Across the globe, corals can survive in temperatures between 18-34°C, however, they adjust to the ‘normal’ temperature of where they live, and so each reef is adapted to its unique climate. On Koh Tao, our corals thrive between about 28-30°C. At temperature greater than this, corals will start to stress and bleach. Over the last few decades, multiple coral bleaching events have impacted our overall reef health. Our program monitoring temperature using in-situ probes, small electronic devices that are placed out onto the reef, and later retrieved. This allows us to take constant water temperature data (every 5 min) from multiple sites and from multiple depths. We have regular data available from multiple sites since 2010.

Salinity, or the salt content of the water, on the reef tends to be stable, unless there are large storms or other sources of freshwater inundation. Even several weeks after large storms, fresh water seeping from the groundwater table can affect the salinity around corals. Our program uses a hand refractometer to measure sea water salinity.

Even though it is a bit counter-intuitive, coral reef actually thrive in areas with low nutrient levels. That is because corals eat plankton, and do not actually absorb nutrients form the water. Having excess nutrients in the water can lead to an increase in the amount of bacteria, biofilm, and macro-algae growth on the reef, which negatively impacts coral health and survival. Sources on nutrients may be sea based (upwelling, changing currents, etc.) But most notably stem from human activities on land such as agriculture, deforestation, erosion, development, sewage, etc. The two most important limiting nutrients concerning human impacts to reefs are phosphates and nitrates/nitrites. In our program, nutrient levels are mostly inferred using indicator species (ie algae growth) of the EMP. We also have a colorimeter to quantitively measure nutrient levels when necessary.

Turbidity is a measurement of the clarity of the water, clear water would have a low turbidity, whereas dirty or unclear water would have a high turbidity. This measurement is affected by many different factors such as the amount of suspended sediment, dissolved solids, micro-algae, etc. For divers, this is usually just known as ‘the viz’. In our program, we use a device called a Secci Disk to evaluate the vertical or horizontal visibility of the sea water on all our dives. This is measured in meters, and represents light penetration. When more accurate values are necessary, we also have a colorimeter to measure turbidity of water samples.

Sedimentation rates fluctuate throughout the year based on currents, upwelling, etc. But generally, in a healthy reef system sedimentation rates would be very low, as corals are easily stressed by sediment. Sediment is also a medium for bacteria growth, and reefs with high sedimentation rates tend to have higher rates of disease. Sources of sedimentation tend to stem from on-land activates such as agriculture, construction, deforestation, development, etc. This is usually most pronounced following storms. In sea sources of sediment include dredging, trawling, and other activities which disturb the sea bed. In our program, we monitor sedimentation rates using sediment traps. These are essentially cups that are left on the reef for a set period of time then collected. The contents are then filtered, dried, and weighed to measure the rate of sedimentation for that reef.

Water currents bring in fresh nutrients, food, and gases to our reefs. Currents can affect bleaching and recovery rates, as well as many other factors on the reefs. In our program, we use a flow meter to measure current speed and direction.