4.2 Phytoplankton
Phytoplankton are microscopic photosynthetic organisms that form the base of marine food webs. They produce ~50% of Earth's oxygen and are crucial for the global carbon cycle.
Major Groups
Diatoms
Silica frustules. Dominant in nutrient-rich waters. ~40% of marine primary production.
Dinoflagellates
Two flagella. Some toxic (red tides). Important in coral symbiosis.
Coccolithophores
CaCO₃ plates. Affect ocean carbon cycle. Emiliania huxleyi most common.
Cyanobacteria
Prochlorococcus, Synechococcus. Smallest but most numerous. Nitrogen fixation (Trichodesmium).
Primary Production
\( \text{6CO}_2 + \text{6H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{6O}_2 \)
Global marine primary production: ~50 Gt C/year
~50 Gt C/yr
Marine NPP
~50%
Of Earth's O₂
days-weeks
Turnover time
Limiting Factors
Light
Photosynthesis limited to euphotic zone (~0-200m). Critical depth concept.
Nutrients
N, P often limiting. Iron limits HNLC regions (Southern Ocean, subarctic Pacific).
Grazing
Zooplankton control phytoplankton biomass. Bloom-bust dynamics.
Python: Chlorophyll Profile
#!/usr/bin/env python3
"""phytoplankton.py - Deep chlorophyll maximum model"""
import numpy as np
import matplotlib.pyplot as plt
def chlorophyll_profile(z, Chl_surface=0.5, Chl_max=2.0, z_max=80, sigma=30):
"""
Typical chlorophyll profile with deep chlorophyll maximum (DCM)
z: depth (m), z_max: depth of DCM
"""
surface_component = Chl_surface * np.exp(-z / 50)
dcm_component = Chl_max * np.exp(-((z - z_max) / sigma)**2)
return surface_component + dcm_component
z = np.linspace(0, 200, 100)
chl = chlorophyll_profile(z)
plt.figure(figsize=(6, 8))
plt.plot(chl, z, 'g-', lw=2)
plt.xlabel('Chlorophyll-a (mg/m³)')
plt.ylabel('Depth (m)')
plt.title('Deep Chlorophyll Maximum (DCM)')
plt.gca().invert_yaxis()
plt.axhline(80, color='r', linestyle='--', label='DCM depth')
plt.legend()
plt.grid(True, alpha=0.3)
# Integrated chlorophyll
integrated = np.trapz(chl, z)
print(f"Integrated chlorophyll: {integrated:.1f} mg/m²")