Lesson 03: Biomes & Aquatic Systems

Warm-Up

Drop a pin anywhere on Earth and two numbers largely predict what lives there: how warm it is and how much rain it gets. Those two variables sort the planet into a few dozen recognizable biomes — the frozen, treeless tundra; the sweltering, hyper-diverse rainforest; the grasslands where the buffalo (and the wheat farm) roam. The exam won't ask you to memorize obscure species, but it will ask you to read a climate graph and name the biome, or to explain why coral reefs are productive but the open ocean is a "biological desert." Today is about pattern recognition: temperature, water, and light decide who lives where.

Core Concept

What determines a biome

A biome is a large geographic region defined by its climate and characteristic plant/animal communities. The two dominant controls are temperature and precipitation, which in turn depend on latitude (angle of sunlight) and elevation.

[DIAGRAM: Biome chart, x-axis = mean annual temperature (cold→hot left to right), y-axis = mean annual precipitation (low→high bottom to top). Regions plotted: tundra (cold, dry); boreal forest/taiga (cold, moderate); temperate grassland (moderate temp, low-moderate precip); temperate deciduous forest (moderate temp, moderate-high precip); desert (any temp, very low precip); savanna (hot, seasonal moderate precip); tropical rainforest (hot, very high precip).]

Major terrestrial biomes

Key exam ideas: Rainforest has the highest biodiversity and NPP but poor soil (nutrients locked in living biomass, not soil). Grasslands have the most fertile soils (great for agriculture). Tundra and taiga store huge amounts of carbon (a Unit 9 climate concern as permafrost thaws).

Aquatic systems

Aquatic systems are classified by salinity (fresh vs. marine) and by light penetration.

Freshwater — lakes/ponds are zoned: - Littoral zone: shallow near shore; rooted plants; high productivity. - Limnetic zone: open, sunlit surface water; phytoplankton; the productive open zone. - Profundal zone: deep water below light penetration; no photosynthesis. - Benthic zone: the bottom; decomposers and detritivores.

Lakes are also described as oligotrophic (nutrient-poor, clear, low productivity, high oxygen) or eutrophic (nutrient-rich, murky, high productivity, low deep-water oxygen).

Freshwater — rivers/streams change from cold, clear, oxygen-rich headwaters to warmer, murkier, nutrient-rich downstream water. Wetlands (marshes, swamps, bogs) are among the most productive systems and provide flood control and water filtration.

Marine systems are zoned by light: - Photic zone: sunlit upper layer where photosynthesis occurs. - Aphotic zone: below light penetration; no photosynthesis. - Intertidal zone: between high and low tide; harsh, variable. - Coral reefs: warm, shallow, clear tropical water; extremely high biodiversity and productivity; sensitive to temperature (bleaching) and acidification. - Estuaries: where rivers meet the sea (brackish water); among the most productive ecosystems; nurseries for fish; include salt marshes and mangroves. - Open ocean: low productivity per area (nutrient-limited) but vast total NPP.

[DIAGRAM: Lake cross-section showing littoral (shore, plants), limnetic (open sunlit surface), profundal (deep dark), benthic (bottom sediment). Separate marine profile showing photic zone (top, sunlight) vs. aphotic zone (dark deep).]

What controls aquatic productivity

Three limiting factors: sunlight (limits the photic zone depth), nutrients (N and P; upwelling brings deep nutrients to the surface, creating rich fisheries), and temperature/dissolved oxygen (cold water holds more O₂). Coral reefs and estuaries win on all three; the open ocean is nutrient-starved.

Why this matters

Reading a climatogram to name a biome, and knowing which aquatic zones are productive and why, are recurring MC items. The soil-fertility contrasts (rainforest poor, grassland rich) reappear in Unit 5 (land use).

Worked Examples

Example 1 (easy): Name the biome

A region averages 27 °C and 350 cm of rain per year, with no dry season. Name the biome and one soil characteristic.

Solution: Hot + very wet + aseasonal = tropical rainforest. Soil is nutrient-poor because nutrients are held in living biomass and quickly recycled, and heavy rain leaches minerals.

Interpretation: High productivity ≠ rich soil — a favorite trap.

Example 2 (medium): Read a climatogram

A climate diagram shows temperatures below freezing for 8 months, a short cool summer, and under 25 cm precipitation. Identify the biome and one carbon-cycle concern.

Solution: Cold, dry, short growing season, low precipitation = tundra. Concern: permafrost stores vast carbon; warming thaws it, releasing CO₂ and methane (a climate feedback, Unit 9).

Interpretation: Tundra's low precipitation surprises students — it's technically a cold desert.

Example 3 (AP-style): Explain a productivity contrast

Explain why an estuary is far more productive than the open ocean.

Solution: Estuaries receive continuous nutrient input from rivers, are shallow (light reaches the bottom, maximizing the photic zone), and have abundant rooted vegetation. The open ocean is deep and nutrient-limited at the surface (nutrients sink), so productivity per area is low despite its size.

Interpretation: Productivity = light + nutrients + suitable temperature. Estuaries maximize all three.

Example 4 (AP-style): Upwelling and fisheries

Coastal Peru has one of the world's richest fisheries. Explain the role of upwelling, and predict the effect of an El Niño year.

Solution: Upwelling brings cold, nutrient-rich deep water to the sunlit surface, fueling phytoplankton and the food web that supports fish. In an El Niño year, warm surface water suppresses upwelling, nutrients don't reach the surface, phytoplankton crash, and fisheries collapse.

Interpretation: This links Unit 1 productivity to Unit 4 ocean circulation (ENSO).

Common Mistakes

• "Rainforest has rich soil." No — high productivity, poor soil. Grasslands have the richest soils.
• Confusing tundra and taiga. Tundra = treeless, permafrost, coldest. Taiga (boreal forest) = coniferous trees, cold but not permafrost-dominated, the largest terrestrial biome.
• Assuming the open ocean is highly productive because it's huge. Low productivity per m²; high total only due to size.
• Mixing up photic/aphotic (marine, defined by light) with littoral/limnetic/profundal (lake zones). Different systems, different vocab.
• Ignoring precipitation when naming a biome. Temperature alone can't distinguish desert from forest at the same latitude — always use both axes.

Practice Problems

11. (FRQ-style) Two lakes have equal surface area. Lake A is oligotrophic; Lake B is eutrophic. Compare their clarity, productivity, and deep-water oxygen, and identify which is more vulnerable to fish kills.

12. (Data) A climatogram shows mean temperature ~10 °C, precipitation ~100 cm spread across the year, and four distinct seasons. (a) Name the biome. (b) State one reason its soils support productive agriculture.

FRQ Practice — Designing an Investigation (10 pts)

A student hypothesizes that light availability limits primary productivity more strongly than nutrient availability in a local pond.

(a) State the independent variable(s) and the dependent variable. (2 pts) (b) Describe a controlled experiment using pond-water samples to test which factor limits productivity more. (3 pts) (c) Identify the control group. (1 pt) (d) Sketch (describe) the expected results if the hypothesis (light-limited) is correct, and if it is wrong (nutrient-limited). (2 pts) (e) Explain one reason results from bottled samples might not represent the whole pond. (2 pts)