230 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



meets the eye of the casual observer. Examples 

 of such occurrences are given in the next section 

 on silica relations. Although a phytoplankton 

 tow in the very clear subtropical Gulf water will 

 usually disillusion and disappoint the investigator 

 used to northern collecting, he must be alert both 

 in observation and method not to miss or lose the 

 delicate and minute forms present. 



The first adequate quantitative studies of 

 diatom plankton productivity in the Gulf were 

 made at Tortugas by Riley (1938). His work, 

 based on Harvey's method of extraction of plank- 

 ton pigments, and on measurements of oxygen 

 changes in suspended clear and dark bottles 

 (method of Marshall and Orr), gives indices of 

 reliable value. It might be added that, espe- 

 cially with the phantom-like diatom plankton, 

 these methods are dependable, while errors and 

 uncertainties of sampling with quantitative plank- 

 ton net and of cell counts would give deceptive 

 results. 



Comparing plankton productivity at Tortugas 

 with that of Long Island Sound, Riley found 

 that the amount of total plant pigment from 

 Tortugas plankton was only one twenty-fifth that 

 of the Long Island area. On the other hand, 

 oxygen production in suspended bottles was one- 

 third to one-half as much in the former as in the 

 latter area. It would seem evident, he says, 

 "that actual productivity is much greater than 

 the standing crop would indicate." The results 

 are subject to qualification with respect to a 

 number of altering factors which are discussed in 

 Riley's report. 



The limited period covered by Riley's observa- 

 tions may or may not have been a favorable one 

 for comparison, and further investigations of this 

 character extended to other places in the Gulf 

 and other times of the year, are clearly needed. 



Another problem about which information to 

 date is both meager and vague concerns the rate 

 of turnover and renewal of the population, espe- 

 cially of the small diatom species such as members 

 of the genera Nitzschia, Synedra, Dimmerogramma, 

 Grammatophora, Amphora, Raphoneu. The esti- 

 mation of the so-called "standing crop" of phy- 

 toplankton growth is a standard and fairly satis- 

 factory practice, but an adequate evaluation of 

 the amount of transformation of organic sub- 

 stance over a specified period of time is fraught 

 with a complication due to the constant break- 



down and renewal of population elements, in 

 which the reproductive rate is rapid and the life 

 span is short. Especially is this true where the 

 plankton is very poor as it is frequently in the 

 Gulf waters. 



Knowledge of the general occurrence and suc- 

 cession of the Gulf flora is too meager to make 

 any reliable statements as to the relative impor- 

 tance of the various diatoms for there are very 

 many of the species mentioned, and perhaps 

 others, that occur at times in large numbers. 

 Just a few, however, that are definitely typical of 

 the region, though by no means restricted to it 

 might be cited: Biddulphia mohiliensis (Bail.) 

 Grun., Terpsinoe musica Elu-., and Hemidiscus 

 hardmannianus (Grev.) Mann. Further study 

 will certainly add many others that periodically 

 occur in abundance in different parts of the area. 



SILICA RELATIONSHIPS 



In the waters of tropical seas, poor in silica and 

 other nutrients, the diatoms of wide or cosmopoli- 

 tan distribution are smaller and have frailer shells 

 than are to be found in the same species from 

 waters of temperate and northern latitudes. The 

 variety of species is not less in the tropics, nor is 

 there necessarily a smaller number of individuals, 

 but the size and robustness of their cells is dimin- 

 ished. Although this observation is applicable, 

 in general, to species found both m tropical and 

 in temperate seas, certain forms may be cited as 

 notable examples, namely, Synedra undulata (trop- 

 ical form often half the length of its northern 

 counterpart), Biddulphia pulchella and B. penta- 

 criniis, Surirella reniformis, Grammatophora ma- 

 rina, Isthmia sp., and others. On the contrary, 

 some typically warmer water forms, like Raphoneis 

 surirella do not appear to grow larger, heavier 

 shells in richer waters of northerly latitudes. 



The plankton diatoms of the waters containing 

 minimum amount of dissolved silica are diapha- 

 nous, as for example, the Chaetoceros, large-celled 

 Hemidiscus and Coscinodiscus species found in the 

 Gulf plankton. These latter, and similar forms, 

 may be both large and numerous, but their bodies 

 are so watery and their shells so lightly silicified 

 that they are very transparent and are easy to 

 overlook. 



Numerous minute-celled forms such as small 

 Nitzschia, Cocconeis, Dimmerogramma, Synedra, 

 Grammatophora, and Amphora species, common in 



