Seagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide.

In an 8-week aquarium experiment, we investigated the interactive effects of waves (present vs. absent) and water-column nutrient level (high vs. low) on the survival, growth, morphology, and biomechanics of the seagrass, Zostera noltii. Survival was reduced when plants were exposed to both waves and high nutrient levels. Wave and nutrient interaction significantly reduced aboveground biomass and leaf lengths, whereas waves independently reduced growth rate, internode abundance, elongation, and appearance rates. Nutrient supply significantly reduced the strength of the leaves. Wave and nutrient interaction was the main driving force affecting survival and morphological properties of seagrass, whereas dynamical characteristics were independently affected by waves, and nutrient supply affected mainly biomechanical properties. In conclusion, this experiment revealed that the combination of exposure to waves and high nutrient levels was detrimental for Z. noltii, which indicates that this could be an important unexplored force involved in seagrass declines.

No abstract

Seagrasses may acclimate to environmental heterogeneity through phenotypic plasticity. In contrast to leaf morphology, which has been a central point in seagrass acclimation studies, plasticity in leaf biomechanics and fibre content is poorly understood, despite being crucial in plant ecological performance, especially regarding physical forces. We hypothesised that mechanical traits (e.g. breaking force, strength, toughness, and stiffness) and fibre content of seagrass leaves vary as morphology does under differential environments. Cymodocea nodosa was seasonally monitored at three locations around Ca ´diz Bay (southern Spain) with hydrodynamic regime as the most noticeable difference between them. Leaves showed plasticity in both morphology and mechanical traits, with wave-exposed individuals presenting short but extensible and tough leaves. Leaf fibre content was invariant along the year and with little spatial variability. Cross-sectional area rather than material properties or fibre content differentiates leaf mechanical resistance. Seagrass capacity to thrive under a range of mechanical forces may be dictated by their plasticity in morpho-biomechanical traits, a key element for the hydrodynamical performance and, hence, for species colonisation and distribution.

No abstract

Coastal vegetated ecosystems are major organic carbon (OC) and total nitrogen (TN) sinks, but the mechanisms that regulate their spatial variability need to be better understood. Here we assessed how superficial sedimentary OC and TN within intertidal vegetated assemblages (saltmarsh and seagrass) vary along a flow gradient, which is a major driver of sediment grain size, and thus of organic matter (OM) content. A significant relationship between flow current velocity and OC and TN stocks in the seagrass was found, but not in the saltmarsh. OC and TN stocks of the saltmarsh were larger than the seagrass, even though that habitat experiences shorter hydroperiods. Mixing models revealed that OM sources also varied along the flow gradient within the seagrass, but not in the saltmarsh, showing increasing contributions of microphytobenthos (17–32%) and decreasing contributions of POM (45–35%). As well, OM sources varied vertically as microphytobenthos contribution was highest at the higher intertidal saltmarsh (48%), but not POM (39%). Macroalgae, seagrass and saltmarsh showed low contributions. Local trade-offs between flow current velocities, hydroperiod and structural complexity of vegetation must be considered, at both horizontal and vertical (elevation) spatial dimensions, for better estimates of blue carbon and nitrogen in coastal ecosystems.

Supplement 1. Variance partitioning analysis. Breaking force (F brk) is expressed as the product of width (w), thickness (t), and tensile strength (σ brk) (eq.1): (eq.1) F brk = w • t • σ brk After logarithm transformation, eq.1 turns additive (eq.2): (eq.2) log 10 (F brk) = log 10 (w) + log 10 (t) + log 10 (σ brk) → F brk ' = w' + t' + σ brk ' The variance (Var) of the breaking force can be then expressed as the sum of the covariances (Cov) between each term and the mechanical resistance (eq.3): (eq.3) Var(F brk ') = Cov(F brk ',w') + Cov(F brk ',t') + Cov(F brk ',σ brk ') The contribution (Cont) of each component to F brk ' is expressed as its covariance divided by the variance of F brk ' (eqs.4): (eqs.4) Cont(w) = Cov(F brk ',w')/Var(F brk ') Cont(t) = Cov(F brk ',t')/Var(F brk ') Cont(σ brk) = Cov(F brk ',σ brk ') /Var(F brk ') Cont(w) + Cont(t) + Cont(σ brk) = 1 Therefore, the contribution of each component is translated as relative importance in mechanical resistance. 2 Supplement 2. Tables Table S1. Sampling sites by bioregion, with geographical coordinates, species collected, date(s) of collection (mm/yy), and habitat details (depth in m). Species ID: Amphibolis antarctica (Aa), A. griffithii (Ag), Cymodocea nodosa (Cn), C. rotundata (Cr), C. serrulata (Cs), Enhalus acoroides (Ea), Halodule pinifolia (Hp), H. uninervis (Hu), Halophila ovalis (Ho), H. ovalis ssp. bullosa (Hb), Posidonia australis (Pa), P. coriacea (Pc), P. oceanica (Po), P. sinuosa (Ps), Ruppia maritima (Rm), Syringodium isoetifolium (Si), Thalassia hemprichii (Th), Thalassodendron pachyrhizum (Tp), Zostera capricorni (Zc), Z. marina (Zm), Z. nigricaulis (Zg), and Z. noltei (Zn). Number of samples per species is included between brackets next to each species code at each site. Sites Geographical coordinates Species ID Date(s) of collection Habitat details TIP: Tropical Indo-Pacific region (8 species, 5 sites, 21 species-at-site sets)

No abstract