SECT. 2] PKODUCTIVITY, DEFINITION AND MEASUREMENT 159 



primary production was found to vary greatly depending on whether the 

 current came from the north, carrying saline Kattegat water, or from the south, 

 carrying brackish (ordinarily rather unproductive) Baltic water. 



Rodhe (1958) has presented a series of daily measurements from a fixed 

 station in the Swedish Lake Erken. The station was situated at the border 

 between the main part of the lake and a much more productive bay of the lake. 

 The variations from day to day were often very considerable depending on the 

 direction of the wind. Jonasson and Mathiesen (1959), on the other hand, have 

 published similar daily measurements from the Danish Lake Esrom So. They 

 showed that the rate of production was nearly identical throughout the lake. 

 Accordingly, the variations from day to day on a fixed station were found to 

 be negligible. 



In certain parts of the tropics the conditions for primary production must 

 be fairly constant throughout the year. In other parts the monsoon shifts affect 

 the conditions for primary production to a great extent. Even upwelling may 

 take place during part of the year. In the big anticyclonic eddies, situated in all 

 oceans between the latitudes of about 10°-35°, winter mixing does not reach 

 the depths where the nutrient-rich water is found. Under these conditions 

 primary production proceeds at a low, rather steady, rate throughout the year 

 (see Steemann Nielsen and Aabye Jensen, 1957; Menzel and Ryther, 1959). 

 In the western part of the North Atlantic eddy (the Sargasso Sea) the northern 

 boundary is situated at about 32°N. North of this boundary the winter mixing 

 reaches down into the underlying, more nutrient-rich water-masses and thus 

 induces the formation of a considerable winter maximum in the primary 

 production (see Menzel and Ryther, 1959). 



7. The Influence of Weather Conditions 



When making in situ or simulated in situ measurements of primary produc- 

 tion, it is necessary to take into account the daily variations in the light energy 

 penetrating the surface of the sea. Ordinarily, however, the influence of such 

 variation is relatively small. 



In the case of an overcast, or partly overcast, sky the radiation received is 

 reduced compared with that on days when the sky is clear. According to daily 

 continuous light measurements over a period of two years in Copenhagen 

 (Romose, 1940) the average radiation measured during May- August was 67% 

 of the average radiation measured on the four brightest days — one from each 

 month. About the same average has been found in Washington for a whole 

 year (Kimball and Hand, 1936). In the course of the measurements in Copen- 

 hagen it was found that 75% of the days during May-August received a solar 

 radiation higher than 50% of that measured during the brightest day of the 

 month, and that 94% of the days received more than 33% of that on the 

 brightest day. 



Fig. 20 presents a schematic curve for a 100 m-deep photic layer in a tropical 

 ocean. The rate of photosynthesis is shown as a function of the depth on a 



