GROWTH OF DUCKWEED 411 



trout (Salmo clarki clarki) occurred under a 24-hr temperature cycle 

 rather than shorter or longer cycles. He suggested that most fish 

 respond to the extremes rather than the mean of the temperature 

 cycle. Heath also summarized several studies indicating that certain 

 organisms have higher thermal tolerances under cyclic than under 

 continuous exposure to high temperatures. Such other processes as 

 larval development in the mud crab, Rhithropanopeus harrisii 

 (Costlow and Bookhout, 1971; Christiansen and Costlow, 1975), and 

 byssal thread formation by Modiolus demissus (Van Winkle, 1969) 

 apparently respond to cyclic temperatures in a mainner similar to that 

 of a constant temperature equal to the mean of the cycle. 



In addition to constant temperatures and cyclic fluctuations, 

 many organisms must contend with randomly varying temperatures 

 or with thermal regimes in which the temperature change is 

 much less predictable. This circumstance is becoming more 

 common as many industrial facilities and power plants release heated 

 water into streams and lakes. Even though effluent temperatures may 

 not exceed the tolerance levels of the organisms, the thermal regimes 

 may change frequently in unpredictable ways. 



A few studies of growth and development in aquatic animals 

 under fluctuating temperatures are available (e.g., Grainger, 1959; 

 Thorp and Wineriter, 1978) but the effects of irregularly changing 

 temperatures have received little attention. This study compares the 

 effects of three types of thermal regimes (constant, cyclic, and 

 acyclic or randomly fluctuating) on survivorship and growth of an 

 aquatic plant. 



METHODS 



A common species of duckweed, Spirodela oligorrhiza (Kurz) 

 Hegelm, was chosen for study because of its abundance in south- 

 eastern aquatic habitats, because of its ease of manipulation in 

 experimental systems (Clatworthy and Harper, 1962; Hodgson, 

 1970), and because of the large body of literature on growth, 

 metabolism, and flowering in the Lemnaceae, as summarized by 

 Hodgson (1970) and Hillman (1976). Populations of S. oligorrhiza 

 were collected from five separate locations at the U. S. Department 

 of Energy's Savannah River Plant in South Carolina. These were (1) a 

 beaver pond along a creek that had received cooling water from 

 nuclear reactor operations 9 years previously, (2) a sewage-effluent 

 pond, (3) a roadside ditch, (4) a pond receiving chemical effluent, 

 and (5) a ponded area along a natural stream. Spirodela oligorrhiza 

 plants from each of these sites were grown under similar conditions 

 in the laboratory (at a temperature range of 15 to 20° C) for 8 weeks 



