I’ve always heard that cold ocean water has ‘more life’ in it than warm water. This is what I’ve observed:
[ul][li]Caribbean, warm water: The water is very clear[/li][li]Channel Islands, California, semi-warm water: The water is clear, but not as clear as in the Caribbean[/li][li]Puget Sound and Strait of Georgia: Cold water. Limited visibility[/ul][/li]I’ve heard that water off the northern end of Vancouver Island sometimes has visibility of 100 feet. But when I dove in Seattle and off of Nanaimo (Vancouver Island) it was like swimming in soup. Plankton and small jellies reduced visibility significantly. The water temperature was 42°F. My first open-water dive was off of Anacapa Island in California’s Channel Islands. I looked at the pebbly bottom of the ocean and marveled at the clarity. (I’d only been accustomed to the beaches where’s I’d swim and body surf.) Turns out the ‘pebbles’ were rather large rocks and they were 65 - 70 feet down. The water temperature was 65°F. And we’re all familiar with the crystal clear warm waters (80°F or 85°F) of the Caribbean.
Question: Why are colder waters ‘richer in life’ than warmer waters?
Almost certainly it’s due to (or at least strongly related to) the fact that gasses, such as oxygen, dissolve more readily in cold water than warm, which is why soda comes out of the fountain tap cold. More oxygen means more microorganisms, more microorganisms mean more things to eat them, ad nauseum.
The correlation is not with cold water per se, but with the amount of dissolved nutrients in the water that are available to support the growth of photosynthetic organisms. For several reasons, these nutrients tend to be more available in colder water. All else being equal - that is, an equal amount of light and nutrients being available - warmer water will be more productive than cold water because organisms grow faster at higher temperatures.
Cold-water zones have some advantages over warm-water zones, but are not uniformly richer in life - just look at coral reefs for a good counterexample.
Here are some of the reasons nutrients are different between warm and cold water in the Northern Pacific & Arctic (just for this example I’m ignoring the rest of the world, so this is not going to be complete).
The Eastern North Pacific has large upwelling (warning: PDF) zones that seasonally bring cold water up from the depths of the ocean. This cold water has high levels of nutrients (there being virtually no primary production down there to remove nutrients) and feeds a large and diverse food web.
Areas surrounding Alaska and Chukotka are covered in ice for large parts of the year, during which primary production goes down considerably and nutrients re not being removed from the water. During the summer, productivity balloons, fueled by lots of leftover nutrients.
Continental margins are very important in the addition of nutrients to the ocean. Most of the western coast of North America can be considered cold water due to the cold California Current running south from the Gulf of Alaska. There is an equivalent (perhaps larger due to the greater land mass) input of nutrients along the warm Kuroshio Current running north on the eastern coast of Asia. If you look at a map, you’ll notice that due to the shape of the ocean basins, large swaths of warm equatorial water are very far from any continent, and therefore have fewer nutrients. Exceptions to this are near the many chains of Pacific islands (like Hawaii) or seamounts.
By and large the difference in productivity between warm water and cold water areas is due to a difference in nutrients, and often that is due to warm water being stuck in certain places (e.g. in the middle of an oceanic gyre) and/or all the nutrients being rapidly taken out of warm water during plankton blooms (note that Colibri mentioned the faster growth rates of most critters in warm temperatures.)
Coral reefs are extremely diverse in the number of species, but the productivity there is not particularly high. Although there are many species, the actual number of individuals of each species is low. In fact, corals prefer areas where there are few nutrients in the water, because clear water provides more sunlight to their symbiotic algae. In areas where nutrient levels are too high, corals can be out-competed and smothered by green algae.
Here in Panama, the Caribbean is characterized by clear, low-nutrient water that promotes the growth of coral reefs (except where there is too much sedimentary run-off from land that clouds the water). On the Pacific, there is seasonal cold-water upwelling from December to April in the Gulf of Panama when the trade winds blow warm surface water offshore and all nutrient-rich cold water to rise from below. At this time there are large blooms of phytoplankton which is the basis of a food chain that extends up from animal plankton, through anchovies and shrimp, to tuna, marlin, seabirds, and marine mammals. The ecosystem is highly productive, but it is not very diverse. There are few species, but very large numbers of individuals.
True. But the OP asked about “richness in life” which doesn’t necessarily mean productivity (or even species richness, although that was one thing that crossed my mind). One thing that is quite remarkable about coral reefs is that the recycling rate of nutrients is high, despite the poor overall level of nutrients in the water.
That’s funny: here in the Gulf of the Farallones when we get our upwelling season (approx. July to September), not only does productivity shoot up dramatically, but species richness does as well. I don’t have overall numbers handy for the ecosystem, but for the marine mammals, we’ll have about seven species commonly sighted without upwelling, but closer to thirty during the upwelling season. Seabirds show a similar increase. I wonder why the Gulf of Panama doesn’t show a similar increase… possibly because you’re too far to get many migrants from the Gulf of Alaska?
Yes, it wasn’t really clear whether the OP was talking about species richness or the total abundance of marine life.
I would guess that species richness probably increases during upwelling in Panama as well - certainly many kinds of seabirds come in to nest during this season, and then disperse later. But I was speaking about general species richness of all forms of life relative to the coral reefs on the Caribbean side of Panama. Seabirds and marine mammals are much more diverse on the Pacific side of Panama, both during upwelling and at other seasons; but the overall diversity of fish and most other forms of aquatic life is far higher on the Caribbean throughout the year.
I don’t think there are too many species shared between the Gulf of Alaska and Panama - Humpback Whales are one - but we do get a fair number of migrants from the Gulf of California.