Variability and transport of suspended sediment, particulate and dissolved organic carbon in the tidal freshwater Hudson River

Abstract

Abstract. Measurements of suspended matter, particulate organic carbon and dissolved organic carbon were made over a three year period at stations spanning -150 km of the tidal freshwater Hudson River. Suspended matter concentrations varied from year-to-year and were not related to freshwater discharge. The increase in suspended matter with depth in vertical profiles suggests that, during medium to low flow conditions, resuspension of bottom sediments was as important a source of sediment as loadings from tributaries. Particulate organic carbon showed significant variability among stations, and both autochthonous primary production and detrital organic matter are contributing to POC standing stocks. Dissolved organic carbon represented over half of the total organic carbon in the water column and showed little variation among stations.Examining downstream changes in transport showed that there was significant production of both suspended matter and POC within the study reach during the ice-free season. Tributary loadings within the study reach do not appear to be the cause of these increases in downstream transport. Dissolved organic carbon behaved conservatively in that there was no evidence for net production or net consumption within the river.The spatial/temporal patterns and analyses of transport suggest that suspended matter and POC, but not DOC, were controlled to a significant extent by processes occurring within the river and were not simply related to loadings from outside.

Bacterial metabolism of organic carbon in the tidal freshwater Hudson Estuary

“…The average bacterial production at the Kingston station on dates where respiration estimates were made was 77 pg C 1-' d-' (2 51, n = 7) which is slightly below the long-term mean of ca 100 pg C I-' d-l (Findlay et al 1991a). Heterotrophic respiration averaged 270 pg C I-' d -' (f 217, n = ?).…”

“…Previously published (Findlay et al 1991a) and subsequent data on DOC concentrations a t the Kingston station show either an increase in DOC over the ice-free season or no significant change in concentration. Total DOC concentrations in longitudinal transects over a 150 km stretch of the Hudson behave conservatively, showing neither accrual nor depletion (Findlay et al 1991a). Given the high bacterial demand, but absence of change in standing stock, it appears that sources of DOC within the river or from local tributaries compensate for the continuous bacterial mineralization of DOC.…”

“…As derived above, our largest est~mate of DOC assimilation is roughly 300 pg C 1-' d -' whereas the lowest estimate is about 140 pg C 1-' d-l. Annual average DOC concentration near Kingston is 3 to 4 m g C I-' with some interannual variability (Findlay et al 1991a). Therefore, the most rapid turnover of DOC would be on the order of 12 d and the slowest turnover would be ca 25 d. During summer flow conditions, water travels only 46 km in 25 d, implying that bacterial assimilation is large relative to downstream transport.…”

“…These amounts were calculated to be equivalent to natural concentrations of particulate organic carbon (1 to 3 mg C 1-l; see Findlay et al 1991a). T. angustifolia litter was collected from the surface of Tivoli North Bay in late summer of 1989.…”

“…This extraction procedure is arbitrary and was intended to examine potential DOC availability. Aliquots of this solution were added to unfiltered Hudson River water so as to roughly double the ambient concentration of DOC (ambient concentrations were 2 to 4 mg C I-'; see Findlay et al 1991a). TdR incorporation and bacterial cell numbers were measured at time zero, 17, 23, 46 and 70 h. Experiments were conducted monthly from May to October 1991.…”