Abstract
Seagrasses may frequently experience a combination of velocity and light stresses, as elevated hydrodynamics often enhances turbidity and the subsequent light reduction. The objective of this study was to investigate the effects that these stressors induce on morphometric and dynamic seagrass features depending on the initial biomass partitioning. For this purpose, a factorial meso- cosm experiment was conducted on plants of Zostera noltii subjected to combinations of 2 contrast- ing light levels (2.5 ± 0.6 and 15.6 ± 2.5 mol photons m -2 d -1 ) and 3 unidirectional flow velocities (0.35, 0.10 and 0.01 m s -1 ). No interactive effects between the 2 variables were recorded, except on plant survival and leaf length, and generally, light effects prevailed over hydrodynamic ones. Plants responded to light reduction regardless of the flow velocity treatments, showing low survival rates (which improved at high velocity), high aboveground/belowground biomass ratios (AG/BG) and a poorly developed root-rhizome system compared to plants under saturating light conditions. Plant morphometry only responded to hydrodynamic stress under saturating light: at high current velocity, plants preferentially allocated biomass into BG structures, bearing short leaves and displaying high internode and root appearance rates. Overall, light reduction promoted similar responses in plants with different AG/BG biomass ratios, but dissimilarities were recorded for current velocity. Thus, it can be concluded that, under simultaneous light and hydrodynamic stresses, light effects prevailed over hydrodynamic ones in Z. noltii, while acclimation to hydrodynamics only occurred under saturating light.
Highlights
Shallow marine habitats, and especially intertidal zones, are characterised by large variability in physico-chemical conditions, which are often influenced by anthropogenic activities (Mann 2000)
The experiment was conducted on the temperate seagrass Zostera noltii Horneman, characterised by high growth rates and large morphometric plasticity, both of which are affected by light conditions (Brun et al 2003b, Peralta et al 2005) and current velocity (Peralta et al 2006)
The net growth rate of plants under limiting light (LL) was only positive at HV, whereas negative values were
Summary
Especially intertidal zones, are characterised by large variability in physico-chemical conditions, which are often influenced by anthropogenic activities (Mann 2000). This environmental variability imposes a strong effect on the surrounding biota, usually being considered as stress when it results in reduced organism growth and/or survival. Zostera noltii Horneman is a temperate seagrass that often inhabits intertidal zones. Being rooted organisms, they cannot avoid the occurrence of abiotic stresses, which must be partly overcome through phenotypic plasticity, i.e. through adjustments in metabolic, physiological and morphological traits (Sultan 1987, Pigliucci 2001). Morphological plasticity in clonal plants has been interpreted as an acclimation mechanism ( called acclimatization) (Sultan 2000, Peralta et al 2005, Brun et al 2006b), improving ecological breadth and fitness and, survival and colonization success of the species (Bazzaz 1996, DeWitt et al 1998, Sultan et al 1998).
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