In August 2010, at the head of Cobbs Creek in Mathews County, schools of young menhaden crowded into the shallow water behind one family’s house and died there on four different days: August 10, August 16, and August 28 and 29. Neighbors said the water looked thick enough to walk across. The local paper reported no official cause. The timing and the place, though, match a pattern scientists measure along the shallow edges of the Chesapeake Bay every summer.

Green water makes oxygen all day

Menhaden are small, silvery filter feeders. They swim in tight schools near the surface and strain tiny algae out of the water, and Virginia’s creeks are their nursery. In late summer those creeks run warm, shallow and green, because the water is crowded with algae. Every algae cell is a producer. In sunlight its chloroplasts carry out photosynthesis: they use light energy to build sugar from carbon dioxide and water, and they release oxygen. That oxygen dissolves in the creek, and by mid-afternoon a crowded creek can hold more dissolved oxygen than the air above it would put there on its own.

Everything in the creek uses oxygen all night

The algae do not stop living when the sun goes down. Like the fish, the crabs and the bacteria, they carry out cellular respiration every hour of the day: their mitochondria break down sugar with oxygen to make ATP, and release carbon dioxide. In daylight, photosynthesis adds oxygen faster than respiration removes it. After dark only respiration is running, and bacteria add to the demand as they decompose the algae that died the day before.

So the oxygen falls all night, and it is lowest just before the sun comes back. At 14 shallow monitoring sites in Maryland and Virginia, including the lower Rappahannock River and Nomini Creek, the daily low came between 6 and 8 a.m. and the daily high between 2 and 6 p.m. At some sites the dawn low dropped below 1 milligram per liter. Monitors call water hypoxic below 2 mg/L; a healthy summer reading in the Bay is about 7.

Cloudy days and hot water make the dawn worse

A string of cloudy days cuts the light, so the algae build less sugar and release less oxygen in the daytime. The creek starts the night with a smaller supply, while the demand from respiration stays about the same. Heat adds a second problem. Warm water holds less oxygen to begin with: sea water at 30 °C (86 °F) can hold at most about 5.9 mg/L, while cold fresh water at 5 °C holds about 12.8. Warm water also speeds up every organism’s respiration, the fish included. Hundreds of menhaden packed into the shallow head of a creek are all respiring at once, in the water with the least oxygen left at sunrise, with no deeper, cooler water to swim to.