216 RILEY 



millipore filters with a pore size of 0.45 M, ° r even 0.2 /i, does not ordinarily 

 sterilize the sample. There are small pleomorphic forms that pass through and 

 subsequently develop into active growth stages of a larger size that can be 

 measured visually or by plate counts. The bacteria go through a growth cycle in 

 which the population increases to a maximum of the order of 30 X 10 3 cells/ml 

 in the first 2 or 3 weeks and then declines to a smaller and relatively constant 

 level of 10 X 10 3 or less. In other experiments in which a small quantity of a 

 culture of Pseudomonas sp. was added to the filtered water, there was a similar 

 growth cycle, except that the maximum and subsequent decline came more 

 quickly. 



Particulate organic carbon passed through a similar cycle. The maximum 

 averaged 0.44 mg C/liter in a series of experiments with inshore Nova Scotia 

 waters. The concentration in the later stable phase averaged 0.20 mg C. Visual 

 examination suggested that living bacteria constituted only a small fraction of 

 the total particulate matter. There were discrete particles and hazy, amorphous 

 masses of material that more or less resembled naturally occurring organic 

 aggregates except that they were more tenuous. Estimates of bacterial biomass 

 based on plate counts and observed sizes indicated that living matter probably 

 constituted 10%, more or less, of the total particulate carbon. 



A few attempts were made to measure total carbon in the filter-passing 

 fraction in order to get a complete carbon balance in the experiments. This was 

 not altogether successful because the method was not precise enough for good 

 results. However, the initial reduction in the filter-passing fraction was roughly 

 twice the increase in particulate carbon, and subsequently there was no change 

 within the limits of error of the method. This work needs to be repeated with 

 better methods now available. However, the results can be interpreted to mean 

 that (a) about half the initial decrease in the filter-passing fraction can be 

 ascribed to utilization by bacteria and the other half to conversion of 

 filter-passing materials to larger particulate matter; (b) the bacteria subsequently 

 utilize some of the particulate matter and reduce it to a lower level; and (c) in 

 the final stable phase, the remaining particulate matter is resistant to utilization 

 (the fact that further reductions in the filter-passing fraction are too small to be 

 detected by the methods used suggests that the bacteria are in a senescent 

 condition). 



The so-called stable phase was observed for periods of up to 4 months with 

 only minor variations. However, in these experiments, as in the ones involving 

 bubbling, removal of part of the particulate matter tended to induce further 

 formation, so that a steady state was maintained with moderate cropping rates 

 of the order of 7% per day. Larger rates of 20% per day could not be 

 accommodated, and the quantity of particulate carbon gradually went down 

 over a period of a month. 



Attempts were made to determine whether bacteria were necessary for the 

 aggregation process, and these were not entirely satisfactory because the drastic 

 procedures necessary to eliminate bacteria may very well introduce artifacts. 



