Why is Grand Banks more biologically active than Caribbean?The COLD water off the Grand Banks of Canada has more algae (known for the green water) and food chains to make fishiing very productive. The WARM water in the Carribean is clear and blue and NOT known for massive algae / fishing. Why doesn't the WARM water support more biology? Minnesota Lakes get all full... | |
Cold water holds more dissolved oxygen. So there's that and... oh hell, I'll give you the whole hog explanation.
Algal (phytoplankton) biomass in a marine environment is dependent on the amount of solar energy received to drive photosynthesis, and the quantity of available nutrients to work with (e.g. nitrogen, phosphorus, calcium, iron, carbon, oxygen, etc). Solar energy (obviously) comes from the sun (duhh) and is greater in tropical areas...
...BUT, the tropics can be very limited in nutrient availability, restricting plankton growth.
Why?
Nutrient sources are riverine outflow, dissolved solids from land and the underlying seabed, and other biomass transported from areas like estuaries, offshore reefs, and rocky shorelines. So we can sort of estimate that there is a finite amount of input that's going to be coming into a given area. Now, in that given area, that finite amount can give rise to X-sized plankton population. With me so far? Once that finite amount is used up, the plankton have to go elsewhere.
But! hallelujah--cold water can hold more dissolved nutrients than warm, AND there's plenty of cold water at deep depths. Sweet! Food! So if you're a nutrient-limited plankton, just swim deeper to collect some goodies. THere's a limit on that too, however--think about being a photosynthesizing phytoplankter. You've got to have a certain amount of sun, or you won't have fixed enough energy to make use of those nutrients. Yes?
So that means, if you're constantly swimming deeper, you're getting less and less sun. What this means is that there is a "critical depth" of how deep you can swim in order to gather up nutrients before you'll be spending more energy swimming than you'll be gaining by photosynthesizing. As in, you've burned all your fixed sugar swimming down deep, so you run out before you can get to the sun with the goods to make some more. SOOO, that means that the upper layers of water (above the critical depth) can become depleted of nutrients, and there is often a rich nutrient source in the deeper waters below.
Fine, you say--why don't the waters get mixed up enough for this deepwater nutrient source to come to the surface? This doesn't happen because cold and warm water don't like to mix much because of their different densities. So unless there is physical disturbance like wave action, mixing won't occur, and that resource can go untapped.
One big fat wonderful way around that mixing problem is upwelling. If there are certain currents directed upward by seamounts or banks, those nutrients can get carried to the surface and lead to very high productivity--as is what happens on the Grand Banks and Georges Bank.
In fact, in the Gulf of Maine and North Atlantic, if you look at plankton productivity for a year's cycle, it's typically pretty low during winter (for lack of sufficient sunlight), high in spring because of improving sunlight and good stormy mixing with the lower layers (and nutrients haven't been depleted yet), lower in summer in open-ocean areas because of depletion of the top layers, but higher in areas such as the Grand Banks that have upwelling from the bottom, and then higher again in the fall when the storm season starts to mix things up a bit more but the sun hasn't disappeared yet.
Caribbean and other tropical areas are quite productive, but not so high in plankton productivity because they mostly operate on a tight nutrient-recycling system. Coral reef (and tropical rainforests for that matter) systems' major source of nutrients is itself, i.e. other animals and plants dying and returning their tissue nutrients to the water; however, rather than being taken up by plankton, the nutrients are taken up so quickly by the "wall of mouths" of filter feeders and photosynthezing zooxanthellae (symbiotic algae in the coral) making up the reef, and blooms rarely occur because the nutrient uptake is so fast.
When blooms do occur, it's usually because of nutrification, i.e. pollutant biological runoff from islands cause by stormy mudslides or human activities. These blooms can lead to reef smothering.
Sorry for the huge answer--your question is a lot bigger and more complicated than I expect you thought! Post again on a specific chunk of it if it needs clarification. Basically it's just a fundamental difference in ecological character of tropic vs temperate environments.
