46 



[The following paper written by Dr. Ryther was submitted for 

 the record:] 



Growth and Yield of Aquatic Plants 



Farmers and ecologists are familiar with the concept of plant productivity or 

 yield — amount of material produced per unit of area and time. Short-term yields are 

 usually expressed as g/m^.day, seasonal or annual crops as metric tons/hectare.year 

 (or the more familiar, to many, British units of short tons/acre.year, which are 0.36 

 times the metric units). Ecologists interested in the comparative productivity of 

 different kinds of plant species or communities usually express yields in dry weight, 

 often in ash-free dry weight, which is to say the strictly organic fraction of the plant 

 production. 



Other botonists concerned with physiological processes of organisms think in 

 terms of the specific growth rate of plants. This may be exressed as g increase/g.day 

 or, more often, percent increase/day or doubling time in days. 



Growth and yield are, of course, closely related, yield being the product of growth 

 rate and plant density. But the relationship is not constant because growth rate is 

 itself a variable function of density. This is illustrated for four quite different kinds 

 of aquatic plants in Fig. 1. Original data were obtained for the marine diatoms, 

 from Goldman and Ryther(l), for the red seaweed Gracilaria, from Ryther et al.(2), 

 and for the two freshwater macrophytes, from DeBusk et al.(3). 



The diatoms were grown in Woods Hole, MA in 2000 1 (2.3 m diameter 0.5 m 

 deep) continuous cultures of seawater enriched with 2° treated sewage effluent. In 

 steady state, growth rate was considered equivalent to dilution rate, density was 

 measured as particulate organic carbon and doubled to give total ash-free dry 

 weight, and yield calculated as the product of density and dilution rate. The diatoms 

 were essentially monocultures of Phaeodactylum tricornutum, Amphipora sp. and 

 Amphora sp. which succeeded each other as dominants during the course of the 

 experiment. 



The Gracilaria was grown in Fort Pierce, FL in 50-1 outdoor cultures in which 

 the plants were suspended by aeration and through which enriched seawater was 

 circulated at an exchange rate of 20 volumes/day. The culture was removed from 

 the water, drained and weighed at weekly intervals. Ash-free dry weight was 

 considered at 5.0 percent of wet weight. Density is expressed as the mean of the 

 starting and final weight for each one week interval. 



The freshwater macrophytes duckweek (Lemna minor) and water hyacinth (Eich- 

 hornia crassipes) were also grown in Fort Pierce, FL in 25,000 1 (30 m^ area, 0.5 m 

 deep) PVC-lined earthen ponds through which enriched well water was exchanged 

 at one volume/day. 



The water hyacinths were held in Vexar-mesh cages ranging in size from one to 

 2.3 m^. At intervals of one week the cages with the contained plants were lifted 

 from the water, allowed to drain, and weighed. Duckweed was grown loose in the 

 pond. Each week, the plants were netted from the water, drained and weighed. Ash- 

 free dry weights of Lemna and Eichhornia were considered to be 9.0 percent and 

 4.25 percent of wet weight respectively. 



Yields of the three macrophytes are expressed as the mean daily increase in ash- 

 free dry weight/m^ for each weekly interval and mean growth rate for that interval 

 obtained by dividing yield by mean density. 



All of the above studies were carried out over a period of four to six months 

 during late spring, summer and early fall. Growth rates and yields were at or near 

 their annual maxima and are not typical or representative of average conditions 

 throughout the year. However, the relationships between the three variables were 

 subsequently found to be the same in winter as in summer. 



In every case, growth rate decreased with increasing plant density. Yield, the 

 product of the two, was greatest at an intermediate density. The reason for the 

 decline in growth rate is not clear. It is tempting to invoke detrimental effects of 

 overcrowding-self-shading, nutrient limitation, accumulation of metabolites, etc. — 

 but the fact that the effect occurred equally at low as well as high densities makes 

 such an explanation unconvincing. One can say only that the phenomenon appears 

 to be a general characteristic of aquatic plants, possibly of all plants. The photosyn- 

 thetic portions of both duckweed and water hyacinth are, after all, air-borne the 

 same as terrestrial plants. Watson(4) obtained a very similar relationship between 

 the yield of kale and its leaf-area index, a unit that is proportional to total plant 

 density. Optimal yield was obtained at an intermediate leaf-area index above and 

 below which it declined rapidly. Davidson and Donald(5) described a similar rela- 



