Abstract
Through respiration and photosynthesis, seagrass meadows contribute greatly to carbon and oxygen fluxes in shallow coastal waters. There is increasing concern about how shallow-water primary producers will react to a near-future climate scenario with increased temperature variation. When modelling primary productivity under high temperature variability, Q10 values are commonly used to predict rate changes depending on biophysical factors. Q10 values are often assumed to be constant and around 2.0 (i.e. a doubling of the rate with a temperature increase of 10 °C). We aimed to establish how the gas exchange of seagrass (Zostera marina) tissues at various maturity stages would respond over a broad range of temperatures. Seagrass shoot maturity stage clearly affected respiration and apparent photosynthesis, and the Q10 results indicated a skewed balance between the two processes, with a higher photosynthetic Q10 during periods of elevated temperatures. When estimating whole-plant Q10 in a realistic maximal temperature range, we found that the overall response of a seagrass plant’s net O2 exchange balance can be as much as three to four times higher than under ambient temperatures. Our findings indicate that plant tissue age and temperature should be considered when assessing and modelling carbon and oxygen fluctuations in vegetated coastal areas.
Highlights
Seagrasses are submerged marine angiosperms colonising areas of soft-bottom sediment with an extensive network of below-ground tissues
We hypothesised that: (a) the respiration rate will be higher in younger seagrass tissues in response to potentially higher energy demand; (b) the basal parts of leaves, where meristematic growth occurs, will have higher respiration rates than more mature leaf parts; and (c) photosynthetic rates will be higher in the leaf apex and middle part than in the leaf base due to a potentially greater abundance of light-harvesting chloroplasts
Q10 values differed depending on both plant tissue age and shoot age, and were higher and more variable for photosynthesis than for respiration
Summary
Seagrasses are submerged marine angiosperms colonising areas of soft-bottom sediment with an extensive network of below-ground tissues (i.e. roots and rhizomes). Seagrass plants fix large amounts of carbon dioxide, i.e. 394–449 gC m−2 year−1 globally[3], while a portion of the fixed carbon is released back into the water through respiration This portion can be large, quite variable, due to the large amount of below-ground seagrass tissue[2]. High seagrass productivity can greatly influence coastal carbon and oxygen fluxes, and subsequently pH4–7 Both photosynthesis and respiration are strongly influenced by abiotic factors such as water temperature that normally cause increased rates up to an optimal level beyond which rates drop[8,9,10,11]. To understand the variability of plant productivity on an individual ramet basis, we assessed how the respiratory and photosynthetic rates of the temperate seagrass Z. marina vary relative to tissue age under ambient temperatures and during higher temperature events. We hypothesised that: (a) the respiration rate will be higher in younger seagrass tissues (i.e. both above- and below-ground parts) in response to potentially higher energy demand; (b) the basal parts of leaves, where meristematic growth occurs, will have higher respiration rates than more mature leaf parts; and (c) photosynthetic rates will be higher in the leaf apex and middle part than in the leaf base due to a potentially greater abundance of light-harvesting chloroplasts
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