Estuarine benthic diversity declines with temperature across a tropical–temperate gradient

Estuarine benthic sediments are important habitats that support ecosystem health and productivity, but the drivers of large-scale patterns in benthic diversity are not well documented. Using 18S amplicon sequencing of 425 sediment DNA samples from estuaries spanning 20° of latitude along Australia's east coast, we assessed tropical–temperate patterns in benthic eukaryotic richness, community composition, and environmental structuring. Contrary to classical expectations, ASV richness declined towards the tropics, with higher richness in temperate estuaries. This inverse gradient was evident for total eukaryotes, microeukaryotes, macrofaunal metazoans, and multicellular algae, while microbial metazoans and unicellular algae showed weaker or no latitudinal trends. Temperature was one of the strongest predictors of richness and community composition, although its high correlation with latitude means that thermal effects cannot be separated from other latitudinally structured processes. Sediment properties were also important, particularly sediment heterogeneity, which was consistently positively associated with richness. Nutrients and water-column physicochemical variables, including salinity, dissolved oxygen and pH, had more group-specific effects on richness but contributed more clearly to shifts in community composition with latitude and taxon-level abundances. Over the latitudinal gradient, differences in habitat conditions may have contributed to these observed patterns. Compared with higher-latitude temperate estuaries, lower-latitude tropical estuaries tended to have more heterogeneous sediments but lower nutrient levels, lower dissolved oxygen and lower pH, suggesting that warmer environments were associated with less favourable benthic habitat. These findings show that estuarine benthic eukaryotic diversity does not necessarily follow classical tropical-richness expectations, but instead reflects interactions between large-scale gradients, local sediment habitat conditions, and taxon-specific environmental responses. More broadly, our results suggest that warming and changes in sediment dynamics could reduce benthic biodiversity and compromise the habitat quality and functioning of estuarine ecosystems.