Sunlit tropical water looks like the ideal place to grow. It is often close to a desert. Productivity in the sea is set by whichever ingredient runs out first, and away from the coast that ingredient is almost always nutrients.
📘 What you need to know
Thermal stratification layers the water by temperature and blocks nutrients from reaching the surface.
Mixing processes — wind-driven currents and seasonal overturning — bring nutrients back up.
The limiting factors are light, temperature and nutrients (especially nitrogen, phosphorus and iron).
Light limits productivity to the photic zone, and turbidity can shrink that further.
Productivity is highest in coastal waters, shallow tropical seas and reefs, and above all upwelling zones.
Temperate seas show spring and autumn blooms, with a summer slowdown caused by stratification.
Why layering limits life
In much of the ocean, warm water sits on cold water and the two barely mix. The nutrients that phytoplankton need — released as dead organisms sink and decompose — accumulate in the deep layer. Light is only available at the surface. Separate the two and productivity falls, however sunny it is.
Mixing is what solves this. Wind-driven currents, seasonal overturning and upwelling all lift nutrients into the lit surface layer, and the response is immediate.
The condition for a bloom
light + nutrients in the same water = productivity
The three limiting factors
Light. Photosynthesis needs it, and it fades quickly with depth. Productivity is therefore confined to the photic zone, the upper layer sunlight reaches. Turbidity from sediment or plankton itself reduces penetration, so even shallow water can be light-limited.
Temperature. Warmer water speeds up metabolism and nutrient cycling, which helps. But if it gets too warm the surface layer becomes strongly stratified, mixing stops and productivity falls. Temperature cuts both ways.
Nutrients. Nitrogen, phosphorus and iron are essential for phytoplankton growth, and across much of the open ocean they are the factor in shortest supply.
Where the productive water is
Reefs are the exception that proves the rule. They are productive despite nutrient-poor water because they recycle nutrients internally, helped by symbiosis between corals and their algae.
Geographic patterns
Coastal areas. Shallow seas receive nutrients from rivers and from upwelling, so productivity is high. Estuaries in particular act as nurseries for marine species.
Tropical shallow areas and reefs. Constant sunlight supports high productivity where nutrients are available, and reefs sustain themselves through tight nutrient recycling and symbiosis.
Upwelling zones. The most productive regions in the ocean. Winds and currents push surface water away from some coastlines and cold, nutrient-rich water rises to replace it, feeding vast phytoplankton populations.
The temperate seasonal cycle
This graph is the one to be able to read. It shows why temperate seas bloom twice a year and go quiet in between.
Read the two blooms as collisions. Spring: rising light meets winter’s stored nutrients. Autumn: cooling and wind break the stratification, returning nutrients while some light remains.
Season by season
Winter. Surface cooling and storms mix the column thoroughly, so nutrients are plentiful. Light is too weak for much growth.
Spring. Light increases while nutrients are still high, so phytoplankton multiply rapidly — the spring bloom, the largest of the year.
Summer. Surface waters warm and stratify. The bloom has used up surface nutrients and mixing cannot replace them, so productivity slows despite maximum light.
Autumn. Cooling surface water and stronger winds restore mixing. Nutrients return while there is still some light, giving a smaller autumn bloom before winter.
If you can explain the summer dip you have understood the whole page. Plenty of light, plenty of nutrients in the system, but a thermocline keeping them in different places.
Worked examples
WORKED EXAMPLE 1
Explain the summer fall in productivity [4]
Use the graph above.
Link 1
Strong summer sun warms the surface, creating thermal stratificationLink 2
The layers no longer mix, so deep nutrients cannot reach the photic zoneLink 3
The spring bloom has already used up surface nutrientsLink 4
Nutrients, not light, become the limiting factorPhytoplankton biomass falls even though light is at its peakthe phrase “limiting factor” is what the mark scheme is looking for
WORKED EXAMPLE 2
Why is the tropical open ocean unproductive?
It receives more sunlight than anywhere else on the planet. Explain the apparent contradiction. [3]
Point 1
Warm surface water is permanently stratified — there is no seasonal cooling to break it up
Point 2
Nutrients released by decomposition stay in the deep layerPoint 3
Light is abundant but useless without nutrients, so nutrients are limitingOften described as a marine desertcontrast it with a nearby upwelling zone at the same latitude to show the difference mixing makes
WORKED EXAMPLE 3
Predicting the effect of a warmer ocean
Suggest what stronger, longer-lasting stratification would do to a temperate sea’s seasonal cycle. [3]
Point 1
Stratification would start earlier and last longer, shortening the mixed period
Point 2
Less mixing means fewer nutrients returned to the surface, so blooms are smaller
Point 3
Lower primary productivity means less energy entering the food webSmaller fish stocks and weaker fisheries over timethis links straight back to the warming page — a good cross-topic point in an essay
💡 Exam tip
Name the limiting factor explicitly in every productivity answer.
Say that stratification separates light from nutrients. That one sentence covers most of this page.
Learn the sequence: winter mixing, spring bloom, summer stratification, autumn bloom.
List the key nutrients: nitrogen, phosphorus, iron.
Use photic zone and turbidity when light is discussed.
Rank productivity by region and be ready to explain why upwelling zones top the list.
⚠ Common mix-up
Assuming more sun means more productivity. In stratified water, nutrients run out first.
Confusing stratification with pollution. It is a natural physical process, though warming intensifies it.
Thinking the autumn bloom is as big as the spring one. It is smaller, because light is fading.
Forgetting turbidity. Shallow water is not automatically well lit.
Saying temperature always raises productivity. It helps metabolism but drives stratification.
Mixing up nutrient mixing and upwelling. Upwelling is one specific form of mixing, driven by wind along coasts.
Up next: Measuring a Fish Stock — how anyone actually knows how many fish are out there, and why the number always comes with an error bar attached.
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