Passive Transport 311 



Passive Transport 



A second, and entirely different, mode of origin of groups of the 

 same species results from the passive transport provided by the 

 medium. Wind often sweeps mosquitoes and other insects from ex- 

 posed terrain and thus indirectly concentrates them in sheltered places. 

 Most commonly this mechanical action of the medium in bringing 

 about aggregations is seen in the aquatic enviionment. Inhabitants 

 of streams tend to be swept together in eddies, and the currents of 

 lakes and oceanic areas often concentrate phytoplankton and zoo- 

 plankton in the same way, but more slowly. The current system in 

 the Gulf of Maine, for example, plays a major role in the accumula- 

 tion of certain types of plankton in the water mass overlying Georges 

 Bank. The clockwise circulation of water around the margin and 

 the relatively quiet eddy of homogeneous water over the center of the 

 Bank have produced conditions in the central area that favor the de- 

 velopment of Sagitta and of the copepod Pseudocalanus but are un- 

 favorable for Calanus, a closely related copepod. The current sys- 

 tem tends to concentrate the two former species in the central part 

 of the Bank and to exclude immigrants from populations of Calanus 

 and other species developing beyond the margins of the Bank ( Clarke, 

 Pierce, and Bumpus, 1943). 



Another example of the concentration of animals by passive trans- 

 port is the occurrence of "plankton traps" found particularly in Scandi- 

 navian fjords but also to a lesser extent in other estuaries. Charac- 

 teristically, a considerable amount of fresh water enters from rivers 

 at the head of the fjord, and this flows out at the surface over the 

 more saline water beneath. Water from offshore moves in at the 

 bottom, gradually rises at the head of the fjord, and mixes with the 

 outward flowing water. Plankton organisms that tend to remain in 

 deeper water layers because they are positively geotactic or negatively 

 phototactic are drawn into the fjord by the deep circulation but are 

 not carried out again by the surface flow. The result is that a me- 

 chanical concentration of the plankton takes place in the lower layers 

 of the fjord. In any species in which tactic responses cause the young 

 stages to move toward the surface, a horizontal separation of the age 

 groups results from this current action. The transport of the larvae 

 of sessile invertebrates to the upper portions of tidal estuaries, as de- 

 scribed in Chapter 2, is another manifestation of the concentrating 

 action of this type of differential water movement. 



