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

摘要

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 (C-org) and nitrogen (N-org) concentrations in new leaf tissue ranged from 25 to 35 mmol C g(-1) and 2 to 5 mmol N g(-1). Their stable isotopic composition ranged from -11.0 parts per thousand to -6.9 parts per thousand (delta C-13(leaf)) and +3.3 parts per thousand to +7.57 parts per thousand (delta N-15(leaf)), while the stable isotopic composition of sulphur in the new leaf (delta S-34(leaf)) ranged from -3.0 parts per thousand to +6.4 parts per thousand. The young seagrass tissues had lowest N-org, highest C:N, most depleted delta C-13(leaf), and most enriched delta N-15(leaf) at the standing biomass maximum (approximately 150-200 g dry weight m(-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 delta S-34(leaf) was recorded in the same season. The isotopic composition of the seagrass tissues reflected that of the external inorganic source. The delta C-13(leaf) correlated (r(2) similar to 0.4) with the delta C-13 of total dissolved inorganic carbon (DIC) in the surface waters (delta C-13(DIC) range: -0.4 parts per thousand to +1.2 parts per thousand). The apparent carbon isotope enrichment factor of new leaf relative to the bulk seawater DIC (epsilon(seagrass-DIC) range: -11.2 to -8.1 parts per thousand) indicated reliance on direct HCO3- uptake, especially early in the growing season (spring). The delta N-15(leaf) reflected the delta N-15 of pore water ammonium (delta N-15(NH4)+ range: +6 parts per thousand to +10 parts per thousand; average: +7.4 +/- 0.8 parts per thousand) 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: 8-14 mmol N m(-2) d(-1)) relative to conservative, biomass-based estimates of plant demand (1.5-4.0 mmol N m(-2) d(-1)) during the growing season. The delta S-34(leaf) suggested influence on the seagrass plants of the benthic sulphur cycling under anoxic conditions. (c) 2006 Elsevier Ltd. All rights reserved.