Spatiotemporal variability and drivers of modeled primary production rates in the Northern Humboldt Current System

Abstract

A coupled physical-biogeochemical model was employed to explore the spatiotemporal dynamics of primary production (PP) rates within the Northern Humboldt Current System (NHCS). The coastal zone spanning 250 km from the shore, from 3°to 18°S, stands out as a highly productive upwelling region, exhibiting an average surface PP value of 2.5 mol C m⁻³ yr⁻¹. Correspondingly, the average vertically integrated PP within the euphotic layer amounts to 13 mol C m⁻² yr⁻¹. In this context, summer emerges as the peak of productivity, yielding 18 mol C m⁻² yr⁻¹, while winter signifies the period of least productivity, with 9 mol C m⁻² yr⁻¹. Our study revealed that surface PP variability is primarily driven by changes in surface chlorophyll and phytoplanktonic biomass (mainly diatoms), followed by changes in photosynthetically active radiation (PAR) levels. During summertime, these three drivers contribute to substantial positive anomalies in surface PP. However, the reduction in nutrient availability resulting from weakened upwelling-favorable winds has a slight negative impact on surface PP rates. Yet, this decline is offset by a positive thermal effect during the warmer season. In contrast, during the winter season, a significant decrease in surface chlorophyll concentrations due to a vertical redistribution into a deeper mixed layer significantly diminishes surface PP. Furthermore, the reduction in both PAR levels and biomass concentrations has a comparable effect, further contributing to the decrease in surface PP rates during wintertime. At a depth of 20 m, changes in PP are primarily driven by variations between the opposing influences of PAR and chlorophyll concentrations. While PAR adheres to the seasonal cycle of warming and cooling throughout the year, chlorophyll-driven anomalies exhibit an inverse pattern to those at the surface, influenced by the vertical dilution effect within the mixed layer. Overall, this study provides valuable insights into the complex interplay of drivers that govern PP dynamics across various depths within one of the world’s most productive marine regions.