As mentioned above, open pond cultivation is currently considered the most viable option for large scale cultivation of algal biomass for energy. According to LCA research conducted by Jorquera et al. [55], open ponds provide a much higher net energy ratio in comparison to PBRs, both tu­bular and flat plated. Jorquera et al. [55] investigated the amount of en­ergy consumed and produced using the three different cultivation methods for a yield of 100,000 kg of biomass annually. Productivities and energy yields in PBRs were found about three times higher than ponds, due to differences in efficiency. The energy consumption of PBRs to produce the equivalent amount of algal biomass compared to cultivation in ponds was however considerably higher (ca. ten times for tubular PBRs). The high energy consumption of the PBRs is mainly due to the air pumping, water pumping and the caloric content of the equipment used. On the whole, it can be assumed that despite low productivity requirements, open ponds provide a higher biomass yield for the energy consumed. One limitation of this reported research [55] is that it assumed that the cultivation of algae and the estimated productivity in the pond is possible all year round.

It has been previously reported that, under environmental conditions, wild strains of algae are likely to dominate and the strain of algae will also change depending upon the season [91]. Before discussing controlled conditions further, it is important to stress that certain strains of algae may be controlled by their requirement for extreme conditions. Spirulina and Dunaliella, for example, require a high pH and raised salinity to survive, most invasive species would be intolerant of these conditions [91]. The majority of species, however, require less extreme conditions and compe­tition by native algae (and possibly other living microorganisms) remains a problem. During the summer seasons the dominant algae will be those that thrive in higher temperatures and conversely in winter those species that survive colder weather will dominate. Tseng et al. [92] found that at temperatures between 17 to 22 °C, Chlorella vulgaris dominated whereas at higher temperatures from 22 to 27 and even up to 32 °C, Scenedesmus ellipsoideus, S. dimorphus and Wastella botryoides were dominant. Al­ternative observations were, however, made by Professor Shelef finding that in Israel, Chlorella and Micratinium were dominant in the summer whereas Euglena and Scenedesmus were the common species in winter [23]. Clearly the location of cultivation has a large impact upon which species will dominate in each season. What is most important, however, is that when growing algae in environmental conditions, selectivity of strain is not currently possible.

Recently research has investigated methods of species control in cul­tivation ponds. For example, two high-rate algae ponds have been com­pared, one which included recycling of algae and one which did not [91]. The species under investigation was Pediastrum spp. and algal biomass was collected every day and settled in algal settling cones. One litre of biomass was returned to one of the ponds and not the other. The pond with recycling provided over 90% dominance of Pediastrum sp. in one year whilst the non-recycled pond provided only 53% dominance. It is sug­gested that this method of recycling may provide a useful method of spe­cies control in open pond cultivation, however it may not be successful for every strain required for cultivation. Further research is required to assess whether recycling will be beneficial for any considered strains.