Diurnal and Tidal Variability in the Photobiology of the Seagrass Halophila johnsonii in a Riverine Versus Marine Habita
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Diurnal and Tidal Variability in the Photobiology of the Seagrass Halophila johnsonii in a Riverine Versus Marine Habitat Amanda E. Kahn & Jeffrey L. Beal & Michael J. Durako
Received: 9 April 2012 / Revised: 16 October 2012 / Accepted: 22 October 2012 / Published online: 15 November 2012 # Coastal and Estuarine Research Federation 2012
Abstract The threatened seagrass Halophila johnsonii grows intertidally to 3 m deep in river-influenced and marine-influenced habitats. In this study, environmental parameters and photosynthetic characteristics of H. johnsonii were measured hourly for populations from adjacent riverine and marine habitats under opposite tidal regimes (high tide at midday, low tide at midday). The two populations exhibited habitat-specific diurnal responses, which indicate long-term acclimatization to their different environments. During periods with similar bottom irradiances, effective photochemical efficiencies and chlorophyll concentrations were comparatively greater in the riverine population, indicative of low-light acclimation. In addition, ultraviolet pigment absorbance (340–345 nm) was generally greater in the riverine plants and it generally increased following ebb tides and decreasing salinity, suggesting a stress response. Multivariate analyses indicated that photosynthetic characteristics were most dissimilar when environmental conditions were most dissimilar, i.e., riverine plants when low tide occurred at midday compared to marine plants when high tide occurred at midday. Salinity, photosynthetically active radiation, and optical water quality (K0PAR, SCDOM, and a412) were most correlated with variability in photosynthetic characteristics. As there is no A. E. Kahn : M. J. Durako (*) Department of Biology and Marine Biology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403, USA e-mail: [email protected] J. L. Beal Florida Fish and Wildlife Conservation Commission, FAU HBOI, 5600 US1, Ft. Pierce, FL 34946, USA
significant genetic variation among populations of H. johnsonii, the photosynthetic characteristics of the riverine and marine populations we examined reflect acclimation to their respective habitats through a highly phenoplastic physiology. Keywords Halophila johnsonii . Seagrass . Acclimation . Physiological plasticity
Introduction Organisms that inhabit estuarine environments are subject to wide fluctuations in water quality. Tidally induced variations in water depths and water influxes from marine and freshwater systems also change the quantity and quality of light available to benthic organisms (Kenworthy and Haunert 1991; Gallegos and Kenworthy 1996). In order to successfully establish in such habitats, submerged aquatic vegetation must be able to acclimatize to short-term changes in a variety of environmental parameters. Halophila johnsonii Eiseman is a seagrass that occurs from the upper intertidal, where it can be exposed during low tides, to 3 m deep and occupies diverse habitats in close proximity to both freshwater (riverine) and m
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