A World of Water Under One Roof

When we think of aquatic habitats, we often fixate on the different lifeforms living in them. The water flowing through these ecosystems has its own diversity, too, though. Walking through the National Aquarium is a journey across the world, from the tropical reefs of the Indo-Pacific to the cold seaside cliffs of the northern Atlantic. These exhibits mimic their natural counterparts, down to the water flowing through them.

Behind the scenes, an aquatic circulatory system is at work. Its network of pipes, valves and reservoirs spans each portion of the building, transporting water to and from exhibits. Aquarium Life Support staff and aquarists oversee its operation to ensure all 2.4 million gallons of water are just right. And it all starts with the same water that flows from faucets around the city: clean, fresh water that goes on a journey all its own once it enters our building.

Salt Water Solutions

Most Aquarium exhibits replicate marine habitats and contain hundreds of thousands of gallons of salt water. Life Support crews first divert some incoming fresh water to a special mixing vat to make a salty solution. The fresh water passes over activated carbon filters on its way there, where charcoal pellets (like those found in household Brita filters) pull chemicals like chlorine out. (While chlorine is good for disinfecting our drinking and tap water, it and other chemicals are irritating and harmful to aquatic animals.)

This dechlorinated water then heads to a 30,000-gallon mixing tank where, over the course of a day, 8,000 pounds of added salt turns it into a solution twice as salty as the ocean. Then, Life Support shunts the super salty water into enormous storage tanks and dilutes it, adding another 30,000 gallons of filtered fresh water. Now, they have a good reserve of salt water, ready to pump into many habitats.

The Salty Seas

More than 70% of the Earth's surface is covered with water, 97% of which is salty. This salt water provides habitats for almost 80% of our planet's life. It's only fitting then, that more than two-thirds of the Aquarium's exhibits—totaling more than 2 million gallons in volume—exclusively house marine life.

Salt Water Bound for Blacktip Reef

Blacktip Reef is one of the first exhibits guests encounter. It transports them to an Indo-Pacific reef submerged in over a quarter million gallons of balmy salt water. This corner of the globe experiences seasonal monsoons, so Life Support staff mimic this by adding filtered fresh water or salt water as needed. Opening a few valves brings salt water up from storage tanks in the depths of Pier 3. Over three months, new salt water replaces about 15-20% of the exhibit's total volume.

Although the added water is clean, it doesn't stay that way for long. After all, more than 600 animals live in the exhibit—and they need to eat! Targeted or broadcast feedings add nutrients to the system. This, combined with fish excreting waste, affects water quality. In actual reefs, detritivores (organisms that eat waste or dead material) and bacteria break this fecal matter down, or ocean currents dilute it by whisking water away. In the Aquarium, the water enters a filtration system instead.

Water's Journey Through This Reef

Once out of the exhibit, Blacktip Reef water pushes through a series of filtration chambers. In one, it squeezes through heaps of sand that catch large clumps of waste. In another, air bubbles through it, trapping dissolved waste in a frothy, floating foam (just like waves on a beach do). Ozone—another gas molecule that breaks through cell walls—also churns through it, killing germs and further clarifying the water. In yet another filter, water crashes over honeycomb-like material that houses bacteria, which actively feast on any remaining waste particles. This "waterfall" also acts as a gas exchanger, where exhaled carbon dioxide from the animals can escape the cascading water while oxygen from the surrounding air gets mixed back in. The final product emerges cleaner than before, is warmed to the right temperature (about 75-80 degrees Fahrenheit) and pumped back into the habitat.

To keep the filtration system running effectively, Life Support will sometimes run parts of the system in reverse to reroute water flow and expel built-up waste into a separate vat. Like wastewater in your home, this water and residue need more intensive processing. Instead of flushing it all, it's separated into two categories: intensively recycled and cleaned water that can be used elsewhere in the Aquarium, and a sludgy residue that is sent to the city's sanitary system.

Corals take minerals and ions from seawater, like dissolved calcium and carbonate ions, to build their skeletons. Two factors affect how many calcium and carbonate ions the corals can pull from the water: the water's acidity (pH) and alkalinity. Alkalinity measures how well a body of water can resist changes in pH. If alkalinity isn't high enough in reef seawater, acidity increases, which makes it difficult for coral to take up and hold onto carbonate ions. These chemical reactions are what make ocean acidification so concerning. In the Aquarium's controlled environment, staff track these and other factors, tweaking the chemistry to keep it in an optimal range and adding calcium carbonate so the corals get the minerals they need.

Coastal and Estuarine Waters

Coastal habitats make up about 8% of the Earth's land area. Fresh- and salt water combine in these regions, creating brackish water. Brackish water is technically any water with more salinity than fresh water, but less than that of salt water. A handful of Aquarium exhibits are filled with brackish water; what artificial seawater isn't used in the marine exhibits flows to these habitats instead.

Our Aquarium's Marsh

Open ocean habitats tend to have more stable water quality. Minerals, nutrients and sediments are less concentrated in these large bodies of water. Closer to land, though, water quality is more dynamic and aquatic life has to tolerate more extremes. Here, rivers drain into estuaries and lower coastal salinity, receding waters leave behind tide pools that quickly warm in the sun's rays, and nutrients spike and fall with heavy rainfall or plankton blooms.

The Chesapeake Marsh in Maryland: Mountains to the Sea is a good (if simplified) example of such a coastal habitat. Similarly to how staff manage salinity in Blacktip Reef, they make a 50-50 mix of filtered, dechlorinated fresh- and salt water to mimic the brackish conditions of Maryland salt marshes.

This Marsh's Ebb, Flow and Filter

Smaller exhibits like Chesapeake Marsh experience faster water quality changes, especially if many animals live in them. Staff frequently check water quality, monitoring levels of waste like ammonia, nitrates and nitrites. Exhibit water also circulates through an attached filtration system. This system has a sand filter and gas exchanger but lacks an ozone bubbler. While an excellent sterilizer, ozone can be too volatile in a small system, so this system uses ultraviolet light instead.

Despite all the cycling and filtration, waste levels still slowly climb; changing out some of the water and replacing it with a new batch keeps the overall water quality in check.

A Freshwater World

Although freshwater habitats cover only about 2% of the Earth's surface, they house almost as many different species as marine environments. And that isn't even counting all the terrestrial life that relies on this precious resource! About one-third of the Aquarium's exhibits contain fresh water, and just like with artificial seawater, the process of making and filtering it is intense.

Recreating River Water

Rivers carry the marks of plant life and soil as their water flows toward the ocean. In the Amazon River, the water chemistry in the river basin's hundreds of tributaries is as diverse as the surrounding forest. For the Amazon River Forest's much smaller volume, though, Life Support chose to replicate the conditions specific to the Rio Negro tributary.

Pure water, like rainwater, is slightly corrosive. In many rivers around the world, rainwater lands on rocks and pulls minerals from the stones. However, the land the Rio Negro flows through is covered in centuries' worth of thick layers of plant material that act like a barrier and keep rainwater away from rocks. This river, therefore, has low alkalinity and cannot resist changes in pH very well. As tea-colored tannins seep from tree bark and leaves, the water becomes stained and more acidic.

Following Amazon River Forest's Route

The best way to replicate this unique aquatic habitat starts with lots of fresh water that has gone through reverse osmosis. Reverse osmosis is a process whereby fresh water is pushed through a membrane that stops impurities from passing through. Stripped of minerals and impurities, it already has a slightly lower pH. Life Support further lowers the pH by injecting carbon dioxide, mimicking the natural process of rainwater absorbing this gas as it falls through the atmosphere. As carbon dioxide mixes with water, it reacts to form carbonic acid—the same chemical reaction at play in ocean acidification. Excess carbon dioxide would spell disaster in most marine exhibits, but here, it helps create the conditions to which Amazon River species have adapted.

Staff monitor alkalinity and pH as the water continues flowing through the filtration system. Like with Blacktip Reef, the water squeezes through large sand filters before the ozone bubbler thoroughly cleans it. What extra carbon dioxide makes it through the system escapes via the gas exchanger or is absorbed by the live plants growing throughout the exhibit. Routine water quality checks reveal if waste levels are gradually increasing, after which a partial water change resets the levels.

An Evolving Practice

As with most science, the technology and methods behind maintaining aquarium water quality continue to improve. Creating a simplified version of a habitat's natural counterpart is not easy, and replicating these habitats has led to innovation in resource management and conservation, energy efficiency and more.

Maintaining an aquarium is a humbling and challenging exercise. It's all about finding the balance in a system where every animal, plant and mineral can tip the scale. Through it, though, we can better appreciate our planet's awesome ability to circulate these diverse bodies of water that make life possible.