Abstract
Population reconstruction techniques was used to assess for the first time the population dynamics of a seagrass, Cymodocea nodosa, exposed to long-term elevated CO2 near three volcanic seeps and compared them with reference sites away from the seeps. Under high CO2, the density of shoots and of individuals (apical shoots), and the vertical and horizontal elongation and production rates, were higher than at the reference sites. Nitrogen limitation effects on rhizome elongation and production rates and on biomass were more evident than CO2 as these were highest at the location where the limitation of nitrogen was highest. At the seep where the availability of CO2 was highest and nitrogen lowest, density of shoots and individuals were highest, probably due to CO2 effects on shoot differentiation and induced reproductive output, respectively. At the three seeps, there was higher short- and long-term shoot recruitment than at the reference sites, and growth rates was around zero, indicating that elevated CO2 increases the turnover of C. nodosa shoots.
Highlights
Seagrass photosynthesis may be limited by dissolved inorganic carbon concentration of the water column, which has led part of the research community to suggest that, seagrass could benefit from the global increase in oceanic CO2 levels (Koch et al, 2013; Russel et al, 2013; Borum et al, 2018)
The presence of seagrass at the CO2 seeps of Adamas and Paleochori bay of Milos island, Greece and of Levante bay at Vulcano island, Italy, provided an opportunity to test the effects of longterm CO2 enrichment on seagrass population dynamics
Confirming our initial hypothesis, we observed a positive response of C. nodosa population to high CO2 at the three seep sites, with higher total and apical shoot density, and higher vertical and horizontal rhizome elongation and production rates than at the reference sites
Summary
Seagrass photosynthesis may be limited by dissolved inorganic carbon concentration of the water column, which has led part of the research community to suggest that, seagrass could benefit from the global increase in oceanic CO2 levels (Koch et al, 2013; Russel et al, 2013; Borum et al, 2018). Several CO2 enrichments studies (ex-situ and in-situ) on seagrasses reveal that the response is species-specific and more complex than originally thought Long experiments, such as that carried out by Alexandre et al, (2012), which lasted for five months reported positive effects on the photosynthetic production of Cymodocea nodosa under experimental CO2 enrichment conditions, but not effects on growth. Short-term experiments have shown increased photosynthetic rate and shoot productivity for Z. marina (Pajusalu et al, 2016; Zimmerman et al, 1997), Thalassia hemprichii (Jian et al, 2010) and Zostera muelleri (Collier et al, 2018) and increased community production for Zostera noltei (Mishra et al, 2018). Recent studies on three tropical (C. serrulata, T. hemprichii and Z, muelleri) and nine temperate (including Z. polychlamys) seagrass species showed significant increase in net productivity with increased CO2 (Ow et al, 2015; Borum et al, 2016; Collier et al, 2018)
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