Hui O Ka Wai Ola Research Report
Changes in Water Quality During a 12-Hour Period at Two Maui Beaches
This research report was prepared by the Hui O Ka Wai Ola ocean water quality monitoring team. To learn more about this program, please visit huiokawaiola.com.
To better understand how tide change influences water quality, a group of Hui O Ka Wai Ola Team Leaders spent a full day in December 2019 (approximately 12 hours) measuring water quality parameters hourly at Cove Park in South Maui and Hanaka‘ō‘ō Park in West Maui – a deviation from the Hui’s usual practice of monitoring 41 sites once every three weeks only in the morning.
During this one-day study, we measured nutrient levels. We also measure nutrient levels as part of our regular ocean water quality monitoring year-round. Nutrients, including nitrogen and phosphorus, can indicate pollution from wastewater, run-off from agriculture, landscaping and/or golf courses. Once in the ocean, these nutrients can lead to an overgrowth of algae. Algae, particularly invasive species, quickly out-compete corals for sunlight and overrun large tracts of living reef.
Nutrient values can vary a lot over the course of a tide cycle — low nutrient levels at high tide and higher nutrient levels at low tide. Sites with higher nutrient levels acquire these nutrients via ground water that comes into the ocean as underground “springs.”
The fact that there is fresh water coming in from underground springs over the course of a day is shown by the change in salinity readings. This explains the ever-shifting levels that we see from the data we collect once every three weeks at each site.
During our one day experiment we took 4 nutrient samples, with the first sample at high tide and the last sample near low tide. The accompanying chart shows how salinity decreases as the tide goes out and the fresh water from underground springs comes in, while the nitrate levels increase as the amount of fresh water increases.
Our ongoing Hui O Ka Wai Ola water quality sampling program was designed to catch a variety of tide states so that we can understand not only the average level of nutrients but also the range of nutrient levels each site exhibits.
Another interesting observation is that other parameters vary during the day as well — temperature rises as the sun comes up and heats the top layer of water and then starts to decrease as the sun goes down or if clouds come over. During our one-day experiment, temperatures varied over 2 degrees centigrade during the course of the day.
With our ongoing water quality monitoring, we are careful that we measure this parameter at each site at approximately the same time of day every time we sample so that seasonal temperature variations can be distinguished from diurnal variations.
Finally, dissolved oxygen (DO) and pH levels also vary across the course of a day. Dissolved oxygen is needed for aquatic plants and animals to survive. Corals are very sensitive to changes in ocean chemistry, including increased ocean water acidity (which results in a drop in pH levels).
During our 12-hour study, we found that in Cove Park and Hanaka‘ō‘ō Park these diurnal variations are not as obvious because of the influx of fresh ground water as the tide is going out which also affects levels of pH and DO. In general, pH and DO are at their lowest levels just as the sun rises due to the fact that photosynthesis has not been taking place overnight. CO2 levels rise at night as marine organisms respire, taking in oxygen and producing CO2. There is no photosynthesis at night so as CO2 levels rise, the water becomes more acidic and pH levels drop. Dissolved oxygen also drops because of a lack of photosynthesis that generates oxygen as a bi-product. Once the sun begins to rise, photosynthesis begins again, and the pH and DO levels rise.
Please visit again for more reports from the field. Learn more about Hui O Ka Wai Ola at www.huiokawaiola.com.