Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot Acute and prolonged effects of variable salinity on growth, gas exchange and photobiology of eelgrass (Zostera marina L.) Jordi Sola a,c, *, Brian K. Sorrell a , Birgit Olesen a , Martin Søndergaard Jørgensen a , Lars Chresten Lund-Hansen a,b a Aquatic Biology, Department of Biology, Aarhus University, Ole Worms Alle 1, 8000 Aarhus C, Denmark b Arctic Research Center, Department of Biology, Aarhus University, Ny Munkegade 114, 8000 Aarhus C, Denmark c Department of Evolutionary Biology, Ecology and Environmental Sciences, University of Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain ARTICLE INFO Keywords: Seagrass Baltic Climate change Photobiology Leaf age Fluorescence imaging ABSTRACT Sea level rise and more frequent storm and precipitation events associated with climate change are predicted to increase salinity fluctuations in estuarine and inshore areas, where foundation species such as eelgrass (Zostera marina L.) will be exposed to more frequent salinity changes. Effects of acute hyposalinity exposure on seagrasses remain poorly understood compared to the effects of more prolonged, constant salinity. Here, we examined growth and photo-physiological responses of Z. marina to 5 levels of stable salinity (5, 12 19, 25, 33) and compared effects of prolonged (16 days) versus acute (24 and 48 hours) exposure to hyposalinity (salinity 5 and 12) using fluorescence imaging. We also examined if fluorescence kinetics were affected by age differences across leaves. Growth reached an optimum at salinity 19 and was more affected by hyposalinity than hy- persalinity. Rapid reduction from salinity 25 to 5 decreased the maximum quantum yield (F v /F m ) after just 48 h. In contrast with prolonged exposure, non-photochemical quenching processes were not increased at salinity 5 after 48 h. Young leaves were more susceptible to extreme hyposalinity than older leaves (e.g., lower photo- synthetic quantum yield), which emphasizes the importance of considering shoot-scale and within-shoot var- iations in studies of stress response patterns. Differences between hyposalinity and hypersalinity responses were generally replicated in the literature, but we were not able to detect any differences across studies. Overall, these results suggest that eelgrass is tolerant to large fluctuations in salinity, but sudden extreme reductions may act as a severe co-stressor, and contribute to accumulated stress-exposure effects (chronic or lasting effects). 1. Introduction Salinity is a critical factor affecting the distribution and physiology of macroalgae and seagrasses (Larsen and Sand-Jensen, 2006; Short et al., 2007; Olafsson 2016). To cope, seagrasses have physiological salinity tolerance mechanisms ranging from photosynthetic to osmor- egulatory adjustments (Touchette, 2007; Olsen et al. 2016). Fluctua- tions and changes in salinity may impair these physiological processes, decreasing growth and increasing resource demand (Touchette and Burkholder, 2000). Salinity fluctuations are usually gradual in coastal environments (e.g., seasonal fluctuations), but short-term decreases following heavy rainfall, and catchment runoff may also be important in inshore and estuarine regions (Cardoso et al., 2008). A major forecast global change in coastal hydrology is an increasing frequency of storm events and, in consequence, catchment flooding events that can sud- denly lower coastal salinity (Millennium Ecosystem Assessment, 2005). In Denmark, climate change is indeed predicted to increase the fre- quency of such acute salinity changes due to increasingly frequent and extreme climatic events (Trenberth, 2011; Olesen et al., 2014). Contrasting long- and short-term salinity fluctuations are especially important for foundation coastal taxa such as eelgrass, Zostera marina L. (e.g., Garrote-Moreno et al. 2014). Eelgrass is a temperate meadow- forming seagrass that inhabits a wide range of saline environments throughout the northern hemisphere (Short et al., 2007; Bostrom et al., 2014). Around Denmark, it is distributed throughout coastal waters in salinities ranging from ∼5 in inner parts of fjords, to over 30 in the Skagerrak region (Krause-Jensen et al., 2011; Bostrom et al., 2014). Along the east Danish coast, Z. marina meadows thrive in the Atlantic- Baltic transitional zone, where salinity levels can be very variable across seasons due to ocean currents and wind speed (e.g., Omstedt and Axell, 1998). This fluctuating salinity environment, together with the physiological plasticity of eelgrass and its frequent use in previous https://doi.org/10.1016/j.aquabot.2020.103236 Received 26 August 2019; Received in revised form 10 March 2020; Accepted 17 March 2020 ⁎ Corresponding author at: Aquatic Biology, Department of Biology, Aarhus University, Ole Worms Alle 1, 8000 Aarhus C, Denmark. E-mail address: jsolacod@gmail.com (J. Sola). Aquatic Botany 165 (2020) 103236 Available online 05 May 2020 0304-3770/ © 2020 Elsevier B.V. All rights reserved. T