By CLARA VAUGHN
ANNAPOLIS—In April 2011, more than 3,000 fish died in a pond in Upper Marlboro.
High demand for oxygen triggered a lethal drop in oxygen levels in the pond, investigators from the Maryland Department of the Environment found.
Three months later, 1,500 fish died in a storm water management pond in Charles County, also the result of lethally low levels of dissolved oxygen.
Over the last three decades, low dissolved oxygen has been the likely culprit behind more fish kills reported to the Maryland Department of the Environment than any other cause, including disease, toxins and discards from commercial and recreational fishing.
Low-oxygen conditions accounted for 519 of 1,770 lethal events investigated by the Maryland Department of the Environment since it began collecting fish kill data in 1984, a Capital News Service analysis of the data found.
Thats just shy of one-third of the kills.
Dissolved oxygen describes the amount of oxygen available in a body of water.
It is also an indicator of water quality and waters ability to support aquatic life.
There is a definite correlation between fish kills and low dissolved oxygen. Fish have to breathe, said Capt. Diana Muller, a South Riverkeeper who has been collecting water quality data in Anne Arundel County for more than 20 years.
The good news is that fish kills linked to low dissolved oxygen dropped below the long-term average in 2011 - possibly showing that cleanup efforts in the bay and its watershed are yielding positive results.
"It's a really good sign and is probably indicative of the amount of progress we've been making to reduce those pollutants. And it's indicative of the efforts we need to continue making, said Bill Goldsborough, the Chesapeake Bay Foundation's director of fisheries.
That includes measures like the state flush tax, a fee enacted in 2005 to fund upgrades to wastewater treatment plants.
Muller, though, warned there might be a less rosy reason for the decrease in fish kills linked to low dissolved oxygen.
There are fewer fish in the bay, so there are fewer fish to kill, she said.
And with more people moving to the bay watershed, sheer numbers might have eclipsed progress.
Most of the advances we've made in nutrient reduction have been cancelled out by the increasing population," said Chris Luckett, a natural resources planner for the Maryland Department of the Environment.
Nutrients are the building blocks underlying problems caused by low dissolved oxygen. Though plants need them to thrive, an overload of nutrients like phosphorus and nitrogen can throw an entire ecosystem off kilter.
Activities from driving cars to fertilizing crops add nutrients to the environment, which leak into waterways during storms.
When that nutrient-rich water stops flowing in tidal creeks or the Chesapeake Bay, algae flourishes.
It grows into this giant, floating algae monster, Muller said.
Eventually, that "monster" dies, devouring huge amounts of oxygen during decomposition.
Its totally out of whack. Thats not a normal ecosystem, not a healthy ecosystem at all, Muller said.
To combat the problem, restoration projects must be carried out, she said.
That includes restoring streams, planting buffer zones and cover crops to reduce runoff, and enforcing controls on the amount of fertilizer added to fields.
"We're holding our own in a situation where it (water quality) could have gotten worse, Luckett said. Ideally, it would have gotten better.
Muller agreed: We need to do something further to make it better.
To report a fish kill, algae bloom or oil spill, call 1-877-224-7229.
Visit http://www.mde.maryland.gov to learn more about water quality and fish kills in the state.