Abstract
Seagrasses are distributed across the globe and their communities may play key roles in the coastal ecosystems. Seagrass meadows are expected to benefit from the increased carbon availability which might be used in photosynthesis in a future high CO2 world. The main aim of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Zostera marina in a brackish water environment. The short-term mesocosm experiments were conducted in Kõiguste Bay (northern part of Gulf of Riga, the Baltic Sea) in June-July 2013 and 2014. As the levels of pCO2 naturally range from ca. 150 μatm to well above 1000 μatm under summer conditions in Kõiguste Bay we chose to operate in mesocosms with the pCO2 levels of ca. 2000, ca. 1000 and ca. 200 μatm. Additionally, in 2014 the photosynthesis of Z. marina was measured outside of the mesocosm in the natural conditions. In the shallow coastal Baltic Sea seagrass Z. marina lives in a highly variable environment due to seasonality and rapid changes in meteorological conditions. This was demonstrated by the remarkable differences in water temperatures between experimental years of ca. 8°C. Thus, the current study also investigated the effect of elevated pCO2 in combination with short-term natural fluctuations of environmental factors, i.e. temperature and PAR on the photosynthesis of Z. marina. Our results show that elevated pCO2 alone did not enhance the photosynthesis of the seagrass. The photosynthetic response of Z. marina to CO2 enrichment was affected by changes in water temperature and light availability.
Highlights
Over the last 20 years, it has been established that the pH of the world’s oceans is decreasing which is caused primarily by the uptake of carbon dioxide (CO2) from the atmosphere, known as ocean acidification (OA)
This study focuses on the seagrass Zostera marina, which is a widely distributed seagrass of the temperate coastal waters in the northern hemisphere
The main goal of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Z. marina in the brackish water conditions
Summary
Over the last 20 years, it has been established that the pH of the world’s oceans is decreasing which is caused primarily by the uptake of carbon dioxide (CO2) from the atmosphere, known as ocean acidification (OA). Models project that the surface water pH in the central Baltic Sea may decrease 0.3–0.4 units by 2100 (Omstedt et al, 2012; Schneider et al, 2015). Besides light and water temperature, pH and partial pressure of carbon dioxide (pCO2) show a substantial amplitude of natural variability, especially under summer conditions. These daily pH changes may be of a larger magnitude than the scenario modeling suggests for the surface-water pH decrease in the Baltic Sea by the end of this century (personal measurements, 2013, 2014). Biological responses to the elevated water CO2 concentrations in combination with these short-term natural fluctuations of environmental factors have generally received little attention
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