The Kidneys of the Estuary EarthCache
The Kidneys of the Estuary
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Waypoint 4 of 10 on Going Coastal’s NY-NJ Harbor Estuary Earthcache Discovery Trail in Inwood Hill Park, caches developed by Going Coastal, Inc. (www.goingcoastal.org) as a special project in affiliation with Groundspeak and support from the NY-NJ Harbor Estuary Program and the New England Interstate Water Pollution Control Commission.
The NY-NJ Harbor Estuary Earthcache Discovery Trail is meant to help visitors develop a better understanding of the Estuary, make connections between earth and environmental science, and foster stewardship.
The Inwood Hill Salt Marsh is built on mud and dependent on tides. There used to be salt marshes surrounding all of Manhattan Island but unfortunately most of them are now gone. In fact there is only one natural salt marsh left in all of Manhattan, and you’re in it. What remains is this narrow strip of vegetation on the lagoon. (visit link)
Below the tidewater, a small fenced-off patch of cordgrass is in the process regenerating. The field is returning to tall grasses and other wetland vegetation. The ground is softening. The field is sinking. If left alone, the patch will spread across the ball field. (visit link)
Salt marshes depend on sedimentation. Sediments from eroded rocks and minerals formed a fine silt and clay that was carried from the land by rain and rivers. Sand was moved by the rise and fall of the tides. These sediment deposits settled as mud in the sheltered low-lying lagoon. This mud-type substrate (base soil) supports life on the salt marsh.
Salt marshes are transition zones between the land and the sea that are alternately inundated by the tides. Only about 1% of North America is salt marsh habitat. The marsh has discrete zones of increased gradient from the mudflats to the high marsh, characterized by the amount of tidal inundation.
Tides from the Atlantic Ocean drive the sediment process. The pull of the moon’s gravity and the sun’s gravity on the ocean causes seawater to rise and fall twice a day in New York City, about every six hours. The mean tidal range (the difference between high and low tide) at Spuyten Duyvil is 3.8 feet, high enough to cover the low marshland. The tidal range goes up to 4.6 feet in spring due to melting snow and seasonal rains when the high tide reaches the high marsh zone. (visit link)
You probably know tides best from the beach. When the tide comes in you have to move your towel up the sand and when it goes out, there is a farther walk to the water. Different parts of the estuary experience different tides. Tide information is available in tide tables so you can gauge the tide wherever you are in the harbor.
When the sea level rises with the flood tide it brings nutrients in. The mudflat and salt marsh are submerged, the level of salt rises (called salinity) and the temperature cools. When the tidal current rushes out to its lowest at the ebb tide it carries away waste. The intertidal zone (the band between high and low tide) is exposed.
The mudflat is left with winding channels of water that carry detritus and deposit sediment, the salinity level lowers and the heat rises. The salinity level is also determined by the amount of freshwater entering the marsh from the Hudson River, rain and runoff. This salty mix is called brackish water.
Once mud starts to build up, smooth cordgrass (Spartina alterniflora) takes root. This seeding of cordgrass is when the salt marsh is born. The flora (plants) of the salt marsh has to be tolerant to salt, because most plants cannot live in salty soil. Native saltmarsh cordgrass has tough, thick blades that absorb water and excrete excess salt. Touch a blade and you can taste the salt on your fingers.
Within the salt marsh is a complex food web (visit link) producers and consumers. Newly hatched fish (fish larva) drift with the tidal currents feeding on plankton. There are two types of plankton – free-floating microscopic plant-like phytoplankton (algae) and animal-like zooplanktons (fish larva, hatchling crabs and shrimps). Like all green plants, phytoplankton contain chlorophyll and capture sunlight, water and carbon dioxide (Co2) to grow and produce oxygen, called photosynthesis. Phytoplankton produce about 50% of the oxygen that you breathe every day! One teaspoon of seawater holds about a million phytoplankton. They are at the base of the world’s food chain providing food for small fish that larger creatures eat it.
The salt marsh is a factory for building land from open water. Marshes require a base of tidal sediments, usually silt/mud. The colonization of the mudflat border by grasses is called succession of plants. The salt marsh stores huge quantities of organic carbon in the form of marsh plant material. During warmer months you can see the cord grass. It naturally anchors mud, steers tidal flow and traps sediment expanding the marsh. Each autumn, the shoots of cordgrass die back accumulating into salt marsh peat that builds marsh elevation and can mitigate the effects of sea level rise from climate change. The plant roots survive the winter with fresh shoots pushing up each spring, starting the cycle anew. An established marsh can keep up with a rising sea; and, the marsh spreads out landward as it grows vertically as the water level rises.
Saltmarsh cordgrass provides a nursery for young aquatic life that lives in and around its stalks, finfish like stripped bass. It is the primary producer of the salt marsh and the base of a vast food web. The food web starts with the sun. Live cordgrass is not a source of food, but bacteria, algae and fungi feed on the decaying plants, leaf litter, and other floating matter breaking it down to detritus, also called marine snow. The detritus feeds insects, worms, ribbed mussels, marsh snails, and fiddler crabs that burrow and crawl over the loose surface of the mudflats. These in turn feed the wading birds, such as Great blue herons and snowy egrets. The feces from the birds fertilize the growth of new cordgrass.
Salt marshes are the kidneys of the estuary by purifying water and filtering nonpoint source pollution from land runoff, precipitation, atmospheric deposition, drainage, seepage or hydrologic modification.
Logging Tasks:
1. What is the tide level at the time of your visit? Given the present tidal flow, are nutrients being added to the salt marsh or is waste being removed? Include the date and time in your answer.
2. Describe in detail the mud at the salt marsh. What is the color of the mud? Do you see bubbles, holes, snaking channels, etc. Is it firm ground? Could you easily walk across it at low tide? How does mud act like a liquid and a solid at the same time?
3. What is trapped in the mud? How did it get into the lagoon?
4. (Optional) The salt marsh is naturally regenerating on the ballfields. Look closely at the fenced area and describe how this land, ground and vegetation differ from the surrounding landscape. How will rising sea level affect this landscape?
To log a find on this earthcache, email the cache owner (DO NOT POST IN YOUR LOG). Use your GPS device to locate the next cache - GC2RWJ1. (visit link)
Remember, to upload a photo and let us know in your log ways we can improve the trail.
Data Sources:
• Alien Teacher - (visit link)
• Going Coastal - (visit link)
• Harbor Estuary Program - (visit link)
• NYC Dept. of Parks & Recreation – (visit link)
• “Salt Marshes of New York City” NYC Parks Natural Resources Group
• Wikipedia “Salt Marsh”
• Welikia: Beyond the Manhatta Project - (visit link)
Data Collected: September 26 – March 11, 2011
Name and Type of Land
Inwood Hill Park
W 218 Street & Seaman Avenue, Manhattan
Phone: (212) 304-2365
www.nycgovparks.org
Additional Hints
(No hints available.)