406 



MARINE DIXOFLAGELLATES AND RED WATER CONDITIONS 



which would tend to carry them down again into deeper layers. 

 Harvey ( 1917 ) mentions that Noctiluca could not be observed at the 

 surface during windy days. A re-examination of Table II will reveal 

 that most of the occurrences of red water throughout the world were 

 accompanied or preceded by periods of calm weather and smooth 

 seas. Such conditions, together with the high temperatures which 

 usually accompany them, may further stabilize the water through 

 thennal stratification, providing additional resistance to the vertical 

 mixing of the organisms. 



PREVAILING WIND 



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Fig. 3. Accumulation of floating material along shore by prevailing onshore 

 wind. 



Once the organisms have accumulated at the surface of the water, 

 there are several means by which they may become further con- 

 centrated. Three such mechanisms will be discussed briefly below. 



(1) Prevailing onshore winds: Surface water driven shoreward by 

 prevailing onshore winds establishes a circular pattern, sinking at the 

 waters edge and returning seaward at lower depths. Buoyant organ- 

 isms will accumulate in windrows along shore or at the region of 

 descent (Fig. 3). 



(2) Where brackish coastal water, particularly in the vicinity of 

 river mouths, meets open ocean water, there is a mixing and sinking 

 of the two water masses along a line of convergence. Both types of 

 water flow toward this line, and buoyant organisms will accumulate 

 at or near the convergence line, producing streaks of floating material. 

 (Fig. 4). 



(3) Convection cells: Wind-driven vertical convection- cells may 

 be established which rotate alternately clockwise and counterclockwise 



