The geochemical cycles of iron and sulphur in marine sediments are strongly intertwined and give rise to a complex network of redox and precipitation reactions. Bioturbation refers to all modes of transport of particles and solutes induced by larger organisms, and in the present-day seafloor, bioturbation is one of the most important factors controlling the biogeochemical cycling of iron and sulphur. To better understand how bioturbation controls Fe and S cycling, we developed reactive transport model of a coastal sediment impacted by faunal activity. Subsequently, we performed a model sensitivity analysis, separately investigating the two different transport modes of bioturbation, i.e. bio-mixing (solid particle transport) and bio-irrigation (enhanced solute transport). This analysis reveals that bio-mixing and bio-irrigation have distinct—and largely opposing effects on both the iron and sulphur cycles. Bio-mixing enhances transport between the oxic and suboxic zones, thus promoting the reduction of oxidised species (e.g. iron oxyhydroxides) and the oxidation of reduced species (e.g. iron sulphides). Through the reoxidation of iron sulphides, bio-mixing strongly enhances the recycling of Fe and S between their reduced and oxidised states. Bio-irrigation on the other hand removes reduced solutes, i.e. ferrous iron and free sulphide, from the sediment pore water. These reduced species are then reoxidised in the overlying water and not recycled within the sediment column, which leads to a decrease in Fe and S recycling. Overall, our results demonstrate that the ecology of the macrofauna (inducing bio-mixing or bio-irrigation, or both) matters when assessing their impact on sediment geochemistry. This finding seems particularly relevant for sedimentary cycling across Cambrian transition, when benthic fauna started colonizing and reworking the seafloor.
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This study investigates the influence of limnological parameters that are known to influence rotifer species richness and composition in temperate areas on two families of monogonont rotifers in a tropical area. We analysed species richness and diversity of the two families from 133 samples taken from several habitat types in Thailand. Coordinates, elevation, pH, temperature, conductivity, salinity and dissolved oxygen were measured and their effect on rotifers assessed. A total of 60 species in 5 genera was recorded from the two families. Latitude and temperature had an effect on species richness of Brachionidae. None of the variables affected species richness of Lecanidae, except habitat type, albeit not significantly. Habitat type and conductivity had an effect on species composition of the two families, whereas latitude had an effect only on species composition of Lecanidae. Species richness and composition of the two rotifer families are well documented in Thailand and no or only few additional species are expected. The limnological correlates of rotifer diversity seem to be in part comparable to and in part different from those known from temperate areas, suggesting that there may be general trends in rotifer species richness and composition across different latitudes.
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RBINS Staff Publications 2017
