Flotsam and jetsam: a global review of the role of inputs of marine organic matter in sandy beach ecosystems

Abstract

Sandy beaches are iconic interfaces that functionally link the ocean with the land by the flow of marine organic matter. 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 examime the spatial scaling of the influence of these processes across the broader seascape and landscape, and identify key gaps in our knowledge to guide future research directions and priorities. Globally, large quantities of detrital kelp and seagrass can flow into sandy beach ecosystems, where microbial decomposers and animals remineralise and consume the imported organic matter. The supply and retention of wrack 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 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 break-down, and these can return to coastal waters in surface flows (swash) and the aquifier 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, these can be at variance with aesthetic perceptions resulting in widespread activities, such ‘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 on food webs and biodiversity. Similarly, future sea-level rise and stormier seas are likely to profoundly alter 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.

Human-Induced Hydrological Connectivity: Impacts of Footpaths on Beach Wrack Transport in a Frequently Visited Baltic Coastal Wetland

“…Changes in water level and wind speed were most extreme during storm Malik, resulting also in the largest beach wrack deposition during our monitoring period. Both wind and wave climate that influence the biogeographic patterns of beach wrack (Reimer et al, 2014;Hyndes et al, 2021) will change in the next decades and alter the beach wrack depositions patterns. Wind and wave projections for the Baltic Sea are highly uncertain due to large natural variability; but in areas no longer covered by sea ice, wind fetch and wind speeds could increase (Ahola et al, 2021).…”

“…Beach wrack increases the surface roughness and has been identified as a key factor in trapping wind-blown sand (Nordstrom et al, 2011). Furthermore, beach wrack enhances plant colonization at more seaward-located positions because it elevates the backshore and provides essential nutrients (Dugan et al, 2011;Biel et al, 2018;Hyndes et al, 2021). For Baltic coast wetlands, Ward et al (2016) have shown that already small alterations in elevation and thus inundation frequency can lead to substantial vegetation changes.…”

“…However, this refers to the submarine part with the storage of organic matter in the seafloor. Despite the ecological importance, so far, most studies have focused on sandy beaches (e.g., Dugan et al, 2011;Reimer et al, 2018;Hyndes et al, 2021;Pan et al, 2022) and the transport and fate of beach wrack in vegetated coastal ecosystems has not been studied yet.…”