The plastic bags and bottles that litter streets, rivers and beaches create an eyesore and a threat to wildlife that may ingest or become entangled in them. But increasingly, scientists around the region, and the globe, wonder whether tiny bits of plastic that go unseen may pose an equal or even greater threat.

Much of the concern about plastic marine debris has stemmed from emotionally charged photos of birds wrapped in plastic fishing line or tangled in six-pack plastic rings, or reports of plastic bags found in the guts of turtles.

But plastic doesn't easily go away. Over time, it typically breaks down into smaller and smaller particles known as microplastics, eventually becoming invisible to the human eye.

Now, scientists are worried that the growing concentrations of those unseen particles in the nation's waterways may be consumed by filter-feeding fish and oysters, or by tiny zooplankton, which form the base of the aquatic food chain.

"The smaller they are, the greater the population of critters" that can consume them, noted Robert Hale, a professor in the Department of Environmental and Aquatic Animal Health at the Virginia Institute of Marine Science.

There  is reason to suspect that portions of waterways and the oceans are becoming a soup of microplastic particles. Despite the increased global production of plastics, the amount of floating plastics observed in the North Atlantic over the last two decades has not notably changed, "indicating that loss through fragmentation, sedimentation, ingestion and deposition may be significant and should be quantified," said a report from a 2010 workshop on microplastics sponsored by National Oceanic and Atmospheric Administration's Marine Debris Program.

But the issue is so new — and has received so little study — that scientists are at a loss to estimate the level of risk posed by tiny bits of plastic. "There was high uncertainty surrounding many of the potential sources, stressors, effects and impacts," the 2010 report summed up.

Water-sampling equipment often uses nets with a mesh that is too large to capture microplastics. When they are caught, they are often discarded as debris. But in the few places where scientists have looked at microplastics over the years, they see an increasing trend.

Studies in Puget Sound by the University of Washington-Tacoma have found that as much as 10 percent of the total mass of suspended solids in water samples consists of microplastics, with the average being about 1.8 percent — and those samples don't include the smallest particles. The study indicated that limited work in the Chesapeake had similar results.

Interest about the impact of plastics on aquatic environments has grown in recent years since the discovery of large plastic-filled "garbage patches" in the Pacific and Atlantic oceans — areas where currents sweep together huge accumulations of plastics that wash off the continents and into the sea. The patches are a vivid reminder of how long plastics stay in the environment.

The qualities that make plastics useful —durability, light weight and long life — make them dangerous when they end up in the water, where they can last for decades, perhaps centuries.

To address those problems, manufacturers often design things like bags to degrade. But that doesn't mean the plastic goes away — it just breaks into smaller and smaller pieces. "From a chemical toxicology standpoint, maybe you traded one problem for another," Hale said.

Depending on their size and composition, as they break into particles plastics may release previously bound-up chemicals into the water. At the same time, limited research suggests the changing composition of the smaller plastic bits may make it easier for them to absorb other chemicals in the water. Tests have shown that some small plastic particles readily take up PCBs and DDT.

When something comes along and eats the plastic bit, Hale said, "they have basically ingested a pill of chemicals."

No one knows how long small plastic particles may stay afloat in a place like the Chesapeake. The particles might be flushed out of the Bay quickly, or they may attract a biological film that weighs them down and causes them to bind with other particles and sink. They might be buried by sediments, possibly releasing "microbursts" of chemicals, or continually be re-suspended in the water over time by waves and wind, Hale said.

There are two main sources for microplastics in aquatic systems. The first is from plastic trash which finds its way into waterways, where it gradually breaks down into smaller particles.

The second source is tiny plastic beads which are increasingly found in many personal care products such as scrubs, shampoos, soap, toothpaste, lip gloss, deodorants and sunblocks. People who never think of themselves as litterbugs may be washing plastic microbeads down the drain every time they wash their hair or take a shower. (Unilever, which manufactures such brands as Ponds and Axe, recently announced it would phase out the use of plastic microbeads in its products.) Microbeads are particularly difficult to measure because they are typically less than 0.3 millimeters (less than 1/80th of an inch).

"They were designed to go down the drain. All of our plankton tow nets are too big," said Kirk Havens, assistant director of the Center for Coastal Resources Management at VIMS. "Once they're in the system, you can't get them out."

Limited studies in other places have found accumulations of microbeads in the guts of mussels, and microplastics in some fish and even whales. Their ingestion could introduce toxins into the food chain; but even if chemicals were not a problem, the materials can create obstructions that keep the organisms from taking in enough food, leading to malnutrition or starvation, the studies suggested.

VIMS scientists are partnering with the Hampton Roads (VA) Sanitation District to determine whether microbeads could be made of a plant-based substance, polyhydroxyalkanoate, which rapidly biodegrades in the marine environment.

But for the most part, out of sight has meant out of mind, Hale said. "To the general public, the idea of microplastics is not sexy. It is like another invisible threat," he said. "But from the scientific standpoint, I think it is something that needs to be evaluated to establish how big a problem it really is."