17 



liiver with a view to (letermiiiiiig the diurnal tluctuatioii in the 

 amount oi' the dissolved gases. In all these operations parallel 

 collections of the plankton have been made at the same time 

 and- from water collected in the same manner, the plankton 

 pump being used for the collection both of the plankton and 

 the water for gas analysis. The twenty-four-hour series col- 

 lected in 1898 can be brought into correlation with the move- 

 ments of the water-bloom, w'hich is a marked feature of the 

 plankton of the river during the warmer months of the year. 

 About one hundred bottles are comprised in the collections 

 l»elonging in this series. 



The serial plankton work rests upon the supposition that a 

 single catch made in a typical locality of a lake or river will 

 give a fair sample of the microscopic life of the water, both as 

 to its quantity and constituent organisms. With a view to 

 testing the validity of this supposition, Thompson's, Quiver, and 

 Mantanzas lakes and the river have in previous years been sub- 

 ject to extended examinations, collections being made on the 

 same date at a considerable number of localities at regular 

 intervals throughout the body of water examined. In 1897 

 Thompson's Lake was reexamined on this plan and a biological 

 cross-section of the river was made at Havana. This series of 

 collections has been increased by twenty-five bottles during the 

 period covered by this report. 



The tests looking toward the detection and correction of 

 sources of error in the plankton method and the justification of 

 changes in it which we have made, have been continued during 

 the past two years. Of the collections made in these tests about 

 one hundred and fifty have been preserved. Tests have been 

 made of the errors resulting from leakage, from the progressive 

 clogging of the drawn net and the consequent variable coefficient, 

 and from the active escape of the larger organisms of the plank- 

 ton. Several types of funnels for the in- take of the plankton 

 pump have also been devised and tested. Tests of the leakage 

 through the silk and efforts to correct it by some form of micro- 

 filter or precipitation method have also been continued. A 

 variety of filters have been examined, including the Sedgwick- 

 Eafter sand filter, using sharp Berkshire sand according to the 



