EPPLEY: PHYTOPLANKTON AND TEMPERATURE 



of Glooschenko and Curl (1971). These authors, 

 and Thomas (1969, 1970a), found no enhance- 

 ment in waters in upwelhng regions, but assim- 

 ilation numbers were increased in response to 

 nutrient additions in oligotrophic subtropical 

 water. Malone (1971a, b, c) found assimilation 

 numbers in eutrophic waters to be nearly an 

 order of magnitude greater than those in oligo- 

 trophic surface waters of the subtropical and 

 tropical Pacific. 



It has so far proved difficult to sort out the 

 effects on assimilation number of low light and 

 low temperature in seasonal studies of natural 

 waters. Phytoplankton cultures grown with 

 either low light or low temperature show low 

 maximum photosynthetic rates per chlorophyll 

 a at light saturation (Pmax) and low saturating 

 intensity (h) for photosynthesis (Tailing, 1957; 

 Steemann Nielsen and Hansen, 1959, 1961; 

 Ichimura, 1960; Yentsch and Lee, 1966). Thus 

 some of the effects on assimilation number usu- 

 ally attributed to low light levels may, in cold 

 waters, result also from low temperature. Bunt 

 and Lee (1970) were able to sort out the two 

 variables in their study of diatom growth under 

 the ice in Antarctica by comparing a station with 

 clear ice to one with snow cover. Photosyn- 

 thetic rate and growth rate were considered to 

 be light -limited at the station with snow cover 

 but temperature-limited at the clear ice station 

 (see also Saijo and Sakamoto, 1964, for photo- 

 synthesis vs. depth curves in ice-free and ice- 

 covered lakes). 



Assimilation numbers in Antarctic waters are 

 low. Many values are less than 1.0 per hour 

 (Mandelli and Burkholder, 1966; Home, Fogg, 

 and Eagle, 1969; Bunt and Lee, 1970). Saijo 

 and Kawashima (1964) found an average value 

 of 1.2 mg C/mg Chi. a/hr which they attributed 

 to low temperatures and to a deep mixed layer 

 (resulting in a low average irradiance seen by 

 a cell) . Water temperature in these studies was 

 usually in the range — 2° to 1°C. El-Sayed and 

 Mandelli (1965) gave a range of 1.0 to 3.6 for 

 the assimilation number in surface samples over 

 a temperature range — 1.75° to 6.0°C. Assim- 

 ilation numbers of 4-5 were found in Drake 

 Passage and Bransfield Strait where water tem- 

 perature was usually about 1°C (El-Sayed, 



Mandelli, and Sugimura, 1964). All these val- 

 ues are compatible with assimilation numbers 

 predicted by Figure 9. 



Besides shifts in carbon/chlorophyll a ratios 

 with temperature and the effects of nutrient lim- 

 itation and light on assimilation number there 

 is yet another factor which tends to obscure the 

 expected seasonal changes in assimilation num- 

 ber with temperature. This comes about as a 

 result of the variation in growth rate and assim- 

 ilation number with cell size. By passing a water 

 sample through netting one can conveniently 

 separate the phytoplankton into two size cate- 

 gories: the larger cells and diatom chains re- 

 tained by the net (the netplankton) and the 

 smaller cells and chains which pass through the 

 net (the nanoplankton). Malone (1971a, b, c) 

 has recently compared assimilation numbers of 

 the two size fractions and cites earlier studies. 

 Invariably, the nanoplankton showed higher 

 assimilation numbers than the netplankton, as 

 would be expected from laboratory studies (cited 

 earlier) which show a regular diminution in 

 growth rate with increasing cell size. He fur- 

 ther showed that netplankton are relatively more 

 abundant during upwelling in coastal waters off 

 California (Malone, 1971b). Chain-forming di- 

 atoms seem to be characteristic of the rich waters 

 of temperate regions during the spring bloom. 

 Yentsch and Ryther (1959) have shown a pro- 

 gressive increase in the ratio nanoplankton/net- 

 plankton with seasonally increasing temperature 

 off New England. Tropical, warm, oligotrophic 

 waters have been shown repeatedly to contain 

 a high proportion of nanoplankton (see refer- 

 ences cited by Malone and by Sutcliffe et al., 

 1970). 



The causes of such seasonal succession of phy- 

 toplankton species is one of the significant 

 problems in the study of marine phytoplankton. 

 One can only speculate on possible contributing 

 factors. For example, the high (relative) sink- 

 ing rates of large-celled species and long diatom 

 chains suggest that suspension and buoyancy are 

 more significant problems for large cells than 

 small (Munk and Riley, 1952; Smayda, 1970). 

 Hence stratification, reduced mixing, and the im- 

 position of a seasonal thermocline would tend to 

 discourage large forms. Perhaps the most ele- 



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