Elemental and stable isotope composition of Zostera noltii (Horneman) leaves during the early phases of decay in a temperate mesotidal lagoon

The aim of this study was to assess the changes in the elemental and stable isotope composition of Zostera noltii leaves during the early phases of decay. The litter biomass, the C, N and S elemental contents, the C:N ratios and the δ¹³C, δ¹⁵N and δ³⁴S values were monitored in a litterbag experiment for 60 days at Ria Formosa lagoon, southern Portugal in two different field conditions, permanently submerged in a salt marsh pond and at high intertidal. The elemental and stable isotope composition of both attached leaves and leaf wrack of Z. noltii was also determined. The overall decay rate of Z. noltii leaf material throughout the whole experimental period was k = 0.016 ± 0.001 d^?1. Even though the biomass loss of the Z. noltii leaves after 30 days was about 25% higher within the pond than at the high intertidal, at the end of the experiment there were no significant differences between sites. The decay of the C and N content was of the same magnitude and thus the C:N molar ratio did not vary significantly in the experiment. The elemental content of the leaf wrack was in the range of the element content of leaf litter after 30–60 days of incubation. Contrary to other vascular plants rich in lignin, which degrades slowly, and thus forces the decrease of the δ¹³C values during decay, the δ¹³C values of Z. noltii leaves did not vary significantly over time or between incubation sites. The δ¹⁵N values of leaves did not increase as has been reported for vascular plants with high C:N ratio. This suggests that during the early phase of the Z. noltii decomposition the community of decomposers relied mostly on the endogenous nitrogen of the plants, not needing to immobilize exogenous N. Stable isotope studies to assess the contribution of this species to the food web can rely on the natural δ¹³C and δ¹⁵N abundances of living tissues without any correction to account for decomposition. The analysis of the δ³⁴S values of Z. noltii detritus was not useful probably due to the contamination of the samples with pyrite, which has a depleted δ³⁴S signal.