Every May – October, volunteers from our Citizen Water Monitoring Network (CWMN) program go out with dissolved oxygen (DO) monitoring probes to several sites throughout the watershed. We carefully look over this data to see if there are any sites with unusually low dissolved oxygen levels. We then follow up on these sites as part of our Hotspot program to see if we can get to the bottom of the issue. This year, Turner Pond in Walpole was one of these sites.
Dissolved oxygen (DO) is a measure of the amount of free oxygen (O2) present in water. It gets incorporated into water bodies through diffusion from the atmosphere and photosynthesis of aquatic plants. In lakes, rivers, and ponds, the amount of DO is really important to monitor, as it’s an indication of general water quality. Fish and other aquatic organisms need DO to live. Water bodies with DO levels of 5 mg/L or above are favorable for fish and their eggs. Below 5mg/L, conditions are considered stressful for fish. Below 2 mg/L, conditions are not suitable for aquatic life at all.
We installed a DO sensor that continually takes measurements every 15 minutes in Turner Pond. We go to retrieve the logger and download the data once a month. What we saw was concerning – DO levels were consistently below 5 mg/L, and they would even sometimes drop to 0 at night. These conditions would be incredibly stressful conditions for fish living in Turner Pond. Curiously, there had been no reported sightings of dead fish in Turner Pond, and we even spoke to a fisherman who said he’d caught a fish that day.
DO conditions are known to fluctuate seasonally as water warms in the spring and summer and cools in the fall and winter. Temperature and DO have an inverse relationship, because the solubility of DO decreases as temperature increases. Warmer water holds less DO, so lower levels in summer are not always cause for alarm. Lower DO levels at night are also expected due to photosynthetic activity halting in the sun’s absence. This being said, DO levels below 5 are still concerning, any time of day or year.
How were fish surviving in these conditions? We took a morning to investigate. On canoe, we traveled in transects across Turner Pond, taking DO readings every foot in depth at 20 different points throughout the pond. What we found was interesting. At a one foot depth, the average DO level was almost 7 mg/L. At two feet in depth, it dropped to 5.5 mg/L. At 3 feet, it fell below the 5 mg/L boundary to 3 mg/L, and by 4 ft the DO levels were almost at 0. This stratification, or layering, of DO is a documented phenomenon in lakes and ponds. There is a point at which temperature in a lake sharply drops – called the thermocline. The layer of water under this line is called the hypolimnion, and is characterized by very low DO levels. The DO logger we had installed in the pond lay right at this 3 foot boundary. This means that fish could survive in the upper 3 ft of the water column in Turner Pond.
For comparison, we also went to nearby Willett Pond and conducted a similar survey. Willett Pond is larger and deeper than Turner Pond, and we found the DO readings to average around 8 mg/L throughout the water column until about 11 feet, where the DO levels steeply dropped off to zero. This confirmed the presence of a well defined hypolimnion layer here, too.
This assuaged some of our concerns about the bottom of Turner Pond being very low in DO, but still left us with questions about the upper water column. Only the very top 1 foot of the pond is over 5 mg/L and comfortable for fish to live in. One of the contributing factors to the low DO levels is likely runoff from neighboring lawns containing chemical fertilizers. The influx of excess nutrients leads to eutrophication, a state of over-enrichment that leads to low DO levels. Smaller ponds like Turner Pond are especially susceptible to this, and can often become overwhelmed with algae. We took measurements with a secchi disk, used to determine how deep light is penetrating in to water column. In Turner Pond, light was only getting about 2 feet into the water column. This is because of all of the algae present as a result of excess nutrients. For comparison, light penetrates Willet Pond to almost 8 feet.
Neighbors living near bodies of water should be conscious of the amount and type of lawn fertilizers they use in order to avoid harming local fish populations. With less runoff, algae amounts will decrease, allowing DO levels to return to normal above 5 mg/L levels.
Annie O’Connell, Environmental Science Fellow