Abstract
Gas ebullition from aquatic systems to the atmosphere represents a potentially important fraction of primary production that goes unquantified by measurements of dissolved gas concentrations. Although gas ebullition from photosynthetic surfaces has often been observed, it is rarely quantified. The resulting underestimation of photosynthetic activity may significantly bias the determination of ecosystem trophic status and estimated rates of biogeochemical cycling from in situ measures of dissolved oxygen. Here, we quantified gas ebullition rates in Zostera marina meadows in Virginia, U.S.A. using simple funnel traps and analyzed the oxygen concentration and isotopic composition of the captured gas. Maximum hourly rates of oxygen ebullition (3.0 mmol oxygen m−2 h−1) were observed during the coincidence of high irradiance and low tides, particularly in the afternoon when oxygen and temperature maxima occurred. The daily ebullition fluxes (up to 11 mmol oxygen m−2 d−1) were roughly equivalent to net primary production rates determined from dissolved oxygen measurements indicating that bubble ebullition can represent a major component of primary production that is not commonly included in ecosystem‐scale estimates. Oxygen content comprised 20–40% of the captured bubble gas volume and correlated negatively with its δ18O values, consistent with a predominance of mixing between the higher δ18O of atmospheric oxygen in equilibrium with seawater and the lower δ18O of oxygen derived from photosynthesis. Thus, future studies interested in the metabolism of highly productive, shallow water ecosystems, and particularly those measuring in situ oxygen flux, should not ignore the bubble formation and ebullition processes described here.
Highlights
Experimental work used bubble ebullition from cut aquatic plants to measure carbon dioxide assimilation and photosynthetic rates in the laboratory (Reinke 1883; Blackman and Smith 1911)
Later improvements produced “Wilmot’s Bubbler,” which allowed for more accurate measurements of ebullition and identified several difficulties associated with estimating ebullition including variations in bubble gas concentrations, bubble size, pressure, and gas saturation states (Wilmott 1921)
Substantial qualitative evidence exists for the formation and ebullition of bubbles in photosynthetic systems, little attention has been applied to quantifying seagrass oxygen ebullition rates and the in situ conditions that promote bubble formation
Summary
Experimental work used bubble ebullition from cut aquatic plants to measure carbon dioxide assimilation and photosynthetic rates in the laboratory (Reinke 1883; Blackman and Smith 1911). The simplest design employs an inverted funnel to capture gas, followed by manual collection and measurement of the gas volume (Odum 1957; Martens and Klump 1980; Keller and Stallard 1994; Cheng et al 2014; Koschorreck et al 2017) These bubble traps can be deployed to quantify seagrass ebullition as conditions favorable for bubble formation have been identified (e.g., shallow water, high irradiance, low flow, high oxygen saturation) (Zieman 1974; Hargraves 1982)
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