Epibenthic community variation along an acidified tropical estuarine system
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The benthic communities associated with hard substrata in tropical estuaries (rocky surfaces and mangrove roots) are underexplored compared to sediment-associated communities. Being unaffected by within-sediment chemistry, rocky surface communities are exposed to water-column chemistry. Natural and anthropogenic acidic inflows into estuaries are common, yet understanding of how low pH estuarine water impacts communities is limited. This study investigated variation in a rocky substratum benthic community along a steep pH and carbonate saturation gradient in a tropical estuary. Samples (n=72) were collected from four stations in the Brunei estuarine system, South East Asia (pH 5.78 - 8.1, salinity 0.1 - 29.5 psu). Species richness, diversity and abundance were greatest at the seaward end of the estuary (where pH and salinity were high), reduced in the middle estuary, and relatively high again in the upper estuary. A total of 34 species was recorded, with station abundances varying between 95 and 336 individuals/100 cm2. At a coarse taxonomic level (class/order), multivariate analyses revealed three distinct communities, a tanaid–polychaete dominated community, a mussel–dipteran community, and a mussel–amphipod–dipteran community. The observed shift from amphipod-dominance to polychaete-dominance along a decreasing pH gradient is consistent with the community changes seen in open ocean systems influenced by elevated pCO2. This study is the first description of community structure variation for hard-substratum invertebrates in an old-world tropical estuary. It shows that acidified estuaries offer insights into community-level responses to marine acidification in general.
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