Marine plankton also naay be classified in 

 general ternns as oceanic and neritic (coast- 

 bound). Oceanic plankton contains forms that 

 normally inhabit areas of the open sea, such 

 as certain species of copepods, diatoms, and 

 peridinians that thrive best in intercontinental 

 areas (arctic, tennperate, and tropical sea 

 forms differ from each other). Neritic plankton 

 is made up of forms that remain connparatively 

 close inshore and may include: (1) the pelagic 

 larvae of the bottom dwellers such as oysters 

 and starfish; (2) pelagic eggs and larvae of 

 fish; and (3) organisms held inshore by spe- 

 cific requirements for food, temperature, 

 water chemistry, and possibly other environ- 

 mental demands related to land masses. The 

 distinction between oceanic and neritic plank- 

 ton becomes difficult when members of either 

 type are transported by wind or water cur- 

 rents from their usual habitat into that of the 

 other. 



DISTRroUTION 



The distribution of planktonic organisms is 

 almost universal in aquatic environments. They 

 are in the oceans, rivers, lakes, and ponds the 

 world over. Some cosmopolitan forms of plank- 

 ton are found at all latitudes, while other 

 forms have disconnected distributions and 

 occur in scattered discrete areas where con- 

 ditions favor their existence. No climate is 

 too warnn or too cold to support plankton of 

 one type or another. The greatest concentra- 

 tions of phytoplankters occur in the upper 

 layers of water down to the limit of effective 

 penetration by light (100 to 130 feet). Maxi- 

 mum density of zooplankters also occurs in 

 the surface strata where the herbivorous 

 species feed on the phytoplankton. Some forms, 

 prinnarily zooplankton, inhabit the bottom zones 

 of lakes and streanns, and also the profound 

 depths of the oceans. 



REPRODUCTION 



Reproduction of planktonic forms, as of 

 land creatures, provides for the nnultiplica- 

 tion of individuals to ensure preservation of 

 species. The various groups in the plankton 

 reproduce in different ways. The resulting 

 diversity of methods to accomplish this very 

 necessary biological function nnakes it pos- 

 sible to trace nature's gradual development 

 of the reproductive process from asexual 

 (without sex) to sexual. The nnost primitive 

 method is simple asexual division, in which 

 the protoplasm of one-celled plants and ani- 

 mals divides into two equal parts to form two 

 new individuals. In slightly more advanced 

 organisms an intermediate stage of sexual 

 behavior is found. Individuals that are alike in 

 size and structure reproduce only after they 



fuse and exchange naicronuclear nnaterials. In 

 general, morphologically connplex forms have 

 more highly developed sexual organs and 

 behavior. Rarely, individual animals of all 

 species may have both male and female organs 

 (hermaphroditism), and at least one group of 

 common animals, the oysters, may change 

 sex from time to time. 



SWARMS AND BLX)OMS 



Especially dense concentrations of a single 

 species of zooplankton or phytoplankton are 

 population explosions known as swarms, and 

 flowers or blooms. They occur seasonally or 

 at irregular intervals when environmental 

 conditions are exceptionally favorable for the 

 growth and reproduction of a particular spe- 

 cies. Swarming or flowering comnnunities 

 occur most often when the water is enriched 

 by runoff from agricultural land, by upwelling 

 from the bottom, by overturn of a body of 

 water due to wind and thermal changes, or by 

 the addition of small quantities of organic 

 wastes from urban communities. 



A bloom in the Vermillion River, S.Dak., 

 lasted 2 or 3 days after the spring runoff in 

 1923. It formed a thick red scum from bank to 

 bank for many miles along the river's course. 

 Under the microscope, the scum was seen to 

 be made up of Euglena, a single- celled or- 

 ganism. Some authorities believe Euglena is 

 a plant because it contains chloroplasts (green 

 bodies containing chlorophyll); others believe 

 it is an animal because its body structure and 

 habits of life are more similar to those of 

 animals than to those of plants. 



The bloon-iing or swarming of Euglena was 

 not an annual occurrence, and large crowds 

 gathered to view this unusual phenomenon. 

 Most of these people had lived near the river 

 n^any years but had never seen the red scum 

 before. A similar occurrence at some earlier 

 time probably gave the river its name. 



Ostracods (small zooplankters belonging to 

 the Crustacea) swarmed in the Pamunkey 

 River, Va.,^ in April 1940. They formed a brown 

 coating on the surface of the water and clogged 

 fine-meshed collecting nets fished near the 

 bottom. The Chief of the Pamunkey Indians, 

 who lived nearby, had never before seen such 

 a swarming. 



Blooms of filamentous algae form dense 

 green rafts on bodies of enriched fresh water 

 in stagnant pools, small lakes, and protected 

 places in larger bodies of water during the 

 summer. Nearly everyone has seen them. They 

 sonnetimes interfere with boating and swim- 

 ming at resorts, where the bloonns are a 

 nuisance. The bloonris usually last only a few 

 days, for the life span of filamentous algae is 

 short. 



Most such rafts of filamentous algae are 

 formed of Spirogyra, an alga that has two 



