It takes a microscope to get a good look at these minuscule eelgrass grazers: Caprella penantis (above), Gammarus mucronatus (below, top), and Bittium varium (below, bottom). Photographs by Paul Richardson.

EMMETT DUFFY IS GOING HUNTING for " 'pods."

He lowers himself from his boat into the water off the Goodwin Islands near the mouth of Virginia's York River. The scientist sinks waist-deep in his wetsuit, but he's pleasantly surprised. It's been a cool spring, and he was expecting the water to be chillier. "Not too bad," he says to the handful of researchers out with him today. "Alright, let's have some fun."

The water's crisp and clear today, so you can make out the clumps of eelgrass (Zostera marina) poking up from the sandy bottom, around three feet below the surface. It's the start of what by summer will be a thick and green Chesapeake Bay grass bed. And this is where Duffy, a marine scientist at the Virginia Institute of Marine Science (VIMS), will be looking for a kind of biodiversity that few people have the patience or wherewithal to see.

To get a better look, Duffy's lab manager, Paul Richardson, scrapes a net along the Bay bottom, then dumps the contents into a tray. There are scores of small creatures amid the blades of eelgrass and ribbonlike algae.

Richardson fondly refers to them as bugs or 'pods because most of the creatures are types of crustaceans called amphipods. The most numerous are the caprellids. If you peered at them through a microscope, these skinny, green animals (Caprella penantis) would look like tiny plant stems. Right now, they're clinging to the eelgrass blades, curling and unfurling like inchworms. Then there are small snails, called gastropods (Bittium varium), sporting shells that look like peppercorns. Richardson counts more crustaceans that resemble petite shrimp (Gammarus mucronatus).

In all, it's a world that takes attention and good eyesight to appreciate. "Who cares about bugs?" Richardson jokes. "We do."

They have good reason to care about them. Duffy and his colleagues have discovered that the creatures they collect in their trays do a lot to keep these underwater grass beds clean. The small animals consume harmful algae that otherwise would grow on the plants and stunt their growth.

Research like Duffy's shows that preserving biodiversity in the Bay and elsewhere is important, not just for its own sake. It's also important to people. Even small, hard-to-find organisms like these 'pods are part of the web of life that stretches up from eelgrass beds to crabs and striped bass and, eventually, to people.

Despite its importance, biodiversity in the Bay and other marine habitats is fragile and threatened. Duffy says there's a lot to learn about it before more is lost.

"Biodiversity is not just an aesthetic issue," Duffy says. "It's fundamental to how nature's life support system works for us."

On this spring morning, Duffy looks at home in his mask and snorkel. They've been his window into learning about the beauty of biodiversity. As a graduate student in the 1990s, he spent five weeks diving on Australia's Great Barrier Reef. "It was this absolutely other worldly experience," he says. "Huge fish, tons of coral, clear water."

But since then, that coral reef has lost large portions of its big fish and coral. So have other marine communities worldwide, for many of the same reasons — overfishing, global warming, increasingly acidic waters, invasive species, and nutrient pollution among them, researchers have concluded. As biodiversity has suffered, Duffy says his outlook has expanded to consider how its loss could affect humans and their wallets. "I now understand that that's a really important other aspect," he says. "Since the world is run by economics, you have to talk about that."

Often seen in a mask and snorkel, Emmett Duffy may talk about the economic benefits of maintaining biodiversity, but he also thinks ecosystems like eelgrass beds have a value all their own. Recently, to sing the praises of small eelgrass grazers, Duffy strummed guitar in a music video produced by his lab. Called "Grazer," it was set to the tune of "Loser" by Beck, a staple of the 1990s alternative rock scene. Photograph by James Kealey.

Scientists now agree that diverse ecosystems also tend to be healthy ecosystems — and potentially lucrative ones. They use the term "ecosystem services" to describe the economic benefits created by the web of life. Those benefits include things like an active seafood industry, added value to real estate, and clean drinking water. In Maryland alone, seafood generates an estimated $600 million in economic gains each year. Tourists, many of them drawn to the Bay, spent more than $14 billion in the state in 2011. The full worth of the Bay's ecosystem services is probably much larger.

In the Chesapeake Bay, that value rests partly on some of its tiniest residents, like the algae grazers. The 'pods aren't as captivating as tigers, pandas, and other zoo animals, but they deserve some respect.

To understand, consider the eelgrasses dotting this chain of islands on the York River. Besides providing a home for fish and crabs, these plants deliver other important benefits in the Chesapeake. They trap sediments that would otherwise cloud the water, and they soak up excess nutrients that feed dangerous blooms of algae.

But come back during the summer, Duffy says, and you'll see a layer of brown fuzz that looks like gnarly dryer lint covering much of this eelgrass. It is actually a type of algae that grows directly on plants. Such "epiphytes" are common around the Bay but can be bad news — when nutrients in the water are plentiful, this algae grows a little too abundantly. A thick layer of epiphytes can block sunlight from reaching the underwater plants, essentially causing them to starve. What's more, unlike trees, eelgrasses in the Bay grow in monocultures. So if they die off, there are no other species to come in and take their place.

That same fuzz contributed to the collapse of underwater vegetation across the estuary in the 1960s and 1970s, brought on by increasing nutrient and sediment pollution and other factors. Historically, such plants may have covered more than 600,000 acres of the Bay's bottom. In 2012, that number was less than 50,000 acres. It's hard to put a dollar amount on such a loss, but the decline of bay grasses around the estuary has likely taken a huge toll on the local fishing, seafood, and tourism industries.

Thankfully, bay grasses have a natural ally in the 'pods. They graze on the smothering algae like tiny, helpful cows. Duffy and his colleagues conducted an experiment on the Goodwin Islands that demonstrated the importance of this behavior. The researchers used a special, repelling chemical solution to chase grazers away from certain eelgrass beds. When the grazers were gone, algae growth more than quadrupled on the plants. In other words, these cows make a big dent in algae populations. The team published their results in 2013 in the journal Ecology.

The diversity of grazing species is important, too, Duffy says. In a 2010 paper published in Marine Ecology Progress Series, his team showed that communities of grazers made up of many different species ate a lot more algae than less diverse herds. There are a number of reasons why that could be. It's possible that "if you have more than one species, they may be using resources more efficiently," Duffy says. On its own, each animal might focus its gluttony on only one type of epiphytic algae. Put a lot of species together, however, and they can consume the whole smorgasbord of algae.

That benefit of diversity seems to be a general phenomenon. In a project called the Zen Experimental Network, or ZEN, Duffy teamed up with scientists at more than 15 sites worldwide who also investigated grazers on eelgrass. This notoriously widespread plant grows from Alaska to Portugal and Japan.

The number of species at each of these sites varies. In the Bay, a typical bed will host 15 to 20 species of grazers, including the caprellids, gastropods, and others in Richardson's tray. But you can find dozens of species in San Diego or only a handful in Norway. Still, preliminary results from the project indicate that no matter the site, "when you have more diverse grazers, you see less algae," Duffy says.

Think of the grazers this way: they clean up after humanity's mistakes by providing an important counterbalance to nutrient pollution. But as important as the creatures are, scientists don't have the data to say whether small grazers across marine habitats worldwide are thriving or struggling.

Lush eelgrass beds surround the Goodwin Islands (map above) near the mouth of the York River. Such Bay grass communities support the Chesapeake food web (graphic above), providing food and shelter for many Bay animals, including fish, birds, and crustaceans. But excess nutrients and sediment in the Bay fuel the growth of special kinds of algae known as "epiphytes" (the brown fuzz covering the eelgrass in the photo above) that can overrun grass beds, thinning them out. Goodwin Island detail map, Google Maps, adapted by Sandy Rodgers; inset Bay map, iStockphoto.com/University of Texas Map Library; Photograph, Jonathan Lefcheck; Graphic, U.S. Geological Survey (modified from Perry et al., 2005).

Similarly, there's a lot that scientists don't know about the status of marine biodiversity in the Chesapeake Bay. You could point to certain species that exist only as small reminders of their former abundance: oysters, shad, and Atlantic sturgeon, to name a few. But researchers don't monitor many organisms further down the food chain, or at what scientists call a lower "trophic level," to estimate whether the numbers of these small organisms are climbing or falling. So while Duffy's grazers may be threatened in the Bay — just like shad or oysters — no one knows for sure.

That sort of information could tell you a lot about the health of the whole estuary. It's not just because grazers help bay-grass beds, but also because such small animals are important food for fish. In other words, protecting biodiversity doesn't just mean protecting a lot of different species at one step in the food chain, but throughout the entire ecosystem.

"I think that we need to do this [monitoring] if you want to ensure sustainability and productivity" of the ecosystem, says Edward Houde, a fisheries scientist at the Chesapeake Biological Laboratory at the University of Maryland Center for Environmental Science. "You do want to protect the lower trophic levels in the ecosystem."

What's clear is that biodiversity in the Chesapeake is likely more fragile and easily disrupted than in more diverse ecosystems. As in most estuaries, the salt levels here are rarely steady, thanks to the never-ending tides that send jolts of salt water into and out of the Bay. That makes it a tough environment for many of the plants and animals who live there.

As a result, the Bay hosts more than 2,700 species of plants and animals — a big-sounding but modest number compared to what you'd see in many tropical ecosystems. The Great Barrier Reef, which Duffy visited years ago, is home to an estimated 5,000 or more species of mollusks alone, including clams, octopuses, and oysters.

The bottomline is that the Bay can't stand to lose much of its biodiversity; it has relatively little to begin with. Take small crustaceans: A single shrimplike crustacean — a species of opossum shrimp that grows about as long as your fingernail called Neomysis americana — is a primary source of food for many of the region's juvenile fish. Those include young weakfish, summer flounder, and striped bass. Lose that shrimp, and a lot of animals that people like to eat could go hungry.

"You have less redundancy," says Andre Buchheister, a graduate student at VIMS who studies fish biodiversity in the Bay. "So if something happens to one species or one group, you have the potential for altering the system much more dramatically."

Responding to this lack of knowledge about marine biodiversity, Duffy led a team that proposed the creation of a Marine Biodiversity Observation Network. In a 2013 paper in the journal BioScience, he and his colleagues advocated a wide monitoring program that could be undertaken globally or in a local habitat. It would track the rise and fall of marine life at all levels of the food web. Right down to little grazers.

On the Chesapeake, many scientists have already begun work that could fit into such a network, Duffy says. Scientists at VIMS, for instance, conduct a regular trawl survey of the estuary, pulling up fish that live near the bottom of the Bay to monitor how their populations are doing. By combining this survey with others, including his own work on the Goodwin Islands, Duffy says that it may be possible to get a much better look at the Bay's biodiversity from top to bottom.

"We basically need a health plan for biodiversity," he says. "We need to have a monitoring program so we can keep our finger on the pulse and find out what's wrong so we know what to do."

He'll be among those giving the 'pods their checkup.