Aquatic Biomes

1. Ocean Vertical Zonation

• Photic zone (0–200 m): sufficient light for photosynthesis. Phytoplankton fix CO₂ here; basis of all marine food webs. Contains ~50% of Earth’s primary production.
• Mesopelagic (200–1 000 m): “twilight zone,” faint light but no photosynthesis. Massive diel vertical migrations of zooplankton + mesopelagic fish transport carbon downward (biological pump).
• Bathypelagic (1 000–4 000 m): complete darkness, near-freezing, high pressure. Bioluminescence near-universal. Food from marine snow sinking from above.
• Abyssal (4 000–6 000 m): covers ~60% of Earth’s surface (!). Hydrothermal vents support chemosynthetic communities independent of solar energy — the only ecosystems on Earth that do.
• Hadal (>6 000 m): oceanic trenches. The Challenger Deep (Mariana Trench) at 10 984 m hosts specialised amphipod and fish fauna under ~110 MPa pressure.

2. The Biological Pump

Phytoplankton fix ~50 Gt C yr^-1 at the surface; a fraction sinks as dead cells, faecal pellets, and aggregates (“marine snow”) through the water column, transferring carbon from surface to deep ocean on timescales of centuries to millennia. The biological pump sequesters ~10 Gt C yr^-1; without it, atmospheric CO₂ would be 150–200 ppm higher (Sigman 2000).

3. Coral Reefs

Coral reefs occupy <0.1% of ocean area yet host ~25% of all marine species. The paradox — tropical seas nutrient-poor yet reefs productive — is resolved by extreme internal recycling between coral polyps and their photosynthetic endosymbionts (Symbiodinium dinoflagellates), with tight coupling between benthic and pelagic compartments. Loss of Symbiodinium during thermal stress causes the coral to whiten (bleach):

\[ \text{DHW} \;=\; \sum_i \max(T_i - T_{\text{MMM}}, 0)\cdot \Delta t,\quad \text{bleach at DHW} \geq 4 \]

Mass-bleaching events on the Great Barrier Reef in 2016, 2017, 2020, 2022, and 2024 have pushed >50% of historical coral cover below recovery thresholds across much of the northern and central GBR.

Simulation: Ocean Zonation & Bleaching

4. Freshwater Biomes

Despite covering <1% of Earth’s surface, freshwater systems host ~10% of all known species and ~40% of all fish species. Categories:

The Living Planet Index reports freshwater vertebrate populations have declined ~84% since 1970 — the steepest drop of any ecosystem. Drivers: dam construction, abstraction, pollution, invasive species, and climate change.

5. Threats

Ocean acidification has dropped surface pH by ~0.1 since 1750 (a 30% increase in H⁺) as ~30% of emitted CO₂ has dissolved into seawater. Warming reduces oxygen solubility; ocean deoxygenation reached ~2% since 1960 (Schmidtko 2017). Plastic pollution inputs ~8 Mt/yr; fisheries landings peaked in the 1990s. Marine biodiversity crisis is multi-driver and accelerating.

Key References