In 1930, this station was sar.pled twice in each of the months 

 of July, August and September. The changes in abundance were somewhat 

 erratic, as one might expect from the location of the station, but the 

 counts of the three leading groups were consistently high, with the 

 exception of diatoms on July 3. Curing the period of three months, 

 diatoms were most abundant in September, at the time of the autumn 

 maximum in the Island Section. Presumably there was a spring ma;d.ir.um 

 also, but sampling was begun too late to show it. The abundance in 

 autumn was well above the average abundance in the Island Section in 

 1930 at the same time. The green algae were extremely abundant on all 

 of the six dates; the lowest count was nearly six times as high as the 

 maximum for the Island Section. Moreover, the greens were more abun- 

 dant than the blue-greens on four of the six dates. The blue-greens 

 became abundant later than the greens but outnumbered them in late 

 August and late September. Both greens and blue-greens were most abun- 

 dant in mid-August, rather than in September, as in the Island Section. 

 Both of these groups x;ere more abundant than diatoms on every date. 

 The algae other than diatoms, greens, and blue-greens were absent on 

 several dates, but were rather abundant on others. 



The explanation for the much higher counts of 1930 as com- 

 pared with those of 1929 probably is Dound up, in part, with current 

 reversals in the river. Because of the usual rarity of plankton in 

 rivers, one would expect to find few algae at Station 250 after the 

 current had been out of the river for a long time, and many algae after 

 a long period of inflow. Unfortunately current direction is known 

 only for the time of sampling, so it is not possible to determine 

 definitely whether such a relation exists in the present case. However, 

 on June 26 and July 17, 1929> the current was out when the samples were 

 taken, and the total counts were lower than on August 3, when there was 

 no current, and much lower than on September 7, when the current was 

 upstream. The record for I930 shows outgoing current on August 28 and 

 September 9. The fact that high counts were recorded on those dates 

 would not be surprising if it were known that the current haa just 

 begun to flow out after a long period of inflow. The remaining four 

 samples were tak'-»n when the current was flowing into the riv£r, or 

 when there was no current. The relatively low counts in July probably 

 are to be explained on the basis of seasonal change. 



Granting that direction of current was partly responsible for 

 the apparent difference in abundance of algae in the two years, it 

 seems probable that there was an actual difference. This is suggested, 

 first, by the superiority in numbers of the September, 1930, samples 

 over that of September, 1929, even though the observations on current 

 would inilicate the reverse relationship; and second, by the rather con- 

 sistently larger counts at 3 Nations 252 and 25U in 1930 as compared with 



185 



