Impact of shallow‐water hydrothermal seepage on benthic biogeochemical cycling, nutrient availability, and meiobenthic communities in a tropical coral reef

Anna Lichtschlag,Katja Guilini,Ulrike Braeckman,Soeren Ahmerkamp,Rachael H James,Dirk De Beer

doi:10.1002/lno.12017

Abstract

AbstractWe investigated the influence of high‐CO2 hydrothermal seepage on element cycling, early diagenetic processes, and meiobenthic communities in sediments of a coral reef in Papua New Guinea. Based on fluid flow velocities, determined from temperature gradients, and element concentrations, the solute fluxes from the seeps were estimated, showing that seepage through sediments can be a source of nutrients but also of potentially toxic elements to the reef ecosystem. The sediment pore waters consisted of up to 36% hydrothermal fluids, enriched in As, Si, Li, Mn, Fe, Rb, and Cs relative to ambient seawater. During their ascent to the seabed, the acidic fluids reacted with the sediments, leading to increases in total alkalinity, nutrients, and alkali elements in the fluids. Mixing of hydrothermal fluids with seawater within the sediments lead to precipitation of redox‐reactive species, including Fe‐oxides, but the sediment pore waters were still a source of trace metals to the water column. Presence of the low‐pH fluids in the sediments resulted in dissolution of sedimentary carbonates and left behind finer‐grained volcanoclastic sands containing As, Cr, and Ni in concentrations toxic to biota. These finer‐grained sediments had a reduced permeability, reducing the rate of remineralization of organic matter. Benthic meiofauna and nematode abundance and functional diversity were relatively lower at sites with hydrothermal seepage through the sediment. As benthic and pelagic processes are tightly coupled, it is likely that the changes in benthic biogeochemical processes due to sediment acidification will also affect epibenthic and pelagic communities.

Highlights

Large numbers of hydrothermal seeps are found in the “Ring of Fire” surrounding the Pacific Ocean; many of them are located in coral reefs and are ideal sites for studying the effects of ocean acidification on biota, as reefs harbor an enormous biodiversity and depend on calcification, a process that is challenged by low-pH conditions
Seepage dynamics The pH, temperature, and Eh of the bottom water varied with the tides, with pH and Eh minima and temperature maxima correlating with low tide (Fig. 2)
Due to its high concentration in the hydrothermal fluids, the average Si flux from the seep sediments (125 mmol mÀ2 dÀ1) was nearly 85 times higher than the highest flux we have found from coral reef sediments in the literature (À 0.02 to 1.5 mmol mÀ2 dÀ1, Ullman and Sandstrom 1987)

Summary

Introduction

CO2-rich shallow-water hydrothermal seeps are natural analogues for studying the effects of ocean acidification on ecosystems and biogeochemistry (Aiuppa et al 2021). The released nutrients are subsequently available for the primary producers This intensive recycling of organic material via permeable sediments contributes to the reef productivity and biodiversity. Several studies have explored the impact of hydrothermal fluids on the geochemistry and ecology of shallow-water environments Such sites were used as natural laboratories for ocean acidification and documented its effects on seagrass growth (Hall-Spencer et al 2008), coral diversity (Fabricius et al 2011), bacterial and metazoan community structure (Hassenrück et al 2016; Smith et al 2016; Molari et al 2018), and bivalve populations (Martins et al 2021). The composition of the upwelling hydrothermal fluids can be altered by mixing with oxic seawater below the seabed, precipitating constituents in the subsurface sediments or at the seawater-seabed interface (Pichler et al 1999b)

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Discussion

Conclusion