Using variation in the chemical and stable isotopic composition of Zostera noltii to assess nutrient dynamics in a temperate seagrass meadow

The influence of seasonality in growth and benthic organic matter remineralization on the chemical and isotopic composition of the seagrass Zostera noltii was investigated from March to November over the course of two years in a temperate meadow in North Wales, UK. The carbon (Corg) and nitrogen (Norg) concentrations in new leaf tissue ranged from 25 to 35mmolCg−1 and 2 to 5mmolNg−1. Their stable isotopic composition ranged from −11.0‰ to −6.9‰ (δ13Cleaf) and +3.3‰ to +7.5‰ (δ15Nleaf), while the stable isotopic composition of sulphur in the new leaf (δ34Sleaf) ranged from −3.0‰ to +6.4‰. The young seagrass tissues had lowest Norg, highest C:N, most depleted δ13Cleaf, and most enriched δ15Nleaf at the standing biomass maximum (approximately 150–200gdry weightm−2) in the summer, reflecting the temporal imbalance between inorganic nutrient supply and plant demand imposed by seasonality in the growth rate. The most depleted δ34Sleaf was recorded in the same season. The isotopic composition of the seagrass tissues reflected that of the external inorganic source. The δ13Cleaf correlated (r2∼0.4) with the δ13C of total dissolved inorganic carbon (DIC) in the surface waters (δ13CDIC range: −0.4‰ to +1.2‰).The apparent carbon isotope enrichment factor of new leaf relative to the bulk seawater DIC (εseagrass-DIC range: −11.2 to −8.1‰) indicated reliance on direct HCO3- uptake, especially early in the growing season (spring). The δ15Nleaf reflected the δ15N of pore water ammonium (δ15NNH4+ range: +6‰ to +10‰; average: +7.4±0.8‰) as the growing season progressed into the summer, indicating utilization of benthic remineralized nitrogen with some degree of limitation in the uptake step despite ample supply (benthic net nitrogen remineralization rate: 8–14mmolNm−2d−1) relative to conservative, biomass-based estimates of plant demand (1.5–4.0mmolNm−2d−1) during the growing season. The δ34Sleaf suggested influence on the seagrass plants of the benthic sulphur cycling under anoxic conditions.