Spirulina harvest scheduling.

The spirulina growth curve

Spirulina cultures pass through predictable phases:

• Lag phase:After inoculation or post-harvest dilution, the culture adjusts to new conditions. Minimal growth for 12–24 hours.
• Exponential phase:Maximum growth rate — cultures can double biomass every 3–5 days under optimal temperature, light, and nutrient conditions. This is the target growth phase to stay in continuously.
• Linear phase:As density increases, light penetration decreases. The bottom of the culture becomes light-limited. Growth rate slows but remains steady.
• Stationary phase:Light limitation, nutrient depletion, and/or pH change stop net growth. Culture stabilises at maximum density (~6–8 g/L in dense home cultures). Quality declines — phycocyanin content drops, contamination risk increases.
• Decline/crash:If stationary phase persists, the culture begins dying. Avoid entirely through regular harvesting.

The goal of harvest scheduling is to keep the culture permanently in early exponential to linear phase — never allowing it to reach stationary phase.

Optimal harvest density

Harvest when the culture reaches 2–4 g/L (grams of dry weight per litre). For a 20-litre culture, this is 40–80 grams of dry spirulina in the tank.

Practical density measurement without lab equipment:

• Secchi disc depth:A white disc on a string — when it disappears at 2–4 cm depth, the culture is at approximately 2–3 g/L. Simple and reliable.
• Turbidimetry:A basic optical meter measures light transmission through the culture — calibrated against known concentrations.
• Filtration and weighing:Filter a known volume (100 mL), dry the filter paper at 40°C for 2 hours, weigh. Accurate but time-consuming — use monthly to calibrate visual/secchi methods.

How much to harvest

Remove 30–40% of total culture volume per harvest. This is the “partial harvest” model used in continuous production:

• For a 20L culture at 3 g/L: harvest 6–8 litres (targeting 18–24 g dry weight)
• Immediately replace harvested volume with fresh bicarbonate media to restore volume and replenish nutrients
• The remaining 60–70% culture serves as inoculum for the next growth cycle

Over-harvesting (>50% of volume) reduces the inoculum too severely, extending lag time and increasing contamination vulnerability. Under-harvesting (<20%) allows density to build toward stationary phase.

Harvest frequency

Under optimal conditions (30–35°C, 100–200 µmol/m²/s light intensity, pH 9.5–10.2, adequate nutrients):

• Summer, optimal temperature:Harvest every 3–4 days. The culture rebuilds to target density quickly.
• Cooler conditions (25–28°C):Harvest every 5–7 days. Growth rate is lower; the culture takes longer to rebuild.
• Sub-optimal (below 25°C):Harvest every 7–10 days. Don’t harvest more frequently than growth allows — monitor density, not calendar.

Weekly yield calculations

For a 20-litre home culture under good conditions:

• At 3 g/L, harvest 30% = 6L containing approximately 18g wet-weight dry equivalent
• Harvesting every 4 days: approximately 1.75 harvests/week × 18 g = ~31 g dry spirulina/week
• At 5 g/day personal dose, this covers 6 days of personal use from a single 20L culture
• For higher yield (2–3× personal dose, surplus, or sharing): scale to 40–60L total culture volume

These are idealised estimates. Real-world yields are typically 60–80% of maximum due to suboptimal temperature periods, harvesting variability, and seasonal light changes.

Indicators that harvest timing is off

• Culture turning pale or yellowish-green:Overcrowding — cells at the bottom are light-starved, reducing phycocyanin synthesis. Harvest immediately and reduce density.
• pH persistently above 10.8 during daytime:Dense culture consuming all available CO₂ — harvest to reduce density, add bicarbonate, and increase CO₂ supply or agitation.
• Green film on culture container walls:Different algae species establishing — contamination risk from lowered alkalinity. Correct pH first; if contamination is established, a partial or full culture replacement may be needed.
• Culture not recovering to target density within expected time:Nutrient depletion (add fresh media), temperature too low (check heating), or pH crash (check and correct).

Scheduling with life

For most home growers, strict every-3-day harvesting is impractical. Practical approaches:

• Weekly harvest: acceptable if keeping culture density moderate (2–3 g/L) — cultures at lower density grow more slowly, reducing the urgency of the harvest schedule
• Planned absence: reduce temperature and light to “maintenance mode” (20–25°C, low light) before a 1–2 week absence — the culture survives but doesn’t need harvesting. Resume full conditions on return.
• Harvest journal: a simple log (date, volume harvested, density estimate, temperature) helps calibrate your specific setup over time — the first 3 months are learning the rhythm of your culture.