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 (Canoe Beach) 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, as well as with our regular ocean water quality monitoring year-round, we measure nutrient levels. 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 (Figure 1) 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 (Figure 2). During our one-day experiment, temperatures varied over 2 degrees centigrade during the course of the day.

As of Monday, May 4, a small group of Hui O Ka Wai Ola staff members are monitoring coastal water quality at all 41 of our leeward Maui sample sites every three weeks. Sampling has been on hold since March 16 due to safety concerns during the COVID-19 pandemic. However, permission was recently granted from Maui County officials for our group to continue this essential work.
 
Working in teams of two, the Hui’s Project Manager and Team Leaders are following strict guidelines to maintain a six-foot distance from each other, wearing face masks and sanitizing while collecting and processing samples.
 
We are grateful to be monitoring at this unprecedented time when island activity is at an all-time low. We hope to learn if the decrease in visitors and changes in behavior may impact coastal water quality.
 
We plan to maintain this monitoring protocol for the next two months and hope to get our amazing volunteers back in the field as soon as it is safe to do so.