Protocol to compensate net evaporation and net precipitation in open-pond microalgal massive cultures and permit maximal steady-state productivities

• We model the effect of evaporation, precipitation and harvesting on the salinity of an algal pond.
• A range of management protocols (involving rates of harvesting, water addition and recycling) are considered.
• The model is applied to a nine year long data set of actual weather data.
• It is possible to maintain the pond salinity at no more than 50% above the starting level.

Commercial cultivation of microalgae in open ponds can be strongly limited by evaporation. This is particularly the case in highly evaporative areas where production plants are likely to be located. We present a mathematical model to calculate native solute and exogenous nutrient concentrations as affected by net evaporation or net precipitation. The model takes into account the periodic compensatory addition of new feedstock water and/or removal of rain-originated excess water and the eventual recycling of the culture medium. We present a management protocol in which, for a wide variety of climates, it is possible to stabilise the native solute concentrations and minimise the exogenous nutrient washout. The protocol includes harvesting a minimum of 10% of the pond volume per day and replacing this volume and the evaporated water with new feedstock water. We test the approach against a 9-year daily weather data set for a locality with 1740 mm.y−1 average net evaporation. We find that the native solute concentrations can be maintained between 1.2 and 1.5 times the feedstock water values, while the daily washout of exogenous nutrients is ≤3% and the pond volume is kept nearly constant. This protocol should prove particularly useful for the exploitation of marine microalgae growing in open ponds with brackish or seawater-based growth medium.