Application Story

Onset’s HOBO® water temperature data loggers are helping U.S. National Park Service researchers understand the effects of water temperature on coral reef health at the service’s field station in St. John, U.S. Virgin Islands. More specifically, researchers are using the data loggers to establish a link between elevated water temperature over time and coral bleaching, or the whitening of coral due to the loss of life-supporting algae. Coral bleaching – a phenomenon that has been linked to global warming in recent years – causes coral to become colorless, weak, and vulnerable to a variety of environmental stresses.

“We have witnessed a high number of bleaching episodes involving acropora coral in this region, to the point where the coral is almost an endangered species,” explains Rob Waara, a biological technician with the National Park Service’s Inventory & Monitoring Program. “While there are a number of environmental factors that can cause these events, such as sedimentation and low salinity levels, the increase in sea temperature between the winter and summer months can play a role. Thus, we need a reliable way to continuously monitor water temperature so we can better understand the true impact it is having on the health of these fragile ecosystems.”

To monitor water temperature, Waara and his team have strategically placed HOBO data loggers at several representative sites on St. John, including reef areas off of Lamshur Bay and Hawksnest Bay. Each data logger, which is housed in a buoyant-neutral, durable plastic case designed for extended deployment in water, samples water temperature every 30 minutes.

After three month’s worth of data has been collected, Waara and other researchers retrieve the data loggers and offload the data using a high-speed infrared (IR) communication port. The port is later connected to a computer running Onset’s data graphing and analysis software program, which instantly translates the collected data into easy-to-read graphs that clearly indicate spikes and drops in water temperature over a given three-month period. Temperatures are reported within ±0.2°C accuracy over a measurement range of 0º to 50°C.

According to Waara, the collected data helps the National Park Service build a comprehensive picture over time of the average variations in water temperatures in the region. “While the question of what’s happening to the reefs may be a simple one, there’s a lot that goes into the answer," he explains. "There are just so many variables involved. We do know, however, that water temperature is an increasingly important piece of the pie. The data we get from the HOBO data loggers moves us one step closer to understanding exactly how these temperature variations affect our reefs."

For more information about data loggers, please contact Onset at 1-800-LOGGERS, or visit