"I feel like a fork in spaghetti!" a student commented while standing in a bed of water chestnut. He proceeded to demonstrate by spinning around and winding the plants around his legs.
Despite this student's smile, the water chestnut (Trapa natens) is a nuisance invasive species. Native to Eurasia, this species was first introduced in the late 1800s by botanists who thought it a "handsome" plant. With a floating rosette of leaves attached to the river bottom by a narrow stem, the plant presents a lovely image. I can understand the plant's attraction. By the 1950s, however, this species had lost its allure as it spread throughout the bays and backwaters of the Hudson River Estuary.Water Chestnut grows in thick beds that are difficult for boaters to navigate. The spiny nuts collect on the shore and river bottom and injure the feet of swimmers. This species also has a significant impact on the ecology of the estuary by outcompeting native species and by reducing dissolved oxygen levels.
Last Monday, students from Marist College visited the Norrie Point Environmental Center to assess the Water Chestnut's impact on dissolved oxygen concentrations. The students used hand-held monitoring devices and walked into the bed to make their observations. Using the HRECOS real-time data display, however, you can make similar observations from your home computer.
Like all plants, water chestnuts release oxygen during photosynthesis. Unlike some aquatic plants, however, it releases this oxygen into the air and not to the water. In addition to this, the leaves of the water chestnut block light so other aquatic plants beneath are unable to photosynthesize. All the while, plants and animals are continuously using oxygen for respiration. The end result is a decline in concentrations of dissolved oxygen as it is used by consumers but not replenished by producers.
The HRECOS stations at Tivoli monitor the water flowing in and out of the bay. When the tide rises, it brings in fresh, oxygen-rich waters from the main stem of the river and the sensors detect an increase in oxygen concentrations. When the tide recedes, the sensors observe decreasing dissolved oxygen concentrations as the plant beds deplete the oxygen supply brought in by the high tide. It is for this reason that dissolved oxygen concentrations are so closely linked to the tides in water chestnut beds.
Standing in water chestnut beds at Norrie Point, the students from Marist College were able to observe the same phenomena as we see in the Tivoli Bay data. They began monitoring conditions while the tide was coming in and observed the oxygen levels rise as supplies were being replenished by the water from the main stem. Had they stayed long enough for the tide to recede, they would have then seen the oxygen levels decline just as occurs at Tivoli Bay.
What is interesting is that this pattern was not observed at the Norrie Point HRECOS station, just 300 feet away from the water chestnut beds where the students stood. This is because the Norrie Station is in the mainstem of the river where conditions are buffered by a significantly larger volume of water.
To observe the influence water chestnut is having on Tivoli Bay conditions right now, click on "current conditions" in the menu to the left.