Effect of freshets on Passamaquoddy plankton 329 



water outside the Passage than in the Passage. The Amphipod Crustacean, Euthemisto 

 compressa, another immigrant from the Gulf of Maine, was present in quite small 

 numbers which for the successive days were: 0; 0; 3; 6; 2; 8; I; 2; 7; 7; 22; 26; 

 17. Its numbers definitely increased on August 21. 



Salpa fusiformis and S. zonaria were taken in the hauls on successive days in the 

 following numbers respectively: 44 and 18; 28 and 0; 34 and 0; 71 and I; 54 and 0; 

 27^ and 0; 75 and 0; 22 and 0; 45 and 2; 23 and 1 ; 181 and); 25 and 2; 44 and 0. 

 Since the more abundant species was in the aggregated stage, in which the individuals 

 are attached together in rather long strings of as many as 18 individuals, there is 

 considerable variation in its numbers, which will, therefore, not reflect very well its 

 general abundance at the time and place. However, the numbers in italic mean 

 that on August 16, 17 and 18 the individuals were in poor condition. It is to be 

 expected that such open ocean forms as these would be injured by exposure to low 

 salinity, and indeed those concentrated in the Bay were to be found in considerable 

 numbers dead on the beaches as left by ebbing tide. It may be inferred that individuals 

 that had been damaged by low salinity in the Bay were being carried out as the freshet 

 worked out through the Passage. On this basis, the outflow occurred on August 16, 

 17 and 18, and the resurgence of Salter water began on August 19. This agrees with 

 the behaviour of the Euphausiids. 



DISCUSSION 



The passage of fresh water through Passamaquoddy Bay and Head Harbour 

 Passage from the Digdeguash River to the Bay of Fundy may be expected to vary 

 in time with the forces that effect it. One of these variable forces is the hydrodynamic 

 effect of lighter water at the head of Passamaquoddy Bay than in the Bay of Fundy 

 outside. This will vary with the amount of warm and fresh, and thus relatively light, 

 water discharged by the Digdeguash River. The freshets of September 18, 1933, and 

 of August 9, 1951, would provide a strong force to drive the water through the Bay 

 and the Passage. Correlatively, the heavy outside water would be driven inwards. 

 This force would operate at the outer end of the Passage only when the fresher water 

 reached that far. 



Following the 1933 freshet, the greater force set in motion was clearly acting in the 

 Passage between 8 and 1 1 days after the rainfall, as shown after 1 1 days by high 

 salinity at the inner end and a great abundance of Calanus in the outer part. Following 

 the 1951 freshet, the fresher water was in the Passage 7 days after the rainfall, as shown 

 by drop in abundance of Calanus and Sagitta and by appearance of many Sa/pa in 

 poor condition. The greater force had clearly acted through the Passage by 10 days 

 after the rainfall, as shown by Salpa in good condition, as well as by increased numbers 

 of Euphausiids and Euthemisto. By that same time, there were sharp rises in surface 

 salinity at Mascabin Point and at St. Andrews. 



It would seem to have taken in 1951 9 days, with the main discharge one day after 

 the main rainfall, for the peak of the freshet to pass from Digdeguash Basin to the 

 outer part of Head Harbour Passage. What relation this has to the calculated average 

 flushing time of about 15 days for the Bay alone is uncertain. The latter is based upon 

 average conditions, and certainly not upon the conditions following a freshet, as the 

 authors clearly state (Ketchum and Keen, 1953). 



Whatever may be the complex conditions in Passamaquoddy Bay, it seems clear that 



