4.43 yg 1 , 1 sd 1.35 yg 1 ) when 

 growth rates are high and fairly 

 constant (Malone 1977). 



A few exceptions to this overall 

 pattern may have occurred, but their 

 effect on means calculated over high 

 and low flow periods should be small. 

 Not included in above nannoplankton 

 biomass mean are two occasions when 

 Chlorophyll a exceeded 10 yg 1 . At 

 these times the day to day vari- 

 ability of Chlorophyll a was high 

 (i.e. > factor of 2) probably as a 

 result of downstream advection of a 

 "patch" of Chlorophyll a from the 

 upper estuary (Malone et al. 1980). 

 During low flow variations also oc- 

 curred at the lower layer boundary at 

 MP-7 (mean Chlorophyll a 8.06 yg l" , 

 sd 6.24 yg 1 ). These variations 

 were probably related to inputs from 

 Raritan Bay (O'Reilly et al. 1976), 

 but it is not known how these inputs 

 change with time. 



CONCLUSIONS AND RECOMMENDATIONS 



MODELING APPROACH 



The two-dimensional box model 

 based on suitably averaged and cor- 

 rected salinity data is a valuable 

 tool for studying phytoplankton 

 distributions in estuaries. In addi- 

 tion to describing the essential 

 features of two-layered estuarine 

 circulation and interactions between 

 estuarine and coastal waters, the 

 model provides independent estimates 

 of fluxes due to estuarine circu- 

 lation and fluxes due to other pro- 

 cesses. Flux component estimates for 

 the lower Hudson Estuary compared 

 reasonably with the results of other 

 analysis. The seasonal variation in 

 Chlorophyll a circulation was con- 

 sistent with previous observations 

 on the direction of Chlorophyll a 

 transports into and within the estu- 

 ary (Malone 1977, Malone et al. 

 1980). More importantly, rates of 



phytoplankton growth, sinking, and 

 grazing inferred from source and sink 

 fluxes agreed with rates calculated 

 from experimental data. Hopefully 

 the present success of the two- 

 dimensional box model will motivate 

 its application to other, similar 

 situations . 



FRESHWATER FLOW EFFECTS 



Using the two-dimensional box 

 model we can see the results of 

 natural variations in freshwater flow 

 on circulation and response times in 

 the lower estuary. The estuarine 

 transport and increases in response 

 time are small compared to changes in 

 Q f . This result probably applies to 

 other partially mixed estuaries of 

 fairly constant cross-sectional area, 

 such as the James River (Pritchard 

 1967). However, modifications to the 

 model to study the effect of very low 

 Q f s will not be valid in general, 

 since the Q at which the estuary 

 would no longer be partially mixed 

 and the two-dimensional box model 

 would not apply cannot be determined 

 by such an empirical approach. Inde- 

 pendent estimates of salt transport 

 at MP -7 showed a similar stabilizing 

 tendency, in which estuarine circu- 

 lation is maintained by increasing 

 influx of salt as freshwater flow 

 increased (Hunkins submitted) . 



In light of this stability the 

 direct effect of changes in fresh- 

 water flow on phytoplankton biomass 

 in the lower estuary was small com- 

 pared to the effects of seasonal 

 variations in rates of phytoplankton 

 growth and grazing within the estuary 

 and the development of netplankton 

 blooms in adjacent coastal water 

 (Malone 1977, Malone et al. 1980, 

 Malone and Chervin 1979). 



During both flow regimes the 



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