Effects of flow and colony morphology on the thermal boundary layer of corals

Abstract

The thermal microenvironment of corals and the thermal effects of changing flow and radiation are critical to understanding heat-induced coral bleaching, a stress response resulting from the destruction of the symbiosis between corals and their photosynthetic microalgae. Temperature microsensor measurements at the surface of illuminated stony corals with uneven surface topography (Leptastrea purpurea and Platygyra sinensis) revealed millimetre-scale variations in surface temperature and thermal boundary layer (TBL) that may help understand the patchy nature of coral bleaching within single colonies. The effect of water flow on the thermal microenvironment was investigated in hemispherical and branching corals (Porites lobata and Stylophora pistillata, respectively) in a flow chamber experiment. For both coral types, the thickness of the TBL decreased exponentially from 2.5 mm at quasi-stagnant flow (0.3 cm s(-1)), to 1 mm at 5 cm s(-1), with an exponent approximately 0.5 consistent with predictions from the heat transfer theory for simple geometrical objects and typical of laminar boundary layer processes. Measurements of mass transfer across the diffusive boundary layer using O(2) microelectrodes revealed a greater exponent for mass transfer when compared with heat transfer, indicating that heat and mass transfer at the surface of corals are not exactly analogous processes.

Figure 1.

Schematic of the typical surface features of a coral, showing details of the skeleton corallites secreted by the soft tissue of individual polyps (modified with permission from Kelley [17]). Photographs of the morphologically distinct coral species used in the flow chamber experiments: L. purpurea and P. sinensis were used for mapping of the thermal microenvironment of corals of complex surface topography, while the relatively smooth, branching S. pistillata and hemispherical P. lobata were used to assess the effect of flow velocity on the TBL thickness.

Figure 2.

Typical temperature profile (open circles) over a hemispherical P. lobata exposed to a flow velocity of 0.6 cm s⁻¹ and an irradiance of 1500 W m⁻², fitted hyperbolic tangent curve (solid line) and estimate of the effective thickness of the thermal boundary layer, δ_TBL, as the intersection between the tangent at z=0 and the constant water temperature in the free-flowing water above the coral.

Figure 3.

Contour maps of the thermal boundary layer over (a) two neighbouring polyps of P. sinensis and (b) an individual polyp of L. purpurea, under flow and light conditions of 1 cm s⁻¹ and 430 W m⁻² (approx. 1500 µmol photons m⁻² s⁻¹), respectively. For better illustration, the map for L. purpurea was duplicated by right symmetry. (c) Schematic of polyps of the P. sinensis. and L. purpurea specimens. ΔT (°C): dashed line with open circles, +0.05; solid line with open circles, +0.15; solid line with open diamonds, +0.25; solid line with filled diamonds, +0.35.

Figure 4.

Effect of flow on the tissue surface warming of S. pistillata (open triangles) branches (n = 9) and hemispherical colonies of P. lobata (open circles) (n = 10), measured in a flow chamber under high irradiance (1500 W m⁻²; approx. 2500 µmol photons m⁻² s⁻¹). Symbols with error bars represent averages ± s.e. The least-square power-law regressions are: ΔT = 0.11 u^−0.47, r² = 0.99 and ΔT = 0.31 u^−0.59, r² = 0.98 for S. pistillata and P. lobata, respectively.

Figure 5.

Coral surface warming of S. pistillata (open triangles) branches (n = 9) and hemispherical colonies of P. lobata (open circles) (n = 10) plotted against the thickness of the thermal boundary layer, measured under 1500 W m⁻² (approx. 2500 µmol photons m⁻² s⁻¹) irradiance and flows ranging between 0.3 and 5 cm s⁻¹. Symbols with error bars represent averages ± s.e. The least-square regression lines are: ΔT = 9.9 × 10⁻⁵ δ_TBL–7.8 × 10⁻³, r² = 0.67 and ΔT = 3.4×10⁻⁴ δ_TBL–6.0×10⁻², r² = 0.72 for S. pistillata and P. lobata, respectively.

Figure 6.

Effect of flow on the surface O₂ concentration of S. pistillata (open triangles) branches (n = 7) and hemispherical colonies of P. lobata (open circles) (n = 8), measured in a flow chamber and exposed to 430 µmol photons m⁻² s⁻¹. Symbols with error bars represent averages ± s.e. The least-square power-law regressions are: C_s = 163 u^−0.22, r² = 0.94 and C_s = 180 u^−0.18, r² = 0.99 for S. pistillata and P. lobata, respectively.

Figure 7.

Effect of flow on δ_TBL and δ_DBL over S. pistillata branches (n=9) and hemispherical colonies of P. lobata (n=10). Symbols with error bars represent averages ± s.e. The least-square power-law regressions are: (a) S. pistillata: TBL=1137 u^−0.36, r²=0.98 and DBL=469 u^−0.74, r²=0.99; (b) P. lobata: TBL=1118 u^−0.38, r²=0.96 and DBL=268 u^−0.84, r²=0.97. Open circles, thermal boundary layer; open triangles, oxygen diffusive boundary layer.