Effects of Mixed Layer Depth on Phytoplankton Biomass in a Tropical Marginal Ocean: A Multiple Timescale Analysis

Abstract

In open oceans, changes in mixed layer depth (MLD) may affect phytoplankton growth and biomass variations via the regulation of nutrient supply from deep waters. Estimates of relationships between variability in phytoplankton dynamics and the MLD remain limited, especially at different time scales. We compiled and analyzed averaged euphotic-depth-integrated chlorophyll-a (IChl-a) and surface chlorophyll-a (SChl-a) concentrations collected from 27 cruises during the period of 1999–2019 in the tropical northern South China Sea (SCS). Seasonal differences existed in both averaged IChl-a and SChl-a concentrations, with significantly high concentrations in the cold season. Inconsistent relationships between the averaged IChl-a and SChl-a concentrations between seasons implied that the use of SChl-a concentration as a common indicator of phytoplankton biomass dynamics should be performed with caution. Over the past decades in the northern SCS, the averaged IChl-a, SChl-a, and MLD decreased to a greater extent in the cold season than in the warm season, while sea surface temperature (SST) rose rapidly and dramatically in both seasons. The MLD was observed to have better correlations with the averaged IChl-a and SChl-a concentrations than the SST in the time-series data. Our results highlight the importance of IChl-a concentration, which is an overall measure of phytoplankton responses to euphotic zone conditions, and the MLD could be used as a good index for changes in phytoplankton biomass under climate change.