EFFECTS OF FRESHWATER FLOW ON SALINITY 



AND PHYTOPLANKTON BIOMASS IN THE LOWER 



HUDSON ESTUARY 



Patrick J. Neale, Thomas C. Malone 

 and David C. Boardman 



Lamont-Doherty Geological Observatory, Palisades, New York 



ABSTRACT 



INTRODUCTION 



A two dimensional box model 

 was used to describe variation of 

 phytoplankton biomass in the lower 

 Hudson Estuary under different 

 flow conditions. Both the flux 

 contribution of estuarine circu- 

 lation and other gain or loss pro- 

 cesses are quantified by the 

 model. Lag between gauging data 

 and freshwater flow in the estu- 

 ary, the vertical structure of 

 current velocity and salinity, and 

 the tidal variation of salinity in 

 each layer were considered in the 

 estimation of model parameters. 

 Results indicate that estuarine 

 circulation was strong and flush- 

 ing times relatively short under 

 both high and low flow conditions. 

 Biomass fluxes in terms of chloro- 

 phyll a were dominated by bound- 

 ary inputs during high flow and by 

 growth and grazing during low 

 flow. The good agreement found 

 between independent measurements 

 of specific rates and specific 

 rates estimated from the model 

 indicates that the model gives a 

 reasonable description of estua- 

 rine phytoplankton processes, and 

 may be applicable in other 

 estuaries . 



Phytoplankton biomass variations 

 in estuaries are primarily the result 

 of material fluxes due to estuarine 

 circulation and fluxes due to biolo- 

 gical and particulate processes (e.g. 

 growth, sinking, and grazing). Thus 

 fresh water effects on estuarine 

 circulation can be translated to 

 effects on phytoplankton biomass if 

 the relative contribution of circu- 

 lation-related fluxes is known. In 

 this paper we approach the problem by 

 using models of circulation to make 

 estimates of each flux component dur- 

 ing different freshwater flow condi- 

 tions . 



Simple, first order models have 

 been used successfully to study pol- 

 lutant (Pritchard 1969) and nutrient 

 (Simpson and Hammond unpublished; 

 Taft et al. 1978) distributions in 

 partially mixed estuaries. These 

 models do not include explicitly the 

 dynamics of circulation or details of 

 diffusion processes, but lump all 

 effects into a small number of para- 

 meters. A one-dimensional descrip- 

 tion of the tidally averaged concen- 

 tration of some property C is given 

 by the advection-dif fusion equation: 



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