COMPARATIVE RICHNESS OF ZOÖPLANKTON IN COASTAL AND OFFSHORE AREAS OF THE ATLANTIC

¹ From the Biological Laboratories, Harvard University, and the Woods Hole Oceanographic Institution

1. The abundance of the zoöplankton in the coastal water, the slope water, and the Sargasso Sea at stations between Montauk Point, N. Y., and Bermuda was investigated over a period of a year and a half by means of shallow and deep hauls made chiefly with 75 cm. scrim nets. Analysis of the plankton by volume measurement and by enumeration revealed that wide variations both in the quantity and in the composition of the hauls occurred within short periods of time and within relatively short distances.

2. A noticeable diurnal migration of the crustacean plankton was detected in the coastal water and particularly in the slope water, although the vertical distribution of the plankton as a whole was not significantly affected by diurnal movement. When all observations are combined, the surface layers do not appear to have been generally richer than the deeper strata except in the case of the Sargasso Sea station.

3. An unexpectedly great seasonal difference in the abundance of the plankton was revealed, the volume of the catches in the warm half of the year being as much as 20 times or 40 times greater than in the winter in the coastal area and as much as 10 times greater in the slope water area, but no significant seasonal change was indicated in the Sargasso region.

4. When the shallow and deep hauls throughout the year are combined, the investigation as a whole shows that the average volume of the plankton was about 4 times greater in the coastal area than in the slope water area and that the latter was about 4 times greater than at the Sargasso Sea station. Differences in the same direction but of smaller magnitude were found for the crustacean plankton.

5. Individual study of Calanus finmarchicus and of Centropages typicus showed that the abundance of these copepods, which were confined to the coastal area, fluctuated widely, the former being most numerous in spring and early summer and the latter in late summer and autumn.

6. The actual richness of the plankton in the coastal area as measured by the plankton sampler fell within the approximate range observed by earlier investigators, but all indications point to the period of the present study as one of relative scarcity. In addition, the proportion of the crustacea in the hauls was unusually low (less than half the total plankton in 6 out of 10 cruises) and the number of salpae was unusually high. The superiority in richness of the coastal area over the offshore areas probably is generally even greater than that found in the present case.