The effects of long-term in situ CO2 enrichment on tropical seagrass communities at volcanic vents

Abstract

Abstract The effects of long-term exposure to elevated levels of carbon dioxide (CO2) on seagrass communities are still poorly understood. This study investigates the tropical subtidal seagrass communities at three shallow volcanic CO2 vents in Papua New Guinea. Seagrass cover and biomass increased threefold and fivefold, respectively, from control to medium and high pCO2 sites (average pH = 7.9, 7.7, and 7.5, respectively). The seagrass community composition differed significantly between the pCO2 sites: Cymodocea serrulata, Cymodocea rotundata, and Halodule uninervis were more abundant at high pCO2 sites, whereas Halophila ovalis, Thalassia hemprichii, and Syringodium isoetifolium occurred only at low and mid pCO2 sites. Cymodocea rotundata was the only species common among all pCO2 sites and locations. The δ13C in its leaves significantly declined with increasing pCO2, indicating that additional CO2 influenced seagrass carbon uptake, and specifically, that there was discrimination against the heavier isotope (13C) when carbon was more abundant. Size-specific leaf growth rates (i.e. leaf turnover) also significantly declined with increasing pCO2; however, leaf growth rates showed no consistent difference in response to elevated pCO2 in two of four surveys. Our study suggests that progressive ocean acidification may lead to higher cover and above- and below-ground biomass, but lower size-specific growth and altered species composition in tropical seagrass communities. The effects of co-limiting factors, such as light and nutrient availability, on early-responding parameters, such as growth rates, require further attention to improve projections.