Use of fertilized molten slags to create Sargassum forests in subtropical shallow waters

Recently, a solid fertilizer with condensed nutrients was manufactured using molten slag from municipal solid waste and dried cow dung. This novel substratum was used for the afforestation of Sargassum alternato-pinnatum Yamada in the subtropical Amami Ohshima Island since 2010. Seedlings were attached to discoidal, fertilized, molten slag (DFMS) blocks by hanging fertile thalli within a cage unit (4 m3), deployed at a depth of 4–5 m from March to April. The attached germlings grew rapidly from November and thalli of a total length of >1 m filled the cage by the next reproductive period the following year. Water temperature varied between 17.6 and 31.9 °C in 2013–2014; the water was oligotrophic (i.e., nitrogen-deficient). To measure the nutrients liberated from the DFMS blocks (28 × 5 cm; diameter × width), a batch experiment (N = 3) was undertaken using 15-L seawater plastic containers with weekly water exchange. High concentrations of NH4-N (16.3–90.0 μmol L−1) and SiO2-Si (5.9–256.9 μmol L−1) exuded out of the blocks for a period of 3 months and lower concentrations exuded for another 3 months, even after the block was dried for 7 months. To clarify the fertilizer effects of exudates from the DFMS blocks on algae, S. alternato-pinnatum juveniles (0.7 g) were cultured in 300-mL flasks for 1 month. The juveniles produced new leaves and increased in weight in both enriched seawater and control treatments, but the color of the thallus was darker than normal. To visualize the effects more clearly, introduction of the yellowed (nitrogen-starved) thalli of the red alga, Chondracanthus intermedius, was useful because the color of its thallus turns red under enriched conditions.