No abstract

Hydrodynamics on coral reefs vary with depth, reef morphology and seascape position. Differences in hydrodynamic regimes strongly influence the structure and function of coral reef ecosystems. Submerged coral reefs on steep-sided, conical bathymetric features like seamounts experience enhanced water circulation as a result of interactions between currents and the abrupt physical structure. There may also be similar interactions between smaller pinnacles and regional water currents in offshore locations (crests > 10 m), while shallow reefs (crests <10 m) may be more subject to surface currents driven by wind, waves and tide. Here we tested whether coral pinnacles experienced stronger and more variable currents compared to emergent reefs at the same depth in both nearshore and offshore positions. Current speeds and temperature were monitored for 12 months at 11 reefs, representing the three different reef categories: submerged offshore pinnacles, emergent offshore reefs and emergent nearshore reefs. We found different patterns in current speeds and temperature among reef types throughout the year and between seasons. Submerged pinnacles exhibited stronger, more variable current speeds compared to both near and offshore emergent reefs. We found seasonal changes in current speeds for pinnacle and nearshore reefs but no variation in current strength on offshore reefs. Whilst instantaneous current directions did reflect the seascape position of individual sites, there was no difference in the directional variability of current speeds between reef types. Annual daily average temperatures at all reef types were not strongly seasonal, changing by less than 2 °C throughout the year. Daily temperature ranges at specific sites however, exhibited considerable variability (annual range of up to 6.5 °C), particularly amongst offshore emergent reefs which experienced the highest temperatures despite greater exposure to regional-scale circulation patterns. Additionally, we found a consistent mismatch between satellite sea surface temperatures and in-situ temperature data, which was on average 2 °C cooler throughout the annual study period. Our results suggest that distinct hydrodynamic processes occur on smaller submerged structures that are physically analogous to seamounts. Our findings highlight important nuances in environmental processes that occur on morphologically distinct coral reef habitats and these are likely to be important drivers for the community dynamics of organisms that inhabit these reefs.

Predation is a key ecological process regulating the structure and diversity of biological communities, yet predators do not exist homogeneously in nature. Coral reefs possess diverse assemblages of predatory fishes, the distribution and abundance of which is well documented for coastal and emergent reefs. However, for remote, isolated and submerged reefs, such as those found on pinnacles and seamounts, our understanding of predatory fish communities is limited. These features are ubiquitous in the world’s oceans and frequently targeted by fishers for their presumed fish aggregation properties. Here we describe communities of predatory fishes on a series of pinnacle reefs and contrast these to regional coastal and offshore emergent reefs. Pinnacles supported 2-4× higher abundance, biomass and diversity of predatory fishes compared to emergent reefs. They also supported unique communities, with 32 out of the 63 predator species in our study found only on pinnacle reefs. For species found on all 3 reef types, all were most abundant on pinnacles and the 6 taxa driving differences in community structure were most abundant on pinnacles. Together, our results show that predatory fishes are strongly attracted to pinnacles, although the mechanisms are still unclear. Prioritising the selection of these small ecological magnets in conservation planning would be an effective approach to target the protection of regional reef fish biodiversity.