Spatial variation in the [formula omitted] signature of Ruppia megacarpa (Mason) in coastal lagoons of southwestern Australia and its implication for isotopic studies

Abstract

Ruppia megacarpa was collected from three coastal lagoons in southwestern Australia: Nornalup Inlet, Peel Inlet and Oyster Harbour. In each case, plants were sampled from four sites and δ 13 C values were determined for leaves, rhizomes and roots of replicate plants to assess the variability between lagoons, between sites in lagoons and between plant parts at sites. The δ 13 C values of tissue ranged from −6.6 to −17.8‰ with a mean of −11.15‰. In Nornalup Inlet, there were significant differences in the δ 13 C values of different plant parts, but there was also a significant interaction between plant part and site. In Peel Inlet significant differences were found between plant parts and between sites. Oyster Harbour displayed no significant differences in δ 13 C values between any of these factors. The δ 13 C values of R. megacarpa were highly variable, with only one other marine angiosperm having a higher recorded range in values. Significant differences in δ 13 C values occurred between sites within lagoons and between plant parts, and the pattern of spatial variability was different in each lagoon. The spatial differences within lagoons appear to relate to differences in water quality resulting from proximity to river and ocean inputs. Differences between plant parts may be related to the storage of biochemical products with distinct isotope ratios. Trophic studies, therefore, need to be conducted in a way that explicitly identifies the spatial scales that capture the variability in δ 13 C values relevant to the aims of the study.