Dossier Océan et énergie - Énergie Thermique des Mers
Sommaire IOA News Letters
Ocean Resource Development and the Utilization of Deep Ocean Water:the U.S. Experience with Marine Biology and Mariculture ¡¯
MAR RES Associates
South Beach, Oregon, USA
Kelton R. McKinley
University of Hawaii at Manoa
Honolulu, Hawaii, USA
The vast areas included within exclusive economic zones surrounding the world's coastal states are rich in natural resources: fish, marine minerals, and ocean thermal energy. Perhaps the greatest resource is that represented by the high concentrations of nutrients contained within deep ocean waters. These waters hold potential solutions to man's greatest resource needs, including renewable energy and food production.
This paper will review that potential within the context of the U.S.
experience. Deep ocean water can be used in aquaculture applications, such as fish, shellfish, and marine biomass production. Artificially generated zones of high primary and secondary productivity similar to those encountered within zones of natural upwelling are envisioned and will likely be required, if the full potential of deep ocean water is to be realized. Much research has been completed but much work remains in operationalizing and commercializing the findings.
Man's basic needs for food and energy have not changed substantially over the last several thousand years. World population continues to expand exponentially and will likely double from its present 5 billion people to 9 billion in 40 years. That doubling will be accompanied by increased competition for land, an increased need for food, and a depletion of our fossil fuel reserves. Man, ever hopeful that new sources of food and energy will be readily obtainable, has long looked to the sea as holding that promise.
Covering some 70% of the globe, the world's oceans are often viewed as the last substantial undeveloped area available to us, although the sea has long supplied many natural resources. For example, U.S. coastal fisheries alone produce 10 billion pounds of food each year, adding $23 billion per year to our nation's economy (Vadus, 1990). Certainly, our oceans hold much promise for the future in energy as well as food production.
Most of us here are already quite familiar with the acronym "OTEC" for ocean thermal energy conversion. As we review the history of the U.S. experience it will at times appear that deep ocean water is inextricably linked to biological production from the sea. While such a linkage is suggested by the research to date, and many of the arguments in support of the economic viability of ocean thermal energy conversion processes, it does not hold true in all locations, particularly areas at high latitudes. However, energy from the sea can take another form, and that is through biomass production. This energy pathway is not limited to latitude, but to the availability of sufficient nutrient stocks.
This week we will discuss the various potential uses of deep ocean water. This paper will focus on the previous experience by the U.S. with deep ocean water aquaculture and artificial upwelling and conclude by identifying areas for further discussion and future research and development. For the most part it will address biological and ecological issues only, as others attending this workshop will be addressing physical oceanographic and engineering topics in greater detail.
Deep Ocean Water
Deep ocean water holds much promise for biological production wherever these waters reach the surface since deep ocean water contains considerably more nutrients than surface waters. Table 1 compares the nutrient content of deep ocean water from the vicinity of the Big Island of Hawaii to that of surrounding surface waters. Nutrient concentrations for deep ocean waters from elsewhere throughout the world are similar, e.g., the U.S. Virgin Islands (Roels, 1980). Deep ocean waters contain more than two orders of magnitude more nitrogen than surface waters and more than one order of magnitude more nitrogen than surface waters and more than one order of magnitude more phosphorous. As the majority of the nitrogen and phosphorous contained in deep ocean water is derived from the decay of previous biological production in upper layers of the oceans, deep ocean water represents an ideal source of nutrients for photosynthesis and aquaculture feedstocks production. Indeed, the carbon: nitrogen: phosphorous ratio of deep waters is near the "Redfield ratio" of 106: 16:1 (Redfield, et al., 1963) considered optimal for phytoplankton growth. Areas of natural upwelling represent a scant 0.1 percent of the world's oceans but produce some 44% of the world's fisheries catch (Penny, Bharathan 1987). What can be envisioned is expanding the production of the seas by increasing the available nutrients in surface waters through the use of deep ocean water to create additional areas of upwelling.
Research and development utilizing deep ocean water had its real beginning in the early 1970's and was tied to the energy crisis being experienced in the U.S. at that time. In the U.S. Virgin Islands Oswald Roels of Lamont-Doherty Geological Observatory and his colleagues conducted a series of OTEC-related aquaculture experiments intended to imitate areas of natural upwelling (Roels, 1979; 1980). Water was drawn from a depth of 870 meters. Flow rates were 175 1/min and water temperatures were near surface ambient, between 22¢J and 29¢J. The work focused on chained multiple-use, low-flow aquaculture toward the maximization of biological production and economic return. This work continued for nearly 8 years.
Microalgal species, bivalves, crustaceans, and macroalgae were cultured. The best performing microalgal species was the diatom Chaetoceros sp. Microalgae were fed to a variety of shellfish including: the American and Pacific oysters (Crassostrea virginica and C. gigas), the European oyster (Ostrea edulis), the Japanese pearl oyster (Pinctada martensii), the Japanese little-neck clam (Tapes japonica), the hard and southern clams and hybrids of the two (Mercenaria spp.), and the bay scallop (Argopecten irradians). Crustaceans included the spiny lobster (Panulirus argus) and marine shrimp (Penaeus spp.), and the principal macroalga grown was Hypnea musciformis, a red seaweed (Roels, 1979; Baab, et al., 1973).
Steady-state cultures of diatoms were able to utilize up to 99% of the available nitrogen and 77% of the phosphorous, although on average percent capture was less (Roels, et al., 1975). The transfer of algal protein-nitrogen to clam flesh (i.e., the Japanese little-neck clam) averaged an astounding 33% (Roels, et al., 1978). Beef feed-lots average conversions of approximately 6%, while catfish aquaculture ponds can achieve 10% conversion rates. The conversion of deep ocean water influent nitrogen to clam flesh achieved an overall rate of 23%.
Hypnea, the red macroalga, was cultured in waste discharges, growing 5 times faster in shellfish effluent than deep ocean water alone and 3 times faster than in surface waters. This growth was proportional to the ammonia content in the effluent waters and was likely directly attributable to the availability of this form of nitrogen in the waste stream, a form of nitrogen generally absent, or found in low concentrations in deep ocean water (see Table 1). Under optimal conditions Hypnea doubled its weight every 60 hours (Roels, et al., 1975).
At about the same time that the work above was being conducted in the Caribbean, work was also underway off the West Coast. There the focus was energy production from marine biomass. As envisioned by Howard Wilcox, consultant to the then President Johnson's Commission on Ocean Resources, large open-ocean macroalgal farms would provide the U.S. with food, feed, fertilizer and energy. While precedent setting in many positive ways, the loss at sea and failure of a derivative program, General Electric's Quarter Acre Module (QAM), has probably done more to set back the dream of the utilization of deep ocean water than nearly anything else that can be pointed to with an accusing finger. Like hydrogen's Hindenburg disaster before it, a mere mention of open-ocean mariculture can still set program managers running in some agencies within Washington. That sad fact acknowledged, much good work came as a result of Wilcox's vision.
By 1972 the Navy had initiated work on Wilcox's original concept, a multiproducts aquafarming system featuring the giant brown kelp (Macrocystis prifera). With the arrival of the energy crisis in the early 70's, the Energy Research and Development Authority (ERDA) and the American Gas Association (AGA) through their subsequent counterparts the U.S. Department of Energy and through them SERI (the Solar Energy Research Institute) and GRI (the Gas Research Institute) took over program funding and management. The original multiproduct approach was scrapped and a focus on marine biomass cultivation toward natural gas production via anaerobic digestion was selected (Bird and Benson, 1987). No crop was actually ever obtained at sea, but useful information was generated, not the least of which was the observation by the Institute of Gas Technology in 1978 that kelp was a better source for methane production than any other biomass source tested.
Macrocystis is a very large marine alga. Adult plants can reach a length of 60 meters. Giant kelp is a temperate zone species found to occur naturally in areas of upwelling. It attaches to the bottom with a specialized hold-fast cell and is well adapted to the use of the nutrient regime found in these zones. In a series of experiments North demonstrated that deep ocean water is quite suitable for the growth of Macrocystis (North, 1977; Gerard and North, 1981). Using water from a depth of 300 meters off southern California, 450 meters at the Laguna test farm and 850 meters at St. Croix, he demonstrated that kelp showed superior growth in a 50/50 mixture of deep ocean water than with deep ocean water alone. Wilcox (1979) reported growth rates nearly as great (i.e., 18% vs 19%, as percent change per day) for a mixture of 10% deep ocean water mixed with surface water as compared to the 50/50 mixture.
Kelp is therefore not only well suited to culture using deep ocean water, it grows at phenomenal rates in deep ocean water. Mature plants in dense stands can grow more than 0.3 meters per day. Wilcox projected that a deep water installation upwelling 60,000 m3/ha/day could produce 700 to 800 tons/ha/yr of kelp. An additional piece of important information that came out of this work was the fact that the mechanical attachment of plants to any structure is not a trivial engineering issue. Plants are highly susceptible to abrasion, particularly from attached bivalves and marine crustaceans that are also attracted to any structure placed at sea.
Work has also been conducted in Alaskan coastal waters. Neve and colleagues conducted a series of experiments drawing nutrient-rich deep water to the surface during the period of summer stratification. On-shore tests demonstrated that standing stocks of phytoplankton increased five-fold over that in surface waters during the summer stratification period. Growth of mussels (Mytilus edulis) was also enhanced with growth in the upwelling pond greater than 10 times that achieved in nature (Neve, et al., 1976; Paul, et al., 1978).
Hawaii has also conducted its own research using deep ocean waters. Most of this effort has been OTEC-related and was conducted at the Natural Energy Laboratory of Hawaii Authority (NELHA) facilities at Keahole Point on the Big Island of Hawaii. This work is reviewed by Fast and Tanoue (1988) in their OTEC Aquaculture in Hawaii. The early focus was on seaweed cultures and salmonid production.
The algal species selected for early trials was nori, because nori of high grade and quality commands a premium price in the market place. Nori (Porphyra tenera) is an edible seaweed that is used extensively throughout Japan. It is most commonly experienced formed and dried into paper-like sheets. Nori was successfully cultured in deep sea water tumble cultures, achieving some of the highest daily production rates (60 gm/m2/day (dry weight)) ever recorded for this technique. The seaweed removed more than half of the nitrogen and more than one quarter of the phosphorous from the water. While the growth rates achieved were quite favorable, the quality of the end product was judged to be low, perhaps as a result of the method (tumble cultures) employed, although later trials with a different species, P. yezoensis, produced nori judged to be of much higher quality.
Salmonid research at NELHA focused on Coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) salmon and rainbow trout (O. mykiss). Maturation was enhanced in temperature controlled systems and salmonid broodstocks were readily manipulated for sustained egg production. Other species that were successfully cultured at NELHA included: the American lobster (Homerus americanus), the red abalone (Haliotis rufescens), and giant kelp (Marcrocystis pyrifera).
Commercialization of the biological potential of OTEC-generated deep ocean water has always been a part of the NELHA master plan. True to this aim commercial enterprises have sprung up at the 221 hectare (547-acre), US$18 million Hawaii Ocean Science and Technology (HOST) park at Keahole Point, enterprises hoping to capitalize on the research conducted there and the readily available supply of cold, nutrient-rich, pathogen-free deep ocean water. In recent years firms included:
|Ocean Farms of Hawaii (previously Hawaii Abalone Farms) which grew giant kelp (and diatoms), abalone, sea urchins, oysters, and both coho and chinook salmon;|
|Cyanotech Corporation grew microalgae (Spirulina spp. and Dunaliella spp.) for the health food and pharmaceutical markets;|
|Royal Hawaiian Sea Farms cultured the edible seaweeds nori and ogo (Gracilaria spp.);|
|Dawani Lauro Corporation also cultured ogo;|
|Aquacultures Enterprises cultured American lobsters and served as a cold storage "warehouse" for trans-shipment of lobsters from the U.S. mainland to Japan;|
|Hawaiian Seafood Growers cultured the fish, mahi mahi, (Coryphaena hippuras);|
|Hawaii Culture Pearl grew pearl oysters (Pinctada fucada);|
|Yonizawa Suisan cultured the Japanese flounder (Paralichthys olivaceus); finally|
|D/S Ventures cultured sea cucumbers.|
Not all of these firms were profitable, and even the largest and oldest firms have experienced serious financial difficulties. For the most part, the answer to these firms' financial problems will likely come out of expanded university conducted research and development programs, as none can afford aggressive private sector sponsored efforts.
Along related research lines, the Pacific International Center for High Technology Research (PICHTR), based in Hawaii, is currently supporting research on a different concept called "open-cycle ocean thermal energy conversion" (OC-OTEC) in which warm ocean surface water is vaporized in a depressurized chamber, producing energy in the form of low pressure steam for power generation (Takahashi and Woodruff, 1990). Cold seawater from depth is used for condensing the steam in a continuous cycle. Freshwater, a valuable commodity in many island or remote rural locations, is a product of OC-OTEC.
Implications for the Future
Experience has shown us that deep ocean water is an excellent culture medium. Its large scale use holds enormous potential, both for food and energy production and for economic return. Using the Keahole water quality data as a starting point we can calculate that 69 million m3/day of water, nutrient poor surface water combined with enriched deep sea water on a 1:1 basis, delivers approximately 8000 tons of nitrogen (and other major nutrients) to the euphotic zone in the course of a year. If we assume that kelp is 1% nitrogen on a dry weight basis (Neushul, et al., 1990) and fish 10% nitrogen on a dry weight basis (Kennish, 1989) we can estimate the quantities of both materials that could be generated by such a nutrient resource. Assuming a conversion efficiency of 10% for primary producers and 1% for secondary consumers we can roughly estimate that this quantity of deep ocean water would generate some 80,000 tons dry weight of kelp per year (equivalent to approximately 40,000 barrels of oil), or 8000 tons of fish (fresh weight).
Bailey and Vega (1980 and 1981), in a series of carefully refined calculations referencing the work by Roels and Wilcox discussed earlier, concluded that a similar volume of water would produce:
55,000 dry tons of kelp,
35,000 dry tons of microalgae,
96,000 tons fresh clam meat, or
9,000 tons fresh fish meat.
The clam meat from such an facility would generate a crop valued at nearly $500 million per year given clam meat at $2.50/lb. The electricity from an OTEC plant of this size would produce a revenue stream at 100% capacity equal to $90 million. This quantity of kelp converted to natural gas would produce an energy equivalent of 0.2MW of gas per every 1.0MW of electrical energy produced by the OTEC plant. While this could represent as much as a 10% increase in net power production, this may not represent the best use of the kelp biomass. Alginates, acetone, ketones and other organic compounds would produce an even higher return. This is not to overlook the fact that kelp is also a good source of potash. At 12% dry weight content (Bird and Benson, 1987) the potash in 55,000 dry tons of kelp is worth approximately $3 million at today's $400 per ton prices in Hawaii.
Challenges for the Future: Discussion and Conclusions
The numbers are staggering. While all might agree that the promise of deep ocean water is indeed great, most here would also agree that much work is still required on nutrient biogeochemistry, the physics, and potential environmental impacts associated with deep ocean water discharges.
An earlier look at research and information needs was conducted by the National Science Foundation and the Science and Technology Agency of Japan. They sponsored a workshop in March of 1990 in Kona, Hawaii, to examine these issues (i.e., Japan-U.S. Cooperative Workshops in Ocean Engineering Research). Those attending the artificial upwelling portion of the conference identified the following for further consideration:
Artificial Upwelling Topics in
- Artificial Upwelling Devices
- Ocean Water Discharges and Hydrodynamic Mixing
- Mariculture Facilities Systems Development
- Artificial Upwelling Topics in
- Growth Kinetics of Marine Organisms in Deep Ocean Water
- Species Selection for Mariculture and Open Ocean Ranching
- Environmental Impact Assessment of Deep Ocean Water
In July 1990 an Electric Power Research Institute (EPRI) workshop was held on a related topic, examining the role of macroalgal oceanic farming in global change (Neushul, 1990). Their list of research and development needs was quite similar to those of the NSF workshop noted above.
To these lists we should probably add that we also require a better understanding and practical knowledge of: a) the nutrient dynamics in areas of natural upwelling; b) the utilization of nutrients within natural marine systems; c) multiple-species crop production; d) nearshore, as well as open water, oceangraphic processes; and e) the probable behavior of deep ocean water discharges under different conditions and settings, and across different seasons. Once we understand the ecological efficiencies associated with areas of natural upwelling we will be in a much better position to outline realistic goals for artificially manipulated systems and environmentally sustainable projects. Further, biologists must work hand and hand with engineers to find cost-effective and reliable means of bringing deep ocean water to the surface and keeping it there as well as for the attachment and/or containment of the species of interest.
Many issues for discussion are available. The difficult task ahead of us is to formulate a framework or blue print for the future, a detailed, staged plan for a coordinated effort toward realizing the already demonstrated potential of deep ocean water and open ocean mariculture.
Table 1. Average water quality data provided by the Natural Energy Laboratory of Hawaii. Deep ocean water samples obtained by pipeline from a depth of 600 meters (1995 feet). Surface and deep ocean water samples from the vicinity of Keahole Point on the west coast of the Big Island of Hawaii. Surface mixing zone extends to a depth of approximately 100 meters. All nutrients reported as micromolar units.
Parameter Surface Water Deep Ocean Water Temperature(¢XC) 26.0 8.9 Nitrogen (NO3 plus NO2) 0.2 39.0 Nitrogen(NH4) 0.4 0.2 Phosphorous(PO4) 0.2