186 THE OCEAN RIVER 



Here is where turbulence, rising and falling waters, and the 

 conflict of cold and warm currents play their part in the econ- 

 omy of the sea, by replenishing the vanishing plant food. If the 

 waters remained calm and motionless, the upper layers would 

 be drained of their vital chemicals and life in the oceans would 

 cease. Instead, replenishment of the surface fertilizer comes in 

 many different ways, and the efficiency with which it is brought 

 about has much to do with the yield and location of our com- 

 mercial fisheries. The rivers, draining water from the land, also 

 bring with them fertilizer leached out from the soil, and thus 

 add continually to the fertility of coastal waters. This is one of 

 the reasons why our fisheries are found in the shallow waters 

 along the submerged edges of the continents, out as far as the 

 edge of the slope where the sea floor runs from 100 fathoms or 

 so to the deeps. But there are other reasons. 



Plants cannot live without sunlight, and thus those in the 

 plankton are able to grow rapidly only in depths of less than a 

 hundred feet or so, depending of course on the amount of sun- 

 light and the clearness of the water. It is therefore easy to see 

 that only where the sea floor does not exceed this depth are the 

 chemical foods released by decay at the bottom still available 

 to plankton. In such shallow water there is no great danger of 

 losing fertilizer, for it is brought back into circulation by plank- 

 ton as fast as the dead bodies release it. Even in somewhat 

 deeper water, though the plant life cannot grow rapidly in the 

 lower layers, there are often mixing processes taking place that 

 bring fertilizer back to the surface. When and where this hap- 

 pens we may usually expect to find a good plankton growth 

 and therefore a good crop of fish. 



In waters of moderate depth, as in the English Channel, the 

 mixing of rich bottom water and depleted surface water is by 

 no means continuous. There are sea seasons for growth just as 

 there are on land, but for different reasons. The painstaking 

 sampling of sea water, the making of chemical analyses, and 



