Stable carbon and nitrogen isotope biogeochemistry in the Delaware estuary

Abstract

Seasonal variability in stable carbon (S'XZ) and nitrogen (b15N) 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 6°C and 615N 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 PM resulted in more negative 615N values for organic matter fixed by phytoplankton. As algal growth proceeded, the 615N of seston reached a maximum (+ 1 SY&) because phytoplankton were using a pool of NH,+ enriched in 15N as a result of previous fractionation during assimilation. Similarly, maximal 813C 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, across the cell membrane became increasingly rate limiting.The 613C values in bottom sediments were equivalent to those in suspended particulate matter, but a 2Y60 difference in 615N 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.Differences exist among natural abundances of stable carbon isotopes (813C) and stable nitrogen isotopes (615N) in organic matter from terrestrial and anthropogenic I Present address: Department of Oceanography, Texas A&M University, College Station 77843.

Controls on the seasonal and interannual dynamics of organic matter stable carbon isotopes in mesotrophic Lake Holzmaar, Germany

“…The simultaneously increasing demand on inorganic carbon because of increased phytoplankton growth results in a drawdown of the DIC pool that cannot be forthwith replenished by atmospheric CO 2 at the air-water interface. The decrease in the DIC pool is followed by a reduced phytoplankton discrimination against 13 C, resulting in the observed cumulative isotopic enrichment of POM (Cifuentes et al 1988;Hollander and McKenzie 1991). Because of the long stratification period, commonly lasting from the end of April to October, which is accompanied by a considerable depth expansion of the lake's epilimnion, a major amount of phytoplankton organic matter receives relatively positive d 13 C POM values (Schelske and Hodell 1991).…”

“…The stable carbon isotope composition of particulate organic matter (d 13 C POM ) has proven to be a tool to understand the biologically driven aquatic carbon cycle and to assess allochthonous carbon sources (Cifuentes et al 1988;Bernasconi et al 1997). d 13 C POM values have also been used to trace food chains and to distinguish the ratio of autochthonous and allochthonous organic matter sources from anthropogenic nutrient loadings of lake ecosystems caused by agriculture, settlement, and lake management (Hollander 1989;Ostrom et al 1998;Herczeg et al 2001).…”

Biofilm responses to marine fish farm wastes

“…In ecological studies, stable isotope analyses have emerged as reliable tools for elucidating the trophic niche and inferring pathways of energy flow in food webs (Cifuentes et al, 1988). This method involves the comparison of stable isotope ratios between consumers and food supplies (Deegan and Garritt, 1997).…”

Isotopic evidence for the contemporary origin of high-molecular weight organic matter in oceanic environments

“…The complete -'34Th data set will be published elsewhere Santschi et al, 1995 ). A ~C, which was measured by Accelerator Mass Spectrometry / Trumbore, 1994), and 6 L3C analysis (Cifuentes et al, 1988) were made on COM or POM subsets (i.e.. 10-30 mg of freeze dried material) of the collected water samples. One sigma errors in A ~4C are given explicitly in Table 1.…”