experiments as described by Malone 

 (1977). Netplankton and nannoplank- 

 ton refer to phytoplankton popula- 

 tions that were retained and passed 

 by a 20 ym mesh screen, respectively. 



Freshwater flow of the Hudson 

 River at Green Island (250 km 

 north of Upper Bay) was provided 

 by the Water Resources Division of 

 the Geological Survey, U. S. 

 Department of Interior. Total 

 freshwater flow in the lower estu- 

 ary was calculated by applying a 

 correction for lower basin flow 

 (Hammond 1975; Deck 1980). 

 Cross-sectional areas for each 

 station were obtained from fathom- 

 eter profiles, and volumes north 

 of MP were calculated using 

 linearly varying area. In the 

 upper bay the dimensional data of 

 Quirk, Lawler, and Matusky (1970) 

 was employed. 



RESULTS AND DISCUSSION 



FRESHWATER FLOW 



During 

 water flow 



1977 and 

 at Green 

 .7 



1978, fresh- 



to below 2 

 The 25-year 

 10 m d . 



10 



,7 



Island (Q ) 



3 -1 

 m d (spring) 



m d 



ranged from 37 x 



7 3-1 

 x 10 m d (summer) . 



annual mean is 3.1 x 

 In the spring there 

 were large amplitude peaks of 

 short duration which arrived 

 earlier and were higher in 1977 

 than in 1978 (Figure 2) . Summer 

 flow was much lower and less var- 

 iable (1977 x = 1.23, sd = 0.31, 

 1978 x = 1.32 sd = 0.37). Green 



Island is located at mile point 

 (MP) 154, 136 miles upstream from 

 the northern most station. Conse- 

 quently, two corrections must be 

 applied to Q to estimate fresh- 

 water flow into the lower estuary 



(Q f ) : (1) lower basin inputs must 

 be added to Q and (2) a lag must 

 be used to account for the time 

 required for a change in Q pT to be 

 reflected in Q . Hammond (1975) 

 and Deck (1980) adjusted Q for 

 lower basin contributions to 

 freshwater flow in the estuary 

 (Q ) , but possible lag times and 

 spreading of flow peaks were not 

 accounted for. Lag time estimates 

 range from 5 to 20 days (Stewart 

 1958; Hammond 1975), though the 

 possibility of shorter times in 

 the spring has been noted (Hammond 

 1975). Since adjusted Q can change 



ul 



as much as 25 x 10 d in as lit- 

 tle as 4 d during spring (Figure 2), 

 this is the season when accurate lags 

 are most needed. 



Lag times between Q T and Q f 

 were examined through the corre- 

 lation of < S > , the average sur- 

 face salinity between MP 25 and 

 MP -7. Q pT was adjusted as above 

 and smoothed 1 by a centered moving 

 average to allow for flow peak 

 spreading during transit. The aver- 

 aging period was varied as a 

 function of the lag (Table 1). 



During high flow (Q f > 5 x 



7 3 -1 

 10 m d ) variations in < S >x 



were mainly a function of QHf 

 changes. By comparing variations 

 in Q„ T with variations in < S > 

 a lag of 1 day was found to give 

 the maximum correlation (Figure 2) . 

 The correlation was also examined 

 by fitting < S > = ( < S > ) exp 

 (b Q ) (cf. Eq* la) for lag X t?mes 

 of 11, 7, and 1 days. A one-day lag 

 time gave the best fit 7 for, D 2t.h 

 high and low (Q f 5 x 10 m d ) 

 flow conditions liable 1). Such a 

 short lag under low flow condi- 

 tions indicates the poor sensi- 

 tivity of the model during low flow 

 since this lag is shorter than the 

 minimum low flow lag of 5 d calcu- 



172 



