Laws and Terry (1983; see also Laws 1984a) reported on the further development of the ARPS. Four 9.2 m2 experimental raceways were built to allow replication of experiments and testing of variables, again using P. tricornutum. These raceways were used in a multifactorial experimental design testing eight variables at two levels each:

1. depth (5 and 10 cm),

2. dilution rate (0.2 and 0.4 d-1),

3. night time temperatures (15°or 20°C),

4. flow rates (15 and 30 cm/s),

5. CuSO4 filters (present and absence),

6. pH (8 and 9),

7. N source (ammonia and urea), and

8. salinity (15 and 35 ppt).

Sixteen runs (of 256 possible) in four sequential blocks were carried out, with the assumption apparently being that these variables are non-interacting and additive (probably a poor assumption for biological processes). No block-to-block controls were provided, which could have been affected by light intensity and other variables.

The ponds had been equipped with six sets of mixing foils, but rather suprisingly the presence or absence of the foils was not a variable tested. A complex data evaluation, in terms of “factor effects”, was presented, but no actual productivity data for any of the experiments are available. The authors concluded that “by far the most significant factor affecting biomass production” was culture depth, arguing that the “self-shading effects were more than offset by higher areal standing crops.” This was a rather puzzling conclusion as it is contrary to both theory and experience, which assumes that, everything else being equal, depth should not affect productivity. Of course, depth can affect pH, temperature, pO2, and other variables, which if not held constant will indeed affect productivity. But these should have been second-order effects, not the overriding factor in determining productivity.

Although no actual productivity data were reported, Laws (1984) stated that this factorial experiment demonstrated maximum productivities of about 25 g/m2/d, corresponding to an approximately 5% light conversion efficiency. The author claimed that this was 50% to 100% better than achieved with “conventional culture techniques,” though the basis for such extrapolations or comparisons was unclear.