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Role of pristine and remobilised tephra in coastal manganese cycles and its impact on coral reef physiology

Frank Förster 1, Christine Ferrier-Pagès2, Erouscilla Joseph3, Tom E. Sheldrake1

  • Affiliations: 1Geovolco Team, Department of Earth Sciences, University of Geneva, Genève, Switzerland; 2Ecophysiology Team, Centre Scientifique de Monaco, Monaco, Monaco; 3UWI Seismic Research Centre, St. Augustine, Trinidad and Tobago, West Indies 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 20

  • Programme No: 6.6.8

  • Theme 6 > Session 6

Abstract

On April 9th the La Soufrière volcano on St. Vincent erupted, ejecting copious amounts of ash, and impacting surrounding coral reefs. Upon contact with seawater, ash surface adhering metal salts and acids rapidly dissolve, releasing essential trace metals such as manganese (Mn), which is reported to enhance coral photosynthesis. This study aimed to quantify Mn leaching from four different tephra types (marine tephra, riverbank tephra, fine-grained summit pyroclastics, and pristine ash) deposited during the eruption and to assess its effect on Mn uptake and photosynthetic efficiency in the coral Stylophora pistillata. A three-week coral culture experiment was performed at the  Centre Scientifique de Monaco using 24 beakers (1L), containing either tephra with coral microcolonies or tephra alone. Tephra (1g) was added five times per week, and Mn concentrations in seawater were sampled weekly and measured using inductively coupled plasma mass spectrometry (ICP-MS). The photosynthetic efficiency of the coral was monitored by measuring the effective quantum yield of PSII. All tephra types released substantial amounts of Mn into seawater, with pristine ash leaching the most (12-14 ppb), compared to altered deposits (0.35 -- 5.4 ppb). Despite negligible coral Mn uptake, an increase in photosynthetic efficiency was observed, particularly at Mn concentrations up to 4 ppb, beyond which no further benefits were detected. This study provides the first evidence that tephra, regardless of depositional age or environment, can enhance coral photophysiology. These findings underscore the ecological importance of volcanic ash in marine ecosystems, suggesting subaerial volcanism may support coral photosynthesis via Mn-mediated pathways.