Structural complexity governs seagrass acclimatization to depth with relevant consequences for meadow production, macrophyte diversity and habitat carbon storage capacity

Abstract

Analyses of the integrated seagrass response to depth support the previously documented low plasticity and consistent shade-adapted leaf physiology of a habitat-builder that dominates well-illuminated reef environments. Two structural responses, “canopy-opening” and “below-ground-mass-depletion”, govern the photoacclimatory response and facilitate, respectively, light penetration within the canopy and functional adjustments in whole-plant carbon balances. Conversely, “canopy-closing” may also explain dense canopies formed close to the waterline, as they provide shade and photoprotection to a susceptible leaf physiology under high-light. Canopy light attenuation is primarily regulated by the leaf area index (LAI), which is governed by changes in shoot size and density. Shoot density diminishes non-linearly with depth, while shoot size increases to a maximum followed by a decline. The initial increase in shoot size, which resembles a self-thinning response, increases LAI and meadow production in shallow depths. These seagrass structural adjustments have relevant ecological implications. Canopy-thinning allows macrophyte diversity to increase with depth, while seagrass production and carbon storage diminish exponentially, and are maximal only in a shallow coastal fringe. The results support the universality of plant self-thinning, from phytoplankton to complex canopies, likely the consequence of simple physical laws related to light limitation and pigment self-shading within photosynthetic structures and communities.

Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem

“…The combination of clear water at deeper depths and reduced aboveground biomass, and thus reduced self‐shading, in grazed areas could explain why irradiance was not a significant factor regulating growth at our study site. Aboveground biomass and mass growth of T. testudinum typically decrease with increasing water depth as light becomes limited (Dawes and Tomasko 1988, Lee and Dunton 1997, Dawes 1998, Enríquez et al 2019). However, this response is less pronounced in clear, tropical waters where T. testudinum can grow at depths up to 10 m (van Tussenbroek et al 2006), similar to our study site.…”

“…Changes in seagrass structural complexity can also lead to shifts in nutrient allocation (Medina‐Gómez et al 2016, Enríquez et al 2019). Grazing substantially reduces aboveground biomass, which can affect the production of above‐ vs. belowground tissues (see Table 2) (Thayer et al 1984, Hernández and van Tussenbroek 2014).…”

Variation in Seagrass Carbon Stocks Between Tropical Estuarine and Marine Mangrove-Fringed Creeks

“…Such meadows are known to have lower dissolved organic carbon (DOC) supply, which is likely to affect the percentage of organic carbon through the reduced decomposition of the refractory organic compounds. This difference in species composition and shoot density between the creeks may therefore partly explain the differences in the concentrations of organic matter between them (Serrano et al, 2018;Enríquez et al, 2019), resulting in variations in sediment carbon between the creeks.…”

High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

“…Seagrass meadows have the ability to accrete thick sediment layers and efficiently accumulate belowground carbon deposits (Fourqurean et al, ; Mcleod et al, ), but not all seagrass areas can be considered carbon sinks (Belshe et al, ; Dahl, ; Enríquez et al, ; Morse et al, ). The seagrass Zostera marina is a common species in the temperate zone of the Northern Hemisphere, creating meadows with a high carbon storage potential where the seagrass areas in the Skagerrak‐Kattegat strait and in the Mediterranean show particularly large carbon stocks, exceeding the stocks found in many terrestrial environments (Mcleod et al, ; Röhr et al, ).…”