How large a difference is there with the less depleted 13C data?

Dear Maria

You should consider the possibility that the mussels are taking up dissolved organic carbon. Some DOC will be from terrestrial sources, but algae also contribute to the DOC pool. A couple of papers below give more details.

Best wishes

Simon

AUTORADIOGRAPHIC STUDIES ON THE UPTAKE OF DISSOLVED AMINO-ACIDS FROM SEA-WATER BY BIVALVE LARVAE

By: MANAHAN, DT; CRISP, DJ

JOURNAL OF THE MARINE BIOLOGICAL ASSOCIATION OF THE UNITED KINGDOM Volume: 63 Issue: 3 Pages: 673-682 Published: 1983

Nutrient Uptake by Marine Invertebrates: Cloning and Functional Analysis of Amino Acid Transporter Genes in Developing Sea Urchins (Strongylocentrotus purpuratus)

By:Meyer, E (Meyer, Eli)[ 1 ] ; Manahan, DT (Manahan, Donal T.)[ 1 ]

 BIOLOGICAL BULLETIN

Volume: 217

Issue: 1

Pages: 6-24

Published: AUG 2009

That sounds a good guess to me but you'll need data on resource variability (especially phytoplankton) to confirm it. Is the 13C depletion accompanied by 15N enrichment or depletion relative to your full marine sites? That could be a good next step to amount evidence for either more terrestrial or pelagic diets e.g.

Article Trophic typology of coastal ecosystems based on δ13C and δ15...

We sampled at two pure marine sites and their C isotopic ratios coincide with previous works on filter-feeder at 100 m depth of the same area. The other sites are under the inlfuence of small estuaries and we found 13C, 2 y 5‰ depleted compared with reference, at two of them. From these differences and the pattern found in the marine sites, we deduce (without measurement on marine or freshwater end-members) that a part of mussels DOC and POM consumption is from terrestrial origin. Do you think this is right? Or is there other food source which could change C isotope ratios of these intertidal organisms?

N isotopes do not indicate clear changes.

Thank you very much for your answers. 

Depending on the estuary, other carbon sources besides allochthanous terrestrial input may include phytoplankton (already mentioned), in situ C3 and C4 macrophytes, benthic microalgae, and seagrasses and their epiphytes.  The N signature may not differ greatly among these plants, but the carbon may change substantially depending on location in the estuary.  Here's an example (attached).  We have many more!

May be phytoplankton are good sources, whose growth and biomass depend on nutrients.