Response of Ocean Biology to Future Climate Change

We examine six different coupled climate model simulations of future climate change to determine the range of behavior of those aspects of global warming simulations that are relevant to the ocean biological response. The overall response we infer from examining the physical response of the ocean to global warming is decreased biological production in low latitude upwelling regions and the poleward half of the subtropical gyres, and increased production in the polar regions. Wind-driven upwelling is the dominant mechanism of nutrient supply along the highly productive western margins of the continents and in the equatorial regions. Models predict widely varying results, but the general tendency is towards a reduction of upwelling in these regions, from which we would infer that biological production would decrease. The dominant mechanism for nutrient supply in the subtropical gyres poleward of the subtropical convergence zone is wintertime convection. These regions tend to become more stratified with future climate change, which reduces the depth of wintertime mixing. The expectation, supported by model predictions, is that this would result in reduced biological production. The polar regions generally have a high supply of nutrients due to upwelling and convection, but can suffer from low productivity due to low light supply in deep mixed layers. Increased stratification, which occurs in most models, though with a complex pattern, would thus tend to increase biological production. Exceptions to this would be where low levels of micronutrient supply by dust limit the production, such as is thought to be the case in the Southern Ocean and North Pacific, or where the decreased mixing reduced the nutrient supply to less than the potential biological uptake. The mechanism of nutrient supply to regions between the equatorial upwelling bands and subtropical convergence is poorly understood and poorly simulated in most models. It is difficult to determine how these regions will respond to future climate change. The changes that we have described will also very likely lead to changes in ocean ecology as the major phytoplankton groups such as diatoms, coccolithphorids, flagellates, Phaeocystis, and nitrogen fixers, are sensitive to water column stratification as well as nutrient content.