Stable C and N Isotope Composition of Primary Producers and Consumers Along an Estuarine Salinity Gradient: Tracing Mixing Patterns and Trophic Discrimination

The mixing pattern along a summer salinity gradient in the estuary Odense Fjord was evaluated using nutrient concentrations as well as C-13 and N-15 isotope signatures of suspended and sediment organic matter, immobile macrophytes (Fucus vesiculosus and Ruppia maritima), and benthic fauna (Mya arenaria, Hediste (Nereis) diversicolor, and Arenicola marina). Trophic discrimination (C-13 and N-15) of the infaunal consumers (suspension feeders and detritivores) was assessed from the obtained mixing patterns along the estuarine gradient. Correspondence between salinity, DIC, and DIN in Odense Fjord implies conservative mixing as also evident from linear relationships between salinity and C-13 and N-15 signatures of most living organic pools. Isotope signatures of suspended organic matter (i.e., diatoms) indicate that the river to marine DIC and DIN end-members have daily/weekly C-13 and N-15 averages during summer from -10 to 0 parts per thousand and 10-12 to 0-5 parts per thousand, respectively. Stable isotope signatures of long-lived macrophytes stationary at specific locations in Odense Fjord showed C-13 levels that were about 7 parts per thousand higher than for suspended particles and 3-4 parts per thousand higher than for sediment organic matter, while no such difference was evident for N-15. The food of invertebrate consumers (M. arenaria, H. diversicolor, and A. marina) determined from the estuarine C-13 and N-15 patterns provided the first ever reported trophic discrimination of these animals. Thus, C-13 was 1.9, 1.6, and 1.3 parts per thousand and N-15 was 4.4, 5.0, and 3.5 parts per thousand for the three species, respectively. Accordingly, benthic suspension and deposit feeders in Odense Fjord are largely supported by a diet consisting of benthic and pelagic microalgae, however, with a possible slight shift in diet proportions or to other food sources in the lower reaches of the estuarine gradient.