382 JOHNSON AND BRINTON [CHAP. 18 



prevailing currents, these animals are less used as indicators than the plankton. 

 Therefore, we shall confine our remarks to the latter category. It should be 

 mentioned that certain phytoplankton organisms have also been used as 

 indicators, especially the photosynthetic dinoflagellates. However, the short 

 individual life of unicellular autotrophic organisms and their spontaneous 

 reproductive response to local environmental changes makes their use more 

 complicated, especially as regards indicators of water currents. 



A. Zooplankton Indicators 



In considering the application of organisms in this category, it is necessary 

 to distinguish first between the "permanent" plankton (holoplankton) and the 

 "temporary" plankton (meroplankton), for each division has its own uses and 

 limitations. The permanent plankton consists of organisms that are planktonic 

 throughout their entire life, and includes especially such abundant forms as the 

 chaetognaths, pteropods, euphausiids and most of the copepods. The relatively 

 long life of many of these forms contributes to their value, particularly as 

 indicators of water movements. In addition to precise identification of the 

 species, a knowledge of the life history and information on the breeding season 

 are of great importance in order to ascertain the area in which the animals find 

 optimal or tolerable conditions for reproduction. This is the "home area", 

 which is either a slowly moving water-mass or a section of a continuing, or 

 more or less well-developed, current system. This system may be one in which 

 there is a semi -closed circulation, with some incoming flow balanced by an 

 outward flow at some other point or depth. Thus, depending upon the tolerance 

 of the organisms being swept into the system but not endemic to it, or of 

 endemic forms swept out with outward flow, zooplankton can serve as an 

 indicator of the exchange of water (Bigelow, 1926). Usually these expatriates, 

 if carried far out of their endemic breeding area, are found only as adults or 

 submature animals, whereas in the "home areas" all stages of development may 

 be found during the reproductive season. 



Although no analyses have been made to determine the relative distance 

 expatriates may be carried out of their endemic reproductive area before 

 succumbing to changing conditions, it is reasonable to believe that the border 

 of the endemic area may lie nearer the extreme range of distribution when the 

 transport outward is into gradually increasing temperatures than it would be 

 under cooling conditions; for it is well known that, in general, the optimum 

 temperature lies closer to the lethal high temperature than to the lethal low 

 temperature. The cooler temperatures tend to retard the vital processes but, if 

 not extreme, lead to an extension of the animal's life. 



The temporary plankton is made up of floating larval stages of the benthic 

 or nectonic (swimming) animals. Most benthic and nectonic animals pass 

 through a drifting planktonic stage. The duration of this stage does, how- 

 ever, differ greatly in different species, and only larvae of relatively long- 

 floating existence can be useful as indicators of water movements. 



Not much study has been made of the tolerance of the temporary plankton 



