Anne Louise Cohen

Sea‐level rise (SLR) alters nearshore hydrodynamics, yet its influence on coral reef thermal regimes under climate‐driven ocean warming remains poorly quantified. Using a fully coupled hydrodynamic–wave model validated at Palmyra Atoll, we isolate how SLR modifies temperature variability during a projected 2050 marine heatwave. Results show that while basin‐scale warming raises reef bottom temperatures broadly, SLR deepens shallow habitats and enhances tidal exchange, reducing daily and maximum temperatures by more than 0.5°C across 16% of reef area. Heat budget analysis reveals that SLR shifts the balance between advective and surface flux terms, damping thermal extremes through increased water volume and flow. These results identify physical feedback between SLR and ocean warming that modulates heat accumulation in shallow tropical systems, providing a mechanism for localized thermal buffering amid global ocean warming.

The impacts of El Niño‐Southern Oscillation (ENSO) on salinity and alkalinity in an equatorial coral reef lagoon (Kanton) are investigated using water samples collected in three non‐El Niño years (1973, 2012, and 2018) and one El Niño year (2015). A one‐dimensional, advective‐diffusive model is developed to aid in the interpretation of the sparse observations and make estimates of net ecosystem calcification (NEC) rates. The Kanton lagoon experiences extreme salinity and alkalinity variations driven by ENSO variations in precipitation. During the non‐El Niño years, salinity increases from the ocean (35.5 psu) to the back of the lagoon (38 psu) because evaporation exceeds precipitation, and water resides in the back of the lagoon for ∼180 days. Early in the 2015–2016 El Niño, the back of the lagoon is only ∼1 psu saltier than the ocean because precipitation had begun to exceed evaporation. The model suggests that during El Niño events, when precipitation substantially exceeds evaporation, the back of the lagoon is less salty than the ocean (30–32 psu). Alkalinity variations in the lagoon are primarily due to dilution or concentration driven by the ENSO variations in precipitation and NEC that causes an alkalinity deficit of ∼250 μmol/kg in the back of the lagoon. The estimated NEC rate in 2015 is ∼25% lower (4.1 mmol/day) than in the non‐El Niño years (5.3–5. 7 mmol/day). The NEC rates and coral cover measurements indicate that the Kanton lagoon has recovered from the complete loss of coral cover during the 2002–2003 El Niño.

The impacts of El Niño-Southern Oscillation (ENSO) on salinity and alkalinity in an equatorial coral reef lagoon (Kanton) are investigated using water samples collected in 1973, 2012, 2015, and 2018. A simple advective-diffusive model is developed to aid in the interpretation of the sparse observations and make estimates of net ecosystem calcification (NEC) rates. Salinity and alkalinity variations in Kanton lagoon are primarily driven by ENSO variations in precipitation. During non-El Niño years (1973, 2012, 2018), salinity increases from the ocean (35.5 psu) to the back of the lagoon (38 psu) because evaporation exceeds precipitation and water resides in the back of the lagoon for ~180 days. At the onset of the 2015-16 El Niño the back of the lagoon is only ~1 psu saltier than the ocean because precipitation had begun to exceed evaporation. The model suggests that during El Niños, when precipitation exceeds evaporation, the back of the lagoon is less salty than the ocean (30 – 32 psu). Alkalinity variations in the lagoon are primarily due to dilution or concentration driven by the ENSO variations in precipitation and NEC that causes an alkalinity deficit of ~250 micromoles/kg in the back of the lagoon. NEC rates in the early stages of the 2015 – 2016 El Niño were ~10% lower (4.7 mmol/day) than in the non-El Niño years (5.2 – 5. 5 mmol/day). The NEC rates and coral cover indicate that Kanton Lagoon has recovered from the complete loss of coral cover during the 2002-03 El Niño.

Sr‐U, a coral‐based paleothermometer, corrects for the effects of Rayleigh Fractionation on Sr/Ca by regressing multiple, paired U/Ca and Sr/Ca values. Prior applications of Sr‐U captured mean annual sea surface temperatures (SSTs), inter‐annual variability, and long‐term trends. However, because many Sr/Ca‐U/Ca pairs are needed for a single Sr‐U value as originally formulated, the temporal resolution of the proxy is typically limited to 1 year. Here, we address this limitation by applying laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) to three Porites colonies from Jarvis and Nikumaroro Islands in the central equatorial Pacific (CEP), generating ∼25 Sr/Ca‐U/Ca pairs per month of skeletal growth. Both Sr/Ca and U/Ca vary significantly over small (sub‐mm) length scales and support the calculation of Sr‐U values using the original regression method. Over the represented temperature range of 24–31°C, the Sr/Ca‐U/Ca‐SST relationships are nonlinear, a finding consistent with predictions of the Rayleigh model. To reflect this non‐linearity, we developed a calibration using multivariate nonlinear regression. The multivariate, three‐coral calibration was applied to 20 years of monthly resolved Sr/Ca and U/Ca of a coral interval not included in the calibration, yielding RMSE = 0.73°C and r² = 0.85 (p < 0.05; df = 256). The multivariate calibration performed significantly better than Sr/Ca alone (r² = 0.28). Applying the new calibration to a subfossil Porites from Kiritimati Atoll, CEP (2200 Before Present) yields equivalent phase and amplitude of interannual variability, but water temperatures ∼1.6°C cooler than they are in this region today.

Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long‐lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr‐U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year‐to‐year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow‐growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr‐U values. With the fine‐scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30‐year‐long Sr‐U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr‐U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr‐U can accurately constrain decadal‐to‐multidecadal variability and secular SST trends in regions where this information is urgently needed.

Ocean warming is killing corals, but heat-tolerant populations exist; if protected, they could replenish affected reefs naturally or through restoration. Palau’s Rock Islands experience consistently higher temperatures and extreme heatwaves, yet their diverse coral communities bleach less than those on Palau’s cooler outer reefs. Here, we combined genetic analyses, bleaching histories and growth rates ofPoritescf.lobatacolonies to identify thermally tolerant genotypes, map their distribution, and investigate potential growth trade-offs. We identified four genetic lineages ofP. cf.lobata. On Palau’s outer reefs, a thermally sensitive lineage dominates. The Rock Islands harbor two lineages with enhanced thermal tolerance; one of which shows no consistent growth trade-off and also occurs on several outer reefs. This suggests that the Rock Islands provide naturally tolerant larvae to neighboring areas. Finding and protecting such sources of thermally-tolerant corals is key to reef survival under 21^stcentury climate change.

Ocean warming is causing declines of coral reefs globally, raising critical questions about the potential for corals to adapt. In the central equatorial Pacific, reefs persisting through recurrent El Niño heatwaves hold important clues. Using an 18‐year record of coral cover spanning three major bleaching events, we show that the impact of thermal stress on coral mortality within the Phoenix Islands Protected Area (PIPA) has lessened over time. Disproportionate survival of extreme thermal stress during the 2009–2010 and 2015–2016 heatwaves, relative to that in 2002–2003, suggests that selective mortality through successive heatwaves may help shape coral community responses to future warming. Identifying and facilitating the conditions under which coral survival and recovery can keep pace with rates of warming are essential first steps toward successful stewardship of coral reefs under 21st century climate change.

On coral reefs, flow determines residence time of water influencing physical and chemical environments and creating observable microclimates within the reef structure. Understanding the physical mechanisms driving environmental variability on shallow reefs, which distinguishes them from the open ocean, is important for understanding what contributes to thermal resilience of coral communities and predicting their response to future anomalies. In June 2014, a field experiment conducted at Dongsha Atoll in the northern South China Sea investigated the physical forces that drive flow over a broad shallow reef flat. Instrumentation included current and pressure sensors and a distributed temperature sensing system, which resolved spatially and temporally continuous temperature measurements over a 3‐km cross‐reef section from the lagoon to reef crest. Spectral analysis shows that while diurnal variability was significant across the reef flat—a result expected from daily solar heating—temperature also varied at higher frequencies near the reef crest. These spatially variable temperature regimes, or thermal microclimates, are influenced by circulation on the wide reef flat, with spatially and temporally variable contributions from tides, wind, and waves. Through particle tracking simulations, we find the residence time of water is shorter near the reef crest (3.6 h) than near the lagoon (8.6 h). Tidal variability in flow direction on the reef flat leads to patterns in residence time that are different than what would be predicted from unidirectional flow. Circulation on the reef also determines the source (originating from offshore vs. the lagoon) of the water present on the reef flat.

Ocean acidification (OA) reduces the concentration of seawater carbonate ions that stony corals need to produce their calcium carbonate skeletons and is considered a significant threat to the functional integrity of coral reef ecosystems. However, detection and attribution of OA impact on corals in nature are confounded by concurrent environmental changes, including ocean warming. Here we use a numerical model to isolate the effects of OA and temperature and show that OA alone has caused 13 ± 3% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950. This OA‐induced thinning of coral skeletons, also evident in Porites from the South China Sea but not in the central Pacific, reflects enhanced acidification of reef water relative to the surrounding open ocean. Our finding reinforces concerns that even corals that might survive multiple heatwaves are structurally weakened and increasingly vulnerable to the compounding effects of climate change