# Unit Review: Ecosystems - Interactions and Energy

This review covers everything we learned about Earth's water systems and ecosystem energy flow. Use it to prepare for your unit assessment.

## Topic 1: Water Distribution

Earth is 71% water-covered, but most is NOT usable: - Oceans (saltwater): 96.5%, too salty - Ice caps & glaciers: 1.74%, frozen - Groundwater: ~1%, underground - Surface freshwater (lakes, rivers): <0.01%

Only about 0.5-1% of Earth's total water is readily accessible freshwater.

Analogy: If all Earth's water were 100 liters (a bathtub), only about 2 tablespoons would be the rivers and lakes we depend on.

## Topic 2: The Water Cycle

The hydrologic cycle is powered by the Sun. Six major processes: - Evaporation: Liquid → gas (absorbs energy) - Transpiration: Plants release water vapor (a large tree: 400+ liters/day) - Condensation: Vapor → liquid droplets (releases energy, forms clouds, powers storms) - Precipitation: Rain, snow, sleet, or hail - Infiltration: Water soaks into ground - Runoff: Water flows over land to streams/rivers

Residence Time: How long water stays in a reservoir. Atmosphere = 9 days. Rivers = 2-6 months. Groundwater = 100-10,000+ years.

Why groundwater pollution is critical: It persists for centuries. Prevention >> cleanup.

## Topic 3: Ocean Currents

Surface currents are driven by: wind, the Coriolis effect (curves right in N. Hemisphere, left in S.), and continental boundaries. They form gyres (circular patterns).

The Gulf Stream: Moves 100x the Amazon's flow. Keeps Western Europe 5-10°C warmer than expected.

Deep currents (thermohaline circulation): Driven by density differences (cold + salty = dense = sinks). The global conveyor belt takes ~1,000 years for one circuit.

Climate effects: Warm currents → warmer/wetter coasts. Cold currents → cooler/drier coasts. Upwelling brings nutrient-rich deep water up → great fishing.

El Niño: Trade winds weaken, warm water spreads east. La Niña: Trade winds strengthen, cold water dominates.

## Topic 4: Groundwater & Watersheds

Porosity = empty space. Permeability = how easily water flows through. Clay: HIGH porosity but LOW permeability (tiny pores, poorly connected).

Underground structures: Zone of Aeration → Zone of Saturation → Water Table (boundary) → Aquifer (holds usable groundwater)

Watersheds: All land draining into the same river. Bounded by drainage divides. Everything upstream affects everything downstream.

Human impacts: Impervious surfaces increase runoff. Agriculture adds fertilizer/pesticide runoff. Two types: - Point source: Specific location (factory pipe), easier to regulate - Non-point source: Diffuse (runoff), harder to control

## Topic 5: Food Webs & Energy Flow

Energy enters most ecosystems as sunlight. Producers capture it via photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Roles: Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer; Decomposers recycle nutrients.

Two rules: Energy flows ONE way (sunlight → heat). Matter CYCLES (recycled).

The 10% Rule: Only ~10% of energy passes to the next trophic level. The rest is lost as heat.

Energy pyramid: 10,000 J → 1,000 J → 100 J → 10 J

This is why there are fewer apex predators, less energy available at the top.

Food web arrows point FROM what's eaten TO what eats it (direction of energy flow).