Seasonal and interannual variability in phytoplankton primary to copepod secondary and tertiary production for 18 years (2006–2023) in Sagami Bay, Japan

Seasonal and interannual variations in phytoplankton primary production (PP) and in abundance, biomass, daily production rate (DPR) and food requirement of planktonic copepods (>200 µm) were investigated in the neritic area of Sagami Bay, Kanagawa, Japan, on 426 occasions from January 2006 to December 2023. Monthly/annual mean PP was 0.39–1.81 g C m−2 d−1. Copepod abundance, biomass and DPR revealed distinct seasonal variations, declining in May and remaining low in June–September, related to abiotic/biotic variables. These copepod parameters showed different interannual variations, related to anomalies of abiotic variables and climatic indices. Monthly/annual means of abundance, biomass and DPR of total copepods were 5.70×102–3.17×103 ind. m−3, 3.61–11.83 mg C m−3 and 0.38–1.28 mg C m−3 d−1, while those of depth-integrated copepod secondary (SP) and tertiary production (TP) were 0.018–0.060 and 0.003–0.012 g C m−2 d−1, respectively. Monthly/annual mean transfer efficiency of PP–SP, PP–TP and SP–TP was 1.0–9.9%, 0.2–1.6% and 8.5–22.9%, respectively. Transfer efficiencies of PP–SP and PP–TP decreased exponentially with PP, and these correlations were statistically significant. Monthly/annual mean food requirement by copepod secondary and tertiary producers was estimated to be 0.064–0.216 and 0.009–0.041 g C m−2 d−1, corresponding to 3.5–34.0% and 0.6–5.3% of PP, respectively. Estimation methods for DPR calculated by several empirical growth models were discussed, based on gross growth efficiency (i.e. the ratio of DPR to food requirement).