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Thursday, June 1, 2023

Cooler Waters Draw Marine Life

Some of the coldest waters in more than 20 years have been lurking beneath the warm surface waters of Costa Rica’s Pacific coast. The cold currents, some flowing a chilly 19 degrees Celsius at only 15 meters, did what cold currents do around the world: they brought in a profuse amount of marine life.

Pacific dive sites have been overflowing with fish. From schools of dozens of manta rays to whale sharks and cloud-sized schools of bigeye jack, the cool Pacific has been teeming with life.

“Teeming with life” does not necessarily mean high biodiversity; a lot of life can also mean productivity. A high biodiversity generally means a great number of total species compared to other places, and that there are usually just a few of each species. High productivity usually means great numbers of a few species.

Most biology students know that colder waters are more productive than warmer waters. So, as you might guess, the southern Ocean surrounding Antarctica is the most productive in the world. However, for productivity in warm, tropical waters, Costa Rica’s Pacific is about as good as it gets, especially when the currents are cooler.

Some people confuse the high productivity of Costa Rica’s Pacific with the high biodiversity of the country’s terrestrial forests.

There is no evidence, however, that Costa Rica’s waters, even Pacific and Caribbean combined, have anywhere near the highest marine biodiversity in the world.

The highest numbers of marine species, by far, thrive in the Indo-Pacific, thousands of miles from here. Divers in Costa Rica, with luck, might thrill to a few hundred different species living under the sea here. Very few people can identify that many species.

With enough time in the Indo-Pacific, the trained eye can identify thousands of different species.

Diving in Costa Rica’s Pacific waters lately, you can identify only one species at a time, because there are hundreds in front of your mask blocking the view of anything else – including your fellow divers. One school of bigeye jack, as big as a house and as long as a train, took more than 15 minuets to swim past me. Some days revealed hectares of the sea surface roiling with fish – just the kind of thing most divers love.

As usual, now is the time to go diving in Costa Rica; just make sure you bring your wetsuit, because it’s not as warm as it looks down there.



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