would account for the Chlorophyll a 

 fluxes from the upper layer (Table 

 2) . This is a reasonable sinking 

 rate for netplankton diatoms (Smayda 

 1970) , and comparable to estimates of 

 2 to 4 m d in the apex of the New 

 York Bight (Halone and Chervin 

 1979). Resuspension of phytoplankton 

 from estuarine sediments may contri- 

 bute locally to source fluxes in the 

 lower layer, but since there is no 

 net source flux from the estuary any 

 such contribution must be balanced by 

 settling out elsewhere in the lower 

 layer. 



The source flux in the upper 

 layer of the upper bay segment (MP 

 to MP -3) was unusual as the only 

 high flow upper layer source flux and 

 may have been an input from the ad- 

 jacent East River (Figure 1), which 

 is not included in the model. Since 

 significant growth did not occur 

 elsewhere in the estuary upper layer 

 and there is no evidence that growth 

 rate varied within the estuary, it is 

 unlikely that the source flux was due 

 to phytoplankton growth. Further- 

 more, if the source flux was due to 

 growth, the model fluxes imply that 

 the average doubling times would have 

 been 0.4 d (Table 2). This is short- 

 er than the phytoplankton doubling 

 times (1-2 d) reported for the lower 

 estuary during spring (Malone 1977). 

 Since direct measurements of exchange 

 with the East River (Figure 1) have 

 not been made, this question and the 

 related question of why the source 

 flux anomaly was absent during low 

 flow conditions (i.e. upper bay 

 fluxes were similar to fluxes in ad- 

 jacent boxes) cannot be resolved. 

 Clearly the details of Chlorophyll a 

 circulation in the upper bay merit 

 closer study. 



The pattern of source and 

 sink fluxes during low flow was 

 consistent with high phytoplankton 

 growth rates during the summer, in 



the upper layer and high grazing 

 rates by zooplankton in both layers. 

 The average growth rate estimated 

 from source and sink fluxes was 1.22 

 d" (Table 2). This is high but 

 comparable to a growth rate of 1.27 

 d estimated from C primary pro- 

 ductivity (Malone 1977). Estimates 

 of potential fluxes due to copepod 

 grazing were computed from July per 

 copepod grazing rates in the New York 

 Bight apex (Chervin et al. in press) 

 and copepod abundances in the estu- 

 ary. These estimates were in rough 

 agreement with lower layer sink 

 fluxes (Figure 4) . The above calcu- 

 lations assume that copepod abun- 

 dances and grazing rates are the 

 same in the upper and lower layers. 

 Any vertical variations would strong- 

 ly affect rate estimates, and this 

 recommends the separate sampling of 

 each layer in future studies of estu- 

 ary zooplankton. 



N0N- STEADY -STATE VARIATIONS 



IN CHLOROPHYLL a 



The above discussion assumes 

 that source and sink fluxes were in 

 "local" steady state, i.e. that the 

 rate of significant variation in the 

 boundary conditions and processes 

 contributing to within box fluxes is 

 slower than the response rate of the 

 lower estuary. Generally, the 

 response times were shorter than the 

 weekly sampling interval and it is 

 not known how much variation occurred 

 on time scales shorter than 7 days. 

 However, the time scales of phyto- 

 plankton blooms are generally longer 

 than the estuary response times given 

 here. Netplankton blooms in offshore 

 waters during winter-spring typically 

 last 1 to 2 weeks (Malone and Chervin 

 1979, Malone et al. in press). Nano- 

 plankton biomass at the upstream 

 upper layer was fairly constant dur- 

 ing most of the low flow period (mean 



180 



