Fbbruaey 16, 1900.] 



SCm'NCK 



257 



or have occurred but sparingly in other 

 years. As a rule the numbers of individ- 

 uals and their seasonal range are alike sub- 

 ject to considerable fluctuations from year 

 to year. The qualitative differences in the 

 plankton from season to season are due to 

 the adaptations of different species to dif- 

 ferent seasonal conditions, especially in the 

 matter of temperature, and the differences 

 from year to year are perhaps to be cor- 

 related with the fluctuating environment. 



4. At times of high vrater all plankton 

 stations at which examinations have been 

 made are in one continuous body of water, 

 and the plankton is quite similar through- 

 out, especially in spring floods. As the water 

 recedes and the several lakes and the river 

 emerge as distinct units of environment, 

 the planktons are very quickly differenti- 

 ated by the disappearence of certain species 

 and the multiplication of others as seasonal 

 changes progress, so' that in a few weeks 

 each locality presents its own peculiar as- 

 semblage of organisms. This phenomenon is 

 repeated from year to year and occasionally 

 several times in one year, depending on the 

 fluctuations in the river levels. The differ- 

 ent localities examined thus present at 

 times of low water great local difiterences 

 in the amount and constitution of the plank- 

 ton, corresponding to contrasts in the en- 

 vironment, especially in respect to vegeta- 

 tion, source of water and temperature. 

 The assemblages of organisms characteristic 

 of a given locality, though varying some- 

 what, present many points of similarity from 

 year to year. 



5. "Waters in which aquatic plants, such 

 as Ceratophyllum, Elodea, Naias, Potamogeton 

 and Nymphwa are abundant, are, in the lo- 

 calities examined, as a rule poor in plankton. 

 The removal of the vegetation by wind and 

 flood or by flshermen's seines is followed by 

 an increase in the amount of plankton. 

 The same lake may then be plankton-rich 

 one year and plankton-poor the next, ac- 



cording to the amount of vegetation it con- 

 tains. The phyto-plankton thus apparently 

 replaces the coarser aquatic flora. 



6. The local distribution of the plankton 

 in lakes with somewhat uniform conditions 

 of bottom, water supply, and vegetation is 

 remai'kably even, in so far as the volume of 

 the plankton is concerned, the variation 

 from the average falling within the 30 per 

 cent, observed by Apstein in the lakes of 

 northern Germany. The distribution of 

 many species is far less uniform. In the 

 river, except near the mouths of large 

 tributaries the distribution of the plank- 

 ton is even more uniform than it is in the 

 lakes, the variation in the amount of the 

 plankton from shore to shore is less than 

 10 per cent. Under even conditions the 

 amount of plankton in the current of the 

 river (J to 2h miles per hour) varies but 

 little from hour to hour and day to day — 

 being often within 10 per cent, and rarely 

 exceeding 20'per cent, at one point of obser- 

 vation at Havana. Catches made at inter- 

 vals of 10 miles in the lower 220 miles of 

 the river show a considerable uniformity in 

 amount and constitution, except where 

 modified by local conditions such as sewage 

 on tributary waters. 



7. The plankton of tributary streams is 

 relatively verj' small in amount and is com- 

 posed ofa different assemblage of organisms. 

 The immediate effect of tributary waters is 

 to dilute the plankton. The river is thus 

 a unit of environment with its own pecu- 

 liar fauna and flora. 



8. Flood waters quickly and profoundly 

 affect both the amount and the composition 

 of the plankton, diluting it and washing it 

 rapidly away toward the sea and to a slight 

 extent replacing it with the more or less 

 sessile forms torn away by the current of 

 the creeks and of the river itself. The silt 

 of flood waters is often disastrous to many 

 species of Entomostraca. The recovery in 

 the volume of the plankton after floods is 



