“Hotter surface water can slow upwelling, a phenomenon that brings nutrients from deep in the water like nitrogen and phosphorus compounds toward the surface, feeding the plankton that form the foundation of the food pyramid. Roughly half of the fish in the world are caught in upwelling zones.
Hotter water also holds onto less oxygen, which can suffocate sea life. Earlier this year, thousands of dead menhaden fish washed up on the shores of Texas, due in large part to high water temperatures. The water itself becomes more acidic as it heats up, which can disrupt coral formation. And when the temperature rises too high, coral will expel the symbiotic algae that live in their tissues, turning completely white. This is called coral bleaching. All these effects of hotter water temperatures can then make coral more vulnerable to disease.
Sargassum, a type of algae, has seen massive and growing blooms in the Caribbean Sea and Atlantic Ocean in recent years, but there’s been a record quantity this summer, enough to be seen from the sky, washing up on beaches from South America to Florida. As it rots, it emits smelly, toxic hydrogen sulfide gas. Higher water temperatures are likely a factor here as well.It’s not clear yet exactly how the alarmingly hot waters of the Caribbean will affect seafood catches, but scientists warn that extreme marine heat has proven devastating to fisheries in the past.”
https://www.vox.com/climate/23801858/caribbean-heat-wave-dominica-ocean-marine-sea-coral-fish