Measuring Water Temperature: The need to dig below the surface
When considering the actual temperatures impacting the marine environment, it is important to remember that the shallow sea surface, if reasonably calm, can become much warmer than the underlying water just 5-10 cm below. This “skin” temperature can often be several degrees C higher than the “bulk” water just below it. Unfortunately satellites used for measuring remote sea surface temperature (SST) measurements “see” only the skin layer of the ocean. Unless there is vertical mixing of the skin and bulk water below it, that SST value may not be the same as the temperature even 1 m below where reefs occur. It is incumbent on the researcher, whenever possible, to actually measure the water temperature on reefs with accurate in situ instruments to check for any skin temperature errors from satellite SST data.
The thermal structure of the water column below the surface can not be accurately measured via satellite remote sensing. It is necessary to place either instruments at various depths or have some type of profiling instrument (see ocean observations). In Palau we place recording thermographs at selected depths along the reef profile by diving, leave them to collect data and then recover later and download. Such a deployment of thermographs is called a “vertical array”, each unit recording at its given depth on the same sampling schedule (up to once per minute). The disadvantages of such a system are that you have to find and recover the instruments to get any data, but such arrays can be set up relatively cheaply. Thermographs can also be set up on a mooring, distributed along a line with a float at the top and a heavy weight at the bottom. This has disadvantages in that the mooring can break and float away or the float can fail and then the array sinks to the bottom. Currents also bend the mooring downward, so the depth of each thermograph can change often, producing variable data not at a consistent depth. The array placed along the reef is the best way to get solid data over a period of many months to years.