Beyond beach width: Steps toward identifying and integrating ecological envelopes with geomorphic features and datums for sandy beach ecosystems

Geomorphology 199 (2013) 95–105

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Beyond beach width: Steps toward identifying and integrating ecological envelopes with geomorphic features and datums for sandy beach ecosystems Jenifer E. Dugan a,⁎, David M. Hubbard a, Brenna J. Quigley b a b

Marine Science Institute, University of California, Santa Barbara, CA 93106-6150, USA Department of Earth Science, University of California, Santa Barbara, CA 93106, USA

a r t i c l e

i n f o

Article history: Received 2 August 2012 Received in revised form 30 April 2013 Accepted 30 April 2013 Available online 23 May 2013 Keywords: Intertidal zones Tidal movement Mobile animals Intertidal invertebrates Wildlife habitat Total water level

a b s t r a c t Our understanding of ecological responses to climatic and anthropogenic forcing lags far behind that of physical or geomorphic responses for beach ecosystems. Reconciling geomorphic features of beaches with ecological features, such as intertidal zones and mobile biota that are not described by beach width alone, could help address this issue. First, although intertidal zones characterized by distinct groups of mobile burrowing animals are described for beaches, the locations and elevations of these zones do not coincide with standard shoreline datums. Second, intertidal zonation on beaches is extremely dynamic due to the combination of unstable sandy substrate and a highly mobile biota; shifting strongly with tides, waves, storms, and beach conditions. We propose that beach biota use ecological “envelopes” of cross-shore habitat to cope with constantly changing beach conditions. We estimated the extent of these “envelopes” for a variety of taxa on tidal to daily, semi-lunar and seasonal to annual time scales, using literature values on cross-shore animal movements and a field study of the positions of intertidal beds of two species of typical mid and upper shore beach invertebrates. Daily or tidal cross-shore movement varied most (1 m to 100 m) with daily “envelopes” covering 7% to 85% of the available beach width. Semi-lunar movement (12 m) and envelopes (28%) were relatively small, while estimated annual “envelopes” were large, averaging 61% of beach width. The large scope of annual ecological envelopes relative to beach widths reflects how intertidal animals escape seasonally extreme or episodically harsh conditions. Intertidal bed positions of a talitrid amphipod and an opheliid polychaete correlated well with selected beach features in our field study suggesting that incorporation of ecological envelopes in models of shoreline evolution may be feasible. Describing ecological zones in terms of more dynamic shoreline features, such as total water level (TWL) that incorporate wave setup and runup, may be particularly applicable to upper intertidal biota whose distributions closely followed the high tide strand line (HTS), a feature which tracks total water level (TWL). Developing a TWL approach may also provide new insights on habitat availability for beach nesting wildlife and coastal strand vegetation. Conservation of beach ecosystems could be enhanced by incorporating sufficient beach habitat to accommodate the dynamic ecological envelopes used by mobile intertidal invertebrates and wildlife. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Characterized by unconsolidated sand, a wealth of mobile burrowing animals and a lack of attached intertidal plant life, sandy beaches are a challenging environment for biota and ecologists alike. Management of sandy beaches traditionally focuses on creating and maintaining recreational opportunities, protecting coastal development and industry, and sustaining coastal economies. Far less effort has been applied to maintaining the ecological values, habitats, and functions of beaches as ecosystems. Even in the absence of human interventions and management, dynamic coastal processes can produce rapid and significant changes in shorelines and in the ecological communities inhabiting beach ecosystems. Coastal evolution associated ⁎ Corresponding author. Tel.: +1 805 893 2675; fax: +1 805 893 8062. E-mail address: [email protected] (J.E. Dugan). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.04.043

with management activities and climate change has the potential to profoundly affect biodiversity, community composition, ecological function, and wildlife populations of sandy beach ecosystems (Defeo et al., 2009). Our understanding of ecological responses to climatic and anthropogenic forcing, however, lags far behind that of physical or geomorphic responses for these widespread coastal ecosystems (Schlacher et al., 2007). Reconciling key geomorphic features of beaches with ecological features that are not described by standard beach widths or datums, such as intertidal zones and their characteristic mobile biota, could help address this gap. The standard tidal datums and elevations which are used as boundaries and delineations of tidal or ocean influence can be challenging to apply on beaches where dynamic geomorphic features on the profile including wet/dry lines, berms, terraces, cusps and horns, and vegetation lines, as well as the biota, move with tides, waves, storm events and sediment supply. The ability and ease of measuring

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elevations and locations of dynamic geomorphic features of beaches have improved greatly with the application of technologies that significantly increase accuracy, accessibility and geographic scope (LIDAR, GPS, e.g. Ruggiero and List, 2009). These approaches enhance our capacity to understand and predict responses of shorelines and geomorphic features to climatic conditions and coastal evolution. They also have potential applications to predicting the responses of key ecological components of beach ecosystems to climatic forcing. Intertidal zonation can be discerned on sandy beaches; however, its character differs profoundly from rocky or muddy shores (Peterson, 1991). Although far less obvious than for other types of shores, a general agreement exists that three intertidal zones, defined by characteristic burrowing animals, can be identified on most sandy beaches during static low tide surveys (see review by McLachlan and Jaramillo, 1995). The three zones are generally termed supralittoral, littoral and sublittoral, however, because these zones are all functionally intertidal and strongly affected by waves, tides and currents over longer time scales, we will consider them as upper, mid and low intertidal zones here (Fig. 1). Highest on the beach profile is an upper beach zone located above and below the 24 hour high tide strand line (HTS) or driftline (Table 1) extending up to the landward boundary of wave and tide-influenced sandy habitat (e.g. foredune toe, rocky bluff, or man-made coastal infrastructure). This upper beach zone includes the backshore or supralittoral zone but can extend below the berm crest. Surf-cast material, such as macrophyte wrack, driftwood and carrion, accumulates in this zone and indicates the daily as well as the highest seasonal reach of tides and waves. Coastal strand vegetation can develop near the landward edge of this zone (Feagin et al., 2005; Dugan and Hubbard, 2010) although it may be functionally annual because of seasonal inundation and erosion by waves and tides. This upper beach zone is often defined as critical or essential habitat required for wildlife, including nesting shorebirds and sea turtles, many of which are threatened or endangered. The mid-intertidal zone extends from below the high tide strand line across the damp sand and to or below the water table outcrop depending on the slope and shape of the beach profile (Table 1). The low intertidal zone consists of saturated sand that includes the lowest retreat of the tides and the upper and lower bounds of the active swash zone (Fig. 1). These two zones include the foreshore. Although intertidal zones can be identified on beaches, over the course of even a few hours, the distributions of characteristic mobile

animals of these zones can move significant distances across or along the shore in response to water motion and other coastal processes, such as erosion and accretion of the beach profile. Animals burrowed in one zone of a beach at low tide emerge from the sand to migrate up the shore as the tide floods then move down the shore again on the ebb tide. The position and activity of mobile burrowing animals also respond to wave energy, temperature, light, and beach erosion and accretion. The distinctive mobility of the intertidal fauna of beaches and of the sand itself mean that some of the classical tenets of intertidal zonation, useful for exposed rocky shores, simply do not apply (Peterson, 1991). On intertidal shores with stable rocky substrates, many of the characteristic plants and animals survive the action of tides and waves by strongly resisting movement, using a variety of adaptations and behaviors, such as attaching firmly to the substrate or in crevices using roots, holdfasts, permanent cement or byssal threads, or by clinging with large muscular or even with many tiny feet. Even on sheltered muddy shores, plants take root and many animals build and inhabit relatively permanent burrows in the well-consolidated fine sediments. In contrast, all intertidal animals on sandy unconsolidated beaches must move to survive. Attaching to the sandy substrate or to plants is not an option for beach animals. Consequently, these intertidal animals swim, scud, crawl, run, hop, surf, and burrow rapidly to adjust to the ever-changing conditions of tides, waves, storms, and to dynamic beach profiles. Although burrow occupancy can range from minutes to days, intertidal animals of sandy beaches cannot inhabit permanent burrows. Many beach animals exhibit regular migrations across the beach in response to tide height and phase, with patterns ranging from daily or semi-diurnal tidal to semi-lunar tidal migrations described in the literature (McLachlan and Jaramillo, 1995). Over the course of larger seasonal or event-driven erosion and accretion typical of beaches, intertidal animals move across the shore to adjust to changing profiles and conditions. Data on intertidal zonation and tidal movement of animals are available for sandy beaches from around the world (see review by McLachlan and Jaramillo, 1995 and citations in Table 2abc); however, relatively few of these data have been integrated with standard tidal datums. Further, the majority of analyses to date have necessarily focused on static distributions of the biota during daytime low tide conditions when the majority of animals are burrowed temporarily into the sand. These existing analyses provide a useful starting place but

HIGH TIDE STRAND

Upper Intertidal

BERM Mid-Intertidal Low Intertidal

Fig. 1. Schematic beach profile with the relative locations of the mobile zones and features under study including the coastal strand vegetation, the berm, the high tide strand (HTS), the water table outcrop (WTO) and the upper, mid and lower intertidal zones on a California beach during a low tide, see also Table 1. The scope of the upper intertidal beach inhabited by talitrid amphipods, oniscid isopods, insects, and other air-breathing invertebrates extends from the high water boundary of the beach (bluff, foredune, or man-made coastal infrastructure) to below the HTS. The mid intertidal zone inhabited by polychaetes and cirolanid isopods consists of damp sand located below the HTS and extending to the WTO. The low intertidal zone is saturated sand, which includes the lowest retreat of the tides and the active swash zone, is inhabited by hippid crabs, donacid bivalves, gastropods, amphipods, mysids and polychaetes.

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Table 1 Definitions of terms used for mobile beach features appearing in Fig. 1 and measured in field study and for the shoreline datums discussed. Term

Type

Abbreviation

Definition

Vegetation line High tide strand Berm crest Water table outcrop

Feature Feature Feature Feature

na HTS na WTO

Swash zone Total water level Mean high water Mean sea level

Feature Datum Datum Datum

na TWL MHW MSL

The lowest seaward extent of coastal strand or dune vegetation The highest reach of the tides in a 24 h, often the boundary between damp and dry sand, also called the driftline A noticeable break in beach slope between the beach face and the berm The highest level where the water table reaches the sand surface indicated by the boundary of damp and saturated sand, also called the effluent line. The zone of active wave wash across the sand located inshore of the surf break Tide level (ET), plus the additional elevation due to wave runup and wave setup The elevation of mean high water obtained from local observed tide records The elevation of mean sea level obtained from local observed tide records

do not capture the full range of intertidal habitat used by and required for mobile beach animals to cope with changing beach widths and conditions. In this paper, we introduce the concept that the cross-shore range of intertidal habitat required by different taxa of mobile beach animals represents an “ecological envelope”. This “envelope” encompasses typical biotic distribution widths, regular and episodic movements, and annual cycles in beach widths and profiles in response to changing beach conditions. Defining these ecological envelopes for key biota, then interpreting them in terms of tidal datums, drivers, models and predictions has the potential to provide new insights that could be applied to conserve ecosystem values and functions on sandy beaches. Reliable predictions of the consequences of beach management, shoreline armoring, erosion, and impacts of climate change on beaches are critically needed to inform the conservation of threatened beach ecosystems and wildlife. To accomplish this, approaches that translate high quality 21st C beach process studies and datum mapping to shorelines in ways that are applicable beyond overall beach widths to describing the changing distributions of beach biota are required. Expanding and merging the current understanding of beach ecology and animal movements with the latest science on shoreline dynamics has the potential to provide new tools for evaluating the ecological consequences of coastal evolution associated with erosion, episodic events, and climate change. To begin developing approaches to these issues for beach ecosystems, we explore and discuss 1) the positions of intertidal zones defined by characteristic biota relative to standard tidal datums and 2) the concept of ecological envelopes of habitat used by mobile beach animals using a compilation of cross-shore movements observed on several temporal scales from the literature and a field study of biotic distributions for two characteristic intertidal invertebrate species on a microtidal sandy beach. 2. Methods 2.1. Data sources Data from an unpublished thesis on southern California beaches (Clark, 1969) were used to describe the zonation of the dominant intertidal invertebrates across the beach profile relative to elevations above Mean Lower Low Water during spring low tides. The average position and the range of distribution of intertidal species above Mean Lower Low Water (MLLW) from surveys conducted at two beaches in San Diego County were calculated. The elevations of the standard tidal datums of Mean High Water (MHW) and Mean Sea Level (MSL) were extracted from the current tide gauge record for San Diego (Station ID 9410170, NOAA, 2012) and compared to the distributions of intertidal animals. To develop estimates of the scale of cross-shore movements and the ecological envelopes used by intertidal fauna on beaches, we compiled information on animal movement across the beach on 1) tidal to daily, 2) semi-lunar and 3) seasonal to annual scales from published

literature and incorporated into Table 2. For papers where the amplitude of movement was not specifically reported in the text, we estimated these values from figures provided. The ecological envelope used by different taxa was estimated as the total cross-shore distance between highest and lowest individuals during the time period specified. Intertidal widths (maximum) were compiled where information was available in publications. 2.2. Field study Data on the locations of shoreline features and the distribution of beds of selected intertidal invertebrates were collected regularly (~ every 2nd day) from Summer 2011 to Summer 2012 on a low intermediate type sandy beach in southern California. Surveys of the elevations of shoreline features and of beds of selected intertidal invertebrates were conducted at the study beach during low tides on five dates between March and August 2012. 2.2.1. Study site The field component of this study was conducted on the western portion of Isla Vista beach, a narrow bluff-backed shoreline in Santa Barbara County, California (Hubbard and Dugan, 2003) (Fig. 2ab). Beach dynamics in the region are dominated by longshore processes and observed seasonal variability in sand thickness and beach width lags wave energy by 7 to 8 months (e.g. Barnard et al., 2012). 2.2.2. Field study organisms To obtain detailed information on animal movement, we regularly measured zonal dynamics of the visible beds of two important intertidal invertebrates that inhabit the upper and mid beach zones, talitrid amphipods (Megalorchestia spp.) and a worm, the opheliid polychaete (Euzonus mucronata), respectively. The burrows of these two taxa create beds that are readily observed at the sand surface and can be easily measured in daytime surveys during neap and spring low to mid tides, allowing frequent repeatable measurements. California beaches support five species of talitrid amphipods in the genus Megalorchestia (Light and Carlton, 2007). Talitrid amphipods can dominate intertidal community abundance and mean densities can exceed 90,000 individuals m−1 at the study beach (Lastra et al., 2008). The distributions of highly mobile talitrid amphipods change substantially over daily cycles. They are nocturnal and generally active on the sand surface primarily at night with distributions extending as low on the beach as the top of the swash zone. They feed on freshly deposited macroalgal wrack, then move back upslope to spend daylight hours burrowed in dense beds in damp sand near the 24 h high tide strand (HTS) or drift line (Bowers, 1964). The positions of their burrows are marked by mounds on the sand surface near the high tide strand (HTS) (apparently determined by moisture level, Craig, 1973). A mid intertidal worm, the opheliid polychaete, E. mucronata, creates characteristic narrow, dense beds in damp sand with burrows visible at the surface of the mid intertidal zone during low tides. These worms are not active on the sand surface but move deeper (vertically) into the

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Table 2 Beach widths, cross-shore movement, estimated ecological envelopes and percentage of beach width used by intertidal animals and observed for nests of one shorebird on sandy beaches compiled from the literature and the current field study for a) tidal or daily b) semi-lunar and c) seasonal to annual time periods. Cross-shore movement was calculated as the distance between the landwardmost position of a particular attribute of a population distribution (e.g. mean) and the most seaward extent of that same attribute during a specified time period. The estimated ecological envelope incorporates movement and the total beach width occupied by the population distribution, equivalent to the distance between the position of the most landward individual observed and the most seaward individual observed during the specified time period. Beach width (m)

Movement (m)

% Width

Envelope (m)

% Width

Reference

a. Tidal to daily movement Euzonus mucronata Upper edge of beds, annual mean Lower edge of beds, annual mean Emerita analoga Chile, adults, reflective beach Chile, adults, dissipative beach La Jolla, juveniles Hippa australis Hippa australis Gastrosaccus psammodytes Megalorchestia spp. Upper edge of beds, annual mean Lower edge of beds, annual mean Surface active/burrows-downshore Megalorchestia corniculata, pitfall traps Pseudorchestoidea braziliensis Juveniles, pitfall traps Adults, pitfall traps Talitrus saltator, adults active/burrows Tylos punctatus, burrowed Bullia rhodostoma Donax semignosus Donax semignosus (neap tide) Donax variabilis Donax sordidus Phalerisida maculata Juvenile Adult

60 60 60

4 1.1 1.0

28 175 150 35 29 100 60 60 60 60 27

16 107 23 9 5 to 10 60

57 61 15 26 26 60

2.3 1.4 12.8 12

4 2 21 44

56 56

46 36

45

100

17 to 35 16 to 24 18 0.8 45 >30 3 to 6 45 25

68 68

18 6 to 12

26 13

100

7

2 2

This study This study This study

20 115

71 66

10

17

48 45 to 52

86 88

Cardoso (2002) Cardoso (2002) Fallaci et al. (2003) Hamner et al. (1969) McLachlan et al. (1979) Mori (1938) Mori (1938) Turner and Belding (1957) McLachlan et al. (1979)

33 27

49 40

Jaramillo et al. (2000) Jaramillo et al. (2000)

6

10

33

22

This study This study This study Klapow (1972)

17

28

14

28

35

58

100 72 43

61 72 72

25

Jaramillo et al. (2002) Jaramillo et al. (2002) Efford (1965) Shepard et al. (1988) McLachlan and Hesp (1984) McLachlan et al. (1979) This study This study This study This study Craig (1973)

b. Semilunar movement Euzonus mucronata Upper edge, annual mean Lower edge, annual mean Excirolana chiltoni Megalorchestia spp. Upper edge, annual mean Lower edge, annual mean Ocypode ceratophthalmus burrows Talitrus saltator Donax serra Thinopinus pictus

60 60 60 150 60 60 20 100 50

3.5 3.2 12.8

6 5 9

10.5 6.5 2.3 6 to 7 5 to 7 8

18 11 12 6 12

33 31 80 68

55 52 48 68

34 35

57 58

12 14 to 16 19 15 14 to 18 9 62 42 58 11 36 22

NA 94 38 30 32

This study This study Barrass (1963) Williams (1995) Donn et al. (1986) Craig (1970)

c. Seasonal to annual movement Euzonus mucronata, bed location Upper edge, annual mean Lower edge, annual mean Euzonus sp. Glycera alba Megalorchestia spp., bed locations Upper edge, annual Lower edge, annual Talitrus saltator Isle of Man, bed locations Italy, pitfall traps Tunisia, adults, pitfall traps Talitrid juveniles, pitfall traps Tylos europaeus, pitfall traps Tylos punctatus, burrowed Emerita holthuisi Eurydice sp. Bullia melanoides Donax faba Donax incarnatus Charadrius nivosus nests

60 60 60 165 100 60 60 60 NA 16 50 50 50 100 100 100 100

sand during high tides and return to the sand surface during low tides for respiration and feeding. This polychaete can reach high densities at the study site with mean values that exceed 28,000 individuals m−1 of shoreline (Dugan, unpublished results).

62 42

66 52 66

66 52 66

36

44

44

This study This study This study Seike (2008) Ansell et al. (1972) This study This study This study Williams (1995) Pavesi et al. (2007) Colombini et al. (2002) Colombini et al. (2002) Colombini et al. (2002) Hamner et al. (1969) Ansell et al. (1972) Ansell et al. (1972) Ansell et al. (1972) Eshky (1998) Ansell et al. (1972) Dugan, unpublished results

2.2.3. Intertidal sampling methods To investigate dynamics of the distributions of the two typical upper and mid-beach macroinvertebrates described above, we measured the extent and locations of the beds of these invertebrates, as

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2.2.5. Ancillary data We obtained physical data associated with total water levels during the study. We tabulated the maximum astronomical high tide elevation for the previous 24 h to the nearest 3 cm for each morning of the study. We tabulated daily mean significant wave height (m) and significant wave period (s) for the Goleta Point buoy (located 10.5 km from the study site, water depth 180 m, Scripps Institution of Oceanography, 2012, Fig. 2b), and calculated mean daily wave energy as kW m−1. 3. Results and discussion 3.1. Intertidal zones

Fig. 2. (a) Overview of the study region in California, including the location of the Goleta Point buoy and (b) the location of the west Isla Vista beach study site.

indicated by characteristic surface burrows in frequent daylight surveys. We also measured the positions of shoreline features, including the water table outcrop (WTO), the high tide strand (HTS = highest water level in previous 24 h), the berm crest, and the limits of the coastal strand vegetation zone. All locations and positions were measured to the nearest meter relative to the toe of the sea bluff. These measurements were repeated 195 times on five replicate shore-normal transects during morning hours over 366 days (June 2011 to June 2012).

2.2.4. Elevational data Elevation surveys were conducted on the five transects described above on five dates: March 15, April 14, May 6, May 27, June 10 and August 23, 2012. For these surveys, the locations and elevations of prominent physical features and of the upper and lower limits of the biotic zones of talitrid amphipods and bloodworms were recorded on each transect using a Topcon HiPerlite GPS total station (two piece base and rover GPS system). On each transect, the latitude, longitude, and elevation of the upper and lower limits of the beds of Megalorchestia spp. and Euzonus spp., the lower limit of coastal strand vegetation, if present, were recorded, relative to the WGS84 and NAVD88 projections. Physical features recorded on each transect included the toe of the coastal bluff, the high tide strand (HTS), and water table outcrop (WTO). A tie-point was set up on a nearby beach access staircase allowing the GPS to calibrate for a minimum of 3 h, and then recording a point of constant elevation. This information was then sent into the OPUS (Online Positioning User Service) National Geodetic Survey, which corrected the data according to the National Spatial Reference System (NSRS) to yield accurate data for the point of constant elevation (referred to as the “tie point”). That information was used to correct the elevations for each survey. The resulting data were analyzed using Topcon Tools, which calculates corrections according to the tie points, and then exported into Microsoft Excel. Latitudes and Longitudes were converted to decimal degrees, then to meters (based on local conversion factors of 110,998.18 and 91,957.98 for latitude and longitude, respectively) to obtain horizontal distances from the bluff toe. The distance of each point from the bluff toe was then calculated.

Our example of the elevations of intertidal invertebrates from a California beach, using the data collected by Clark (1969) (Fig. 3), shows that two commonly used tidal datums, mean high water (MHW) and mean sea level (MSL), did not coincide well with the static distributions of dominant beach invertebrates measured during a winter spring low tide. The means of the distributions of characteristic upper intertidal taxa (talitrid amphipods, oniscoid isopods and intertidal insects) were located above these two major datums entirely. For the mid-intertidal zone, only the lower tail of the distribution of the typical cirolanid isopod, Excirolana, extended below MHW. The mean distribution of the low intertidal hippid crab, Emerita analoga, fell between the MHW and MSL but the range of elevations occupied by this species extended above and well below these datums. The low intertidal bean clam, Donax, occurred primarily below mean sea level (MSL). One approach or datum that we propose may be useful for application to ecological distributions on wave exposed beaches, particularly for defining a seaward boundary of upper beach zones, is total water level (TWL) as recommended by Moore et al. (2006) and Ruggiero and List (2009) for estimating shoreline dynamics and for shoreline change analyses. Total water level on a beach at any time is the sum of the tide level (ET), plus the elevation above ET reached by wave runup, including wave setup (Ruggiero and List, 2009). The TWL datum, where available, appears to provide a closer approximation of the 24 hour high tide strand line (HTS) feature that is followed by key upper beach species, such as talitrid amphipods, than MHW. That dynamic feature also defines a critical seaward boundary for beach nesting vertebrate species, including snowy and piping plovers and sea turtles, many of which are threatened or endangered. Ruggiero and List (2009) estimated that the proxy data bias between MHW and TWL averaged 18 m with a bias uncertainty of 9 m for California beaches. Assuming a moderate beach slope (est. 4–8°), the mean elevations of the typical upper beach species from Clark (1969) data would yield an estimated TWL of 11 to 22 m above MHW datum, bracketing the estimates given by Ruggiero and List (2009). This comparison suggests that TWL could potentially be applied to improve mapping of ecologically relevant upper intertidal zone features in this coastal region. 3.2. Animal movement and biotic zone dynamics Animal movement, in response to changing conditions of tide, surf, shore morphology, and season, has been observed in many forms and time frames on beaches around the world (e.g. Trueman, 1971; McLachlan et al., 1979). Examples of several types of regular movement or migration by intertidal invertebrates on beaches sorted by time period and taxon are presented in Table 2. This summary allows comparisons of the relative distance and magnitude of animal movements, which could be used to estimate the ecological envelopes of cross-shore habitat used by a variety of typical beach animals. 3.2.1. Tidal and daily movement Daily cross-shore movements of many intertidal species, especially those of the mid and lower beach, are generally in response to changing

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Fig. 3. Mean elevations and ranges of the distributions of dominant intertidal macroinvertebrates with respect to Mean Lower Low Water (MLLW) from two San Diego beaches during a spring low tide survey (from Clark, 1969). The elevations of the datums, Mean High Water (MHW) and Mean Sea Level (MSL), are indicated by dotted lines.

tide level in combination with daily activity patterns (often nocturnal). Daily tidal migration (e.g. moving up the shore with the flooding tide and down the shore with ebbing tide) has been observed and measured in many characteristic beach invertebrate taxa (McLachlan and Brown, 2006). The observed cross-shore movement associated with tidal migration of intertidal invertebrates ranges widely, from an average of one meter by a polychaete, Euzonus, on a narrow steep low energy beach, to a peak of 107 m for a highly mobile hippid crab, Emerita, on a wide flat dissipative beach during spring tides. During these tidally generated cross-shore movements, animals can cover from 2% to 60% of the intertidal width of the beach between a high and a low tide (Table 2). The amount of cross-shore movement associated with tidal migration can be strongly affected by the tide phase (>30 m spring vs. 3–6 m neap for Donax) (Mori, 1938) and by beach width or type (16 m reflective vs. 107 m dissipative for Emerita) (Jaramillo et al., 2002). Not surprisingly, the greatest cross-shore movements were observed in rapidly burrowing and swimming crustaceans of the swash zone, such as Emerita and Gastrosaccus, inhabiting wide, flat beaches. Swash-riding clams in the genus Donax and snails in the genus Bullia, however, have been reported to migrate up to 45 m with the daily tidal cycle (Table 2a). Along with responding to tides, many upper beach animals (amphipods, isopods, beetles, ghost crabs) are nocturnal and respond to changing light levels by emerging and moving down the beach face to feed on the lower beach at night. The semi-terrestrial upper shore talitrid amphipods are surface active primarily at night and exhibit regular downshore movements of 13 to 35 m to feed on fresh wrack then retreat landward, burrowing in the upper intertidal in daylight (Table 2a). Ocypodid ghost crabs that burrow at or above the high tide strand are also primarily active at night, moving downshore to the swash zone after dark then returning to upper intertidal burrows for the day (Wolcott, 1978). Even intertidal insects can exhibit downshore movements of up to 18 m in a night (tenebrionid beetles, Phalerisida Jaramillo et al., 2000). Intertidal cross-shore movement of beach animals can also have a distinct seasonal component. For example, movement associated with tidal migration occurs in certain seasons and ceases for other seasons e.g. winter, for clams Donax variabilis, (Leber, 1982a,b) and ghost crabs Ocypode quadrata (Wolcott, 1978). Hippid crabs in the genus Emerita

have been reported to move from the intertidal zone to shallow subtidal habitats during the winter returning to the intertidal beach in the spring (e.g. E. talpoida, Edwards and Irving, 1943; Bowman and Dolan, 1985). The ecological envelope or cross-shore amount of habitat used by tidally or daily migrating species (the total distance spanning the positions of the highest and lowest distributed individuals during the tide period) was greater than the amount of movement observed for any one part (top bottom or middle) of the distribution. This ecological envelope varied considerably among taxa, ranging from 4 m for a mid zone polychaete, Euzonus (this study), on a narrow sloping beach to 115 m for the lower zone hippid crab, Emerita, (Jaramillo et al., 2000) on a wide flat dissipative beach. Daily ecological envelopes encompassed as little as 7% of the intertidal beach width for the polychaete (Euzonus) to as much as 88% for a talitrid amphipod (Pseudorchestia). Overall, the tidal or daily ecological envelope used by beach biota averaged almost twice the tidal or daily movement observed, 28% vs. 53% of the beach width. 3.2.2. Semi-lunar movement Along with tidal and daily migration patterns, observations of the cross-shore distances spanned by semi-lunar migrations were available for several taxa for comparison (Table 2b). Some intertidal beach species, such as Donax serra (Donn et al., 1986), do not exhibit daily tidal migration but change position up and down the beach with the spring to neap tide phases with a periodicity of close to 14 days. Daily tidal migrants, noted in the previous category, particularly the swash zone species, also change position on the shore with semi-lunar frequencies. The cross-shore distances, represented by the semi-lunar migrations reported here, were much lower than that exhibited by many daily tidal migrants, ranging from 2.3 m for a ghost crab (Ocypode, Barrass, 1963) to 12.8 m for a swimming mid beach isopod (Excirolana, Klapow, 1972). Generally all types of biota moved less than 10 m in response to an individual semi-lunar cycle. For example, the vertical spring to neap shift of the center of gravity of the band of the clam, Donax serra, was 22 cm in elevation on a South African beach, corresponding to 5 to 6.5 m of cross-shore shift in position (Donn et al., 1986). The ecological envelope of cross-shore habitat used by species exhibiting semi-lunar movement or migration was also considerably

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greater than the cross-shore distances of the movement. The semilunar envelope ranged from 6 m for Euzonus to 33 m for the swimming isopod Excirolana, encompassing as little as 10% of the beach width for the polychaete to as much as 28% for both a talitrid amphipod and a flightless rove beetle (Thinopinus). Overall, the semi-lunar ecological envelope averaged more than twice the semi-lunar movement, 10% vs. 22% of the beach width. 3.2.3. Seasonal, annual and episodic movement Cross-shore movements that occur on seasonal, annual or episodic scales need to be considered to capture the full ecological envelope of habitat used by intertidal beach species to cope with major shifts in wave energy and beach morphology associated with seasonal beach dynamics and major storms. Although data on this type of cross-shore movement were more limited, available estimates for seasonal or annual scale cross-shore movements greatly exceeded tidal or semilunar movement, ranging from 9 m for an upper shore isopod, Tylos punctatus, (Hamner et al., 1969) to 80 m for a polychaete, Euzonus sp. (Seike, 2008) (Table 2c). The most comprehensive data for seasonal cross-shore movement of a variety of dominant species of an intertidal beach community were provided by Ansell et al. (1972) for two beaches in India affected by strong seasonal monsoons. Intertidal distributions of several types of important macroinvertebrates of the mid and lower intertidal zones (including Emerita, Eurydice, Excirolana, Donax, Glycera, and Bullia) responded strongly to major reductions in beach widths (50–55 m) from erosion during the monsoon season (June to November) by moving landward and by compressing their cross-shore distributions (Ansell et al., 1972). Landward shifts of up to 52 m were observed for Donax species and up to 72 m for Emerita holthuisi during the monsoon season on Indian beaches (Table 2) and some species disappeared entirely from the beach seasonally (Ansell et al., 1972). The upper and mid shore species we studied on the California beach over one year, the talitrid amphipods (Megalorchestia spp.) and the polychaete (E. mucronata), also exhibited large seasonal/annual cross-shore movements (> 30 m) relative to their daily (1 m to 2 m) or semi-lunar cross-shore movements (3 m to 10 m) (Table 2c). Interestingly, no patterns in the relative scope of these annual or seasonal envelopes were apparent among taxa or tidal levels in our comparisons (Table 2c). This suggests that the morphological characteristics (slope, width, profile) of individual beaches and the strength of the seasonal erosion signal may be more important than the characteristic mobility or behavior of individual taxa. The cross-shore envelope of habitat used by beach species on an annual or seasonal scale was not much greater than the distance spanned by cross-shore movement on that time scale for most of the species where information was available. The estimated annual envelope ranged from 35 m for E. mucronata to 80 m for a congener, encompassing 44% of the beach width for a clam (Donax) to 72% for a talitrid amphipod and a polychaete (Glycera sp.) Overall, the average annual envelope was similar to the average annual movement reported, 52% vs. 61% of the beach width. Wildlife, such as birds, sea turtles, marine mammals as well as fish, may also respond to beach dynamics seasonally and over longer time scales, particularly species that nest on beaches. For example, the nest locations of the endangered Western Snowy Plover, Charadrius nivosus nivosus, exhibited a seasonal shift in average nest position (n = 400 nests with GPS coordinates) moving seaward an average of 22 m between March and July 2004 along two central California beaches (Table 2c, Dugan, unpublished results). Nest loss from the effects of high tides and wave runup also tended to be higher in the spring than later in the nesting season at this site (Dugan, unpublished results). This type of analysis on nest position and wave runup dynamics could provide valuable conservation and habitat information for vulnerable and threatened beach nesting wildlife, including plovers, oystercatchers and other bird species and sea turtles (see Schlacher et al.,

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in press for list), as well as beach nesting fish, such as the California grunion, Leuresthes tenuis (Smyder and Martin, 2002). Although relatively little information is available, the effects of coastal dynamics operating over longer time scales on the ecological envelopes used by beach animals needs to be considered. Episodic phenomena, such as ENSO events, hurricanes, and storms, can dramatically erode or rotate beaches, and cause migration of barrier beaches, altering the amount of habitat available for beach animals to use in response to daily, semi-lunar and seasonal coastal processes and potentially excluding biota by eliminating intertidal habitat and resources (e.g. Slott et al., 2006; Revell et al., 2011). One example of a multi-decadal shift in the availability of an ecological envelope is provided by a beach nesting fish, the California grunion. These fish lay eggs to incubate in the vicinity of the spring high tide strand line (HTS) a few days after the full moon (Martin et al., 2004). Grunion nested regularly on a section of West Isla Vista beach in the 1970s (Straughan, 1982) but no longer nest at that location because of erosion and the resulting loss of the upper beach zone above the HTS (Schooler et al., unpublished results). On the other hand, climatic phenomena that cause beach widths to increase, such as the cold phase of the Pacific Decadal Oscillation (Revell and Griggs, 2006) or overwash fans caused by storms (Schupp et al., 2013), could potentially allow biota, including plants, invertebrates, and wildlife (e.g. shorebirds, nesting plovers and sea turtles), to recover and recolonize beaches. A potential application of our results is the use of ecological envelopes in developing a more mechanistic understanding of the impacts of coastal armoring and of sea level rise on sandy beach ecosystems. We suggest that these impacts could be expected to occur where the scope of intertidal animal movement and resulting ecological envelopes are restricted. These restrictions could include 1) the loss of habitat associated with coastal armoring structures, including placement loss and passive erosion, 2) habitat loss from rising sea levels and increased erosion associated with global climate change and 3) a combination of the preceding that increases the interaction of existing armoring structures with waves and tides further reducing habitat. An illustration of the restriction imposed by a seawall on tidal movement and the ecological envelope of a mid intertidal cirolanid isopod, Excirolana chiltoni, was evident in the results of Klapow (1972) (Fig. 4). This time series shows how a population of these mobile mid intertidal animals “hits the wall” on spring tides increasing their effective density and potential for negative biotic interactions and physical stress (Fig. 4). Evidence is accumulating on the ecological effects of reduced cross-shore habitat and ecological zones associated with coastal armoring on open coast beaches. Significant impacts of armoring have been reported for upper, mid and lower shore beach biota (Dugan et al., 2008; Jaramillo et al., 2012), ghost crabs, (Barros, 2001; Lucrezi et al., 2009), coastal birds (Dugan and Hubbard, 2006; Dugan et al., 2008), and sea turtles (Rizkalla and Savage, 2011). Upper shore biota and wildlife that depend on upper beach zones for nesting appear to be particularly vulnerable to a loss of cross-shore habitat and the resulting restriction of ecological envelopes associated with armoring. 3.3. Results of the field study The cross-shore positions of the shoreline features (WTO, HTS, berm crest) and of the two intertidal invertebrates observed at our west Isla Vista study site all changed frequently, as illustrated by the time series (Fig. 5ab). These changes in position covaried significantly among features and animals as they responded to tides, waves, and beach accretion and erosion during our one year study period. The distributions of the animals and shore features we measured exhibited distinct spring-neap tide variability year round, and responded to wave and rain events (Fig. 5ab). The total intertidal width of the Megalorchestia spp. beds increased with increasing

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Fig. 4. Semi-lunar cross-shore movement of the mid intertidal isopod, Excirolana chiltoni, on an armored beach in San Diego, California during summer 1970. On spring tides, especially in June and early July, the wave wash interacted with the seawall causing truncation of the upper distribution of the isopods as this upper edge literally hit the wall during cross-shore movement (e.g., 17, 19, 21, and 29 June). As the beach accreted after mid-July, the effect of the seawall on the distribution of the isopods was muted (redrawn from Klapow, 1972).

a

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Fig. 5. Locations of shoreline features and the distributions of two characteristic intertidal macroinvertebrates and coastal strand vegetation (expressed as distance in meters from the bluff) on west Isla Vista beach from June 2011 to June 2012. (a) Upper and lower edges of talitrid amphipod beds (Megalorchestia spp.) and (b) Upper and lower edges of mid intertidal polychaete beds (Euzonus mucronata) and the seaward limit of coastal strand vegetation. Note the bluff is located at zero on the vertical axis.

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maximum 24 h tide height (r2 = 0.449, p b 0.001, n = 185) and the upper edges of the beds showed greater landward movement than the lower edges during spring tides (see Table 2). Average movement of the upper edge of Megalorchestia beds was 2.3 m per day (lower edge 1.4 m) but large surf events could cause the distribution of burrows to shift up to 20 m landward in a single day (for example September 29 and November 12 in Fig. 5a). Following a large surf event, a lag time of 2–4 days was required for the beds to recover to a pre-event position. Large landward shifts in Megalorchestia spp. distributions of up to 20 m were also observed after rainfall events that increased sand moisture, resulting in a wide scatter of burrows above the HTS (For example December 12 in Fig. 5a). The maximum seaward movement of Megalorchestia beds observed was 10 m per day during our one year study. For beds of the mid intertidal zone polychaete, Euzonus, the width was weakly correlated with the maximum 24 h tide (r2 = 0.02, p > 0.05, n = 98). Lower edges of Euzonus beds tracked the movement of the WTO, and the upper and lower edges of the beds moved at similar rates (upper 1.1 m d−1, lower 1.0 m d−1). The distributions of the shore features and intertidal animals measured on Isla Vista beach exhibited similar seasonal patterns, reaching seaward maxima in late summer then retreating landward in the late spring (Figs. 5ab, 6ab). The two photos taken from a standard point in August 2011 (Fig. 6a) and in June 2012 (Fig. 6b) illustrate the large changes in the locations of shore features, such as the HTS, and the width of the upper beach zone observed in our study. In particular, note the absence of dry sand and an upper beach zone in June when the HTS had retreated to the toe of the bluff. The narrow zone of coastal strand vegetation, located just seaward of the bluff, responded differently, with maxima and minima in the spring and summer (Fig. 5a). The position of the narrow band of coastal strand vegetation responded strongly to seasonal forcing, including winter rainfall and the spring erosion associated with short period wind swell. Rainfall in the fall and winter stimulated a combination of germination,

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recruitment and growth of coastal strand vegetation that expanded seaward from a starting position of 2 m from the bluff toe (June 2011) to maximum of 8 m from the bluff toe in early May 2012. The strong spring erosion signal, typical of this beach (Hubbard and Dugan, 2003; Barnard et al., 2012), rapidly caused the vegetation zone to retreat landward to 1 m of the bluff toe by early June (Fig. 5b) and eliminated vegetation from two of the five transects by 4 June 2012. These results indicate the seaward edge of coastal vegetation was strongly controlled by the extreme high water (TWL) of the year in agreement with the projected vulnerability of coastal strand vegetation to erosion and climate change impacts highlighted by Feagin et al. (2005). The movement and positions of the shoreline features we measured (HTS, WTO and berm crest) were affected differently by forcing factors of erosion, accretion, waves and tides. At the temporal scale of our surveys, the observed cross-shore movement of the HTS was greater than the WTO, which was greater than the berm crest. All of these features showed greater movement across the beach profile than did the position of the seaward edge of coastal strand vegetation (Fig. 5ab). Seasonal beach erosion and accretion (estimated by the 30 d running mean HTS position) were associated with 55% of variance in daily HTS position, and 67% of the variance in the position of the WTO. The semi-lunar tide signal (estimated as maximum 24 h tide excursion on 4 degree slope) was associated with 23% of variance in HTS position, and only 4% of the variance in the position of the WTO. Wave energy accounted for only 5% of the variance in the position of the HTS and 3% of the variance in the position of the WTO on West Isla Vista beach during our study. The majority of variation we observed in the position of the HTS (interpolated to daily values) was explained by the combined effects of the position of the berm crest and the semi-lunar tide signal (r2 = 0.70, p b 0.001, n = 366). The upper limit of talitrid amphipod, Megalorchestia spp., burrows was generally located near the HTS (mean distance above HTS =0.6 m ± 2.8 m) (Fig. 5a) and was strongly correlated with the position of the

Fig. 6. Photographs of the west Isla Vista beach study site taken from a standard point (a) near the seasonal maximum width of upper beach zone on August 29, 2011 and (b) near the seasonal minimum width of the upper beach zone on June 8, 2012. The location of the HTS is indicated by the black arrow on each image. These photos correspond to day 241 and day 524, respectively, in Fig. 5a and b.

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Fig. 7. Examples of ecological envelopes (gray shaded areas) for two characteristic intertidal macroinvertebrates, upper intertidal talitrid amphipods, Megalorchestia spp. (closed circles) and a mid intertidal polychaete, Euzonus mucronata (open circles) on west Isla Vista beach between March and August 2012. These envelopes were estimated by plotting the locations of the upper and lower edges of the beds of these invertebrates as a function of elevation above NAVD88 and distance from the bluff toe for five survey dates that spanned spring to summer extremes in beach profiles in 2012. The beach profiles for June and August 2012 are indicated by dotted lines and represent the extremes in profile observed in the five surveys. The elevations of the datums, Mean High Water (MHW) and Mean Sea Level (MSL), are indicated by arrows on the vertical axis.

HTS of the previous 24 h (r2 = 0.879, p b 0.001, n =188). The lower limit of E. mucronata beds was generally slightly landward of the WTO during low and mid-tide surveys (mean distance above = 2.5 m ± 2.6 m) (Fig. 5b) and tracked movements of the WTO (r2 = 0.933, p b 0.001, n = 99) closely. These strong correlations between the locations of beds of characteristic upper and mid intertidal biota and beach features suggest that it may be possible to include these types of ecological distributions in models and analyses of shoreline dynamics and evolution. These integrative models could generate predictions applicable to the conservation, management of sandy beach ecosystems. Estimated ecological envelopes, including the elevations, of Megalorchestia and Euzonus on west Isla Vista beach from late winter to summer are illustrated in Fig. 7. The shaded ellipse sketched around the cluster of points for each species depicts the cross-shore envelope used during that period as well as shifts in elevation. The envelope for the upper intertidal talitrid amphipod spanned 36 m across the beach and 1.4 m in elevation whereas that of the mid-intertidal polychaete spanned 28 m and 0.5 m in elevation. Average widths of the distribution of these species on any single survey date were 8.1 m for the amphipod and 2.9 m for the polychaete (~22% and ~ 7% of the respective cross-shore envelopes). The greater range of elevation used by the upper intertidal amphipod relative to the mid intertidal polychaete may be related to differences in mobility and behavior as well as in the widths of their respective distributions. The winter to summer envelopes, depicted in Fig. 7, are 6 to 7 m less than those estimated for the dataset from the full year (Table 2c, Fig. 5ab). In agreement with Clark (1969; Fig. 3), these results indicate that the tidal datums of mean sea level (MSL) and mean high water (MHW), were located well below the ecological envelope of the upper intertidal talitrid amphipods. In addition, MHW coincided with the lowest elevation of the mid intertidal polychaete, Euzonus, on only one survey date. 4. Conclusions The intertidal zones inhabited by major groups of ecologically distinct mobile animals did not coincide with standard shoreline datums, such as MSL and MHW, even in static low tide surveys of beaches. Our results suggest, however, that total water level (TWL) may be a useful datum for predicting the distributions of biota that respond strongly to the dynamics of the high tide strand (HTS), a beach feature which tracks TWL, including daily and semi-lunar variation and extreme events. In particular, 1) the invertebrates typical of upper beach zones (ghost crabs, beetles, talitrid amphipods and isopods), 2) the lower limits of coastal strand vegetation and 3) the habitat required by beach nesting wildlife (sea turtles, shorebirds and

fish) appear to be most amenable for inclusion in geomorphic models of shoreline dynamics that use TWL. This approach could provide new insights and tools needed to conserve a number of ecosystem values and functions of sandy beach ecosystems. As we have described here, the responses of mobile sandy beach animals to tides, swell events, erosion/accretion episodes and other disturbances result in rapid large changes in the cross-shore positions of these animals and their respective intertidal distributions and zones. Beach dynamics and coastal processes can also result in rapid changes in the alongshore distributions of mobile beach biota which likely interact with the cross-shore envelopes we have described. Although intertidal animal distributions responded to changes in beach profile driven by physical processes, our field observations demonstrate they can also respond strongly to processes that do not alter the beach profile, including tidal and semi-lunar periodicity. Our results on cross-shore movements suggest that many mobile beach fauna use a proportionally broad envelope of available sandy beach habitat on even a tidal or daily basis. The size of these envelopes increases with time. On an annual basis, ecological envelopes of intertidal biota averaged > 60% of the maximum beach width. Over multiyear time scales we expect these ecological envelopes to span a greater proportion of the beach width. The large scope of annual ecological envelopes relative to the beach width reflects how intertidal animals can escape seasonally extreme or episodically harsh conditions by moving across the beach face. The availability of sufficient sandy habitat for mobile animals to move to and occupy in response to storms, rapid erosion, winter conditions and coastal evolution over time may be critical to their survival on a beach. As sandy shorelines retreat, the bounds of these ecological envelopes will move inland as well. Where and when these ecological envelopes are constrained because of topography or coastal armoring, we predict that biota will be lost from the ecosystem as has been shown on armored beaches (e.g. Dugan et al., 2008, 2012; Jaramillo et al., 2012). How the ecological envelopes of different biota interact with erosion, SLR, coastal armoring and altered sediment supply will play a role in determining the biodiversity, function and wildlife support provided by beach ecosystems. Our results suggest that conservation of beaches as ecosystems could be enhanced by incorporating sufficient beach habitat to accommodate the dynamic ecological envelopes used by mobile invertebrates and wildlife. Acknowledgments The ideas for this manuscript benefited greatly from discussions with D. Revell and P. Barnard. We thank N. Jackson and K. Nordstrom for their

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help and insightful suggestions for improving earlier drafts and N. Schooler for his assistance with figure preparation. We gratefully acknowledge M. Clark for his dataset on California beaches and A. Simms for the use of his total station for collecting elevation data. This research was supported by funding from the California Sea Grant Program Project # R/MPA-24B (Grant 10-049) sponsored by the California Ocean Protection Council and (2) the Santa Barbara Coastal LTER funded by the National Science Foundation (award no. OCE-0620276). References Ansell, A.D., Silvadas, P., Narayan, B., Sankaranarayanan, V.N., Trevallion, A., 1972. The ecology of two sandy beaches in south west India. I. Seasonal changes in physical and chemical factors, and in the macrofauna. Marine Biology 17, 38–62. Barnard, P.L., Hubbard, D.M., Dugan, J.D., 2012. Beach response dynamics of a littoral cell using a 17-year single-point time series of sand thickness. Geomorphology 139–140, 588–598. Barrass, R., 1963. 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