Spatial analysis of carbon isotopes reveals seagrass contribution to fishery food web

Rod M Connolly,Nathan J Waltham

doi:10.1890/es14-00243.1

Rod M Connolly, Nathan J Waltham

Open Access

https://doi.org/10.1890/es14-00243.1

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Abstract

Despite the widespread use of carbon stable isotopes to distinguish among potential energy pathways in food webs, their usefulness is limited where potential basal carbon sources are numerous and diverse. We measured carbon isotope values of the major fisheries species, the mostly carnivorous Scylla serrata (giant mud crab), and potential basal, autotrophic sources supporting the food web. Conventional mixing modelling of autotroph and crab isotope data could not differentiate contributions from different sources. Pooling of modelled contributions from sources with similar isotope values indicated a role for organic matter from seagrass meadows or saltmarshes, but still did not define contributions well. Crab isotope data from a subsequent, spatially explicit survey of 14 sites, selected to represent different distances from key habitats, were analyzed using multiple regression. Crab isotope values showed a significant relationship with distance from seagrass (R2 = 0.87), but not with distance from mangroves or saltmarsh grass. Alongside seagrass meadows, crabs had very enriched isotope values, demonstrating their reliance on sources with enriched isotope values (seagrass and algae epiphytic on seagrass, 65–90% of their energy intake). At the site furthest from seagrass (21 km), crabs assimilated carbon primarily from depleted sources such as mangroves and terrestrial organic matter from coastal catchments (70–85%). Explicit spatial analysis of isotope data following a comprehensive survey revealed energy pathways not evident in conventional analyses.

Highlights

Elucidation of energy pathways in food webs remains a central component of ecology, important in its own right for understanding ecosystem function, and an essential element underpinning disciplines such as habitat conservation and restoration (Howe and Simenstad 2015), animal migration (Hobson et al 2012), biotic uptake of contaminants (Jardine et al 2012) and resilience research (Rooney et al 2006).Stable isotope analysis, of carbon, continues to provide a useful avenue for tracing energy pathways
Baseline surveys Carbon isotope values of the six autotrophs were well spread in both surveys, with mangroves and saltmarsh succulents having the most depleted d13C values, seagrass, epiphytes and saltmarsh grass the most enriched, and MBP intermediate (Table 1)
The remaining carbon was assimilated from MPB and depleted sources

Summary

Introduction

Elucidation of energy pathways in food webs remains a central component of ecology, important in its own right for understanding ecosystem function, and an essential element underpinning disciplines such as habitat conservation and restoration (Howe and Simenstad 2015), animal migration (Hobson et al 2012), biotic uptake of contaminants (Jardine et al 2012) and resilience research (Rooney et al 2006).Stable isotope analysis, of carbon, continues to provide a useful avenue for tracing energy pathways. Carbon isotopes can be used to determine the ultimate autotrophic sources of nutrition supporting food webs (see reviews by Layman et al 2012, Ramos and Gonzales-Solıs 2012). The usefulness of carbon isotopes, is limited where the numbers and variety of potential basal sources for a food web are large. This situation arises frequently enough to have been one of the key drivers behind the rapid development and proliferation of mathematical mixing models to analyze isotope data (Phillips and Gregg 2003, Moore and Semmens 2008, Parnell et al 2010). Even the most sophisticated models struggle to distinguish unequivocally among potential pathways where food webs have numerous sources, often with overlapping isotope signatures (Fry 2013)

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