Contrasting hydrodynamic regimes of submerged pinnacle and emergent coral reefs

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.

Strong hydrodynamic drivers of coral reef fish biodiversity on submerged pinnacle coral reefs

“…We suggest that these mechanisms are like the enhanced biophysical‐coupling and energetic focusing found to occur on seamounts (White et al 2007; Lavelle and Mohn 2010). The pinnacles in our study have been shown to possess distinct hydrodynamic regimes, characterized by markedly higher current speeds compared to the nearshore and offshore emergent reefs (Galbraith et al 2022). Through localized hydrodynamics generated at submerged and steep‐side reef morphologies, we propose that the habitat quality is enhanced for reef fishes on pinnacle coral reefs, despite small overall area, greater depths and relative isolation in offshore positions.…”

“…Temperature variables were somewhat important for fish species richness and abundance but were some of the least influential variables for fish biomass. Although average annual temperatures on the pinnacles are slightly cooler than emergent reefs in Kimbe Bay (Galbraith et al 2022), the magnitude of this difference is small and seasonal patterns are remarkably low here (Srinivasan and Jones 2006). Our previous investigation of hydrodynamic regimes and thermal profiles on the pinnacles in Kimbe Bay hints at potential thermoregulatory processes at these sites but this has not been empirically shown.…”

“…(2) Establish the relative influence of a combination of habitat, hydrodynamic and biogeographical variables on reef fish communities below 20 m. (3) Examine the nature of the relationship between fish communities and hydrodynamic variables and test for differences in these relationships between reef types. Current speeds and associated temperature regimes differ between emergent reefs and submerged pinnacle reefs in Kimbe Bay (Galbraith et al 2022). We therefore expect that changes in magnitude of important hydrodynamic variables may result in different associations with fish species richness, abundance, diversity, and biomass.…”