Nantucket Coastal Plain Pond Hydrology and Globally Rare Plants

*Please note, this blog post was originally published in The Inquirer and Mirror July 14th 2016 pg 4B in the article series Island Ecology. The Foundation’s Science staff will be regularly contributing to our local newspaper and reprinting articles here in the following week.*

Alamanc Pond, a large Coastal Plain Pond on Nantucket, full of water in mid-summer

Alamanc Pond, a large Coastal Plain Pond on Nantucket, full of water in mid-summer

After a long spring drought with weeks of no rain in June, Almanac Pond in the Middle Moors sat full to the brim with water.

Wait, what?

Of all of the questions I hear about wetlands each year, the water levels in the smaller ponds throughout the island tops the list, usually because the pond levels seem so different from expectations based on weather. On Nantucket Island, our thoughts about water focus on the harbors and the Great Ponds where water levels make intuitive sense – high with high tide, low with low tide or, in the case of the Great Ponds, low when the beach opens and high when closed. But the little ponds throughout the inland parts of the island; Almanac, No-bottom, Wigwam, Jewel, the Pout Ponds and many more, are an entirely different kind of pond.

Coastal plain ponds: small, shallow freshwater ponds found throughout the Northeastern US and Canada and concentrated in coastal areas of southeastern Massachusetts and Rhode Island are home to one of the highest concentrations of rare plants in the region!

Why do these ponds behave so differently from the other water bodies on Nantucket? Located in areas covered thousands of years ago by the last glacier, these ponds are depressions in sandy or gravelly soil and receive all of their water because they intersect the water table, meaning that groundwater runs directly into these depressions, independent of tides, streams or other surface water. Groundwater? On an island, fresh drinking water is key so you’ve probably heard of our sole source aquifer, the reservoir of freshwater contained under the island. This aquifer is our groundwater which is, on average, 10-20ft below the soil surface but can be as shallow as 0ft and as deep at 100ft below the soil. Where groundwater intersects the soil, water can well up out of the ground – you may have encountered a small burbling pool in Squam Swamp or seen water trickling out of the side of Sconset Bluff – these are springs of groundwater reaching the surface. Where the groundwater seeps up in depressions, coastal plain ponds form. Because of the glacial history of our island, which created pockets, valleys and kettlehole depressions, these groundwater-dependent coastal plain ponds are abundant on Nantucket.

Groundwater fed ponds behave differently than primarily surface water fed ponds (ponds that depend mostly on precipitation, streams or a coastal connection). Surface water pond levels change almost instantly because their water source is instantaneous water, leading to short term changes in water levels. Basically, when it rains, water levels in surface water fed ponds rise almost immediately, when it’s dry for a few weeks, water levels drop. Groundwater, on the other hand is a bit more complicated. Groundwater levels draw down due to pumping for drinking water and uptake by plant roots, but this response is buffered by soil and the amount of groundwater present. Our aquifer is so large, that drawdowns in the aquifer happen slowly over time – although they do happen! Because of the sandy soil on Nantucket, which allows water to move quickly through it, our aquifer can get quickly recharged (refilled so to speak) by precipitation as rain or snow. Water levels in groundwater fed ponds like Almanac Pond don’t change in the same way as more predominantly surface water fed ponds like Hummock Pond. Which is why this summer, after weeks of drought, Almanac Pond is still full to over flowing!

Almanac Pond with lower water levels and abundant coastal plain pondshore vegetation.

Almanac Pond with lower water levels and abundant coastal plain pondshore vegetation.

Typically, groundwater fed coastal plain pond water levels are highest in spring and begin drawing down very slowly over the summer and into early fall. As fall rains begin and recharge the groundwater, these ponds will start filling again in the early winter months. How full a pond is in the spring depends on the previous fall and winter precipitation, not that spring’s rain – so some years the ponds are very full and possibly over flowing their banks and in some years they can start out the spring already 5ft lower than usual.

This variability in water levels over a season is what makes coastal plain ponds so ecologically interesting. Coastal plain pond shores – the edges of these ponds that are variably wet or dry depending on the season and year – host one of the highest concentrations of both locally and globally rare plant species in northeastern US and Canada. The state of Massachusetts has more than 40 state-listed plants and animals that are found almost exclusively in these ponds. The plants that emerge, flower and set seed each year depend almost entirely on water levels, meaning the seeds of plants that come up when the pond is at its lowest might have to survive 5 or more years underwater, a unique adaptation for most plants. In a low water year – rare plant stalkers in New England rush to the coastal plain ponds hoping to see some of the more unique and seldom seen plants in flower. This year, our ponds seem extra high so the possibility of seeing some of these rare plants this summer is not likely but in low pond years you might get a chance to glimpse the lovely, very rare and unique Rhexia mariana (Maryland meadow-beauty). Most years you can find this plant’s cousin Rhexia virginica (Virginia meadow-beauty) higher up on the pond shore.

Rhexia virginica (meadow beatuy) found on the pondshores.

Rhexia virginica (meadow beatuy) found on the pondshores.

What else might you find along the edge of these unique wetlands? Carnivorous sundews often hide along the shoreline, waiting to capture tasty ants or flies for a snack – Drosera filiformis (Thread-leaved sundew) and Drosera intermedia (Spatulate-leaved sundew) can both be found on Nantucket! And, I’m still waiting to stumble upon the very rare and beautiful Sabatia campanulata (Slender rose gentian) – maybe next year will be a low water year!

The rare and carnivorous Drosera filiformis (thread-leaved sundew).

The rare and carnivorous Drosera filiformis (thread-leaved sundew).

These unique coastal plain ponds provide a direct link to our precious aquifer, hosting a range of rare and unique species. The canary in the coal mine of the quality of our island’s freshwater resources, these ponds are susceptible to any groundwater contamination through fertilizer and other nutrient inputs as well as over pumping of the islands aquifer. Luckily, most of these ponds are on protected conservation lands, allowing us to monitor and study them into the future.

Jen Karberg, Ph.D., is a wetland and plant ecologist and the research program supervisor in the Science and Stewardship Department of the Nantucket Conservation Foundation

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The Nantucket Conservation Foundation is a private, non-profit land trust that depends contributions from our members to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us!  www.nantucketconservation.org

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