372 RYTHER [CHAP. 17 



off the coast of West Greenland (62°-67°N). Hansen (1959) found high rates of 

 production over the Faeroe-Iceland Ridge (0.65-2.70 g carbon/m 2 /day), 

 between the East Greenland Polar Current and the Irminger Current (0.55 g 

 carbon/m 2 /day) and over the Reykjanes Ridge (0.53-1.30 g carbon/m 2 /day). 

 By contrast, the productivity of the surrounding waters (in summer) generally 

 ranged between about 0.15 and 0.25 g carbon/m 2 /day. Thus a significant part 

 of the entire region's fertility would appear to stem from numerous local areas 

 of upwelling and hydrodynamic mixing. 



8. Plankton Production in Coastal and Inshore Waters 



When the depth of the wind-mixed layer exceeds that of the euphotic zone 

 beyond a certain point (Sverdrup's "critical depth"), the phytoplankton does 

 not receive sufficient light to grow. The situation obtains in temperate and sub- 

 polar seas during most of the winter, as we have seen. In such cases, it is usually 

 the onset of thermal stability, together with increasing radiation, which 

 provides conditions suitable for the initiation of the spring flowering. 



However, mixing may be confined within the critical depth by a factor other 

 than thermal stratification, the depth of the ocean bottom. In shallow water, 

 the plants may receive enough light to grow throughout the year. In somewhat 

 deeper water, the low radiation in mid-winter may prevent growth, but in- 

 creasing sunlight in the early spring may be sufficient to stimulate plant 

 production even though the waters are still mixed to the bottom. 



Gran (1932) noted that phytoplankton blooms first appeared in the shallow 

 coastal waters of Norway (60°N) in early March, a week or two later at the edge 

 of the continental shelf and several weeks later still further offshore. Hart (1942) 

 made similar observations in the Antarctic. 



Riley (1957) has expressed the critical depth concept in quantitative terms 

 as the average light intensity (/) within the mixed layer. 



t /o(l-e-fcz) 

 kz 



where Iq is the incident radiation and k the extinction coefficient. Table III 

 from Riley shows the mean radiation in the water column for varying depths, 

 times of year and transparency, as calculated from the above expression. He 

 concludes that a mean radiation of 0.03 g cal/cm 2 /min is critical for plant 

 production. Thus reference to Table III will show the depths of water, or 

 mixing, where initation of the spring flowering could be expected at different 

 times of year. 



In the shallow coastal waters off New York and New Jersey (50 m or less) we 

 have observed dense phytoplankton populations and high rates of production 

 throughout the winter (Fig. 7). The relationships between productivity, 

 season and water depth is more clearly shown in Fig. 12 from Ryther and 

 Yentsch (1958). The data here are mean values for separate groups of shallow, 

 intermediate and deep stations. They were obtained from chlorophyll and light 



