Sargassum alternato-pinnatum seedlings were attached to fertilized, molten slag blocks, by hanging fertile thalli within a cage unit (4 x 4 x 4m), deployed at a depth of 4–5m from March to April. A total of 25 units were deployed, each containing 36 blocks. Laboratory tests were also carried out comparing the growth of Sargassum alternato-pinnatum and Chondracanthus intermedius in unfertilised and fertilised seawater.
Fertilisation did not enhance Sargassum growth in laboratory conditions, as Sargassum grew more rapidly in unfertilised treatments. There was no control treatment used in outplanting experiments, so the effect of fertiliser in the field is unclear.
The attached germlings grew rapidly from November, and thalli of a total length >1m filled the cage by the next reproductive period the following year. S. alteronato-pinnatum germlings occasionally appeared on the coral rubble around the cages but never exceeded 5 cm in height. The fertiliser blocks exuded high concentrations of NH4-N and SiO2-Si for a period of 3 months and lower concentrations were exuded for another 3 months. Growth of S. alternato-pinnatum was higher unfertilised seawater compared to fertilised, while growth of C. intermedius was higher in the fertilised treatment. Fertilisation affected colour in both species.
Nature of Disturbance:
In spring and summer, S. alternato-pinnatum juveniles
were browsed by herbivorous fish.