Samoa Guide, 6. Our deep blue ocean

What are some of the marine resources in our offshore waters? Three ocean resources of potential interest to American Samoa are fish, minerals and the water itself. Several kinds of food and sport fish are present in modest numbers: tuna, masimasi, marlin, wahoo, sharks, and flying fish. Surveys indicate, however, that the abundance of oceanic fishes within our 200-mile limit is probably not high enough to warrant significant commercial development of offshore fisheries. That's the main reason why the big tuna boats that deliver to American Samoa's canneries travel far beyond our 200-mile zone to other locations where the tuna are more abundant (the canneries are located here only to gain duty-free access to US markets).

Another resource mentioned from time to time are mineral deposits, such as manganese nodules, that lie on the seafloor. However, these nodules, even if present in our waters, are too deep for economic extraction by current technologies.

Perhaps a more exploitable resource in the future involves the temperature of the ocean's cold bottom layer. Scientists are working on a technology that extracts energy (to produce electricity) through a heat-exchange mechanism that is made possible by the large temperature difference between the tropical ocean's warm surface and cold bottom layers. A demonstration facility for this technology has been operating in Hawaii since the 1970's, but it has been an uneconomical venture so far. The two requirements for this technology -- a large temperature differential in the ocean, and easy access to this temperature difference by land-based facilities -- are met in American Samoa. Will our future electricity needs be powered by our own blue ocean?

Shallow coastal habitats, with their coral reefs and colorful fish, are quite limited in total area because our islands slope steeply down into deeper water and depths of 2000 feet are reached within 0.5-2 miles from shore. So, most of our coral reefs are restricted to a narrow ring around each island. There is also some coral on the tops of several offshore seamounts in the Territory.

You might wonder, how is it that non-tropical oceans are much more productive? The answer is, again, temperature. Away from the tropics, seasonal changes in water temperature cause the water to mix. Winter temperatures cool the upper layer causing it to sink and mix with the bottom layer, and when this occurs, some nutrients are brought up to the surface. The nutrients in shallow sunlit waters stimulate phytoplankton growth, thus fueling a more productive foodweb.

In the tropics, the 2-layer stratification of the ocean persists year-round because hot sun keeps the surface layer warm. The tropical ocean has been called a 'biological desert' for this reason. That's an exaggeration, of course, because all the tuna out there are finding something to eat. And, many other species live out there as well, from an occasional whale, dolphin, sea turtle or seabird, to numerous species of fish and invertebrates such as jellyfish and shrimp-like crustaceans.

The rest of our marine environment consists of deep blue ocean with a fairly flat seafloor 2-3 miles below the sea surface. The reason for the blue color of the ocean is an interesting one and it is a key factor to understanding our ocean ecosystem, so we need to get technical for a moment. Water by itself is highly transparent with a bluish tinge. What adds other colors to the ocean are, in large part, small marine plant-like cells called phytoplankton. The more phytoplankton in the water, the greener the water becomes. Phytoplankton require two main ingredients to grow well: sunlight and nutrients (fertilizers). If they have both, they grow in abundance. This, in turn, supports a productive food web: phytoplankton provide food for the microscopic shrimp-like animals (zooplankton), and the zooplankton provide food for the fish to eat.

Tropical oceans are not green because conditions are generally not good for phytoplankton growth. Although phytoplankton have all the sunlight they could ever want in the surface layer, nutrient levels there are too low to support much plant growth. This occurs because the deep tropical ocean is typically stratified into two layers with very different temperatures. The sun heats up the surface layer, which is about 300 feet deep, to a pleasant 840 F, but the deeper layer remains a chilly 420 F. Because warm water is lighter than cold water, the warm ocean water generally stays on top, the cold water stays on the bottom. The two layers do not mix.

That's the rub. The bottom layer is where the nutrients are, but because of the 2-layer stratification, the nutrients can't get up to the surface layer where they are needed to combine with sunlight for plant growth. So, conditions for phytoplankton are not very good in the tropical ocean. The surface layer has lots of light but few nutrients, while the bottom layer has lots of nutrients but no light. It's pitch black down there. This arrangement doesn't support a very productive foodweb, so there are generally fewer fish in tropical oceans than in non-tropical oceans.

American Samoa is much larger than you might think it is. The whole Territory covers 117,500 square miles, which is about the size of New Zealand or the state of Oregon. The Territory is big because we claim jurisdiction of the ocean that surrounds American Samoa, from the shoreline out to 200 miles offshore. That is standard procedure throughout the world (each country with marine coasts wants to protect its coastline and marine resources from others). To be more precise, American Samoa has jurisdiction over territorial waters out to 3 miles, while the US federal government maintains control of the zone from 3 to 200 miles from shore.