Field and modeling insights into carbon cycling in coastal marine ecosystems [electronic resource] : understanding natural variability and preparing for the future
Keywords: Earth System Science.

This dissertation combines field and modeling studies in coastal ecosystems to better understand high resolution ecosystem metabolism, the resulting natural variability in seawater carbonate chemistry, and to gain insights into the future of coastal ecosystems in a warmer and more acidic future. Chapter 1 uses high-resolution physical oceanography and biogeochemistry to quantify spatial and temporal variability in net community calcification and production on Palmyra Atoll, an isolated coral reef atoll in the central Pacific. I show that environmental controls, such as light and temperature, are the primary predictors of calcification and primary production. However, the ratio of inorganic to organic production, the ultimate control on the overlying water chemistry, is much more closely controlled by benthic community composition. Chapter 1 suggests that overlying water chemistry is very sensitive to the benthic community composition on a coral reef and that the biogeochemical variability is likely to change following human disturbances to the benthic community. Chapter 2 uses high-temporal resolution physical oceanography and biogeochemistry to study the back reefs of Ofu, American Samoa. This is the first study to document the biogeochemical variability on this coral reef. I show that the reef regularly experiences pH minima not expected until near the end of the century and ecosystem metabolism has a very low inorganic to organic production ratio. This study highlights the biogeochemical variability of back reef sites and argues for their continued study as a window into the future extremes on coral reefs in a changing climate. Chapter 3 complements the field-based approaches of Chapters 1 and 2 by using numerical modeling to explore a localized solution to combat nearshore acidification. I use an empirically validated, process-based model to explore the efficacy of nighttime bubble stripping in shallow water ecosystems as a means to reduce carbonate chemistry variability. I demonstrate that bubbling ventilates coastal ecosystems by one to two orders of magnitude faster than wind-speed based estimates of gas exchange. By coupling bubble stripping to a coastal ecosystem metabolism model, I demonstrate that strategically timed nighttime bubble stripping results in small reductions to present-day diel carbonate chemistry variability and that this variability is further reduced in simulations of end-of-century conditions. This chapter concludes with a consideration of the engineering requirements for scaling bubble stripping from laboratory-based test tanks to field-based coastal settings. Chapter 4 applies the field-based observational skills I developed in Chapters 1 and 2 to a central California kelp forest. I present a carbonate chemistry time series of six locations within a kelp forest in Pacific Grove, California spanning July 2013-August 2014, each with samples collected near the surface and near the bottom. I describe the spatial and temporal patterns of biogeochemical and hydrographic variability in the kelp forest. I then analyze the biological and physical controls on the observed carbonate chemistry variability. Finally, I discuss how the observed spatial and temporal differences within the kelp forest create a unique opportunity for studying organismal adaptation to biogeochemical variability as well as a unique challenge for future coastal ocean acidification monitoring efforts. Together these four chapters provide new observations and tools to better understand coastal biogeochemistry so that we may manage our coastal resources in an era of unprecedented global environmental change.