While there is growing interest in understanding how marine life will respond to future ocean acidification, many coastal ecosystems currently experience intense acidification in response to upwelling, eutrophication, or riverine discharge. Such acidification can be inhibitory to calcifying animals, but less is known regarding how non-calcifying macroalgae may respond to elevated CO2. Here, we report on experiments performed during summer through fall with North Atlantic populations of Gracilaria and Ulva that were grown in situ within a mesotrophic estuary (Shinnecock Bay, NY, USA) or exposed to normal and elevated, but environmentally realistic, levels of pCO2 and/or nutrients (nitrogen and phosphorus). In nearly all experiments, the growth rates of Gracilaria were significantly increased by an average of 70% beyond in situ and control conditions when exposed to elevated levels of pCO2 (p<0.05), but were unaffected by nutrient enrichment. In contrast, the growth response of Ulva was more complex as this alga experienced significantly (p<0.05) increased growth rates in response to both elevated pCO2 and elevated nutrients and, in two cases, pCO2 and nutrients interacted to provide a synergistically enhanced growth rate for Ulva. Across all experiments, elevated pCO2 significantly increased Ulva growth rates by 30% (p<0.05), while the response to nutrients was smaller (p>0.05). The δ13C content of both Gracilaria and Ulva decreased two-to-three fold when grown under elevated pCO2 (p<0.001) and mixing models demonstrated these macroalgae experienced a physiological shift from near exclusive use of HCO3- to primarily CO2 use when exposed to elevated pCO2. This shift in carbon use coupled with significantly increased growth in response to elevated pCO2 suggests that photosynthesis of these algae was limited by their inorganic carbon supply. Given that eutrophication can yield elevated levels of pCO2, this study suggests that the overgrowth of macroalgae in eutrophic estuaries can be directly promoted by acidification, a process that will intensify in the coming decades.

While ocean acidification has different effects on herbivores and autotrophs, how acidification may influence herbivory is poorly understood. This study examined how grazing by the gastropod Lacuna vincta (hereafter Lacuna) on the macroalgae Ulva spp. (hereafter Ulva) is influenced by ocean acidification. Herbivory by Lacuna was significantly reduced under elevated partial pressure of carbon dioxide (pCO 2 ; 1500−2000 µatm) relative to ambient pCO 2 (~400 µatm). This significant decrease in herbivory was unrelated to the physiological status of Ulva but rather was specifically elicited when Lacuna was exposed to elevated pCO 2 in the absence of food for 18 to 24 h prior to grazing Ulva. The negative effects of elevated pCO 2 on Lacuna were absent at 400 to 800 µatm pCO 2 or when fed but persisted for up to 72 h following a 24 h exposure to elevated pCO 2 without food. Depressed respiration rates in Lacuna following exposure to high pCO 2 without food indicated these conditions produced metabolic suppression potentially associated with acidosis. Collectively, the lasting (72 h) nature of grazing inhibition of Lacuna following brief exposure (18 h) to moderate pCO 2 levels (>~1500 µatm) when food was not available suggests this process could have broad effects on the dynamics of macroalgae in estuaries where Lacuna is a dominant grazer; these effects will be amplified as climate change progresses.

Abstract. Coastal ecosystems can experience acidification via upwelling, eutrophication, riverine discharge, and climate change. While the resulting increases in pCO2 can have deleterious effects on calcifying animals, this change in carbonate chemistry may benefit some marine autotrophs. Here, we report on experiments performed with North Atlantic populations of hard clams (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), bay scallops (Argopecten irradians), and blue mussels (Mytilus edulis) grown with and without North Atlantic populations of the green macroalgae, Ulva. In six of seven experiments, exposure to elevated pCO2 levels (∼1700 µatm) resulted in depressed shell- and/or tissue-based growth rates of bivalves compared to control conditions, whereas rates were significantly higher in the presence of Ulva in all experiments. In many cases, the co-exposure to elevated pCO2 levels and Ulva had an antagonistic effect on bivalve growth rates whereby the presence of Ulva under elevated pCO2 levels significantly improved their performance compared to the acidification-only treatment. Saturation states for calcium carbonate (Ω) were significantly higher in the presence of Ulva under both ambient and elevated CO2 delivery rates, and growth rates of bivalves were significantly correlated with Ω in six of seven experiments. Collectively, the results suggest that photosynthesis and/or nitrate assimilation by Ulva increased alkalinity, fostering a carbonate chemistry regime more suitable for optimal growth of calcifying bivalves. This suggests that large natural and/or aquacultured collections of macroalgae in acidified environments could serve as a refuge for calcifying animals that may otherwise be negatively impacted by elevated pCO2 levels and depressed Ω.

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Fossil fuel combustion, eutrophication, and upwelling introduce excess CO2 into coastal zones. The extent to which marine autotrophs may benefit from elevated CO2 will be a function of their carbon limitation and, among other factors, competition with other primary producers. Here, we report on experiments performed with North Atlantic species of Ulva and Gracilaria grown in situ or exposed to ambient (~400 µatm) and elevated pCO2 (~2500 µatm) and/or subjected to competition with each other and/or with natural plankton assemblages. Elevated pCO2 significantly increased the growth rates of Gracilaria and Ulva and yielded significant declines in tissue δ13C, suggesting that increased growth was associated with increased CO2 use relative to HCO3 −. Gracilaria growth was unaffected by competition with plankton or Ulva, while Ulva experienced significantly reduced growth when competing with Gracilaria or plankton. Dinoflagellates experienced significantly increased growth when exposed to elevated pCO2 but significantly slower growth when competing with Gracilaria. Elevated carbon-to-nitrogen ratios among macroalgae suggested that competition for nitrogen also shaped interactions among autotrophs, particularly Ulva. While some estuarine autotrophs benefit from elevated pCO2, the benefit can change when direct competition with other primary producers is considered with Gracilaria outcompeting Ulva and dinoflagellates outcompeting diatoms under elevated pCO2.