Small Size, Huge Impact: Disproportionate Effects of Ponds on Aquatic Carbon Cycling and Atmospheric Greenhouse Gases

Abstract

The carbon cycle is essential for all life and is a major driver of Earth’s climate. Freshwater ecosystems (such as lakes, ponds, rivers, wetlands, etc.) play an outsized role in the global carbon cycle, acting in the transport, storage, and emission of carbon to the atmosphere. Freshwaters are significant sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4) to the atmosphere despite only covering 1% of Earth’s surface area, and they have become critical a piece in global greenhouse gas models. Surprisingly, the freshwater ecosystems that may contribute the most to global emissions are small ponds, though causes of emissions and variation across ponds are not well understood. My dissertation aims to more fully understand carbon cycling in ponds and identify key factors that influence greenhouse gas emissions. Through measurements of ecosystem metabolism (i.e., the balance of photosynthesis and aerobic respiration), I found that production and respiration rates in ponds are some of the highest across all freshwater ecosystems, with shallow depths influencing many factors that lead to high rates. By measuring greenhouse gas emissions from a wide range of ponds, I found that the absence of oxygen and high phosphorus concentrations can combine to lead to elevated CH4 emissions. In addition, duckweed on the surface of ponds can exacerbate these conditions and is potentially a target for management strategies. Finally, I monitored greenhouse gas emissions in four ponds throughout an entire year and found that water column mixing and stratification greatly impacts the seasonal timing and magnitude of greenhouse gas emissions. Overall, this work emphasizes that ponds are dynamic ecosystems with high rates of carbon cycling and greenhouse gas emissions, with implications for management and global greenhouse gas dynamics.