GEOCHEMISTRY AND LOADING HISTORY OF PHOSPHATE AND SILICATE IN THE HUDSON ESTUARY

The loading history and geochemistry of soluble reactive phosphorus (SRP) and dissolved silica (DSi) are evaluated in the Hudson estuary using 16 years of axial transect data. SRP behaves atypically in the estuary. Profiles show conservative mixing between a large mid-salinity source and the freshwater and seaward end members. Order of magnitude calculations indicate that waste water treatment facilities (WWTFs) are the dominant mid-salinity SRP source. DSi profiles display behaviour more typical of other estuaries in the northeastern United States, showing conservative mixing during periods of high flow and a mid-salinity source during periods of low flow. A single layered multi-box model is used to evaluate the loading history of SRP and DSi. Shortly after the New York State phosphate detergent ban of 1972, the SRP load dropped to two-thirds of that typical of the early 1970s. Loading of SRP remained at this level until the mid-1980s when construction began at the largest point source. During the construction phase (1984-1986), SRP loading returned to the early 1970s level. Upon completion, the total load declined once again and by the end of the 1980s it reached a level approximately one-third of that existing prior to the detergent ban. Model calculations of observed DSi profiles do not show a similar time-trend. They suggest that during summer months dissolution of diatom tests is a major source of DSi; however, WWTF DSi loads also appear to be a significant source to the Hudson estuary. © 1992 Academic Press Limited.