Monitoring seasonal reproductive cycles in seaweeds is crucial for effective population and ecosystem management, and mariculture seedstock collection. Traditional methods, such as SCUBA diving or snorkelling, are costly, labour-intensive, and limited in spatiotemporal coverage. This study explores substituting these methods with environmental DNA (eDNA) techniques for giant kelp (Macrocystis pyrifera), using quantitative polymerase chain reaction (qPCR). This laboratory study aims to determine the minimum detectable concentration of zoospores and sporophyte tissue needed for detecting the reproductive phenology of M. pyrifera and 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 qPCR analysis with species-specific primers. There was strong positive correlation between zoospore concentration and eDNA copies per μL (ρ = 0.982, P<0.001), and 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 substantial influence of sporophyte tissue on detected eDNA quantity. Sporophyte tissue obscures the zoospore signal especially at lower concentrations (<37 zoospores/mL), 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.