A slightly larger-than-average dead zone is predicted for the Chesapeake Bay during the early portion of this summer, scientists say.

The prediction, which means fish and shellfish will find substantial portions of the Bay off-limits because it has too little oxygen, is driven largely by a slightly higher-than-average amount of nitrogen reaching the Bay from the Susquehanna River during the first five months of the year.

The forecast is based on computer modeling and is made annually by scientists from the University of Maryland Center for Environmental Science and the University of Michigan, and supported by the National Oceanic and Atmospheric Administration.

This year, they predict 8.2 cubic kilometers, or about 16 percent of the Bay’s mainstem, is likely to suffer hypoxic conditions, according to the forecast. Hypoxic water has less than 2 parts per million of oxygen.

In an average summer, about 6.3 kilometers are hypoxic, or about 12.4 percent of the Bay.

A smaller area will be even worse off. The scientists predict about 2.12 cubic kilometers, or 4 percent of the Bay, will be anoxic in early summer, meaning it essentially has no oxygen. That is slightly worse than the 29-year average.

But, by late summer, the anoxic volume is expected to drop to 1.34 cubic kilometers, or about 2.6 percent of the mainstem. That’s slightly better than average for late summer.

The volume of the mainstem Bay — which does not include the tidal portions of its tributaries — is about 51 cubic kilometers.

The hypoxic and early summer anoxic forecasts are driven largely by the amount of nitrogen reaching the Bay from the Susquehanna River from January through May.

Excess amounts of nutrients spur blooms of algae which eventually die, sink to the bottom and are decomposed by bacteria in a process that depletes the water of oxygen.

Above average rainfall increases the amount of nutrients washed off the land and into the Bay. Figures from the U.S. Geological Survey show that flows into the Bay during most of that period were in the normal range, but heavy rains in late April and early May resulted in significantly higher-than-average river flows during May.

High flows, particularly from the Susquehanna, create a barrier, known as a pycnocline, between freshwater on the surface and saltier water near the bottom of the Bay. The pycnocline prevents water on the bottom from mixing with surface water, which typically has a greater amount of oxygen. Thus, the oxygen consumed in deep water by decaying algae is not easily replaced, exacerbating hypoxic and anoxic problems in deep water.

By late summer, though, other factors such as wind and large storms, can increase the mixing of top and bottom layers of water, improving deepwater oxygen conditions.

The predictions only cover the portion of the Bay in the worst shape — those with 2 ppm of oxygen or less. Many other areas would be off-limits to some of the Bay’s most iconic species because they require significantly higher levels of oxygen.

Blue crabs, for instance, typically need 3 ppm or more, while striped bass need 5-6 ppm. As a result, far greater swaths of the Bay would not be available to those species.