During the final year of the Hawaii ARPS project (Laws 1987), the goal was to screen for additional algal species in the smaller flumes and to further study the effect of dilution intervals. Four species were tested in the 9.2-m2 flumes: Navicula sp., C. cryptica, C. gracilis, and Synechococcus sp. From prior work (see earlier Section, and also Laws et al. 1986, 1987a), photosynthetic efficiencies of 9.1% were reported with T. suecica, during a 78-day period, and 9.6% for 122 days with C. cryptica. With the three other organisms listed above, somewhat lower efficiencies were noted during shorter time periods: 7.8 % for Navicula sp., 8.5% for C. gracilis, and 8.6% for Synechococcus. Somewhat “surprisingly” (their characterization), they observed that in a 2-day batch growth mode, initial cell concentrations ranging from about 50 to 400 mg/L (AFDW) had no major effect on productivity. For C. cryptica, at an initial concentration of 40 mg/L at a depth of 12 cm, this would give an areal cell density of about 5 g/m2. For an equal daily productivity of 30 g/m2/d, averaged over 2 days, this would require the cells to divide 2.5 times the first day, and once the second day. Not impossible, certainly, but somewhat problematic. There is indeed some likelihood that some systematic measurement error influenced their productivity measurements (John Ryther, private communications, circa. 1986).

This report also described lipid induction by Si limitation by C. gracilis and C. cryptica. In both microalgae Si limitation greatly reduced overall productivities, and lipid productivities, even though lipid contents increased. Laws (1987) concluded that lipid productivities would be maximized by maximizing total biomass production.

In the final paper, Laws et al. (1988), reported on long-term (13-month) production of C. cryptica in the large flume, with a 9.6% solar conversion efficiency reported with the foils and 7.5% without the foils, similar to earlier results with T. suecica. For 122 days, at optimal dilution (2- day batch cycle) productivity of about 30 g/m2/d was measured. This is, indeed, a high sustained productivity.