Top-down and bottom-up control of phytoplankton in a mid-latitude continental shelf ecosystem

Abstract

It is widely recognized that ecosystems are influenced by both bottom-up (resource-driven) and top-down (predation-driven) controls. However, in the context of marine phytoplankton, there is often a strong emphasis on resource availability, with less attention given to the significant impact of top-down control on phytoplankton patterns. Studies in the Gulf of Cádiz have mainly focused on resource control perspectives, overlooking the role of zooplankton predation as a driving force in phytoplankton dynamics. This limitation in research approaches has resulted in an incomplete understanding of phytoplankton biomass patterns, as they cannot be fully explained by resource control alone. This study makes a first attempt at studying the combined bottom-up and top-down controls of phytoplankton in the Gulf of Cádiz and aims to test their relative influence on the variance of phytoplankton biomass. For this purpose, monthly samples of chlorophyll-a, mesozooplankton and environmental variables determining phytoplankton growth were taken over a six-year period. The variables involved in phytoplankton growth were compiled into a single factor (primary production limiting factor, pplf) representing the bottom-up control. Grazing (G) by mesozooplankton represented the top-down control of phytoplankton. Different statistical tools (multiple linear regression, decision trees and random forest) were used to analyse the relative importance of top-down vs. bottom-up control on the variance of phytoplankton biomass (using chlorophyll-a as a proxy). All three techniques found that the top-down control had more relative weight on the variance of phytoplankton biomass for the continental shelf as a whole. Resource availability (i.e., bottom-up control) determined phytoplankton growth rate, although grazing (i.e., top-down control) defined the maximum chlorophyll values in the studied region. Additionally, the results revealed a spatiotemporal pattern in the relative importance of top-down and bottom-up control on phytoplankton dynamics. These findings underscore the significance of considering the impact of top-down control for a comprehensive understanding of phytoplankton dynamics in the Gulf of Cádiz.