Spatio-temporal variation in macrofauna community structure in Mediterranean seagrass wrack

Hydrodynamic conditions of Posidonia oceanica seagrass berm formation and dismantling events

Seagrass berms are a natural accumulation of seagrass leaves along the shoreline, which play an important role in coastal protection. Seagrass berms have been shown to reduce wave energy, helping to shield the coast from erosion. However, their protective role is debated in coastal management, where there are contrasting views on whether berms should be left in place or removed to improve beach aesthetics or accessibility. Although seagrass berm accumulation and erosion are a globally recognized phenomenon, the physical processes underlying their formation and dismantling remain poorly understood, limiting insights into their role as coastal defense. This study fills a critical knowledge gap by analyzing the first long-term dataset that directly connects Posidonia oceanica seagrass berm dynamics to specific wave conditions. Measurements were collected at Cala Millor beach in Mallorca over almost a decade and the research utilized topo-bathymetric surveys, video monitoring, and wave measurements. Our research reveals that berm formation follows an annual cycle, driven by wave height, period, and direction, alongside seasonal availability of seagrass material. Numerical modeling further shows that high-energy waves facilitate berm formation, while low-energy, spilling waves contribute to their dismantling. These findings highlight the significance of wave conditions in berm dynamics, advancing our understanding of their potential role in coastal protection.

An integrated mapping approach highlights extended distribution and high environmental status of Irish seagrass meadows

To address the remaining knowledge gap regarding the distribution of seagrasses in Ireland, this study aimed a) to create an updated seagrass (Zostera spp.) distribution map, and b) to evaluate the environmental quality to which seagrass meadows are exposed. To achieve the first objective, we (i) combined the available data on seagrass distribution published to date, and (ii) mapped additional meadows by implementing an integrated method based on species distribution models, satellite-derived images, and snorkelling-based surveys. We mapped 209 new seagrass meadows (14.98 km2), representing a 37.03 % increase over previously reported extents. Consequently, the total extent of Irish seagrass meadows is estimated to be at least 54.85 km2. To address the second objective, we assessed the level of anthropogenic pressure of seagrass meadows based on the index provided by the Water Framework Directive of the European Environment Agency. This study demonstrates that Irish meadows are primarily located in areas with 'HIGH' and 'GOOD' water status.

Quantifying seaweed and seagrass beach deposits using high-resolution UAV imagery

Macroalgae and seagrass wash ashore by tidal waters and episodic events and create an ocean-to-land transport of carbon and nutrients. On land, these deposits (beach wrack) are consumed by macrofauna, remineralized by microorganisms, or washed back to the sea, during which recycling of carbon and nitrogen affect the biochemical cycles in coastal zones. Manual quantification of beach wracks is time-consuming and often difficult due to complex topography and remote locations. Here, we present a novel method using Unoccupied Aerial Vehicle (UAV) photogrammetry combined with in situ measurements of carbon and nitrogen contents of wrack to quantify marine carbon and nutrient deposits in beach zones. The UAV method was tested against placed cubes ranging from 125 to 88,218 cm³ and demonstrated a high accuracy (R² > 0.99) for volume acquisition when compared to manual measurements. Also, the UAV-based assessments of the cross-sectional area of beach deposits demonstrated a high accuracy when compared to manual and high-precision GNSS (Global Navigation Satellite System) measurements without significant differences between the methods. This demonstrated that UAVs can provide detailed spatial maps, three-dimensional (3D) surface models, and accurate volumetric assessments of beach wrack deposits. In three case studies, combined with carbon and nitrogen measures, total organic carbon and nitrogen deposits in beach wracks were quantified ranging from 4.3 to 9.7 and from 0.3 to 0.5 kg per meter coastline, respectively. In conclusion, this UAV method demonstrated an effective tool to quantify ecosystem carbon and nitrogen deposits relevant to ecosystem assessments and quantification of blue carbon stocks. The method is optimal when the terrain below beach wrack deposits is known, as in the case with before-and-after or repeated surveys. Further, UAVs display strong time- and cost-effective advantages over manual methods which is amplified with increasing project scale. We propose it as a valuable method for multiple scientific and commercial applications related to environmental monitoring and management, including marine resource exploration and exploitation.

The role of inputs of marine wrack and carrion in sandy-beach ecosystems: a global review

Sandy beaches are iconic interfaces that functionally link the ocean with the land via the flow of organic matter from the sea. These cross-ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed 'wrack', on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source ('carrion') for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy-beach ecosystems (sandy beaches and adjacent surf zones), which typically have little in situ primary production. We also examine the spatial scaling of the influence of these processes across the broader land- and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy-beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach-cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi-aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach-cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy-beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as 'beach cleaning and grooming'. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow-on effects for food webs and biodiversity. Similarly, future sea-level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi-faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach-cast organic matter on innumerable ocean shores worldwide.

Small-scale patches of detritus as habitat for invertebrates within a Zostera noltei meadow

Seagrass detritus can attract numerous invertebrates as it provides food and substrate within the meadow or in adjacent environments. Nonetheless, several factors could modify the invertebrate response to this habitat. In this study, we tested if epifaunal colonisation of Zostera noltei detritus was related to substrate availability rather than food and whether colonising assemblages were similar according to the meadow structural complexity. Litterbags filled with natural or artificial detritus were deployed within an eelgrass meadow in a Mediterranean coastal lagoon (Thau lagoon, France). Colonisation appeared to be driven by the presence of detritus, with similar assemblages in natural and artificial substrate, but with more individuals than the empty bags, used as controls. There were also no differences according to habitat complexity. These findings show that detritus, acting as a faunal magnet, plays an important role in maintaining biodiversity, as epifauna is a critical trophic link between primary producers and consumers.