Applications of environmental DNA monitoring for seaweed reproductive phenology: A case study with giant kelp (Macrocystis pyrifera).

Monitoring the seasonal reproductive cycles of seaweeds is crucial for effective population and ecosystem management, as well as mariculture seedstock collection. Traditional methods, such as visual monitoring by SCUBA diving or snorkeling, are costly, labor-intensive, and limited in temporal and spatial coverage. This study explores substituting these methods with environmental DNA (eDNA) techniques for giant kelp (Macrocystis pyrifera, order Laminariales). This laboratory study aimed to determine the minimum detectable concentration of zoospores and sporophyte tissue needed for detecting the reproductive phenology of M. pyrifera and to assess the ability and sensitivity to discriminate between life stages. The study involved syringe-filtering seawater samples through 0.45-μm pore-size filters before quantitative polymerase chain reaction (qPCR) analysis with species-specific primers. There was a strong positive correlation between zoospore concentration and eDNA copies per μL (ρ = 0.982, p < 0.001), and a weak correlation for sporophyte wet weight (ρ = 0.367, p = 0.134). There was a significant difference between zoospore and zoospore + sporophyte treatments (p = 0.010), indicating the substantial influence of sporophyte tissue on detected eDNA quantity. Sporophyte tissue obscures the zoospore signal, especially at lower zoospore concentrations (<37 zoospores · mL-1), highlighting that eDNA analysis is suitable for monitoring reproductive peaks and broader patterns in seasonal reproduction cycles of giant kelp when zoospore concentrations are high.