Light distribution in tanks with the green seaweed Ulva ohnoi: Effect of stocking density, incident irradiance and chlorophyll content

11 pages, 6 figures, 4 tablesSeaweed farming is an interesting technique to meet the future global food demand. However, it is essential to increase productivity in order to reduce the land surface required and make tank-based production more cost competitive with other food systems. Seaweed productivity is strongly dependent on irradiance, stocking density and nutrients. When nutrients are non-limiting, culture management to increase the productivity has to include the handling of the stocking densities and irradiance at the tanks surface. In this work the light distribution and the average irradiance (named seaweed irradiance Iav,Ulva) in aerated tanks with and without the green alga Ulva ohnoi were studied. Two tanks with opaque walls (small and large tank, 42 and 64 cm diameter, 35 and 30 cm deep, respectively) with stocking densities (SD) from 1 to 5.6 kg m−3, different incident irradiances I0 (from 66 to 696 μmol photons m−2 s−1 at the tank surface) and U. ohnoi with different chlorophyll content (from 77 to 154 µmol chlorophyll m−2) were studied. In the measuring of the Iav,Ulva, statistically significant differences between the different stocking densities, incident irradiances and chlorophyll contents were detected with variance tests. The Iav,Ulva increased with the incident irradiance I0, and decreased with stocking density and chlorophyll content. The percentage of the incident irradiance experienced by the seaweed (%Iav,Ulva) is less than 27% of the incident irradiance in all the configurations and this percentage decreases with increasing stocking density and chlorophyll content. Also, it is shown that Iav,Ulva is much lower than the one predicted with the Lambert–Beer law, the commonly used equation to determine it in tanks with algae. Multiple regression analysis was used to fit a model with three continuous predictors (I0, SD and chlorophyll content), their interactions and polynomial terms of second order to obtain Iav,Ulva in the small and large tank with U. ohnoi. The standardized effects of the terms involving chlorophyll content in the multiple regression model have been greater than the standardized effects of the terms involving SD. The overall evaluation of this final model showed a high goodness-of-fit, with a standard deviation of the distance between the data values and the fitted values equal to 0.4, R2 = 0.994 and R2(adjusted) = 0.992. The inspection of residual plots showed an adequate fit to the data. Furthermore, the value of the predicted R2 equal to 0.981 showed a good predictive capability of the model. The continuous variables stocking density, incident irradiance and chlorophyll content proved to be significant variables to explain and predict the average irradiance Iav,Ulva for the macroalga U. ohnoiThis work was funded by SPANISH Ministerio de Ciencia, Innovación y universidades (RTI2018-095062-A-C22)Peer reviewe