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

Abstract

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.

Keywords: carrion; coastal ecosystems; detritus; ecosystem functioning; kelp forests; landscape ecology; seagrass beds; seascape; spatial subsidy; wrack.

Fig. 1

Conceptual diagrams of wrack dynamics in beach ecosystems. (A) The principal sources, transport routes, and biological fates of marine organic material cast upon sandy beaches; (B) disruption of natural processes caused by beach grooming and coastal armouring; and (C) predicted consequence of climate change for the supply, type and biological fates of marine organic matter in sandy beach ecosystems. POM, particulate organic matter. Diagrams created using IAN Image Library (http://ian.umces.edu).

Fig. 2

Wrack, scavengers and human disturbance on sandy beaches. Wrack on beaches in (A) Cape Town, South Africa (photograph L. Harris); (B) Santa Barbara County, USA (photograph J. Dugan); (C) Salina Bay, Malta (photograph M. Mateo); and (D) Perth, Australia (photograph G. Hyndes). Dingo (E) and white‐bellied sea eagle (F) scavenging on carrion on beaches near Brisbane, Australia (photographs A. Olds), and beach cleaning on beaches in (G) Brisbane, Australia (photograph A. Olds) and (H) Carpinteria, USA (photograph J. Dugan).

Fig. 3

Global distribution of studies (N = 336) classified by the main theme with respect to wrack and carrion on sandy beaches and in surf zones. Pie charts illustrate the different themes of published studies, with the size indicating the total number of studies for a region. Note that more than one theme could be covered by each paper, but the number of studies in each region reflects the total number of papers regardless of theme. See Table 1 for more detail on themes.

Fig. 4

Global patterns in wrack composition (N = 43) and wet mass (WW; N = 27) of wrack (kg WW wrack m⁻¹ of coastline) on beaches and in surf zones based on published papers. Numbers in plain text to the right of each pie chart indicate the number of studies. The text in italics below the pie charts indicates the average biomass in each region, while the numbers in parentheses to the right of the text indicates the number of studies that provided the data. Note that for a study to be included, data needed to be provided for at least three sites or times. Wrack composition was based on wet and dry mass and volume data, while biomass data were based on studies where mass was either provided or data could be converted to wet mass per linear metre of coastline. Distribution of seagrass from UNEP‐WCMC seagrass maps based on Green & Short (2003), while kelp distributions are based on Filbee‐Dexter & Wernberg (2018).

Fig. 5

Coastal, oceanographic and atmospheric factors that influence wrack supply and retention on sandy beaches, and the spatial and temporal scales at which they operate. Sources of information are provided in Table 2. Supply = processes that influence the input of wrack to beach ecosystems; retention = processes that influence the ability of wrack to remain in beach ecosystems. The colour of the ellipse for each factor indicates whether the factor affects either supply or retention, or both supply and retention of wrack in beach systems. Beach management refers to management practices such as beach grooming and armouring that influence supply and retention of wrack, while overgrazing refers to impacts on donor systems such as kelp forests due to grazing pressure.

Fig. 6

The numbers and proportions of papers with a focus on different aspects of total invertebrate assemblages (A) and amphipod populations (B) in wrack on beaches and in surf zones, and the relationship between amphipod abundance and wrack biomass (g wet mass m⁻²) (C) based on data extracted from the literature. In (A) and (B), circle quarters represent summaries of correlations of wrack biomass with invertebrate assemblages and amphipod populations (i.e. abundance, biomass, species richness, diversity). In each quadrant, the number of studies is displayed in parentheses, and the percentage of studies reporting a significant effect for each variable is illustrated by the coloured region in each quadrant (e.g. 83% of invertebrate studies reported positive effects on invertebrate abundance). In C, correlations are based on generalised additive models (GAMs) that assess the relationship between amphipod abundance and wrack biomass, with GAMS limited to four knots. See Table S2 for data sources.

Fig. 7

Abundance rank of the main invertebrate taxa found on the beach and in the surf zone based on extracted data from the literature from across the globe. Dots indicate the normalised ranks (i.e. taxa in order of abundance, e.g. 1, 2, 3, etc., converted to values between 0 and 1, where 1 indicates the most abundant) of taxa in individual studies, while the vertical lines denote the mean rank and horizontal lines the 95% confidence intervals. Sample sizes (number of studies) are provided in parentheses. See Table S3 for data sources.

Fig. 8

Proportions of papers with a focus on different aspects of fish assemblages in the wrack in surf zones (A), and the relationship between fish abundance (B) and diversity (C) with wrack volume (litres per 100 m²) based on data extracted from the literature. In (A), circle quarters represent summaries of correlations with fish assemblages (i.e. abundance, biomass, species richness, diet). In each quadrant, the number of studies is displayed in parentheses, and percentage of studies reporting a significant effect for each variable is illustrated by the coloured region in each quadrant (e.g. 67% of studies on surf fish assemblages report positive effects of wrack biomass on fish abundance). In (B) and (C), correlations are based on generalised additive models (GAMs) assessing the relationship between fish abundance and diversity against wrack volume with GAMs limited to four knots. See Table S4 for data sources.

Fig. 9

Conceptual diagram of: (A) decomposition and nutrient cycling; (B) the grazer/detritivore food web; and (C) scavenging pathways as key processes for the fate of stranded organic material in beach ecosystems. Diagrams created using IAN Image Library (http://ian.umces.edu).

Fig. 10

Mean + SE consumption rates by amphipods feeding on different sources of wrack in beach and surf zone habitats. WW, wet mass. Data extracted from Duarte et al. (2008, 2010b ) (Chile), Lastra et al. (2008) (Spain), Gomes Veloso et al. (2012) (Brazil), MacMillan & Quijón (2012) (Canada), Poore & Gallagher (2013) (Australia), Michaud et al. (2019) (USA) and Suárez‐Jiménez et al. (2017a ) (New Zealand).