## 4 Discussion
It was mainly total light and nutrients (nitrate + nitrite, phosphate and silicate) that were correlated to the first axis (Table 5). The second axis correlated with actual measured light in the reactor (measured with spherical and cosine sensors), though weaker than total light and inorganic nutrients. Doubling rate and production surprisingly did not correlate with total light, here indicating that some other factor also contributed as regulator of growth, since the light intensities in the reactor were well below P\({}_{\text{max}}\). Further temperature also seemed to have a minor impact on growth, while biomass concentration in the reactor was in fact more tightly correlated to total light thanproduction. We interpret this as being because during large periods we cultivated without nutrient and/or temperature stress, i.e., with sufficient nutrients and total light. The fact that production related poorly to total light may also be partly due to the fact that large and small cells acted differently with respect to light utilization (Table 6). Another hypothesis that will be tested is if zinc or aluminum anode materials in the reactor may have had deleterious effects on the cultivated diatoms [116]. This is an important issue to consider if underwater lights shall be applied since (nearly) all materials submerged in seawater are prone to corrosion. Biomass peaks in our 300 m\({}^{3}\) reactor were around 1.28 cm\({}^{3}\) L\({}^{-1}\) biovolume, and production ones were ca. 1.5 cm\({}^{3}\) L\({}^{-1}\) Day\({}^{-1}\). In terms of DW this translates into the production peaking at 0.18-0.42 g L\({}^{-1}\) Day\({}^{-1}\) with a mean of ca. 0.22 g L\({}^{-1}\) Day\({}^{-1}\) DW. Obtaining a reliable overview of \"normal\" microalgae mass production values from the literature is difficult, e.g., due to the variability of cultivation and de-watering/processing concepts. Issues related to intellectual property rights (IPR) can also hinder reporting of correct values. Our own attempt to obtain an overview of published production rates yielded values from 0.05 to 3.8 g L\({}^{-1}\) Day\({}^{-1}\) DW (Table 12). The highest values were obtained with thin plate reactors, where associated doubling rates also were highest, i.e., 1.2 doublings day\({}^{-1}\) (values from cited papers or recomputed from same). Tubular reactors came in second, illustrating that reactor types with short light depths gave the highest production rates. The reported values are not necessarily peak ones though. It is also important to take note of the fact that the large part of the above cited values was obtained in small reactors (<10,000 L). Our Finnfjord initiative must be categorized as \"large volume with medium productivity and high light utilization efficiency\". Large volume continuous culture types can of course cause stress on the water intake filtering system as well as the de-watering capacity. Our inlet seawater was attached to the cooling water stream at the factory, and the intake was at depths > 25 m in the outside fjord. During large parts of the year seawater in the area is devoid of microalgae and small zooplanktons due to the winter darkness with no photoautotrophic activity. In addition, the intake is below the surface water spring/summer stratification depth. The main spring bloom takes place above the pycnocline [41; 84]. We therefore have available \"clean\" seawater that for large parts of the year needs minor filter capacity. In addition, despite renewal of 20-50% (25,000-150,000 L) seawater Day\({}^{-1}\), we were able to cultivate 4-6 months without contamination. The addition of factory fume lowered pH ca. 1.5 units below \"neutral\" values in intake water, and this could have contributed to a reduction of contamination of the culture. The mixing rate in the reactor naturally has the potential to influence production rate, since turbulence/mixing moves the cells through gradients in growth factors. This secures homogeneity in the cultivation conditions which is beneficial, but too high turbulence