Spatial and temporal patterns of nutrient concentration and export in the tidal Hudson River

Spatial and temporal dynamics of N and P were examined in the tidal Hudson River between 1992 and 1996. For all seasons and at all locations in the river nutrient concentrations were generally quite high. TN averaged 60 μM and was above 50 μM in 75% of samples. TP averaged 1.7 μM and was above 1.2 μM in 75% of samples. NO₃ was the dominant form of N (60% of TN) while PO₄ comprised about 40% of TP. Seasonal and spatial variation in most N and P components was quite low but patterns were apparent. Seasonally, forms of N (TN, NO₃ and NH₄) and PO₄ showed opposite patterns. All N components showed summertime decreases, but PO₄ increased over the summer. Spatially, along the 200 km fresh to oligohaline stretch, N and P showed similar patterns—declining from upper to mid sections of the river but subsequently increasing in most down river, oligohaline stretches. The down river increase in nutrients is likely caused by a combination of sewage inputs and salinity-related geochemical release of P. A preliminary budget of the upper to the mid section of the river (a 100 km stretch) suggests that the decline in nutrient concentration in this section is due to the net retention of almost 2,000 mT N and 200 mT P per year or about 20% of the N and P input to this section of river. The retention in tidal rivers, like the Hudson, occurs immediately above the estuary and may, therefore, be relatively more significant than retention occurring higher in the watershed.