Variable contributions of seafloor communities to ecosystem metabolism across a gradient of habitat-forming species

Variability of soft-bottom macrobenthic invertebrates at different spatial scales: Comparisons between habitats and seasons

Studies focusing on patterns of spatial variation in marine soft-bottom assemblages suggest that variability is mainly concentrated at small spatial scale (from tens of centimeters to few meters), but there is still a lack of knowledge about the consistency of this spatial pattern across habitats and seasons. To address this issue, we quantified the variability in the structure of macrozoobenthic assemblages and in the abundance of dominant macroinvertebrate species in the Mellah Lagoon (Algeria) at three spatial scales, i.e., Plot (meters apart), Station (10's m apart) and Site (kms apart) scale, in Ruppia maritima (Ruppia) beds and unvegetated sediments (Unvegetated), and in two dates in winter and two dates in summer 2016. Spatial variability of the most dominant bivalve Mytilaster marioni varied significantly between habitats, but consistent across the two seasons, with a more heterogeneous distribution in Ruppia than in Unvegetated at the Station scale. Furthermore, a second-order interaction among the hierarchical nature of spatial variability, season and habitat emerged for the assemblage structure. Spatial variability between habitats varied significantly in winter, with the largest variation at the Plot scale in Unvegetated and more heterogenous assemblages at the Plot and Site scales than at the Station scale in Ruppia, but did not vary in summer when most of the variance was at the Site scale. We demonstrate that the scales of influence of the processes operating in the Mellah Lagoon are contingent on the specific habitat and/or period of the year at which the study was conducted, highlighting the importance of examining all these sources of variation simultaneously to increase the accuracy of explanatory models derived from the observed patterns in sedimentary environments.

A functional perspective on the factors underpinning biomass-bound carbon stocks in coastal macrophyte communities

Coastal ecosystems have received international interest for their possible role in climate change mitigation, highlighting the importance of being able to assess and predict how changes in habitat distributions and their associated communities may impact the greenhouse gas sink potential of these vegetated seascapes. Importantly, the range and diversity of macrophytes within the vegetated seascape have different capacities to store C within their biomass and potentially sequester C depending on their functional trait characteristics. To bridge the present knowledge gaps in linking macrophyte traits to C storage in tissue, we (1) quantified biomass-bound C stocks within diverse macrophyte communities, separately for soft and hard bottom habitats and (2) explored the links between various traits of both vascular plants and macroalgae and their respective biomass-bound C stocks using structural equation modeling (SEM). We conducted a field survey where we sampled 6 soft bottom locations dominated by aquatic vascular plants and 6 hard bottom locations dominated by the brown algae Fucus vesiculosus in the Finnish archipelago. Macrophyte carbon stocks of hard bottom locations were an order of magnitude higher than those found in soft bottom locations. Biodiversity was associated with aquatic plant carbon stocks through mass ratio effects, highlighting that carbon stocks were positively influenced by the dominance of species with more acquisitive resource strategies, whereas age was the main driver of carbon in the mono-specific macroalgal communities. Overall, our results demonstrate that habitat type and dominating life-history strategies influenced the size of the organism-bound carbon stocks. Moreover, we showed the importance of accounting for the diversity of different traits to determine the drivers underpinning carbon storage in heterogenous seascapes composed of macrophyte communities with high functional diversity.

Comparison of macroplastics dynamic across a tidal-dominated coastal habitat seascape including seagrasses, salt marshes, rocky bottoms and soft sediments

Coastal environments are usually composed by heterogeneous coastal-seascape, which can modify macroplastics accumulation dynamic. We evaluated seasonally the litter trapped on tidal-dominated habitats including two seagrass species, salt marsh, sandy beach, bare sediment and rocky bottom. Vegetated habitats showed the highest plastic accumulation in autumn-winter seasons, especially in medium-lower tidal-elevation zones. Seagrasses accumulated most of the degraded macroplastics, whereas averaged smaller sizes of litter were found in the salt marsh. The trapping ability of macrophytes was related to aboveground-biomass properties (i.e., height, width or flexibility) rather than shoot-density. Sandy beaches exhibited the highest plastics accumulation matching with the touristic-peak in the area, whereas rocky bottom was an important sink for macroplastics. This study provides authorities with comprehensible information to address the marine plastic litter problem taking into account the habitat-connectivity, the litter trap-ability of macrophytes and the tidal-elevation influence in order to improve future actions to deal with plastic pollution.

High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems

Coastal methane (CH₄ ) emissions dominate the global ocean CH₄ budget and can offset the "blue carbon" storage capacity of vegetated coastal ecosystems. However, current estimates lack systematic, high-resolution, and long-term data from these intrinsically heterogeneous environments, making coastal budgets sensitive to statistical assumptions and uncertainties. Using continuous CH₄ concentrations, δ¹³ C-CH₄  values, and CH₄  sea-air fluxes across four seasons in three globally pervasive coastal habitats, we show that the CH₄ distribution is spatially patchy over meter-scales and highly variable in time. Areas with mixed vegetation, macroalgae, and their surrounding sediments exhibited a spatiotemporal variability of surface water CH₄ concentrations ranging two orders of magnitude (i.e., 6-460 nM CH₄ ) with habitat-specific seasonal and diurnal patterns. We observed (1) δ¹³ C-CH₄  signatures that revealed habitat-specific CH₄ production and consumption pathways, (2) daily peak concentration events that could change >100% within hours across all habitats, and (3) a high thermal sensitivity of the CH₄ distribution signified by apparent activation energies of ~1 eV that drove seasonal changes. Bootstrapping simulations show that scaling the CH₄ distribution from few samples involves large errors, and that ~50 concentration samples per day are needed to resolve the scale and drivers of the natural variability and improve the certainty of flux calculations by up to 70%. Finally, we identify northern temperate coastal habitats with mixed vegetation and macroalgae as understudied but seasonally relevant atmospheric CH₄  sources (i.e., releasing ≥ 100 μmol CH₄  m^-2  day^-1 in summer). Due to the large spatial and temporal heterogeneity of coastal environments, high-resolution measurements will improve the reliability of CH₄ estimates and confine the habitat-specific contribution to regional and global CH₄ budgets.

Positive contribution of macrofaunal biodiversity to secondary production and seagrass carbon metabolism

Coastal vegetated habitats such as seagrasses are known to play a critical role in carbon cycling, and their potential to mitigate climate change as blue carbon habitats have been repeatedly highlighted. However, little is known about the role of associated macrofauna communities on the dynamics of critical processes of seagrass carbon metabolism (e.g. respiration, turnover, and production). We conducted a field study across a spatial gradient of seagrass meadows involving variable environmental conditions and macrobenthic diversity to investigate (1) the relationship between macrofauna biodiversity and secondary production (i.e. consumer incorporation of organic matter per time unit), and (2) the role of macrofauna communities in seagrass organic carbon metabolism (i.e. respiration and primary production). We show that while several environmental factors influence secondary production, macrofauna biodiversity controls the range of local seagrass secondary production. We demonstrate that macrofauna respiration rates were responsible for almost 40 % of the overall seafloor community respiration. Macrofauna represented on average > 25% of the total benthic organic C stocks, high secondary production that likely becomes available to upper trophic levels of the coastal food web. Our findings support the role of macrofauna biodiversity in maintaining productive ecosystems, implying that biodiversity loss due to ongoing environmental change yields less productive seagrass ecosystems. Hence, the assessment of carbon dynamics in coastal habitats should include associated macrofauna biodiversity elements if we aim to obtain robust estimates of global carbon budgets required to implement management actions for the sustainable functioning of the worlds' coasts. This article is protected by copyright. All rights reserved.