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🎯 Context

Last lesson you looked at how we manage fishing: quotas, marine protected areas and fish farms. All of them rely on the maximum sustainable yield (MSY) that you met in water.3.2 Collapse, limits and warming waters.

This lesson asks three questions. What controls how much life a lake or sea can produce? How do scientists count fish they cannot see, and keep track of what is being caught? And why is it risky to fish right at the MSY? By the end you should be able to explain how productivity, thermal stratification, nutrient mixing and nutrient loading are linked, describe one method of measuring a fish stock and one method of monitoring the harvest, and explain the risks of harvesting at the MSY.

Phase 1: Where water is most productive

4.3.11 Productivity, thermal stratification, nutrient mixing and nutrient loading are interconnected in water systems.

The highest productivity tends to occur near coastlines or in shallow seas, where upwellings and nutrient enrichment of surface waters occurs.

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Four factors, one system

Primary productivity is the rate at which producers, mostly phytoplankton, turn light energy into new biomass. In water it is limited by three things: light, temperature and nutrients such as nitrates and phosphates. All three have to be available in the same place at the same time.

You saw in water.1.3.hl Stratification and Ocean Circulation that this rarely happens by itself. Light and warmth are at the surface. Nutrients build up in the cold, dark deep water, where dead organisms sink and decompose. Thermal stratification keeps the two apart.

So productivity depends on whatever brings nutrients back into the light:

Nutrient mixing. Wind, storms or winter cooling stir the layers, so that deep water and its nutrients reach the surface.
Upwelling. Wind pushes surface water away from a coast, and cold, nutrient-rich water rises to replace it.
Nutrient loading. Nutrients are added from outside the water body: washed in by rivers and runoff from the land, or released in sewage and farm fertiliser.

That is why the most productive waters are near coastlines and in shallow seas. Near coasts, rivers deliver nutrients and upwelling brings them up from below. In a shallow sea such as the southern North Sea, storms can stir the water all the way down to the seabed, so nutrients released from the sediment keep returning to the sunlit surface. In the middle of a deep ocean, the surface is warm and well lit but cut off from the nutrients far below it.

flowchart TD
    L["Light and warmth<br>at the surface"] --> P["Primary<br>productivity"]
    D["Nutrients stored<br>in deep water"] --> M["Mixing or<br>upwelling"]
    M --> P
    S["Thermal<br>stratification"] -. "blocks" .-> M
    R["Nutrient loading<br>from rivers"] --> P
    P --> F["Zooplankton<br>and fish"]
    F --> X["Dead remains sink<br>and decompose"]
    X --> D

1. State the three limiting factors for primary productivity in water.

2. Distinguish between nutrient mixing and nutrient loading.

The spring bloom

In temperate lakes and seas, such as Lake Geneva or the North Sea, the balance between the four factors changes through the year.

Winter. Cooling and storms break up the layers and mix the water, so nutrients reach the surface. But the days are short and the water is cold, so little grows.
Spring. The days lengthen and the surface warms, while the nutrients brought up in winter are still there. Phytoplankton multiply rapidly. This is the spring bloom, the most productive time of the year.
Summer. The warm surface layer becomes stratified. Phytoplankton use up the nutrients in it, and no new ones can cross the thermocline from below. Productivity falls, even though light is at its strongest.
Autumn. The surface cools and storms begin to break the layers again. A smaller bloom can follow before winter returns.

3. Explain why primary productivity in a temperate sea is often lower in midsummer than in spring, even though there is more light in summer.

Where the ocean is green

Satellites measure chlorophyll, the green pigment phytoplankton use to photosynthesise. Where there is more chlorophyll there are more phytoplankton, so a chlorophyll map is a good guide to where the ocean is most productive.

The map shows average chlorophyll from 2002 to 2010. Pale yellow and light green mean a lot of chlorophyll. Dark blue means very little. The map is centred on the Pacific Ocean, and land is grey.

Global map of average ocean chlorophyll, July 2002 to May 2010, centred on the Pacific. High values along coastlines, at high latitudes and along the equator; low values in the centres of the oceans.

Source: NASA Earth Observatory, image by Jesse Allen and Robert Simmon, via Wikimedia Commons (public domain).

4. Use the map to complete the table. Choose three areas with high chlorophyll and two with low chlorophyll. For each, explain the pattern using light, stratification, mixing, upwelling or nutrient loading.

Area	High or low?	Explanation

Local example: Lake Geneva, nutrients and the whitefish

Lake Geneva shows that nutrients, mixing and fish are all linked, and that more nutrients does not simply mean more fish. The fish at the centre of the story is the whitefish (Coregonus), sold locally as féra.

Too many nutrients. Before the 1960s, the lake held less than 15 micrograms of phosphorus per litre. Sewage and phosphate detergents raised this to a peak of 89.5 µg/L in 1979. This nutrient loading fed more plankton: zooplankton roughly quadrupled between the early 1960s and the early 1980s, and whitefish grew faster. But the extra algae sank and decomposed on the lake bed, using up oxygen and smothering the gravel where whitefish lay their eggs. Fewer young fish survived, and the whitefish catch fell to only 30 to 50 tonnes a year between 1974 and 1984.

Cleaning up. Sewage treatment plants were built around the lake from the 1970s, and Switzerland banned phosphates in laundry detergent in 1986. Phosphorus fell to about 16 µg/L by 2021. Whitefish breeding recovered, and the catch rose to a record of about 971 tonnes in 2014.

Now. Since 2014 the whitefish catch has fallen sharply, as the chart shows. Scientists and fishers are still working out why. Three ideas are being investigated:

Less mixing. The lake has not mixed all the way to the bottom since 2012 (water.3.2). Phosphorus released from the bed stays trapped in the deep water instead of returning to the sunlit surface.
An invasive filter feeder. The quagga mussel was first recorded in the lake in 2015. It filters huge amounts of phytoplankton out of the water, leaving less food for zooplankton and so for fish.
Not enough nutrients. Fishers fear that phosphorus plays a part in the decline. In 2023 the University of Geneva and local fishers began measuring phosphorus around the lake together.

Line chart of professional fishers' catch in Lake Geneva, 1992 to 2022. Whitefish rise from 63 tonnes in 1992 to a peak of about 971 tonnes in 2014, then fall to 119 tonnes in 2019 and 170 tonnes in 2022. Perch vary between about 140 and 590 tonnes, reaching 395 tonnes in 2022.

Sources: catch data from the Lake Geneva fishing statistics, Canton of Vaud (2024); phosphorus and whitefish history from Rogissart, Goulon and Guillard (2024), International Journal of Limnology; University of Geneva, February 2023; Eawag on the quagga mussel.

5. Describe the trend in the whitefish catch from 1992 to 2022. Use figures from the chart.

6. Explain how high nutrient loading in the 1970s reduced the whitefish catch, even though there was more food in the lake.

7. The quagga mussel was first recorded in 2015, just after the whitefish catch began to fall. Explain why this timing alone does not prove that the mussel caused the decline.

Phase 2: Counting fish you cannot see

4.3.12 Accurate assessment of fish stocks and monitoring of harvest rates are required for their conservation and sustainable use.

Include at least one method of measuring a fish stock accurately and one method of monitoring harvest rates.