The U.S. Environmental Protection Agency (1974) has developed a marine algal 

 assay procedure designed to evaluate the effects of nutrients on the growth of marine 

 phytoplankton. This is a batch test in which spikes of a suspected limiting nutrient 

 are added to the pure algal culture and the total biomass produced after a fixed 

 period of time, generally 2 weeks, is determined as a function of the added nutrient. 

 The total biomass produced by cultures of Dunaliella teriiolecla in an artificial 

 seawater medium was shown to be directly proportional to the added concentrations 

 of phosphorus or of nitrogen when the basic medium was prepared without these 

 elements. The tests were performed at various salinities, and it was found that there 

 was no significant effect of salinity in the range of 16 to 35 %o. 



The assay test was also applied to estuarine waters at high and low tides with inter- 

 esting results. Phosphorus was consistently the limiting nutrient when the samples 

 were taken from the estuary at low tide, whereas nitrogen was limiting in samples 

 collected at high tide. The salinity showed the expected changes with the stage of the 

 tide, but the relationship of the production of the organic matter with salinity was 

 not significant. The nitrogen content of the water samples at low tide was consist- 

 ently higher than that at high tide, whereas the phosphorus content showed little 

 change. This confirms the generalization that phosphorus is commonly the limiting 

 nutrient in the freshwater that predominates in these estuaries in the low tide sam- 

 ples, whereas nitrogen is commonly limiting in the marine waters that dominated at 

 the time of high tide. 



A change in the concentration of a limiting nutrient could affect the production of 

 organic matter by phytoplankton either by changing the rate of growth or by 

 changing the total biomass that can be supported. Goldman ( 1 979) states that marine 

 phytoplankton have such a high affinity for nutrients such as nitrogen and phos- 

 phorus that these chemicals are frequently below detectable levels over virtually the 

 entire growth rate spectrum. It is impossible to evaluate the effect of an essential ele- 

 ment on the rate of growth in a batch test because the algae are continuously ex- 

 posed to a changing concentration as they assimilate the element and grow. The 

 effect of a limiting nutrient on the growth rate can be evaluated in a chemostat cul- 

 ture in which a continuous supply of fresh medium is provided. The growth rate in 

 such a culture is proportional to the rate of dilution at a steady-state cell concentra- 

 tion. It seems unlikely that the rate of growth of phytoplankton in the natural envi- 

 ronment, particularly in estuaries and coastal waters, is limited by the availability 

 of nutrients, but the biomass that can be produced is certainly proportional to the 

 amount of the limiting nutrient available for the production. 



Composition of Phytoplankton 



The fact that the normal Red field N:P atomic ratio of 15:1 is consistently found in 

 deep ocean waters and in the ratio of change of these elements in surface waters does 

 not imply that the composition of phytoplankton cells is immutable. Under labora- 

 tory conditions, the N: P ratio of the cell can be varied as a function of the composi- 

 tion of the culture medium and other growth conditions. The observations of 

 Ketchum (1939) that cells deficient in phosphorus are produced when grown under 

 phosphorus limitations of growth have been repeatedly confirmed (Goldman et al., 

 1979; Perry. 1976; Fuhs et al., 1972). Goldman et al. ( 1979) conclude that the typical 

 Redfield ratio of C:N:P equals 106:15:1 is approached in culture only at high growth 

 rates relative to the potential maximum growth rate. This implies that, under natural 

 conditions, the phytoplankton that show this typical ratio are growing at or near 

 their maximum rates, even though the environmental concentrations of nitrogen and 

 phosphorus are low as is the phytoplankton biomass at any given time. They discuss 

 the dynamic conditions that make a maximum growth rate possible under these con- 

 ditions. 



72 



