Biosphere Impacts of Ocean Hypoxia in a Warming Climate

Abstract

Earth’s climate and biology mediate the availability of dissolved oxygen (O2) in seawater, a key environmental constraint on energy acquisition for diverse marine life. Energy available for organismal growth, activity, and maintenance becomes limited when the environmental O2 supply falls short of the biological demand, a physiological condition termed ‘hypoxia’, which has far-reaching biogeochemical and ecological consequences beyond fitness detriments to individual species. For instance, in the ocean’s small anoxic zones (O2 < ~5 µM), O2 limitation of aerobic microbes selects for slower anaerobic metabolisms that convert nitrate (NO3-), a critical macronutrient, to biologically inert N2 gas, thereby causing widespread nitrogen limitation of phytoplankton. Marine animals are more sensitive to low O2 conditions than microbes and have no sustainable alternative metabolism to aerobic energy production. Even modest O2 depletion can thus lead to their habitat restriction, both in the long-term by stable aerobic barriers, and dynamically from climate forcing. On geologic timescales, periods of extreme ocean warming and O2 loss have been tied to episodes of global mass extinction. Currently, anthropogenic climate warming risks O2 depletion of the modern ocean, threatening both global marine productivity and biodiversity. In this thesis, I aim to use mathematical models of physiology, ecology, biogeochemistry, and climate to explore the effects of hypoxia on marine ecosystems. In Chapters 2 and 3, I develop and analyze a microbial ecosystem model of the ocean’s low O2 zones, fit to geochemical data, to study how competitive dynamics between aerobic and anaerobic microbes regulate global nutrient loss. I find that species competition along minute O2 gradients at the edges of anoxic zones modulates the long-term rate of ocean N removal, its sensitivity to climate forcing, and causes basin-wide fluctuations in N loss over time even in a stable environment. In Chapters 4 and 5, I focus on marine animal extinction risks from O2 loss driven by warming by combining Earth System Model simulations of past and future climate change events with a model of species’ ecophysiologial limits, calibrated by published laboratory measurements. Focusing on the Permian/Triassic marine mass extinction (~252 million years ago), in Chapter 4, I find that the predicted intensity and biogeographic signature of extinctions arising from ocean warming and O2 loss explain reconstructions of this event from the marine fossil record. This work thus establishes a mechanistic link between global warming, hypoxia and extinction, with dire implications for future climate change. In Chapter 5, I project animal extinction risks from anthropogenic climate warming and O2 loss under divergent future greenhouse gas emissions scenarios. If future emissions continue to accelerate, by the end of the century, extinctions driven by temperature-dependent hypoxia will surpass those from current anthropogenic threats, eventually risking a mass extinction comparable to the “Big Five” biotic crises in Earth’s past. “Flattening the curve” of global greenhouse gas emissions would curtail these extinction risks, protecting the diversity of animal life in the oceans.