Morphological, growth and meadow characteristics of the seagrass Posidonia sinuosa along a depth-related gradient of light availability

Abstract

Morphological and growth characteristics of the meadow-forming seagrass Posidonia sinuosa (Cambridge et Kuo), were measured along a depth-related gradient of light to infer its response to long-term differences in light availability. Morphometric measurements were carried out at 6 depths between 1.6 and 9.0 m in summer and winter at Cockburn Sound and summer only at Warnbro Sound in south-western Australia. The minimum light requirement for P. sinuosa of 8.5% sub-surface light was among the lower range reported for seagrasses. Its slow growth rate (0.5-1.5 mg dry shoot -1 d -1), relative to similarly sized species, may contribute to the low light requirements of this species. Shoot density, leaf area index and biomass showed pronounced and consistent differences among depths (up to 88-fold reduction of above-ground biomass from shallow to deep sites). At the deeper sites, the reduced shoot density probably reduces respiratory demand and alleviates self-shading. Morphological differences (leaf length, width and thickness and number of leaves per shoot) did not follow a clear and consistent trend with depth. Despite a 70% reduction in light availability at the canopy level between the shallowest and deepest sites, leaf growth rate was unaffected by depth during summer, and in winter differed between only a few depths. We propose that the reduction in shoot density partially alleviates the effects of self-shading and permits comparable leaf growth rates across the depth range. These results suggest that for interpreting long-term responses to light availability, shoot density is the most sensitive of the morphological characteristics measured here.

Key biogeochemical factors affecting soil carbon storage in <i>Posidonia</i> meadows

“…It is well established that accumulation of C org in sediments and soils is strongly affected by net primary production (Cao and Woodward, 1998;Serrano et al, 2014). The decline with depth of C org stocks, C org accumulation rates and seagrass-derived inputs into the sedimentary pool that we observed coincides with reduced seagrass abundance and production reported by Collier et al (2007). These authors reported 18-24-fold reductions from shallow (2 m) to deep (8 m) P. sinuosa meadows in shoot density (from 1435 to 80 shoots m −2 ), aboveground biomass (from 899 to 47 g DW m −2 ) and belowground biomass (from 1028 to 43 g DW m −2 ) on the same depth gradient.…”

“…Here we studied Posidonia sinuosa meadows across a depth gradient, aiming to highlight key biogeochemical factors affecting C org storage in seagrass soils that need to be accounted for when attempting to produce regional or global estimates of C org storage in seagrass meadows. Previous research at this site (Collier et al, 2007(Collier et al, , 2008 showed significant variation in plant biomass and productivity, water quality and sediment biogeochemistry parameters across this depth gradient. Bare sediments were also sampled and studied in order to determine the "background" C org stocks and fluxes in the absence of a seagrass meadow.…”

“…The seagrass itself may exert a primary control on C org storage through its biomass, productivity and nutrient content (Lavery et al, 2013;Serrano et al, 2014;Miyajima et al, 2015), all of which are highly variable depending upon seagrass species and habitat conditions (Alcoverro et al, 1995;Collier et al, 2007). Seagrass density, biomass and productivity are strongly related to the underwater light penetration (Dennison, 1987;Duarte, 1991).…”

Light Absorption in Coralline Algae (Rhodophyta): A Morphological and Functional Approach to Understanding Species Distribution in a Coral Reef Lagoon

“…We estimated for A. tribulus intermediate values in thallus light absorption efficiency, metabolic rates and TMA, but photosynthetic production per individual was as high as that of the rhodolith, thanks to its ability to form one the largest light collectors at the organism/canopy level. For the quantification of the size of the light collector of the whole organism, we used a parameter, LAI, common in terrestrial plant ecology, but not so common in marine ecology, although it has been successfully applied for the characterization of seagrass habitats (Olesen et al, 2002;Enríquez and Pantoja-Reyes, 2005;Collier et al, 2007). Seagrass LAI can explain changes in the magnitude of leaf self-shading within seagrass beds (Enríquez and Pantoja-Reyes, 2005) and leaf photoprotection (Schubert et al, 2015).…”

Sustainable Seagrass Restoration in the Virginia Coastal Bays

“…Wicks et al (2009) suggested that increases in Z. marina leaf width and shoot length paired with less root growth in sediments with high organic content introduced more shear stress on the plants in the hydrologic conditions present in Chincoteague Bay which may have caused the plants with more aboveground biomass to be ripped out of the sediment; however, they did not replicate these conditions within mesocosms (Wicks et al 2009). The increase in aboveground to belowground biomass ratios increases respiratory demand which increases light requirements (Collier et al 2007). In the future, it would be beneficial to perform mesocosm experiments in flumes or to pair mesocosm experiments with field experiments as in Wicks et al (2009) …”