methods of reproduction-- vegetative (asexual) 

 and sexual. 



By the vegetative method, Spirogyra rapidly 

 forms long strands of cells while the environ- 

 ment is most favorable. The strands grow by 

 the elongation and division of all cells in the 

 filannent. Division usually takes place at night. 

 After elongation of the cell, the nucleus 

 divides by nnitosis, and then the cell is cut in 

 two by a ring of cellulose which grows inward 

 from the outer wall of the cell between the 

 two nuclei to form two identical cells which 

 usually remain attached to each other. 



Later, when the environment has changed, 

 perhaps by the mere presence of the Spirogyra 

 in it (food consumed, wastes released) or by 

 external factors such as drought or extremes 

 of temperature, the Spirogyra reproduce sex- 

 ually. Conjugation usually takes place between 

 the cells of two vegetative strands, but it may 

 take place between adjacent cells on the same 

 strand. The contents of the cells of one strand 

 pass into corresponding cells of the other 

 strand, thus leaving the cells of one strand 

 empty, or, if the cells are adjacent in the same 

 strand, every other cell is left empty. Although 

 the contents of both participating cells are 

 morphologically alike, the fact that those of 

 one strand remain passive and those in the 

 other strand are motile indicates a slight 

 physiological differentiation of the contents of 

 the cells that at this time become sex organs 

 called gametes. The organisnn resulting from 

 their union is a zygote. The zygote forms a 

 thick three-layered wall about itseli. It is then 

 called a zygospore and is the resting stage of 

 the Spyrogyra . The zygospore is highly resist- 

 ant to extreme cold and to drought. It falls to 

 the bottom of the water on which the green 

 raft had rested a short time before. The cell 

 walls that formed the raft disintegrate and 

 also sink to the bottom. 



The zygotes germinate after an indeter- 

 minate period of rest. The time of germination 

 is entirely dependent upon environmental con- 

 ditions required for the development of the 

 vegetative and sexual reproductive cycles. The 

 length of these cycles also depends on the 

 environment. They will be longer in some 

 places and times than others, but never longer 

 than a few weeks. 



Some species of salt-water plankton also 

 flower or swarm occasionally. The most pub- 

 licized example is the "redtide, " which occurs 

 most often in subtropical marine waters, such 

 as the Gulf of Mexico and the Pacific Ocean off 

 southern California. The red tide has occurred 

 in the Gulf of Mexico at least 20 times since 

 its first recorded appearance in 1844; 17 of 

 these events were off the west coast of Florida 

 and 3 off Texas and Mexico. 



The red tide appears as a dense concentra- 

 tion of dinoflagellates of the genera Gymno- 

 dinium or Gonyaulax . These organisms meas- 

 ure about 1/1,270 to 1/726 inch. The great 



nunnbers of dinoflagellates often impart a 

 reddish amber color to the water, but shades 

 of green interlaced with yellow and brown are 

 also characteristic of the blooms. They may 

 cover hundreds of square miles, and some 

 species produce sufficient toxin to kill mil- 

 lions of fish. Death comes suddenly after the 

 fish enter the area of the bloom. Dead fish of 

 all species and sizes drift ashore, occasionally 

 to the amount of 100 pounds per linear foot of 

 shoreline, causing unpleasant odors and seri- 

 ous disposal problems for cities and resort 

 areas. The deeply colored water sometinnes 

 has an oily appearance; when dipped up and 

 allowed to stand a few minutes, it has a thick, 

 slimy feeling. 



Flowerings and swarmings of plankton are 

 spectacular events, for the clumping of vast 

 numbers of organisms (as many as 15,982,560 

 cells of the dinoflagellate, Ceratium furca , 

 were counted per quart of water off the west 

 coast of Florida in 1955) makes them visible 

 to the human eye. Between population explo- 

 sions the animals reproduce at a greatly 

 reduced rate. Even then, local plankton popu- 

 lations often are dominated by one species 

 almost to the exclusion of others, as shown 

 by the contents of collecting nets. 



SURVIVAL 



Ostracods, cladocerans, copepods, brine 

 shrimp, and many other, but not all, small 

 aquatic animals and plants survive linfavorable 

 seasons, severe droughts, and critically high 

 or low temperatures in a quiescent state known 

 as a resting spore or, in some cases, a winter 

 egg. Resting spores and winter eggs remain 

 viable for long periods, sometimes for many 

 years, and still develop a vigorous new gen- 

 eration of individuals when suitable living 

 conditions return. Because organisms that 

 make up the plankton are able to survive in 

 this manner they appear in temporary ponds 

 formed after rain, although no water may have 

 been there for months or years. This mode of 

 survival also explains why it is possible to 

 obtain many tiny organisms within a few days 

 by the simple nnethodof placing a bit of meadow 

 hay in a shallow dish of water. Windblown 

 spores and eggs that have settled upon and 

 adhered to the vegetation develop rapidly in 

 water with proper light and temperature. 



YIELD 



Rough estimates of the annual yield of phyto- 

 plankton in all the seas approximate 165 

 million tons of organically bound carbon, and 

 estimates of live weight run from 6 billion to 

 6 trillion tons. No reliable method has been 

 developed for measuring the total yield of 

 plankton. The number of organisms in the 



