Sulfide intrusion of seagrass Thalassia hemprichii along a eutrophication gradient with carbonate and terrigenous sediments in tropical coastal sea

Abstract

Seagrasses growing in different eutrophic states in carbonate and terrigenous sediments may exhibit contrasting sulfide intrusion and responses; however, limited information is available. In this study, sulfide intrusion in the tropical typical seagrass Thalassia hemprichii along a eutrophication gradient in carbonate and terrigenous sediments on Hainan Island, South China Sea, was investigated using combined elements, stable isotopes, and photobiology. The sediment porewater sulfide concentration increased with rising nutrient levels, with porewater sulfide as 223.92±25.34 μmol/L when the dissolved inorganic nitrogen concentration was 10.83±0.60 μmol/L and the dissolved inorganic phosphate concentration was 0.39±0.01 μmol/L. The nutrient input significantly enhanced sulfide intrusion in seagrass, resulting in reduced δ³⁴S values in roots from 12.78±1.16 to 2.69±0.46‰, with leaf δ¹⁵N as the greatest explanatory factor. In addition, sulfide intrusion inhibited photosynthesis more strongly in seagrass growing in carbonate sediments than in terrigenous sediments because of the low iron content in carbonate sediments (almost 50% of the iron content in terrigenous sediments), reducing rETR_max and E_k by 43.08% and 36.42%, respectively. Therefore, the synergistic effects of nutrient input, sulfide concentration, sediment substrate, and iron content affected the sulfide intrusion in seagrass.