NO. 9 MARINE INVERTEBRATES, ALASKA — MacGINITIE I05 



diatoms overlapped by a cyclical abundance of larval forms and cope- 

 pods. This does not show up very pronouncedly in the notes, for the 

 investigation of plankton was always secondary to that of the larger 

 animals and the invertebrate fauna of the bottom. The 1950 winter 

 plankton station was 1.6 miles from shore where the water was 80 

 feet deep. During January, February, and March the diatoms ob- 

 tained must of necessity have been ones that were living under the 

 ice or had been carried in by currents from the open leads at least 4 

 miles farther offshore. There were very few during these three 

 months, but in April they began to increase in numbers. Whether this 

 was due to an increase in light under the ice as the sun rose higher 

 in the sky is not known. When the ice went out in July the cyclical 

 phenomenon began. Great abundance followed by decreasing num- 

 bers appeared to take place at about 30- to 40-day intervals during the 

 open season and lengthened to 40 or 50 days in the fall. But several 

 years of sampling would be necessary to establish the certainty of the 

 cycle and the interval. Within one to two weeks following the very 

 abundant flowering of the diatoms, the zooplankton became much 

 more abundant. 



There has always been some difference of opinion among ocean- 

 ographers as to the reason for the diatom cycles in all regions where 

 oceanographic plankton investigations have been carried on. Some 

 maintain that these cycles are caused by a fluctuation in the supply of 

 nutrients — particularly silicates, phosphates, and nitrates — needed by 

 the diatoms; when these nutrients are exhausted the diatoms decrease 

 in number and when the ocean renews them, the diatoms flower again. 

 Others believe that the cycles are due to the increase of zooplankton, 

 made possible by the presence of diatoms, and that the feeding of 

 these hordes depletes the diatom pastures, and the lack of sufficient 

 food in turn depletes the zooplankton. The writer is inclined to think 

 that both factors have their effects ; certainly the feeding cycle is a 

 known phenomenon, and there is convincing evidence that it occurs 

 in the Arctic as well as in other oceans. 



In the lists below particular attention is given to the young stages 

 of such animals as jellyfishes and ctenophores, to copepods, to the 

 larval forms of other groups, and especially to the winter plankton 

 tows. 



Copepods, nauplii, and chaetognaths were the most abundant of 

 the smaller planktonic animals. Any one of these or any combination 

 of these might be so abundant in the material from a plankton tow 

 that it was difficult to sort out the other material. In using the terms 

 "abundant," "very abundant," and "common," the comparison is made 



