Carbon and Nitrogen Isotopes in Suspended Particles and Colloids, Chesapeake and San Francisco Estuaries, U.S.A.

Chesapeake and San Francisco Bays, U.S.A., are both river dominated, temperate estuaries. The organic carbon and nitrogen isotopic compositions of the suspended particles ( 0·4 m), however, show major differences for nitrogen and minor differences for carbon. In northern San Francisco Bay, the carbon isotope values averaged 26·2 0·2‰ 13C for suspended particles, and for Chesapeake Bay and the Potomac River, the average was 24·3 3·2‰. The nitrogen isotope ratios ( 15N) of suspended particles in northern San Francisco Bay in late summer were +0·9 0·4‰, probably reflecting a nitrogen component from agricultural runoff. The values for Chesapeake Bay, and its subestuary, the Potomac River averaged +7·7 3·1‰, with the highest values occurring during summer when the primary source of nitrogen originated from remineralized organic material. Carbon and nitrogen isotope values for colloids ( 0·4 m) were 8·2 1·7 for nitrogen and 26·0 1·6 for carbon (n=17) throughout both estuaries and the Potomac river. Ultrafiltrates, collected after filtration and ultrafiltration, had 15N values of +7·3 0·3 and 13C values of 24·5 1·7. The similarity of isotopic values between suspended particles and colloids in winter samples suggested that these colloids were formed by desorption or dissociation from resuspended sediments and soils. Summer colloids in San Francisco Bay were uniformly heavier by 7‰ than suspended particles suggesting that the lighter isotope was selectively utilized by heterotrophs, leaving an isotopically heavy colloid residual.