Research ArticleMarine Ecology

Rising sea levels will reduce extreme temperature variations in tide-dominated reef habitats

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Science Advances  17 Aug 2016:
Vol. 2, no. 8, e1600825
DOI: 10.1126/sciadv.1600825
  • Fig. 1 Study site and field instrument configuration.

    (A) Location of Tallon Island in the Kimberley region of northwestern Australia [refer to Lowe et al. (22) for details]. (B) Configuration of the field instrument array.

  • Fig. 2 Schematic of the reef heat budget model.

    (A) Reef cross section identifying the reef depth hr(t) that varies between the maximum amplitude (ηtide) above the depth hMSL at mean sea level and the minimum depth hmin of the lagoon or platform (refer to the text for details). The total depth hr at the back of the reef (or lagoon) is composed of the depth over the crest (Embedded Image), if present, and the depth of the back of the reef relative to the crest (Embedded Image). (B) Oblique view of a reef with surface area Areef where water instantaneously exchanges over an area Aopen between the reef and the ocean, with flow velocity Embedded Image and temperature Embedded Image. Air-sea heat exchange on the reef occurs through Qnet.

  • Fig. 3 Observations and model predictions of the reef heat budget for Tallon reef during the study period.

    (A) Time series of the water level variability (relative to mean sea level) measured at a site on the reef platform (red) and at a site offshore on the forereef slope (blue). The solid horizontal black line denotes offshore mean sea level (z = 0 m). (B) Terms that comprise the total net air-sea heat fluxes Qnet (Eq. 6, Materials and Methods), including the combined net short- and long-wave radiation (Embedded Image), the latent (Qlt) and sensible (Qsb) heat flux contributions. Refer to Materials and Methods for a description of each heat flux term. (C) Observed spatially averaged reef temperature Tr compared with the model predictions. (D) Hour of day when minimum low tide measured offshore occurred and when peak solar irradiance occurred each day. The horizontal red dashed line corresponds to the mean hour of peak solar irradiance during the study at approximately noon.

  • Fig. 4 Response of reef temperature to interacting solar heating and a dominant semidiurnal tidal cycle.

    (A) Reef temperature variability (black line; in dimensionless form Embedded Image per Eq. 3) over 30 days for an idealized Tallon reef (Embedded Image and Embedded Image) driven by a dominant M2 tide, illustrating the ~14.8-day modulation of the temperature fluctuations caused by the phase drift between the maximum tidal elevation and solar irradiance. The red line denotes a 1-day moving average. (B) Maximum diurnal temperature variation Embedded Image (defined as the difference between the daily maximum and minimum value of Embedded Image), as a function of the instantaneous phase difference between the tidal and solar cycle (Δφi). Results shown are for an idealized Tallon reef under present-day sea level (Embedded Image), as well as a hypothetical scenario where mean sea level is increased so that the depth at low tide is equal to hmin (equivalent to Embedded Image) and tidal truncation no longer occurs.

  • Fig. 5 Diurnal temperature extremes for various reefs worldwide and their response to sea level rise.

    (A) Magnitude of the maximum diurnal temperature fluctuation (in dimensionless form) as a function of the normalized minimum reef water depth (Embedded Image) and normalized reef depth relative to mean sea level (Embedded Image). Tidal truncation occurs below the black line (Embedded Image). The location of four reefs within this parameter space (Ⓐ to Ⓓ; see Table 1) are shown for three mean sea level scenarios (0 m relative to present, a future +0.7-m rise, and a future +1.5-m rise). Ⓐ, Tallon island; Ⓑ, Warraber island; Ⓒ, Cocos Islands; Ⓓ, Lady Elliot island. (B) Response of Embedded Image for the four reefs with fixed Embedded Image but varying Embedded Image.

  • Fig. 6 Global spatial distributions and frequency distributions of the tidal regimes of warm-water coral reefs.

    (A) Map of the tidal form factor (Ftide) for coral reefs. Tidal regimes with Ftide > 1.5 are classified as mainly diurnal, and those with Ftide < 1.5 are classified as mainly semidiurnal. Note that values are capped at a maximum of 3. (B) Frequency histogram of global coral reef systems based on the tidal form factor. Values >3 are expressed as a summation in the final bar.

  • Table 1 Projected temperature changes due to sea level rise for sample tidally forced reefs globally.

    Labels refer to sites plotted in Fig. 5. Tidal amplitudes (ηtide) represent average values (that is, intermediate between spring and neap). Ftide denotes the tidal form factor (see text for details). Diurnal temperature range changes (% ΔTr change) are relative to present conditions with a mean sea level (MSL) of 0 m. Present temperature ranges (ΔTr) are drawn from literature values and projected for different mean sea level rise scenarios (+0.7 m and +1.5 m) using the model. NA, not available.

    LabelSiteηtideFtideΔTr
    (% change)
    ΔTr
    (range)
    References
    MSL
    +0.7 m
    MSL
    +1.5 m
    PresentMSL
    +0.7 m
    MSL
    +1.5 m
    Embedded ImageTallon Island, Kimberley,
    northwestern Australia
    3.0 m0.13−7%−18%2.5–6.5°C2.3–6.0°C2.0–5.3°CThis study
    Embedded ImageWarraber Island,
    Torres Strait
    1.2 m0.46−23%−72%2.0–5.0°C1.5–3.8°C0.6–1.4°C(51)
    Embedded ImageCocos (Keeling) Islands,
    eastern Indian Ocean
    0.6 m0.48−65%−86%NANANA(52)
    Embedded ImageLady Elliot, Great Barrier Reef0.8 m0.42−39%−78%2.5–5.5°C1.5–3.3°C0.5–1.2°C(19, 53)
    Ofu, American Samoa0.5 m0.15−36%−62%1.5–6.0°C1.0–3.8°C0.6–2.3°C(14, 54)
    Rarotonga, Cook Islands0.4 m0.16−38%−63%NANANA(55)

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/8/e1600825/DC1

    Supplementary Methods

    Supplementary Results

    table S1. Tidal amplitudes and reef morphology parameters for sample tidally forced reefs globally.

    table S2. Sensitivity of the projected temperature changes to spring and neap amplitude variations.

    fig. S1. Response of reef temperature to an interacting solar heating cycle and a diurnal (K1) tidal cycle.

    fig. S2. Response of reef temperature to an interacting solar heating cycle and a diurnal (O1) tidal cycle.

    Reference (56)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Methods
    • Supplementary Results
    • table S1. Tidal amplitudes and reef morphology parameters for sample tidally forced reefs globally.
    • table S2. Sensitivity of the projected temperature changes to spring and neap amplitude variations.
    • fig. S1. Response of reef temperature to an interacting solar heating cycle and a diurnal (K1) tidal cycle.
    • fig. S2. Response of reef temperature to an interacting solar heating cycle and a diurnal (O1) tidal cycle.
    • Reference (56)

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