Land and Water Ecosystems

The James River starts as fresh water in Virginia’s mountains. It empties into the salty Chesapeake Bay at Hampton Roads, near Norfolk.

Along the way, river water mixes with salty water from the Bay. Wild oyster reefs grow in the brackish lower James. The river near Richmond is fresh water, where oysters can die. What changes along the river, and why does that matter? First, decide what you think now.

Explore: take a boat down the James

In this simulation you see a side map of the James River, from the mountains to the mouth of the Bay. A slider moves a sampling boat along the river. A salinity bar shows how salty the water is at that spot, and the organisms that can live there appear or fade. A switch sets the season: a wet spring or summer.

Move the boat: TF5.5 Upper tidal James

Season

Summer (lesson table) Wet spring

Reset

Made up for this sketch: the river positions are not to scale; the spring setting moves the salty water one station down the river; wild celery fits under 1, widgeon grass from 1 to under 5 and eelgrass above 20 (the guide gives the oyster range). The summer numbers are the lesson’s means (Chesapeake Bay Program, June to August 2023, 1 meter below the surface). No tides, no depth layers.

The rule to keep: salinity, an abiotic factor, changes along the river, and each organism fits only where the salinity suits it. The organisms come from guides. Nobody counted them at these stations.

Try this.

1. Predict where along the river oysters first appear. Then move the boat downstream from the mountains and check.
2. In this sketch, does the salty water move up the river or down in a wet spring? Predict, then switch seasons and test.
3. Park the boat mid-river. Predict which organisms live there in each season, then compare.

Explain: the living and nonliving parts of an ecosystem

An ecosystem is all the living things in a place, together with their nonliving surroundings.

A biotic factor is a living part of an ecosystem. Biotic factors include plants, animals, fungi and microorganisms, and the ways they interact. A deer eating acorns is biotic. You saw in Food Webs and Energy that dead parts and wastes of living things count as biotic too.

An abiotic factor is a nonliving part of an ecosystem. The main ones are:

• climate
• sunlight
• water
• soil
• air
• nutrients

Soil is mixed. Its sand, clay and minerals are abiotic. The worms, fungi and bacteria living in it are biotic.

Abiotic factors decide which organisms can survive in a place. Few plants can grow on a forest floor that gets little sunlight. Water and floating mud block light, so few plants grow on the floor of deep or muddy water.

Explain: what shapes climate

Climate is the usual pattern of temperature and rainfall in a place over many years. Earth’s axis is tilted, about 23.4 degrees. As Earth travels around the sun each year, the tilt changes the angle of the sunlight and the length of the day. That is the main reason Virginia has hot summers and cold winters. It is not because Earth moves closer to the sun.

Temperature and precipitation (rain and snow) are the two main parts of climate. These things shape them:

• Latitude. Places farther from the equator usually get less direct sunlight and are colder.
• Elevation. The air usually gets cooler as you climb. A mountaintop in western Virginia is often cooler than the coast on the same day.
• Topography, the shape of the land. Mountains can make one side wet and the far side dry.
• Human actions. Burning fossil fuels adds carbon dioxide to the air, and it traps heat.

Climate, population sizes and the kinds of species all change over time, so ecosystems change too. You will see how living things fit their places in Adaptations for Survival (a later lesson).

Explain: land, fresh water and salt water

Scientists sort ecosystems into three broad groups. A forest, a mountain stream and the Bay each belong to one:

• A terrestrial ecosystem is on land, such as a forest or a field.
• A freshwater ecosystem has water with very little salt, such as a mountain stream, a lake or the upper James.
• A marine ecosystem has salty ocean water.

Salinity is the amount of salt dissolved in water. An estuary is a place where fresh river water mixes with salty ocean water. The Chesapeake Bay is an estuary. Its salinity is in between, and it shifts with rain and season.

Abiotic factor	Terrestrial (Virginia forest)	Freshwater (mountain stream)	Estuary (Chesapeake Bay)	Marine (open ocean)
Salt	Little in most soils	Very little, under about 0.5 parts per thousand	In between; changes with rain	High, about 35 parts per thousand
Sunlight	Blocked by tree leaves	Reaches the bottom of shallow, clear water	Fades fast in muddy water	Fades with depth
Temperature	Changes a lot by season	Cool, often shaded	Changes by season	Changes less
Water movement	Rain and runoff	Flows one way, downhill	Tides and river flow mix	Waves and currents
Soil	Main source of nutrients	Rocks and gravel on the bottom	Mud on the bottom	Sand and mud far below

Elaborate: salt along the James

Crews working with the Chesapeake Bay Program measure the water at the same stations about once or twice a month. The table shows their summer means, from the upper tidal James to the Bay mouth. Salinity is in practical salinity units (PSU). In Bay data, 1 PSU is about the same as 1 gram of salt in 1,000 grams of water.

Station	Place	Samples	Mean salinity (PSU)	Organism that fits this salinity
TF5.5	Upper tidal James	3	0.09	Wild celery
RET5.2	Swann’s Point	3	1.32	Widgeon grass
LE5.1	Near Red Buoy 36	3	4.53	Widgeon grass
LE5.2	Buoy C12-13	3	11.54	Eastern oyster
LE5.3	James River Bridge	3	15.39	Eastern oyster
LE5.4	Hampton Roads	3	20.67	Eelgrass
CB8.1	Between the James mouth and Thimble Shoals	6	23.06	Eelgrass
CB7.4	Chesapeake Bay Bridge-Tunnel	6	25.78	Eelgrass

Source: Chesapeake Bay Program DataHub, water samples taken 1 meter below the surface, June to August 2023 (Virginia DEQ and Old Dominion University). We calculated the means. The organisms come from Chesapeake Bay Program species guides. They were not counted at these stations.

1. Describe the trend in salinity from TF5.5 to CB7.4.
2. How much saltier was LE5.4 than TF5.5?
3. A Chesapeake Bay Program guide says salinity from 10 to 28 is good for eastern oysters, and oysters can die if it stays below 5 for a long time. Which stations fall in the good range, and which fall below 5?

Design problem

Brine shrimp are tiny animals sold as dried eggs for fish food. Plan a fair test of how salinity affects how many brine shrimp eggs hatch.

One way to do it

• Question: How does the amount of salt in water affect how many brine shrimp eggs hatch?
• Independent variable: salt per liter of water: 0, 10, 20, 30 and 40 grams.
• Dependent variable: number of hatched shrimp counted in a 1-milliliter sample after 48 hours.
• Constants: same bottled water, same kind of salt, the same small scoop of eggs, same temperature, same light and same cup size.
• Control: the cup with no salt shows what happens in fresh water.
• Trials: three cups at each salt level. Find the mean for each level.
• Safety: wear goggles, wash your hands, use eggs from a store, and do not pour shrimp into a stream or pond.

If hatching changes a lot from one salt level to another, salinity may limit where brine shrimp live. A cup is a model, not a real bay.

Evaluate: check your thinking

As the James flows toward the Bay, it mixes with salty water, so salinity rises. Oysters can die in water that stays very low in salt for a long time, so their reefs grow in the brackish lower James, and the fresh water near Richmond is poor oyster habitat. Salt, sunlight and climate, together with the living things, decide who lives in each ecosystem.

Science practices you used: making predictions with a model and testing them, reading a data table and finding a difference, making a claim from evidence, and planning a fair test with a control.