Effects of nutrient concentration, germanium dioxide and seeding density on sporophyte production in the kelp Ecklonia radiata

Abstract Overcoming low frequencies of transition from the gametophyte to the sporophyte life history phase is vital to enable efficient hatchery production of the kelp Ecklonia radiata (Phaeophyceae) for aquaculture, restoration and conservation. Therefore, we investigated the effects of nutrient strength (full to 1/20 strength Provasoli enriched seawater (PES)), germanium dioxide strength (full and half strength) and application period (no application, 9-16, 9-30, 16-23 and 16-30 days post seeding), and seeding density (2, 4, 12 and 18 spores mm -2 ) on sporophyte production in E. radiata in three laboratory experiments. Nutrient strength and seeding density significantly affected sporophyte transition frequency, sporophyte formation frequency, and sporophyte length, with all three metrics consistently decreasing with decreasing nutrient strength and increasing seeding density. Sporophyte transition frequencies were more than four times higher in the full-strength nutrient treatment (58.2 %) compared to the 1/8 and 1/20 strength treatments (12.5 and 3.0 %, respectively), and 18% higher in the 2 spores mm -2 seeding density treatment (100.0 %) compared to the 18 spores mm -2 treatment (84.3 %). Germanium dioxide application period but not strength significantly impacted sporophyte transition frequency and sporophyte formation frequency. Transition frequencies were more than two-thirds lower when GeO 2 was applied from day 9 onwards (<25 %) compared to day 16 onwards and no application (>75 %). Based on these results, we recommended using full strength PES, half strength GeO 2 from day 16 onwards only if diatom contamination is an issue and seeding at a density of ≤10 spores mm -2 to maximise sporophyte production.