Stable carbon and nitrogen isotope biogeochemistry in the Delaware estuary

Seasonal variability in stable carbon ( δ 13 C) and nitrogen ( δ 15 N) 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 δ 13 C and δ 15 N 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 µ M resulted in more negative δ 15 N values for organic matter fixed by phytoplankton. As algal growth proceeded, the δ 1 5 N of seston reached a maximum ( + 18 ‰) because phytoplankton were using a pool of NH 4 + enriched in 15 N as a result of previous fractionation during assimilation. Similarly, maximal δ 13 C 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 CO 2 across the cell membrane became increasingly rate limiting. The δ 13 C values in bottom sediments were equivalent to those in suspended particulate matter, but a 2‰ difference in δ 1 5 N 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.