The physiological response of non-calcifying benthic macroalgae to ocean acidification

Oceanic carbon dioxide (CO₂) concentrations have rapidly increased during the last century due to anthropogenic influences, a phenomenon known as Ocean Acidification. Current research has exposed significant threats ocean acidification poses to many marine organisms. Benthic macroalgae serve a multitude of functions within kelp forest communities, though their response to elevated CO₂ is largely unexplored. Light is an important factor influencing benthic algal assemblages, and in kelp-dominated ecosystems where weather-related disturbances are frequent, losses or thinning of the kelp canopies results in dynamic patches of light along the benthos. While much is known about postdisturbance algal succession under present-day conditions, little is known about how succession will occur under future increasing CO₂. Understanding the physiological response of macroalgae to elevated CO₂ and the ecological consequences these responses support are imperative to predict community changes. My research examined the physiological responses of non-calcifying, benthic red macroalgae to present-day and predicted future CO₂ concentrations under local disturbance stress (kelp canopy clearing) by measuring carbon uptake using laboratory and field experiments. Photosynthesis and irradiance curves showed shade-adapted species physiologically depreciate in elevated CO₂, while a clearing-adapted species became more efficient under low light conditions. For all species under both simulated kelp canopy shade and clearing light treatments, short-term exposure to elevated CO₂ reduced carbon uptake. However, species responses varied significantly when lengthening exposure time to elevated CO₂. In the field, elevated CO₂ significantly increased carbon uptake under both kelp canopy shade and clearing, and a clearing-adapted species had the highest carbon uptake values in the clearing. These results demonstrated three main points: (1) light-adapted characterization may predict species responses to elevated CO₂; (2) laboratory methods may create strong artifacts when measuring carbon uptake; (3) elevated CO₂ enhances photosynthesis of non-calcifying benthic red macroalgae in the field. These findings suggest in a high CO₂ world clearing-adapted macroalgal assemblages may potentially inhibit shade-adapted algal recruitment persisting under canopy shade with competitive dominance. The data collected in this study will be useful in future theoretical and experimental investigations of interacting affects of climate change stressors impacting kelp forest ecosystems.

Pesce, A. S., 2012. The physiological response of non-calcifying benthic macroalgae to ocean acidification. MSc thesis. 37 p. San Diego State University. Thesis.

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