Vital connections in tropical marshlands: Using stable isotopes and fatty acids to elucidate the trophic base of production

Background/Question/Methods

Marshlands are shallow freshwater wetlands dominated by emergent macrophytes. While the large standing biomass of macrophytes and their detritus might imply that they play a major role in supporting the secondary production in marshes, algae – despite having low apparent biomass – have been increasingly recognised as a dominant base of production in freshwater wetlands. However, the relative contributions of algal and vascular-plant pathways for supporting secondary production in tropical marshes has not been examined. We studied three small freshwater marshes (range: 150–7,000 m2) in tropical Hong Kong to elucidate (1) the relative importance of the algal and vascular plant pathways in marshes, and (2) the influence of seasonality on food-web architecture. In each marsh, we collected basal resources including algae (periphyton, planktonic microalgae, benthic microalgae, filamentous green algae, and filamentous cyanobacteria) and macrophytes (up to five species of dominant C3 and C4 plants, plus mixed detritus). We also collected consumers including macroinvertebrates, fish, and tadpoles. After conducting bulk δ13C and δ15N isotope analysis, we identified likely basal resources for each taxonomic and functional group of consumers using biplots. Isotopic signatures for each basal resource and group of consumers were then compered between dry and wet seasons. Fatty-acid profiling was carried out for the likely basal resources and up to 10 species of the most abundant consumers per site. Biomarkers specific to algae and macrophyte were identified and their percentage assimilation by each consumer was calculated.

Results/Conclusions

Stable-isotope analysis showed that most consumers in all three sites relied on algae and/or C3 plants, while the contribution of C4 plants and cyanobacteria was low. At all sites, algae (except filamentous green algae) and C3 plants had overlapping δ13C and δ15N values, limiting our ability to resolve their contributions to consumers. There was no seasonality in isotopic signatures of basal resources or consumer functional groups except algae, which were consistently enriched in δ13C and δ15N during the dry season (PERMANOVA; F = 5.6, p = 0.02). Filamentous green algae at one site had a distinct isotopic signature that appeared to be linked to some consumers (larvae of Ephemeroptera, Odonata and Anura) during the dry season. We await the results of fatty-acid profiling to provide a clearer separation between the contribution of algae and vascular plants to consumer productions in these tropical marshes.