234 Gordon A. Rilev 



measuring the latter are not accurate enough for precise estimates, but such informa- 

 tion as is available (Riley, 1952 b and 1955 c) indicates little difference between the 

 Long Island and Block Island Sounds as far as phytoplankton productivity is 

 concerned. 



To be sure, Long Island Sound maintains a slightly higher concentration of nutrients, 

 although again not necessarily a larger total stock per unit area. Two factors tend to 

 maintain this higher concentration. First, freshwater drainage contains a highly 

 variable but generally higher concentration of nitrate (but not phosphate) than Sound 

 waters. Secondly, two-layered transport exchange creates a mechanism whereby the 

 biological system automatically regulates the nutrient supply at a slightly higher level 

 than in the outside waters. During most of the spring and summer, plant growth is 

 sufficiently active to impoverish the nutrient supply in the surface layer. Part of the 

 phytoplankton produced at the surface is utilized elsewhere in the water column, as is 

 sufficiently evident from the existence of a bottom fauna. Thus the surface layer that 

 is transported out of the Sound is relatively poor in total nutrient content, and the 

 latter tends to be conserved and concentrated within the Sound as long as plant growth 

 is sufficiently active to maintain a vertical gradient in nutrients. 



During the autumn, when regeneration of nutrients exceeds utilization, very large 

 concentrations have been found toward the western end of the Sound. These tend 

 to be reduced as the season progresses by the combined effects of lateral diffusion and 

 transport exchange. It has been common in autumn and early winter to find positive 

 vertical gradients in nutrients, where rich western water overrides the inflowing bottom 

 water. It was the frequency of this rather unusual type of vertical distribution that 

 first focussed attention on the subject. 



Preliminary calculations of enrichment by transport exchange have been made, 

 using transport data mentioned earlier and the observed distribution of phosphate 

 and nitrate. The amount of enrichment in the western half of the Sound during the 

 spring-summer season was estimated to equal one-third of the amount of phosphate 

 present at the time of the midwinter maximum and one-tenth of the nitrate stock. 

 More accurate evaluation of this problem, and of the relative importance of river 

 drainage, awaits the completion of work in progress on seasonal variations in the 

 exchange rate and the distribution of total as well as inorganic nutrient stocks. 



ORGANIC PRODUCTION 

 Two one-year series of measurements have been made of the photosynthetic rate 

 of the surface phytoplankton in Long Island Sound (Riley, 1941 ; S. Conover, 1955) 

 using the light and dark bottle technique. The later work also included a few measure- 

 ments at a series of depths from surface to near bottom. Another type of analysis 

 has also been used to estimate the biological rate of change of oxygen and phosphate 

 at various levels in the water column and on the bottom (Riley, 1955 c). Having 

 determined coefficients of vertical eddy conductivity from the vertical distribution 

 and seasonal change in temperature, the coefficients were then appHed to the observed 

 distribution of oxygen and phosphate. This provided information on the rate of 

 accumulation or depletion of the element at a given depth by vertical mixing. Any 

 difference between the calculated rate and the observed change was then ascribed to 

 biological processes. The oxygen and phosphate analyses agreed within narrow limits. 

 The calculated net rate of change of oxygen in the surface layer was about 50% larger 



