3.3 Nutrients & Cycles
Marine nutrients (N, P, Si, Fe) limit primary productivity. Their cycling through biological, chemical, and physical processes drives ocean ecosystems and connects them to the atmosphere and lithosphere.
Major Nutrients
Nitrogen (N)
NO₃⁻, NO₂⁻, NH₄⁺. Often limiting in open ocean. Redfield ratio: N:P = 16:1
Phosphorus (P)
PO₄³⁻. Essential for ATP, DNA. Limiting in some regions.
Silica (Si)
SiO₂. Essential for diatoms (glass frustules). Upwelling regions rich in Si.
Iron (Fe)
Micronutrient, limits productivity in HNLC regions (High Nutrient, Low Chlorophyll)
The Redfield Ratio
\( \text{C}:\text{N}:\text{P} = 106:16:1 \)
Universal ratio in marine organic matter (phytoplankton, detritus)
This remarkable constancy reflects the biochemical requirements of marine life and allows prediction of nutrient cycling from any one measurement.
Nutrient Profiles
Surface
Depleted by phytoplankton uptake
Nutricline
Rapid increase (100-500m)
Deep
High concentration (remineralization)
Python: Nutrient Profiles
#!/usr/bin/env python3
"""nutrient_cycles.py - Typical nutrient profile"""
import numpy as np
import matplotlib.pyplot as plt
def nutrient_profile(z, N_deep=30, z_nutricline=200, dz=100):
"""
Model nutrient concentration vs depth
z: depth (m), N_deep: deep concentration (µM)
"""
return N_deep * (1 - np.exp(-(z - z_nutricline) / dz)) * (z > z_nutricline) + \
N_deep * 0.1 * (z <= z_nutricline)
# Plot
z = np.linspace(0, 2000, 100)
NO3 = nutrient_profile(z, N_deep=35)
PO4 = nutrient_profile(z, N_deep=2.2)
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(10, 6))
ax1.plot(NO3, z, 'g-', lw=2)
ax1.set_xlabel('NO₃ (µM)'); ax1.set_ylabel('Depth (m)')
ax1.invert_yaxis(); ax1.set_title('Nitrate Profile')
ax2.plot(PO4, z, 'b-', lw=2)
ax2.set_xlabel('PO₄ (µM)')
ax2.invert_yaxis(); ax2.set_title('Phosphate Profile')
plt.tight_layout()