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Planktonic Subsidies to Surf-Zone and Intertidal Communities

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MA10CH14-Morgan ARI 24 October 2017 11:56 Annual Review of Marine Science Planktonic Subsidies to Surf-Zone and Intertidal Communities Steven G. Morgan,1,2 Alan L. Shanks,3 Jamie H. MacMahan,4 Ad J.H.M. Reniers,5 and Falk Feddersen6 1Bodega Marine Laboratory, University of California, Davis, Bodega Bay, California 94923-0247; email: [email protected] 2Department of Environmental Science and Policy, University of California, Davis, California 93510 3Oregon Institute of Marine Biology, University of Oregon, Charleston, Oregon 97420 4Department of Oceanography, Graduate School of Engineering and Applied Sciences, Naval Postgraduate School, Monterey, California 93943 5Department of Civil Engineering and Geosciences, Delft University of Technology, 2628CN Delft, The Netherlands 6Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0209 Annu. Rev. Mar. Sci. 2018. 10:345–69 Keywords First published as a Review in Advance on August surf zone, zooplankton, phytoplankton, hydrodynamics, subsidies, larval 28, 2017 recruitment, communities The Annual Review of Marine Science is online at marine.annualreviews.org Abstract https://doi.org/10.1146/annurev-marine-010816- Plankton are transported onshore, providing subsidies of food and new re- 060514 cruits to surf-zone and intertidal communities. The transport of plankton Copyright c 2018 by Annual Reviews. to the surf zone is influenced by wind, wave, and tidal forcing, and whether All rights reserved Annu. Rev. Mar. Sci. 2018.10:345-369. Downloaded from www.annualreviews.org they enter the surf zone depends on alongshore variation in surf-zone hy- Access provided by University of California - Davis on 01/13/18. For personal use only. drodynamics caused by the interaction of breaking waves with coastal mor- phology. Areas with gently sloping shores and wide surf zones typically have orders-of-magnitude-higher concentrations of plankton in the surf zone and ANNUAL REVIEWS Further dense larval settlement in intertidal communities because of the presence of Click here to view this article's online features: bathymetric rip currents, which are absent in areas with steep shores and nar- • Download figures as PPT slides row surf zones. These striking differences in subsidies have profound con- • Navigate linked references • Download citations sequences; areas with greater subsidies support more productive surf-zone • Explore related articles • Search keywords communities and possibly more productive rocky intertidal communities. Recognition of the importance of spatial subsidies for rocky community dy- namics has recently advanced ecological theory, and incorporating surf-zone hydrodynamics would be an especially fruitful line of investigation. 345 MA10CH14-Morgan ARI 24 October 2017 11:56 1. INTRODUCTION Alongshore variation in ocean conditions affects the delivery of planktonic and larval subsidies to Subsidy: plankton, shore, with profound consequences for the dynamics and structure of surf-zone, rocky, and beach detritus, and nutrients communities (Menge et al. 1997a, 2003; Morgan et al. 2016; Shanks et al. 2017a). Variation in phy- from the water column toplankton, zooplankton, and detritus subsidies affects the growth and reproduction of the seden- that sustain benthic tary and sessile filter-feeding invertebrates that form the foundation of these communities (Menge communities et al. 1997a, Leslie et al. 2005) and, together with variation in larval supply, affects the density Surf zone: the region of settlers and the intensity of postsettlement interactions (Morgan 2001, Underwood & Keough between the shoreline and the onset of 2001). Intense density-dependent interactions leading to high mortality occur where planktonic depth-limited wave food and larval subsidies abound, and weaker interactions and lower postsettlement mortality occur breaking where these subsidies are sparse (Menge & Sutherland 1987, Morgan 2001, Menge et al. 2003). Upwelling: aprocess Multiple mechanisms may transport larvae, other zooplankton, and phytoplankton toward in which alongshore shore, including upwelling relaxation events, onshore winds, large waves, tidally generated internal wind and Earth’s waves, and shoreward-flowing bottom waters during upwelling (Shanks 1995, 2006; Pineda 1999; rotation force surface Morgan et al. 2009a,b; Drake et al. 2013). Once plankton are on the inner shelf, the degree to water offshore, resulting in a which they enter the surf zone may depend largely on spatial variation in surf-zone hydrodynamics, compensatory onshore which is determined by the interaction of breaking waves with variation in coastal morphology, flow of cold, ranging along a spectrum from dissipative to reflective (Wright & Short 1984, McLachlan & nutrient-rich bottom Brown 2006) (Figure 1). water that is forced to The goal of this review is to increase awareness of the importance of surf-zone hydrodynamics the surface upon reaching shore for plankton concentrations and the densities, growth, and reproduction of residents of surf-zone and intertidal communities. We first review physical processes that lead to exchange between Relaxation: weakening of the surf zone and the inner shelf. We then evaluate existing evidence for the effect of surf-zone upwelling-favorable hydrodynamics on plankton concentrations and the consequences for surf-zone and intertidal winds communities. We conclude by discussing future directions for this avenue of research. Inner shelf: a region seaward of wave 2. PHYSICAL PROCESSES INDUCING EXCHANGE breaking where the surface and bottom The exchange of water between the surf zone and the inner shelf is induced by a complex circu- boundary layers lation with high variability in fluid velocity, temperature, and potentially salinity at a wide range interact of temporal and spatial scales, which are presently not well understood. This interaction zone Dissipative surf affects the transport of material such as biota, pollutants, and sediments. The three-dimensional zone: a surf zone with inner-shelf circulations are influenced by wind, wave, and tidal (both surface and internal tides) high wave energy that forcing, with the relative importance of each depending on the water depth (Lentz 1994, Lentz dissipates gradually as waves break on et al. 1999, Cudaback et al. 2005, Fewings et al. 2008, Kirincich et al. 2009). Inside the surf zone, Annu. Rev. Mar. Sci. 2018.10:345-369. Downloaded from www.annualreviews.org alongshore bars and current dynamics are driven predominantly by breaking surface gravity waves and depend largely Access provided by University of California - Davis on 01/13/18. For personal use only. gently shoaling on surf-zone bathymetry and morphology classification (Figure 1). Because current dynamics dif- beaches, resulting in fer substantially between the surf zone and the inner shelf, mechanisms for cross-shore transport progressive waves, also differ. Lentz & Fewings (2012) provided an overview of wind- and wave-driven inner-shelf wide surf zones, and fine-grained sand processes. Here, we focus on recent developments that have improved understanding of the trans- port processes at the boundary between the surf zone and inner shelf (out a few surf-zone widths, on the order of 500 m). These processes include wave-driven Stokes drift, wave streaming, rip currents, and internal waves, all of which induce transport of subsidies. 2.1. Cross-Shore Exchange Caused by Vertical Structure (Stokes Drift, Undertow, and Streaming) The flows driving exchange between the surf zone and inner shelf can occur over depth, and therefore the vertical structure of the flows is important in transporting plankton subsidies onshore 346 Morgan et al. MA10CH14-Morgan ARI 24 October 2017 11:56 a DissipativeDissipative andand alongshorealongshore uuniformniform Inner shelf Transient rip current Surf zone 100 m Beach b Dissipative and rip channeled Bathymetric rip current Rip channel Shoal 200 m c Reflective Annu. Rev. Mar. Sci. 2018.10:345-369. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/13/18. For personal use only. 100 m Figure 1 Aerial photographs of three different surf-zone morphodynamic types: (a) dissipative and alongshore uniform, (b) dissipative and rip channeled, and (c) reflective. The beach, surf zone, and inner shelf are labeled in panel a but are also visible in panels b and c.Panela has an 8-m-wide surf zone shown during a Rhodamine WT dye release (Hally-Rosendahl et al. 2014), highlighting transient-rip-current ejection events. Panel b depicts a 150-m-wide surf zone, rip channels and shoals, and bathymetric rip currents. In panel c,thesurf zone is narrow (less than 20 m wide) and the beach is very steep. www.annualreviews.org • Planktonic Subsidies 347 MA10CH14-Morgan ARI 24 October 2017 11:56 abLagrangian Surf zoneflow, Inner shelf u z L( ) Lagrangian flow, uL(z) Mean sea level, z = 0 Eulerian flow, Lagrangian Eulerian flow, Lagrangian uE(z) Stokes flow, uE(z) Stokes flow, uStokes(z) uStokes(z) h z = –h Figure 2 Wave and background flow description of an alongshore homogeneous beach in the Eulerian and Lagrangian frame of reference for (a) typical surf-zone behavior and (b) typical inner-shelf behavior, as observed in the field and described in the literature. The blue dashed line represents the mean sea level, the black solid line at the bottom represents the seabed, and the green dashed lines represent uL(z). (Figure 2a). Within the surf zone, onshore wave-driven flow (Stokes drift) is largely
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