• A fertilization tablet for mitigating oligotrophic coastal seas has been developed by using food waste derived from anaerobic digestate in biogas plants.
• The fertilization tablet enhanced the growth of primary producers in marine ecosystems.
• The fertilization tablet can therefore create a new nutrient pathway connecting biogas power generation to the remediation of oligotrophic seas.

Currently, the Seto Inland Sea, the largest enclosed sea in Japan, experiences oligotrophic conditions. Accordingly, the resulting sharp decrease of fishery production and undesirable discoloration of Nori（Neopyropia）have caused serious economic losses.
Another critical environmental issue is the utilization of anaerobic digestate, which is a byproduct of methane fermentation, namely, anaerobic digestion of livestock manure and food waste. Although the anaerobic digestate is conventionally used as a fertilizer for cultivated land and paddy fields in the spring season, most of the anaerobic digestate is discharged into bodies of water after wastewater treatments. These high costs associated with wastewater treatments become an obstacle towards popularizing biogas power generation.
We developed a fertilization tablet prepared by mixing blast furnace cement and food waste derived from anaerobic digestate with a high nutrient content from biogas power plants (Fig. 1).

In tank experiments to simulate oligotrophic coastal seas, the nutrients dissolved from the tablets were taken up by marine microalgae resulting to 1.4 times increase in the total amount of chlorophyll a compared to the control without the added tablets (p<0.01). At the same time, the flow of dissolved inorganic nitrogen uptake by marine microalgae increased 3–5 times compared to the control.
In Wakame seaweed (Undaria pinnatifida) culture experiments, the nitrogen and chlorophyll a contents of Wakame in the tablet-treated tanks were 1.2 and 2.5 times higher, respectively than those in the control (p<0.01; Figs. 2 and 3). Wakame fronds in the tablet-treated tanks were also 1.4 times longer than those cultured in the control. During the culture experiment, 44% of dissolved inorganic nitrogen from the tablets was taken up by the Wakame. 
As one of the significant results, the tablets developed in this study can enhance the growth of marine microalgae and seaweeds, which are primary producers in marine ecosystems, and can alleviate the undesirable discoloration of seaweeds which otherwise would lead to market rejection.
The fertilization tablets formulated in this study can connect biogas power generation to improving the conditions of oligotrophic seas by the creation of novel nutrient pathway. In the future, we are planning to conduct large-scale and field experiments to optimize effective fertilization strategies applicable for oligotrophic coastal seas using the developed tablets. This study contributes toward the realization of sustainable development goal (SDGs) 2 - zero hunger, 12 - responsible consumption and production, and 14 - life below water.

• Journal: Journal of Environmental Management
• Title: Terrestrial anaerobic digestate composite for fertilization of oligotrophic coastal seas
• Authors: Asaoka, S., Yoshida, G., Ihara, I., Umehara, A., Yoneyama, H.
• DOI: https://doi.org/10.1016/j.jenvman.2021.112944

This study was supported by the Hyogo Eco-town Promotion Conference of Hyogo Prefecture, Japan. We express gratitude to the Fisheries Cooperatives of Hiro, Hiroshima for providing Wakame.