NO. 3 COPEPODA FROM PANAMA — MARSH 3 



would be expected from their environment which produces permanent 

 bottom stagnation. In cold climates the bottom waters of lakes have 

 a more or less complete stagnation in summer and in winter, the 

 stagnation being more complete in the smaller bodies of water in 

 which winds have no opportunity to produce bottom currents, but in 

 spring and in fall, because of the change in temperature, there is a 

 complete overturning of the water. No such change, of course, takes 

 place under the constant temperature conditions of the Canal Zone, 

 so that only the surface waters contain the oxygen which is necessary 

 for the life of the plankton organisms. This has led to a practical 

 difficulty in .the use of the waters for sanitary purposes, and it has 

 been found necessary in some cases to so arrange the outflow pipes 

 from the reservoirs that the water shall always be taken from near the 

 surface and thus avoid the foul odors of the deeper waters. While 

 the foregoing statement is true in regard to the general condition 

 of the waters of the reservoirs, it must not be understood as meaning 

 literally that the oxygen always diminishes in exact ratio to the depth. 

 Local and meteorological causes may produce some modifications of 

 the general statement. This has been discussed by Downes who gives 

 a series of charts of dissolved oxygen (Downes, 1910, 9 and 10). 

 Downes also treats of the general chemical character of the waters 

 and of their bacterial content. 



The present report deals only with the copepods, and, inasmuch 

 as the collections will be put into the hands of other specialists to 

 treat of the other organisms, it is not pertinent to this report to say 

 much of the character of the plankton as a whole until after the more 

 complete examination of the collections has been made. Attention, 

 however, may be called to the fact that the plankton of the Isthmus 

 not only lacks great variety of species, but also with few exceptions, 

 lacks great numbers of individuals. Diatoms were perhaps as abund- 

 ant as would be expected, and in some cases the number of individuals 

 was enormous. The filamentous algae were present, but not especially 

 abundant. The desmids were present in some collections in phenom- 

 enal numbers. This was noticeably true of some species of Micraste- 

 rias and Closterium. One or two of the pond collections consisted 

 largely of Micrasterias. The desmids were quite numerous in the 

 Carabali reservoir, a reservoir which according to Downes has given 

 much trouble. Downes states that Anabceua occurs in this reservoir 

 but it was not noticed at the time these collections were made. 



The Protozoa and Rotifera were present in comparatively small 

 numbers. 



