“What’s that do?”: Measuring water quality on oyster reefs

By Adrienne Breef-Pilz, Guest Writer

Earlier, our lab wrote a post about working in the GTMNERR where we are looking at nonconsumptive effects on oyster reefs. Additionally, we are collecting data on the survival and the growth of the oysters at nine different sites within the reserve. We do this by looking at how oysters grow when predators cannot eat them compared to how they grow when predators can eat them. This is a classic example of what is known as a caging experiment. At each site we are also monitoring water quality similar to the SWMP program at the GTMNERR.

We are collecting two types of environmental data at our sites. We have loggers out which are collecting continuous and long term data, and larger instruments which are only deployed while we are visiting the sites for data collection, but collect data at a higher resolution. For the long-term data, we have two small loggers which take a reading every 15 minutes. The first is a salinity logger, which measures the conductivity of the water as a proxy for salinity. Sea water has sodium ions in it (that’s why it is salty!), which have a positive charge.

The two loggers with the salinity logger on the right and the tidal logger on the left

The more sodium ions in the water the higher the conductance, and the higher the salinity is. The second logger is a water level logger, which compares the pressure from a logger in the air (“air logger”) to the pressure of the logger at each site. When the pressure readings from the “air logger” and the logger at the site are the same, that means the water level is below the level of the logger, and it is under 0m of water.  When the tide comes up, the logger is underwater and that the pressure readings on the logger at the site will be greater. Using the logger software, we can compare the air logger with the logger at each site to find out how high the tides were on a certain day at a certain time. We use this information to see how much time the cages spend out of the water throughout the GTMNERR, because oysters require a certain amount of time both underneath and out of the water. If oysters are underneath the water for too little or too much time, then oyster reefs can begin to deteriorate.

The other instruments we have are ones that we put out to collect data while we check the experiments every month.  They collect a reading every minute so they give us high quality data for a short amount of time, from which we can create a snapshot of the site and compare it to the other sites in the reserve. 

HydroCAT-EP in its housing ready for deployment

The first instrument we have is called a HydroCAT-EP. It collects a water sample every 77 seconds, tests the sample, and then stores the information in its internal memory.  We offload the data from the instrument when we come back from the field. The HydroCAT-EP collects data on salinity, temperature, and depth (just as the loggers do). Several other parameters that this instrument is able to measure are dissolved oxygen, pH, and turbidity. Dissolved oxygen, the amount of oxygen in the water, is important to know because low oxygen or hypoxia can lead to mass die offs of oysters. The instrument also measures pH which is how acidic, neutral or basic the water is. Open ocean water is slightly basic but as the oceans are absorbing more CO2 they are becoming more acidic and change the chemistry of the water.  Turbidity measures the cloudiness of the water; if the water is crystal clear like in tropical waters the turbidity is very low. When the turbidity is high that means there is sediment, algae, bacteria or pollutants in the water.

The turbidity is pretty easy to measure on your own with a Secchi disk and some rope.

Secchi disk under the water.

You tie the disk to a rope and slowly lower it down until you can’t see the disk any more, and then measure the amount of rope you used. If you put the disk in and immediately can’t see it that means the water is pretty turbid. Chlorophyll-a is a pigment found in plants so measuring it can give us and an indication of the amount of phytoplankton in the water. Oysters eat plankton, so this could tell us which sites have more food available.

The last instrument we put out is an ADCP to measure the flow of the water. ADCP stands for “acoustic doppler current profiler”, and the instrument uses the Doppler Effect to measure the velocity at which water is passing the instrument. It emits a sound and measures the echo of the sound back to the instrument to determine how quickly the water is moving.

ADCP in the housing. The signal is emitted and received from the three dots at the right

All of this long-term and high quality environmental data will be inserted into both models: one with the nonconsumptive effects and one without the nonconsumptive effects to see which model best predicts reef success in the reserve.

Keep your eyes out for us in the GTMNERR and we will be happy to show you our experiments and instrumentation.

About the Writer

Adrienne manages the water quality instruments, as well as the outreach projects, for the Kimbro Lab in Florida. Prior to joining the lab Adrienne worked with Dr. Joe Ayers for her master’s project, through Northeastern’s Three Seas Program, along with the Outreach Program at the Marine Science Center in Nahant, MA. During her tour of the MSC she always made sure to point out Boston from the top of Eastpoint along with Deer Island Wastewater Treatment Facility. Prior to her Master’s, Adrienne interned, volunteered and worked at the New England Aquarium where she dove in Ft. Wetherill, Rhode Island; Bimini, Bahamas; and the Phoenix Islands. When not working she can be found diving in the springs or reading on the beach.

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