The Chesapeake Under Pressure

Each summer, water near the bottom of the Chesapeake Bay’s deep channel typically holds too little oxygen for crabs and fish to stay. People call it a dead zone, and part of what feeds it washes in from farms, lawns, wastewater plants and even the air.

No one pours in a chemical that soaks up oxygen. So how can fertilizer far up the James River take oxygen from the Bay’s floor, and can people fix it?

Explore: design a cleanup for a watershed

You get a budget for a watershed that drains to the Bay and spend it on five fixes: forest buffers, cover crops, wastewater plant upgrades, oyster reefs and stormwater ponds. Each fix has a cost and a nitrogen effect. The simulation sketches the summer oxygen and seagrass response in relative units; real responses often lag by years.

Budget: 10 units. Drag a slider to spend on a fix, or press “All in”. Move several sliders to split the budget.

Forest buffers 0 units cuts nitrogen 3 per unit, up to 4 units (then the land runs out)

All in

Cover crops 0 units cuts nitrogen 4 per unit, up to 3 units (then the land runs out)

All in

Wastewater upgrades 0 units cuts nitrogen 3 per unit, no limit within this budget

All in

Stormwater ponds 0 units cuts nitrogen 2 per unit, up to 4 units (then the land runs out); the lesson names these but does not say what they remove; the rate is a placeholder

All in

Oyster reefs 0 units cuts nitrogen 0 per unit; rebuild habitat

All in

Spent 0 of 10 units Reset

Show the Bay: Early years Many years later

• low-oxygen bottom water
• underwater grass
• oyster reef

In this sketch (relative units)	Start	Early years	Many years later
Nitrogen load	100	100
Low-oxygen bottom water, summer	100	100	100
Underwater grass	20	20	20

Load = 100 − 0 = 100

No money spent yet. The nitrogen load stays at 100, so the low-oxygen zone and the grass stay as they are.

In this sketch: costs, cuts and responses are made-up relative units that show the logic, not measurements of any real place. Land limits how much buffers and cover crops can take. Low-oxygen water falls in step with the nitrogen load. Grass starts at 20 and rises 2 for every 1 the load falls, up to 100. Early years show 40% of the final change. Warming and weather are left out.

Rule to keep: less nitrogen feeds fewer algae, so decomposers use less oxygen and more light reaches the grass, and real responses can lag by years.

Try this.

1. Predict which single fix lowers nitrogen most for its cost. Spend the whole budget on it.
2. Split the same budget across three fixes. Predict better or worse, then test it.
3. Spend only on oyster reefs. Predict the change in nitrogen load, then check.

Explain: how extra nutrients use up oxygen

A watershed is all the land that drains into one river, lake or bay. The Bay’s watershed covers parts of six states and Washington, D.C. In Virginia, the Potomac (fed by the Shenandoah), Rappahannock, York and James drain to the Bay.

Runoff is rainwater that flows over the ground instead of soaking in, carrying what it picks up. A nutrient is a substance living things need to grow; here, that means nitrogen and phosphorus. Dissolved oxygen is oxygen gas mixed into water, measured in milligrams per liter (mg/L). Fish, crabs and oysters take it in through their gills.

Here is the chain, step by step:

1. Runoff, wastewater and air pollution carry extra nutrients into the Bay.
2. Algae grow fast. A sudden, thick growth of algae is an algal bloom.
3. The green water blocks sunlight from underwater grasses.
4. The algae die and sink to the bottom.
5. A decomposer is a living thing that breaks down dead matter. Bacteria decompose the dead algae and use oxygen as they do.
6. Bottom water runs low on oxygen, a state called hypoxia. Animals that can swim away leave; others may sicken or die.

Eutrophication is the enrichment of water with nutrients, which feeds too much algae growth and can lead to oxygen loss.

extra nutrients → algal bloom → algae die and sink → bacteria decompose them and use oxygen → low-oxygen bottom water

The fertilizer never takes oxygen itself; living things use it.

Why the deep channel, and why summer? Oxygen enters water from the air and from photosynthesis in sunlit water. In summer, fresh river water floats on saltier, heavier seawater, and the layers mix poorly, so little new oxygen reaches the bottom. Warm water also holds less dissolved oxygen. Shallow, well-stirred water rarely runs as low.

Picture the bottom water as a closed room full of breathers. The analogy breaks because storms can open the door: they mix the layers and bring new oxygen down.

Explain: people and nature both change ecosystems

An ecosystem’s capacity to support life depends on habitat, light, oxygen and food. People change these on land, in the ocean and in the air, and so do natural events. Large disruptions include eutrophication, climate shifts and catastrophes.

Change	Cause	One effect
Clearing forest for farms, roads and houses	People	More runoff and soil reach streams
Intensive farming with fertilizer and manure	People	Extra nutrients, some stored in soil and leaking out years later
Releasing chemicals in wastewater and from industry	People	Nutrients; some contaminants can harm wildlife reproduction
Burning fuel in cars and power plants	People	Nitrogen falls from the air onto land and water
Wildfire	Nature, sometimes people	Storms afterward wash ash and soil into streams
Drought or flood	Nature	Changes how much fresh water and nutrient reach an estuary
Earthquake	Nature	The 2002 Denali Fault earthquake in Alaska sloshed a Nevada desert pool, disturbing spawning areas of the Devils Hole pupfish
Climate shift (warming)	Nature and people	Warmer water holds less oxygen

In one computer-model study of the Bay, warming cancelled roughly 6 to 34 percent of the oxygen improvement from cutting nitrogen.

Explain: fixing a watershed by design

Human activity is not always harmful. Restoration is returning a damaged ecosystem toward health.

In 2010, the U.S. Environmental Protection Agency set a limit on the nitrogen, phosphorus and sediment entering the Bay, the Chesapeake Bay TMDL (total maximum daily load). Each state wrote a Watershed Implementation Plan for its share. Tools include:

• A riparian forest buffer is a strip of trees and shrubs along a stream. It catches runoff before it reaches the water, and roots hold the bank soil in place.
• Cover crops, planted on fields in winter, hold soil and take up leftover nitrogen.
• Wastewater plant upgrades remove more nitrogen. A 2022 study linked large drops in city nutrient pollution to them, and found that nitrogen falling from the air declined across the watershed from 1985 to 2019.
• Oyster reefs: Virginia has rebuilt reefs in the Lafayette, Lynnhaven and Piankatank rivers. Oysters filter particles from water and reefs shelter other animals, but reefs do not replace cutting nutrients.

Submerged aquatic vegetation is underwater grass, usually rooted in shallow bottoms. It shelters young crabs and needs light. A 2018 study found that as nitrogen in Bay water fell, underwater grass regained about 17,000 hectares (roughly 42,000 acres).

Fixing a watershed is an engineering problem. Criteria are what a solution must achieve, such as cutting a set amount of nitrogen. Constraints are the limits it must work within, such as money, land and time. Nitrogen stored in soil can keep leaking out for years, so you build, measure, compare with a place that got no fix, and refine.

Elaborate: read the Bay’s numbers

Measure	Year	Value	Units	Kind of number
Total yearly hypoxic volume	2023	199 (± 26)	km³·days	model estimate
Total yearly hypoxic volume	2024	497 (± 65)	km³·days	model estimate
Total yearly hypoxic volume	2025	462 (± 60)	km³·days	model estimate
Nitrogen load to the Bay	2009	297.1	million lb per year	model estimate
Nitrogen load to the Bay	2025	252.7	million lb per year	model estimate
Nitrogen goal	goal in the 2026 progress release	219.5	million lb per year	target, not a measurement
Underwater grass, areas mapped in both years	2024	76,835	acres	aerial mapping
Underwater grass, areas mapped in both years	2025	82,968	acres	aerial mapping
Lafayette River restored reefs beating oyster targets	2022	2 of 3	reefs surveyed	field survey

Hypoxic volumes are from the Virginia Institute of Marine Science (VIMS) Chesapeake Bay Environmental Forecast System historical table (read October 2026). They come from a whole-Bay model of water below 2 mg/L added up day by day; 1 km³·day is one cubic kilometer of hypoxic water lasting one day. A separate VIMS recent-years table gives different values for the same years, so compare only numbers from one table. Nitrogen numbers are from the Chesapeake Bay Program’s 2026 progress release (whole-watershed model). Grass areas are from the VIMS 2025 preliminary survey. Reef results are NOAA Fisheries’ report of VIMS monitoring.

1. How much did the modeled nitrogen load fall from 2009 to 2025? What percent of the 2009 load is that, and how much more must it fall to reach the goal?
2. Nitrogen loads fell, yet 2024’s hypoxic volume was about two and a half times 2023’s. Name two other things that could explain this.
3. Why compare grass only in areas mapped in both years?

Design problem

Does a strip of plants between a field and a stream reduce the nitrogen in runoff? Plan a fair test with trays of soil, and say how you would refine it if the result is unclear.

One way to do it

Set up six identical trays of the same soil, tilted to the same angle. Three get a strip of grass sod at the low end, the buffer. Three stay bare, the control.

• Independent variable: buffer strip or no buffer strip.
• Dependent variables: nitrate in the runoff (mg/L, from aquarium test strips) and runoff volume. Together they show how much nitrate left each tray.
• Constants: soil, slope, the amount of dilute plant fertilizer on each tray, the water poured and the waiting time.
• Repeated trials: three trays of each kind, repeated on a second day. Compare means.
• Safety: wear gloves and goggles, never taste the water, wash your hands, and dispose of runoff as your teacher directs.

Lower mean nitrate and runoff from buffer trays on both days supports the buffer. If results overlap, refine: widen the strip, add trays, or try another plant. Trays are a model, not a stream, and a real buffer has a constraint: the farmland it takes.

Evaluate: check your thinking

The summer dead zone starts with nutrients, not a chemical that grabs oxygen. Nutrients from farms, lawns, wastewater and air feed algae; the algae die and sink, and bacteria use up oxygen decomposing them, while layered summer water keeps new oxygen from the bottom. People add many of those nutrients, so people can cut them, and Virginia’s buffers, upgraded plants, reefs and returning grass show that recovery is possible, though slow.

Science practices you used: asking a question about a Virginia problem, planning a fair test with a control and repeated trials, reading a data table and calculating a percent change, using a model and naming its limits, and making a claim from evidence.