4.3.11 (HL)—Water system interconnections

High aquatic productivity needs both light near the surface and nutrient supply; stratification can separate the two.

Deep decomposition regenerates nutrients, but a strong density gradient limits upward mixing. Upwelling, seasonal overturn and rivers can restore nutrients to the photic zone.

A shallow coastal site may be more productive than an open stratified ocean because light and renewed nutrients overlap.

Producers need light at the same depth; without delivery to the photic zone, nutrients remain unavailable.

More nutrients can cause eutrophication in a different context; productivity and ecological health are not synonyms.