Widespread inconsistency in logger deployment methods in coral reef studies may bias perceptions of thermal regimes

Ocean warming is the greatest threat to coral reefs, prompting a need to accurately monitor in situ temperatures. Advancements in sensing technologies have led to a proliferation of temperature loggers being deployed globally. However, appropriate deployment of loggers–essential for measurement accuracy in an ecosystem where changes of 1ºC can cause widespread mortality–is often overlooked. For example, loggers deployed in direct sunlight are known to overestimate temperature, but the prevalence of shading loggers is unknown. Here, we survey recent literature to assess the current state of temperature logger use on coral reefs. We then performed lab and field trials on 10 models that span a range of prices and accuracies to evaluate logger performance and assess efficacies of shading techniques. Of the 329 studies we reviewed, >40% of studies deployed loggers shallower than 5 m, yet <5% reported shading loggers, revealing the potential for bias in existing datasets. In field tests, solar bias varied significantly across loggers; the most popular model suffered the largest bias of >2.5ºC, while other models were less affected by irradiance. Wrapping loggers in tape reduced temperature bias, but under high irradiance measurement error still exceeded 0.8ºC. Shading loggers under an opaque object completely eliminated solar bias. We demonstrated a strong linear relationship between in situ irradiance and temperature error and quantified irradiance thresholds for which error >0.5°C can be expected. We then modeled the temperature bias expected for the most popular logger model using in situ irradiance data, showing that errors >2ºC can exist across multiple depths. Our findings reveal pervasive underreporting of logger deployment methods in coral reef studies, highlighting the need to consider the possibility of bias when comparing studies or integrating different in situ temperature records. Future studies should strive to transparently describe deployment methods and appropriately shade loggers.