47 



tionship in clover, and Kasanaga and Monsei(6) developed a numerical model to 

 preduct optimal leaf-area index for maximum plant production. 



The relationship described above is of considerable importance in the context of 

 current research programs investigating the potential of plant biomass as a source 

 of energy. The most sensitive factor in assessing the economic or energy cost- 

 effectiveness of such biomass systems is that of organic yield. In that connection, 

 grow^th rate is often confused with yield. Duckweeds, for example, have, due to their 

 phenomenal reproduction rate" been credited as being "more productive than 

 terrestrial agricultural crops"(7) and able to "grow at least twice as fast as other 

 higher plants"(8). But it may be seen in Fig. IC that the highest growth rate of 

 duckweed occurs at a very low density and that actual yields of the species are 

 relatively low when compared to the better agricultural crops, grasslands, and 

 forests (e.g., (9)). 



The yields of food and fiber crops may be determined simply by weighing the 

 seasonal or annual harvest. No such expedient is possible with plant populations 

 that are not commercially grown and utilized. In order to assess the potential yield 

 of such species there is no substitute to cultivating them throughout the year or 

 growing season in either natural or experimental plots and harvesting and weighing 

 the resulting crop. For species in mild climates that are able to grow continuously 

 in a vegetative mode, such as all four of the examples shown in Fig. 1, it is 

 necessary to harvest the new growth frequently enough to maintain the density of 

 plants at or near its optimum for maximum yield, if the full potential of the species 

 for biomass production is to be determined. 



Such experiments are difficult and time consuming and tend to be replaced by 

 simpler but more crude yield estimates. One such approach has been to measure the 

 growth rate of a given species experimentally, in the field or laboratory, and to 

 apply such grow^th rates to a measured or estimated density of a natural stand of 

 the plants to obtain annual yield values. In some cases short-term growth rates 

 have been used to calculate annual yields, thereby ignoring seasonal effects. This 

 general approach has been used to estimate the annual production of several kinds 

 of aquatic plants including rockweeds and kelps in Novia Scotia(lO), giant kelp off 

 California(ll), seagrasses(12), and water hyacinths(13). 



Reference to Fig. 1 clearly shows how the use of independent values of daily 

 grovvrth rate and density and extrapolation of the resulting daily to annual yields 

 may result in greatly exaggerated projections. Using the maximum growth rates 

 and densities for the seaweed Gracilaria and the freshwater Eichhornia as given in 

 Figs. IB and ID, for example, would result in annual yield estimates in excess of 

 500 ash-free dry tons/ha.yr. Actual measured yields of small, experimental cultures 

 of the two species maintained under the best possible conditions throughout the 

 year in Central Florida were respectively 63 and 75 ash-free dry tons/ha.yr(2, 3). 



To put these figures in perspective, the best commercial yields of sugar cane, the 

 world's most productive agricultural crop, are roughly 63 ash-free tons/ha.yr(14). 

 Commercial seaweed production ranges from about one ash-free ton/ha.yr from the 

 harvest of natural beds of giant kelp off California to some 25 ash-free tons/ha.yr 

 for the small kelp, Laminaria japonica, that is cultivated in Northern China{15). 



Thus Wolverton and McDonald's estimate(13) of 154 dry tons (ca. 131 ash-free dry 

 tons) of water hyacinths/ha. for a seven-month growing season in Mississippi, ob- 

 tained from separate measurements of growth rate and density, must be considered 

 as suspect. Also untenable is the projection, similarly obtained, of up to 262 ash-free 

 dry tons of giant kelp/ha.yr from the ocean energy farm that is presently in the 

 pilot-testing phase by General Electric Corp., under contract from the Gas Research 

 Institute and the Department of Energy(16). 



Such lavish estimates have tended to create an unrealistic opinion of the potential 

 role of aquatic plants as a biomass source for energy. This could prove unfortunate, 

 since many aquatic species are, in fact, comparable to the most productive terrestri- 

 al crops in their organic yields and do not need exaggerated projections to justify 

 their consideration. 



REFERENCES 



1. J. C. Goldman and J. H. Ryther, J. Environ. Eng. Div., ASCE 101, 351 (1975). The same 

 relationship has been demonstrated for these and other data concerning unicellular algae by J. 

 C. Goldman, Water Res. 13, 1 (1979). 



2. J. H. Ryther, J. A. DeBoer, B. E. Lapolnte, Proc. 9th Internat. Seaweed Symp., 9, 1 (1978). 



3. T. A. DeBusk, M. D. Hanisak, L. D. Williams, J. H. Ryther, Aquatic Botany. In press. 



4. D. J. Watson, Ann. Bot. N.S. 22, 37 (1958). 



5. J. L. Davidson, C. M. Donald, Aust. J. Agr. Res. 9, 53 (1958). 



6. H. Kasanaga, M. Monsi, Jap. J. Bot. 14, 304 (1954). 



7. Nat. Acad. Sci. Rep. Ad Hoc Panel of Adv. Comm. on Tech. Innov. "Making aquatic weeds 

 useful", 175 pp. (1976). 



