Upper Ocean Physics as Relevant to Ecosystem Dynamics: A Tutorial

Abstract

Oceanic control of the atmospheric carbon dioxide concentration is the link between the studies of plankton ecology and climate change. Modeling ecosystem dynamics requires some understanding of the physics and chemistry of the upper ocean. In addition, an understanding of the issues involved in predicting climate change can help focus ecological studies. This article is intended as a review of upper ocean physics and chemistry as relevant to ecosystem research, and a summary of climate‐related issues to which ecosystem dynamics might in the future make a contribution. Our picture of the carbon cycle in the upper ocean relies on ecosystem dynamics for an understanding of the efficiency of nutrient uptake and export of carbon in the form of sinking carbon particles, and for the fraction of recycled and exported particulate carbon production. A fundamental variable appears to be the size distribution of phytoplankton. Also, ultimately, an understanding of the partitioning between calcitic‐ and siliceous‐based ecosystems may be important to predicting the long‐term ocean carbon cycle. Ecosystem dynamics of the upper ocean are driven by the interplay between dynamics of the surface ocean mixed layer and the depth of light penetration. This interplay is illustrated by noting the effect of high‐frequency fluctuations in mixed‐layer depth from a physical model (Archer et al. 1993) on an ecosystem model developed at weathership station Papa in the subarctic Pacific ocean (Frost 1987). Three families of surface ocean mixed‐layer models are available for use by plankton ecologists, and although the physical mechanisms by which mixing occurs differ among the model groups, all are generally successful at predicting the observed ocean mixed‐layer depth. This paper explores behavioral distinctions between the three types of models, and summarizes previously published comparisons of the generality, accuracy, and computational requirements of the three models. Nutrients are supplied to the euphotic zone by the exchange of water between nutrient‐depleted surface waters and nutrient‐rich deeper waters. Current understanding of this process is still problematic, with rates of mixing required to balance nutrient uptake estimates higher than values predicted based on turbulence studies. I also review evidence that episodic mixing, driven by frontal and mesocale motions, may be responsible for a significant fraction of vertical nutrient transport.