Review of the oceanography of Long Island Sound 233 



102,000 per m^ in 1952 and 62,000 in 1953. A rapid decline followed, and the species 

 disappeared from the water in early August. A. tonsa appeared in June and increased 

 to 82,000 per m^ in August, 1952 and 46,000 in 1953. There was a more or less steady 

 decrease during the autumn and early winter, a slight rise in early spring, and then 

 virtual disappearance from the water. Thus A. clausi was dominant in winter and 

 spring and was replaced by A. tonsa in summer and autumn, but both species persisted 

 through almost the entire seasonal temperature range. 



The data did not completely rule out the possibility that seasonal succession is 

 controlled by direct, lethal temperature effects, but the form of the seasonal cycle 

 suggested the more likely possibility of a temperature controlled inter-species com- 

 petition. During the winter, when A. clausi was replacing A. tonsa. the latter was 

 found mainly in the bottom waters. In laboratory experiments at low temperature, 

 it was sluggish, but mortality was not excessive. The possibility of temperature- 

 controlled competition was explored through the medium of grazing and respiratory 

 experiments. The grazing experiments determined the quantity of water that each 

 species could filter in a given length of time, and this, combined with the observed 

 quantity of phytoplankton in the Sound, provided an estimate of total food intake. 

 Respiratory experiments in turn determined how much of the food intake was required 

 to satisfy immediate metabolic needs. In A. tonsa these two quantities were about 

 equal during most of the winter. Only at the time of the flowering did the intake 

 appear to be substantially larger than the requirement. A. clausi, on the other hand, 

 was able to feed at a materially higher level than its rate of metabolic loss throughout 

 the winter and hence was a favoured competitor for food at this season. 



In summer A. tonsa was found in greatest abundance near the surface and A. clausi 

 in the slightly cooler bottom layer, so that there was no close and immediate compe- 

 tition for food. Despite the preference for low temperature, A. clausi also remained 

 a more effective feeder in summer than A. tonsa. Thus the summer succession involved 

 a different mechanism, and one which has not been worked out in detail. But since 

 adult A. clausi remained in the Sound for some time after immature stages disappeared, 

 Conover concluded that the replacement was due either to failure of the animals to 

 reproduce at high temperatures or to competition among the juveniles. 



HORIZONTAL DISTRIBUTION OF PLANKTON AND NUTRIENTS 



Maximum concentrations of both phytoplankton and zooplankton are ordinarily 

 found in the western end of the Sound and in the shallow waters along the north 

 shore. Here the average concentration of phytoplankton is about three times as large 

 as in the eastern end of the Sound or in Block Island Sound and ten times the con- 

 centration in the outer coastal waters. Horizontal gradients in zooplankton and 

 nutrients are similar but not so extreme. 



Maximum concentrations are found in very shallow water, and the differences in 

 the total population underlying a unit area of surface are not so marked. Ratios 

 of the total populations in the Long Island and Block Island Sounds average about 

 1-5:1 for phytoplankton and 1:1 for zooplankton. 



It has often been stated that inshore waters are very fertile and that this is primarily 

 due to enrichment by freshwater drainage. However, it is apparent that there are no 

 large regional differences in the total plankton population in southern New England 

 waters, nor is there evidence of much difference in rates of production. Methods of 



