Flotsam and Jetsam: a Global Review of the Role of Inputs of Marine Organic 2 Matter in Sandy Beach Ecosystems 3 4 Glenn A
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1 Flotsam and jetsam: a global review of the role of inputs of marine organic 2 matter in sandy beach ecosystems 3 4 Glenn A. Hyndes1*, Emma L. Berdan2, Cristian Duarte3, Jenifer E. Dugan4, Kyle A. Emery4, 5 Peter A. Hambäck5, Christopher J. Henderson6, David M. Hubbard4, Mariano Lastra7, Miguel 6 A. Mateo1,8, Andrew Olds6, and Thomas A. Schlacher6 7 8 1 Centre for Marine Ecosystems Research, School of Science, Edith Cowan University, 9 Western Australia. [email protected] 10 2 Department of Marine Sciences, University of Gothenburg, Göteborg, Sweden. 11 [email protected] 12 3 Departamento de Ecología y Biodiversidad, Facultad de Ciencias de la Vida, Universidad 13 Andres Bello, Santiago, Chile. [email protected] 14 4 Marine Science Institute, University of California, Santa Barbara, CA 93106, USA. 15 [email protected], [email protected], [email protected] 16 5 Department of Ecology, Environment & Plant Sciences, Stockholm University, Stockholm, 17 Sweden. [email protected] 18 6 School of Science, Technology, and Engineering, University of the Sunshine Coast, 19 Maroochydore, Qld, Australia. [email protected], [email protected], 20 [email protected] 21 7 Department of Ecology and Animal Biology, University of Vigo, Vigo, Spain. 22 [email protected] 23 8 Centro de Investigación Mariña, Edificio CC Experimentais, Universidade de Vigo, Campus 24 de Vigo, 36310 Vigo, Spain. [email protected] 25 26 27 28 *Address for correspondence 29 30 31 32 ABSTRACT 33 Sandy beaches are iconic interfaces that functionally link the ocean with the land by the flow of 34 marine organic matter. These cross-ecosystem fluxes often comprise uprooted seagrass and 35 dislodged macroalgae that can form substantial accumulations of detritus, termed ‘wrack’, on sandy 36 beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on 37 beaches form a rich food source (‘carrion’) for a diversity of scavenging animals. Here, we provide a 38 global review of how wrack and carrion provide spatial subsidies that shape the structure and 39 functioning of sandy beach ecosystems (sandy beaches and adjacent surf zones), which typically 40 have little in situ primary production. We also examime the spatial scaling of the influence of these 41 processes across the broader seascape and landscape, and identify key gaps in our knowledge to 42 guide future research directions and priorities. Globally, large quantities of detrital kelp and seagrass 43 can flow into sandy beach ecosystems, where microbial decomposers and animals remineralise and 44 consume the imported organic matter. The supply and retention of wrack are influenced by the 45 oceanographic processes that transport it, the geomorphology and landscape context of the 46 recipient beaches, and the condition, life history and morphological characteristics of the taxa that 47 are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots 48 of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. 49 Nutrients are produced during wrack break-down, and these can return to coastal waters in surface 50 flows (swash) and the aquifier discharging into the subtidal surf. Beach-cast kelp often plays a key 51 trophic role, being an abundant and preferred food source for mobile, semi-aquatic invertebrates 52 that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach-cast 53 marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile 54 scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in 55 transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst 56 deposits of organic matter on sandy beach ecosystems underpin a range of ecosystem functions and 57 services, these can be at variance with aesthetic perceptions resulting in widespread activities, such 58 ‘beach cleaning and grooming’. This practice diminishes the energetic base of food webs, intertidal 59 fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) 60 are predicted to cause substantial reductions in the amounts of marine organic matter reaching 61 many beach ecosystems, likely causing flow-on effects on food webs and biodiversity. Similarly, 62 future sea-level rise and stormier seas are likely to profoundly alter the physical attributes of 63 beaches, which in turn can change the rates at which beaches retain and process the influxes of 64 wrack and animal carcasses. Conservation of the multi-faceted ecosystem services that sandy 65 beaches provide will increasingly need to encompass a greater societal appreciation and the 66 safeguarding of ecological functions reliant on beach-cast organic matter on innumerable ocean 67 shores worldwide. 68 69 Keywords: Landscape ecology, Seascape, Spatial subsidy, Ecosystem functioning, Coastal ecosystems, 70 Kelp forests, Seagrass beds, Detritus, Wrack, Carrion 71 72 73 CONTENTS 74 I. Introduction .................................................................................................................................. 4 75 II. The current state of knowledge ................................................................................................... 5 76 (1) Thematic areas and the geography of the global research effort .......................................... 5 77 (2) Global patterns in standing stock of wrack on beaches .......................................................... 6 78 (3) Factors influencing standing stocks of wrack .......................................................................... 6 79 (4) Links between wrack and fauna in beach ecosystems ............................................................ 8 80 (a) Invertebrates ................................................................................................................... 8 81 (b) Fishes ............................................................................................................................. 10 82 (c) Birds, Mammals and Reptiles ........................................................................................ 11 83 (5) Biological processing and fate of wrack ................................................................................ 11 84 (d) Decomposition processes .............................................................................................. 11 85 (e) Consumption by detritivores ......................................................................................... 12 86 (f) Interactions between detritus, microbes and detritivores ............................................ 13 87 (g) Nutrient fluxes and chemical transformation ............................................................... 14 88 (h) Scavenging ..................................................................................................................... 15 89 (6) Connectivity with adjacent ecosystems ................................................................................ 16 90 III. Human use, impacts and management ...................................................................................... 18 91 (1) Beach grooming and harvesting ............................................................................................ 18 92 (2) Shoreline armouring and coastal development .................................................................... 19 93 (3) Invasive Species ..................................................................................................................... 20 94 (4) Climate change impacts to recipient and donor ecosystems ............................................... 20 95 IV. Conclusions and future research directions ............................................................................... 21 96 V. Acknowledgements .................................................................................................................... 22 97 VI. References .................................................................................................................................. 23 98 99 I. INTRODUCTION 100 Shorelines are formed by a diversity of coastal landforms, including sandy beaches, surf zones, 101 various dune landscapes, subtidal and intertidal rocky reefs, and sea cliffs. Shorelines are also highly 102 dynamic due to being the interfaces between the ocean and the land (Pilkey et al., 2011). The 103 world’s coastlines have been widely transformed for human uses, providing valuable ecosystem 104 services to society (Small & Nicholls, 2003; Bowen & Davis, 2006). Iconic sandy beaches cover 31% of 105 the world’s ice-free shoreline (Luijendijk et al., 2018), and represent highly valued economic, 106 ecological, recreational and cultural assets for societies around the world (Barbier et al., 2011). Yet, 107 beaches are also one of the most vulnerable coastal landforms due to the recurring action of waves, 108 tides and wind on mobile sediments, are among the most heavily altered coastal features (Schlacher 109 et al., 2014) by coastal development, rising sea level, and increasingly frequent storms attributed to 110 climate change (Dugan et al. 2010; Vousdoukas et al., 2020). 111 Beaches form an ecotone between the ocean and land, which is influenced by land and 112 ocean inputs and processes. Beaches and adjacent surf zones along open coasts are often 113 considered to have little in situ primary production,