Why Does the Sea Turn Green in Spring? Growth Limited by What Runs Out

Phytoplankton are microscopic photosynthetic organisms drifting in the upper ocean, and they multiply by division, which means their numbers can double in a day or less when conditions allow. A bloom is a period of such rapid multiplication that the population rises by orders of magnitude within days or weeks, producing concentrations high enough to change the colour of the water across areas measured in thousands of square kilometres and visible in satellite imagery. The organisms are the base of the marine food web and produce a large share of the oxygen in the atmosphere, so blooms are not an anomaly but the main event in ocean productivity, concentrated in short periods rather than spread evenly.

Growth requires several things at once and the scarcest one controls the rate:

• •Light, which limits growth in winter and at depth
• •Nitrogen and phosphorus, which are the usual limiting nutrients in most of the ocean
• •Iron, which limits growth across large regions of ocean that have everything else
• •Silicon, required specifically by diatoms for their glass shells
• •A stable enough surface layer that cells are not mixed down below the light
• •Temperature, which affects rates without usually being the limit

The temperate spring bloom is the standard case and its timing is explained by the interaction of light and mixing. Through winter, storms mix the upper ocean deeply, so cells spend most of their time below the depth where light is sufficient and cannot accumulate despite nutrients being abundant. As the surface warms in spring, a layer of lighter water forms on top and mixing shallows, which holds the cells in the light, and growth begins immediately because the nutrients mixed up over winter are still there. The bloom continues until something runs out, which is usually nitrogen, or until grazing by small animals catches up, and both happen within weeks. A smaller autumn bloom follows when mixing deepens again and returns nutrients.

The observation that iron limits growth across large ocean regions produced a proposal and a series of experiments. Because phytoplankton take up carbon dioxide as they grow, and because some of the resulting material sinks to the deep ocean, fertilising iron-limited water was suggested as a means of removing carbon from the atmosphere, on the reasoning that a small quantity of iron triggers a great deal of growth. More than a dozen experiments have added iron to patches of ocean and measured the result, and blooms follow reliably while the quantity of carbon actually reaching the deep ocean has proved small and difficult to verify. The approach also raises questions about unintended ecological effects and about governance of the open ocean, and an international moratorium restricts commercial deployment.

Some blooms are damaging and the mechanisms differ. A few species produce toxins that accumulate in shellfish and poison anything eating them, including people, which is why shellfish harvesting is closed when those species are detected and why monitoring programmes exist in most coastal countries. Blooms that are not toxic can still kill by sheer quantity, since when the cells die and sink, the bacteria decomposing them consume oxygen, producing a layer with too little oxygen for fish and invertebrates, which is what creates seasonal dead zones in several enclosed seas and river mouths. Nutrient runoff from agriculture and sewage drives that process directly by supplying nitrogen and phosphorus far above natural levels, which is the main reason such zones have grown.

Phytoplankton can double daily, so populations rise by orders of magnitude within weeks and colour thousands of square kilometres visibly from orbit. The spring bloom starts when warming shallows the mixing and holds cells in the light with winter nutrients still present, and ends when nitrogen runs out or grazing catches up. Decomposition after a bloom consumes oxygen, which is what produces dead zones.