A watershed is where one of the longest running battles in the planet’s history is fought on a daily basis. The power of water and gravity seek to cut and erode the soil, as biological resources seek to hold their ground. The degree to which the powers of erosion emerge victorious in this daily struggle is a reflection of what is happening on the land throughout the watershed.

This is a story that has been told at numerous meetings recently by Nick Carter, a biologist with the Maryland Department of Natural Resources’ Tidewater Administration, in an effort to explain why protecting watersheds is essential to water quality and to the land’s ability to support life.

“Gravity and running water are the two principal forces that erode the earth — and anything on it — downhill, downstream,” Carter said. “Barring mechanisms that act against that, sooner or later all the mineral wealth, or the Earth’s crust, would end up at the bottom of the deep sea.”

The mechanisms that keep that from happening are the living organisms — the plants and animals — that capture, store, and use energy from the sun and materials from the land. Those organisms are made up of the basic materials or life that are of limited availability: carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and others.

Because these elements are limited, they must continually be reused — recycled — in a given area. As a result, the species that evolve within each potential habitat —sunlit or shaded, wet or dry, hot or cold —are the ones that are the most efficient at absorbing energy, and retaining and recycling materials.

“Generally speaking, anything less than these species — anything other than those which have evolved to fill their particular niches — is less efficient in that given situation at carrying out those basic functions,” Carter said. “When the system efficiency is lowered in a watershed, the watershed begins to excessively export materials down hill, down gradient, and downstream.”

This is why, Carter said, that not only are living resources important, but also the diversity of those living resources. Each is specially adapted to its situation: The hemlocks, beeches and dogwoods that are shade-tolerant, and the willows and black locusts that require lots of sunlight. Each occupies its own niche, absorbing energy and retaining nutrients, reducing the amount that runs off the land.

Other mechanisms work to capture nutrients as they escape. When the leaves from the trees enter the water, mayflies, stoneflies, and caddisflies and other insects break them down and absorb nutrient minerals. The insects, in turn, are consumed by other predators, which ultimately are eaten by brook trout in headwater areas Freshwater clams and other filter feeders collect phytoplankton, retaining additional minerals upstream. Wetlands further reduce the loss.

The watershed is filled with many similar complex interactions that serve to retain nutrients and sediment in the uplands. Each change of the landscape alters this mix. That is why, Carter noted, that runoff increases when forests — the native ecosystem for most of the Bay watershed — are removed. Trees are particularly important because — being large — they bind up lots of material.

But even smaller, more subtle changes have impacts. When a degraded stream loses its fish diversity, the nutrients that would have been consumed by specialized feeders are left to flow downstream.

“A specialist is always going to be more efficient in that narrow situation than some generalist that may be able to use it some of the time,” Carter said. “If you eliminate that specialist, then you have an available amount of nutrient and energy that this critter was able to utilize and exploit that is no longer going to be exploited as efficiently as before. And if not exploited to the natural efficiency, if not bound up, then that site is a candidate for exporting that mineral and that fixed energy down drainage. And this is exactly what happens as we alter the watershed.”

What is happening in the Chesapeake is nothing new, Carter said. Over a period of 900 years, the coastal Mediterranean city-state of Ephesus sustained its development on logging and agricultural activities upstream from the city. Over time, these changes resulted in a loss of the natural retention mechanisms, and the land eroded downstream to the Aegean Sea. By the 2nd Century AD, Ephesus was 2 miles inland.

What happened to Ephesus in 900 years has happened to Joppatown, Md., in 400, Carter noted. Once a seaport, Joppatown is now landlocked, the result of landscape-altering activities upstream.

While humans have engineered techniques to reduce runoff, none are as efficient as the natural processes they replace, Carter said. “The best that we can do engineering-wise,” he said, “looks like it’s kind of a poor shadow in terms of relative efficiencies.”