Week 5 Lecture 3 — Eutrophication, Limiting Nutrients & Dead Zones

Analogy: chocolate-chip cookies
- A recipe yields as many cookies as the ingredient that runs out first; algae behave the same way with nutrients.
Algal nutrient needs
- Carbon (from $CO_2$ ), nitrogen, phosphorus, water, micronutrients
- Used in a fixed atomic ratio discovered by Alfred Redfield
- Redfield Ratio: $106\,C : 16\,N : 1\,P$
- Whichever element is in shortest supply relative to this ratio is the limiting nutrient and caps algal biomass.

Fertilizer spill: $8{,}000{,}000\,N$ atoms & $4{,}000{,}000\,P$ atoms
- Required ratio $16\,N:1\,P$ ≈ $16:1$ ; supplied ratio $8{,}000{,}000:4{,}000{,}000 = 2:1$
- N is proportionally scarcer ⇒ Nitrogen is limiting.
Maximum carbon fixation
- Each "set" of nutrients = $16\,N$ + $1\,P$ + $106\,C$
- Available sets = $\frac{8{,}000{,}000\,N}{16} = 500{,}000$
- Carbon removed = $500{,}000 \times 106 = 53{,}000{,}000$ atoms $C$

Word roots
- eu- = good/well (euphoria)
- troph = nourishment/food (trophic level)
- -ication = process/state (multiplication)
Formal definition
- "Process by which a water body becomes enriched in dissolved nutrients that stimulate aquatic plant growth and usually deplete dissolved oxygen."
Despite the prefix "eu" (good), the outcome is generally harmful.

Natural (geologic) cycle
- Weathering of phosphate-bearing rock → rivers → lakes/ocean → sediment → uplift (time-scale: $10^3$ – $10^6$ yrs)
- No significant atmospheric reservoir (contrast with nitrogen).
Biological loop
- Plants absorb $PO_4^{3-}$ → animals eat plants → decomposers return P to soil.
Human actions
- Mining phosphate rock & bird guano → concentrated fertilizers
- Excess application → runoff to water bodies → unprecedented P loading
- Long-term concern: finite mineable phosphorus threatens future agriculture.

High inputs initially boost algal/plant growth but lead to:
- Light blockage → death of submerged vegetation
- Shifts in species composition; often dominance of fast-growing or toxic algae
- Massive die-offs → decomposition surge → oxygen consumption
- Resulting hypoxia/anoxia kills fish, shellfish, and benthic fauna.

Oligotrophic state
- Low nutrients, clear water, deep light penetration, healthy seagrass/kelp, high biodiversity.
Transitional (increased loading)
- Elevated $N$ & $P$ → surface phytoplankton bloom → water turbidity rises.
- Submerged aquatic vegetation (SAV) shaded out and dies.
Eutrophic / Dead-Zone state
- Continuous blooms & die-offs; thick detritus layer
- Decomposers strip $O_2$ ; hypoxia (< $2\,mg\,L^{-1}$ ) or anoxia ( $\approx 0$ )
- Fish/shellfish mortality → more detritus → positive feedback loop.

Hydrologic: stagnant or slow-flowing water (lakes, bays) hampers re-oxygenation.
Thermal stratification (summer): warm, buoyant surface prevents mixing with cooler, deeper layers.
Overfishing & other stresses weaken ecosystem resilience.
Seasonality: worst in warm months when metabolic & decomposition rates peak.

Map shows >400 documented coastal dead zones; hotspots near densely populated coasts (e.g., U.S. Gulf & Atlantic, Baltic Sea, South Korea/Japan).
Threats
- Biodiversity loss & collapse of local fisheries
- Cultural traditions at risk (e.g., Maryland blue crabs cannot survive hypoxia)
- Recreation & potable water quality degraded (toxic blooms).

Balancing agricultural productivity (fertilizers) vs. aquatic ecosystem health.
Phosphorus as a non-renewable resource: current wastage could imperil future food security.
Interdisciplinary solutions: improved land management, wastewater treatment, circular nutrient economy.

Complete: "Week 5 Lecture 3 Assignment" (limiting-nutrient math + eutrophication questions).
Finish Problem Set 2 by week’s end.
Contact Dr. Ni for assistance (email → Zoom meeting).