High Sulfide Intrusion in Five Temperate Seagrasses Growing Under Contrasting Sediment Conditions

Abstract

Five temperate seagrasses (Amphibolis antartica, Halophila ovalis, Posidonia australis, Posidonia sinuosa and Zostera nigricaulis) were surveyed along the south-west coast of Western Australia. These morphological different seagrasses grow in contrasting sediments with large variations in sedimentary organic matter, carbonate and iron contents. We tested if sulfur composition in the plants responded to sulfur dynamics in the sediments and if plant morphology affected the sulfur composition of the plants. The sediments were characterized by low sulfate reduction rates (<9 mmol m−2day−1), low concentrations of dissolved sulfides in the pore waters (<74 μM) and low burial of sulfides (total reducible sulfur <0.8 mol m−2) in the sediments. However, all seagrasses showed high intrusion in the below-ground parts with up to 84 % of the sulfur derived from sedimentary sulfides. There were no direct links between sulfur in the plants and sulfur dynamics in the sediments, probably due to low iron contents in the sediments limiting the buffering capacity of the sediments and exposing the plants to sulfides despite low rates of production and low pools of sulfides. The intrusion was linked between plant compartments (roots, rhizomes and leaves) for the two small species (H. ovalis and Z. nigricaulis), whereas the intrusion into the leaves was limited for the larger species (P. australis and P. sinuosa) and for A. antarctica, where extensive rhizomes and roots and the long stem for A. antarctica separate the leaves from the sediment compartment. Elevated intrusion was observed at two study locations, where natural deposition of organic matter or nutrient enrichment may be contributing factors to enhanced sulfide pressure.