Abstract
Global change has been acknowledged as one of the main threats to the biosphere and its provision of ecosystem services, especially in marine ecosystems. Seagrasses play a critical ecological role in coastal ecosystems, but their responses to ocean acidification (OA) and climate change are not well understood. There have been previous studies focused on the effects of OA, but the outcome of interactions with co-factors predicted to alter during climate change still needs to be addressed. For example, the impact of higher CO2 and different hydrodynamic regimes on seagrass performance remains unknown. We studied the effects of OA under different current velocities on productivity of the seagrass Zostera noltei, using changes in dissolved oxygen as a proxy for the seagrass carbon metabolism, and release of dissolved organic carbon (DOC) in a four-week experiment using an open-water outdoor mesocosm. Under current pH conditions, increasing current velocity had a positive effect on productivity, but this depended on shoot density. However, this positive effect of current velocity disappeared under OA conditions. OA conditions led to a significant increase in gross production rate and respiration, suggesting that Z. noltei is carbon-limited under the current inorganic carbon concentration of seawater. In addition, an increase in non-structural carbohydrates was found, which may lead to better growing conditions and higher resilience in seagrasses subjected to environmental stress. Regarding DOC flux, a direct and positive relationship was found between current velocity and DOC release, both under current pH and OA conditions. We conclude that OA and high current velocity may lead to favourable growth scenarios for Z. noltei populations, increasing their productivity, non-structural carbohydrate concentrations and DOC release. Our results add new dimensions to predictions on how seagrass ecosystems will respond to climate change, with important implications for the resilience and conservation of these threatened ecosystems.
Highlights
Over the last century, humans have produced large amounts of CO2 through activities such as fossil fuel burning, intensive agriculture and deforestation [1]
Lowering the pH led to significant responses in gross primary production (GPP) and R, with values under forecasted pH (FpH) about 1.5 times higher than those under current pH (CpH) for both GPP and R (Fig 2A & 2B)
The interaction between pH and current velocity resulted in significant differences in R, with the treatment combination of FpH + LV showing higher R than the treatment combination of CpH + high velocities (HV) (Fig 2 and Table 3)
Summary
Humans have produced large amounts of CO2 through activities such as fossil fuel burning, intensive agriculture and deforestation [1]. A portion of the CO2 absorbed by oceans is stored in living biomass and sequestered in sediments but a large amount remains in its inorganic form. This increase of inorganic carbon stored in the oceans has driven a reduction in seawater pH and promoted changes in the seawater chemistry in a process commonly referred to as “ocean acidification” (OA) [6,7,8,9,10]. The global ocean pH is expected to fall to between 8.05 and 7.6 by the end of this century (IPCC, 2013; [11]) This raises concern about the possible impacts of these changes on marine organisms. Studies have underscored the crucial role of shallow coastal ecosystems, which function as transition zones between land and ocean and as filters for carbon sequestration [15,16]
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