Stable carbon and nitrogen isotope biogeochemistry in the Delaware estuary

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Abstract
Seasonal variability in stable carbon (δ13C) and nitrogen (δ15N) isotope ratios was observed in suspended particulate matter of the Delaware estuary. Two major pools of organic matter were found in the estuary—phytoplankton growing in situ and a mixture of planktonic and terrestrial detritus. In general, the δ13C and δ15N of suspended particulate matter reflected planktonic dominance. With the background chemical and physical information available for the estuary, it is evident that biogeochemical processes influence isotopic distributions in the estuary to a greater extent than does physical mixing. During spring, we postulate that isotopic fractionation of ammonium assimilated at concentrations >20 µM resulted in more negative δ15N values for organic matter fixed by phytoplankton. As algal growth proceeded, the δ15N of seston reached a maximum (+ 18‰) because phytoplankton were using a pool of NH4+ enriched in 15N as a result of previous fractionation during assimilation. Similarly, maximal δ13C values were related to high rates of primary productivity associated with algal growth. Decreased isotopic fractionation occurred at high rates of production, implying that diffusion of CO2 across the cell membrane became increasingly rate limiting.The δ13C values in bottom sediments were equivalent to those in suspended particulate matter, but a 2‰ difference in δ1 5N was found between suspended and bottom sediments. With nitrogen isotopic differences between water‐column seston and surficial sediments, we estimate that 15– 30% of planktonic production is deposited in the sediments during spring. If this organic matter is remineralized in late summer and fall, it could support up to 20% of primary production at that time.