structure; changes at one level can trigger trophic cascades (for example, removing top predators can cause prey populations to explode, which then overgraze primary producers)

Interactions between nutrient cycling and energy flow

These two processes are tightly linked. Efficient nutrient recycling supports high primary productivity, which in turn enables greater energy flow to higher trophic levels. When either process is disrupted, the effects cascade through the ecosystem. Coral bleaching, harmful algal blooms, and fishery collapses are all examples of what can happen when the balance between nutrient supply and energy transfer breaks down.