Cold ocean water is sometimes described as rich in nutrients

Cold ocean water is often described as rich in nutrients for sea creatures. Why is warm ocean water not even richer in nutrients?

Remember that in aquatic biology, “nutrients” can easily mean inorganic (nutrient) salts, i.e. fertilizer salts, not organic material. Cold ocean water which is rich in nutrients is often an upwelling of deep water.

Deeper in the ocean you’re closer to the seabed, which can leach the various nutrient salts that promote plankton growth into the water. Also, deep down there’s not much plankton which can consume the nutrients.

2square4u has an excellent point about the colder, deeper waters being more nutrient-rich. Note that deep waters are dark, prohibiting the photosynthesis that phytoplankton would perform, therefore phytoplankton can’t live in those deep waters and incorporate the nutrients into their bodies.

I should also add that many warm-water environments are nutrient-poor because what nutrients they do have are already locked up in organisms and biomass. Coral reefs, for example, are a warm-water system where nutrient content in the water is very low both due to nutrients incorporated into living organisms and an extremely efficient recycling rate of nutrients released once those organisms die.

It is occasionally possible to find warm-water environments that are nutrient-rich, but this is often related to disturbance rather than a long-term stable situation, since warm waters that are shallow enough will support plant, algae or bacterial life that moves in to take advantage of available nutrients quickly.

Is there a significant difference in amount of dissolved oxygen? IIRC gasses are more soluble in colder water but I don’t know what difference this would have in practice.

It’s complicated.

Cold-water fish have more oils/fats because of the cold-water environs, and much has been made about the benefits of these fats/oils.

Cold water sinks and creates more upwelling as warm water rises, bringing up nutrients from ocean depths.

In warm water environments, the warm water is on the top, and there is less convection currents happening, thus not as many nutrients are dragged up.

Crazy huh?

Anecdote: Scuba diving in Southern California, the 60º water was quite clear. Diving in the 42º waters off Nanaimo was like swimming in soup.