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What's That in the Water?

This spring you may have noticed a new aquatic plant taking hold around the Boathouse and in the Lullwater area. We have identified the plant as Azolla caroliniana (synonym Azolla cristata Kaufmanii), more commonly known as water fern. Though it looks much like the duckweed (Lemna minor) growing around it, it actually is a fern. This species of Azolla is thought to be native in this part of the US.

The taxonomy of Azolla species can be challenging.  Even highly trained scientists have difficulty accurately comparing different species of Azolla without access to the reproductive parts, which can be very difficult to find without a very powerful microscope. After consulting taxonomists at both Brooklyn Botanic Garden and New York Botanical Garden we had a pretty good idea of what we had, but wanted to be sure that it was the native species and not the exotic A. filiculoides. New York City, being a major international port, can also be an introductory pathway for new invasive species. We wondered if this plant could have come in when someone released an animal from or cleaned out their aquarium.
(Please do not release unwanted pets in the Park. This includes: turtles, fish, frogs and toads, birds, snakes and lizzards,rabbits, hamsters, guinea pigs, etc.)

To further verify our identification, we worked with CUNY PhD candidate Stephen Harris and an intrepid group of high school students from Manhattan’s High School for Environmental Studies. These students have been trying to learn genetic techniques to assess the flora and fauna of NYC. Using DNA barcoding, they isolated the DNA for this plant and compared it to a national database of species.  Their DNA results confirmed what we suspected, identifying it as the native A. caroliniana.

In the picture below, the Azolla is growing alongside and mixed in with Lemna (Duckweed). The darker green blotches are the Azolla, while the lighter green plants are the Lemna.



We expect and hope that A. caroliniana, being a native species, will not have harmful consequences for our ecosystem.  That being said, we are watching closely since it is relatively new here. It has shown up in Prospect Park Lake in the past decade, but the population was apparently knocked back by ensuing cold, harsh winters. In some parts of the world, Azolla is considered an invasive species that local and federal governments are attempting to manage. In other parts, Azolla is hailed as a “super plant” or “green fertilizer” in rice-growing systems because of its fast rate of growth and its nitrogen-fixing relationship with a symbiotic  blue-green alga (http://en.wikipedia.org/wiki/Azolla). It should be further noted that the symbiotic blue-green algae  are only found in the fronds of the Azolla and are not toxic. In fact, we expect that the Azolla, much like the duckweed in previous years, will discourage blue-green algae blooms where it is dense and can cover the surface of the water; monopolizing all the sunlight.

The rapid expansion of Azolla in our aquatic system could be a symptom of nutrient enrichment. Azolla thrives in phosphorus-rich environments where it can take advantage of its nitrogen-fixing capabilities. Prospect Park does not use any fertilizers in the landscape. However, the water course and lake are fed by the New York City water system, which is treated with phosphorus in an effort to chemically stabilize the lead in old pipes and keep it out of the drinking water supply.

We’ll be watching this plant closely this growing season. Once a week we will take as much as we can get with the weed harvester, just as we have done with the duckweed. Mainly we do this to keep the watercourse around the Lullwater and in the area around the boathouse from getting so clogged that the electric boat can’t run. We will be interested to see the degree to which it behaves similarly or differently to the duckweed, and to see if they coexist or if one begins to crowd out the other. If it thrives and expands over the course of this summer, likely it will take on a reddish hue in the autumn.  As next winter comes closer, we’ll be curious to see how it reacts to a harsher winter. Last winter was quite mild, and we suspect a more intense winter could once again reduce the population of this plant.


Special thanks to Steve Glenn at Brooklyn Botanic Garden, Dr. Robbin Moran at New York Botanical Garden, Dr. Brett Branco at Brooklyn College, Ph.D. candidate Stephen Harris from the CUNY Graduate Center, and the students in Stephen’s group from Manhattan High School for Environmental Science.


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