Estimating latitudinal variability in extreme heat stress on rocky intertidal shores

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<ul><li><p>ORIGINALARTICLE</p><p>Estimating latitudinal variabilityin extreme heat stress on rockyintertidal shoresJustin A. Lathlean*, David J. Ayre and Todd E. Minchinton</p><p>Institute for Conservation Biology and</p><p>Environmental Management &amp; School of</p><p>Biological Sciences, University of Wollongong,</p><p>Wollongong NSW 2522, Australia</p><p>*Correspondence: Justin A. Lathlean, Institute</p><p>for Conservation Biology and Environmental</p><p>Management &amp; School of Biological Sciences,</p><p>University of Wollongong, NSW 2522,</p><p>Australia</p><p>E-mail: jlathlean@gmail.com</p><p>ABSTRACT</p><p>Aim Broad-scale patterns of heat stress play an important role in shaping the</p><p>geographical distributions of many species and may differ from large-scale</p><p>changes in average temperatures. For species living on rocky intertidal shores</p><p>extreme heat stress occurs when hot dry aerial conditions coincide with midday</p><p>low tides. We used empirical and modelled temperature data, and estimates of</p><p>cumulative aerial exposure and solar radiation, in order to test the hypothesis</p><p>that heat stress on Australian rocky intertidal shores decreases with increasing</p><p>latitude.</p><p>Location Rocky intertidal shores of south-eastern Australia spanning</p><p>&gt; 1500 km and 13 of latitude (2624023 S to 3907047 S).</p><p>Methods In situ temperature measurements, hourly tidal elevations and daily</p><p>solar radiation taken over three consecutive summers (December 2009Febru-</p><p>ary 2012) were used to quantify latitudinal variability in extreme heat stress,</p><p>cumulative aerial exposure and solar radiation, respectively. Comparisons</p><p>between hourly in situ temperatures and meteorological data were used to pro-</p><p>duce a large-scale statistical model capable of estimating intertidal substratum</p><p>temperatures during daytime low-tides, which was then extrapolated across 22</p><p>locations.</p><p>Results Heat stress estimated using in situ loggers deployed across five east</p><p>coast locations typically did not decline with increasing latitude and neither</p><p>did midday exposure or solar radiation. The meteorological model proved to</p><p>be a successful method for estimating rocky shore heat stress and in contrast</p><p>to the empirical data displayed strong latitudinal trends in mean daily maxima</p><p>and cumulative heat stress. Modelled acute heat stress (i.e. summer maxima),</p><p>however, did not decline with increasing latitude, as there was greater thermal</p><p>variability at higher latitudes.</p><p>Main conclusions The meteorological model developed in this study repre-</p><p>sents a useful approach for estimating broad-scale patterns of heat stress on</p><p>rocky intertidal shores. Results also indicate that latitudinal patterns of acute</p><p>and chronic heat stress may differ from average temperatures, which are com-</p><p>monly assumed to decline with increasing latitude. Such broad-scale patterns</p><p>of thermal stress as described in this study will significantly contribute to our</p><p>ability to understand the impact of climate change on vulnerable rocky inter-</p><p>tidal communities.</p><p>Keywords</p><p>Biogeography, climate change, climatology, ecological forecasting, heat stress,</p><p>intertidal zone, south-eastern Australia, temperature.</p><p>1478 http://wileyonlinelibrary.com/journal/jbi 2014 John Wiley &amp; Sons Ltddoi:10.1111/jbi.12311</p><p>Journal of Biogeography (J. Biogeogr.) (2014) 41, 14781491</p></li><li><p>INTRODUCTION</p><p>Temperature has long been recognized as a major factor influ-</p><p>encing broad-scale patterns of distribution and abundance</p><p>(Shelford, 1911; Grinnell, 1917; Orton, 1920) because it affects</p><p>virtually all physiological processes (Helmuth, 2009).</p><p>Although ecologists have typically been concerned with quan-</p><p>tifying large-scale changes in average temperatures and the</p><p>associated effects on biological communities, more recent</p><p>attention has been given to understanding whether infrequent</p><p>extreme heat events produce longer lasting impacts than con-</p><p>tinual gradual change (Gaines &amp; Denny, 1993; Denny et al.,</p><p>2009; Harley &amp; Paine, 2009; Wethey et al., 2011b). This ques-</p><p>tion has become increasingly important, as global climate</p><p>change is expected to increase the number and frequency of</p><p>extreme weather events, reducing the time ecosystems will</p><p>have to recover from such disturbances (Bertness et al., 2002).</p><p>Despite this, very few studies have attempted to quantify</p><p>broad-scale patterns of extreme heat stress to determine the</p><p>relative importance of local and regional influences in struc-</p><p>turing geographical patterns of temperature variation.</p><p>Rocky intertidal ecosystems have emerged as excellent</p><p>study systems in which to investigate the effects of extreme</p><p>heat events on biological populations and communities (Hel-</p><p>muth et al., 2006b; Denny et al., 2009; Harley &amp; Paine, 2009;</p><p>Wethey et al., 2011b; Lathlean et al., 2012). Living at the</p><p>interface between the marine and terrestrial environment,</p><p>rocky intertidal organisms experience dramatic fluctuations</p><p>in daily temperature, at times in excess of 30 C in less than12 h (Firth &amp; Williams, 2009; Lathlean et al., 2011). These</p><p>daily fluctuations in temperature are primarily driven by the</p><p>tidal cycle (Helmuth et al., 2006b). During aerial exposure</p><p>an organisms body temperature will be the product of sev-</p><p>eral climatic and non-climatic factors, including most nota-</p><p>bly solar elevation and intensity, air temperature, wind</p><p>speed, wave height and humidity (Helmuth et al., 2011).</p><p>These factors can vary over biogeographical scales to produce</p><p>complex spatial and temporal patterns in thermal variability.</p><p>For example, Helmuth et al. (2002, 2006a) demonstrated</p><p>along the west coast of the USA that, as a result of spatial</p><p>variability in the number of midday low tides in summer,</p><p>body temperatures of the mussel Mytilus californianus were</p><p>unexpectedly higher in cooler northern locations than in war-</p><p>mer southern locations. Likewise, using only variability in tidal</p><p>patterns and solar elevation, Mislan et al. (2009) produced a</p><p>simple model which predicted that thermal stress on rocky</p><p>intertidal shores along the west coast of the USA would vary</p><p>independently of latitude in the coming decade. Such studies</p><p>contradict the widely reported inverse relationship between tem-</p><p>perature and latitude (Sorte &amp; Hofmann, 2004; Schoch et al.,</p><p>2006; Jones et al., 2010). They also challenge the common bio-</p><p>geographical assumption that across a species distribution indi-</p><p>viduals situated at higher latitudes will experience lower</p><p>temperatures and reduced heat stress (Sagarin &amp; Gaines, 2002).</p><p>Sea-surface temperatures along the east coast of Australia</p><p>typically decline with increasing latitude due to the weaken-</p><p>ing East Australia Current (EAC) as it flows from the Coral</p><p>Sea in Queensland to the Tasman Sea in New South Wales</p><p>and Victoria (Lough, 2009). It remains unclear, however,</p><p>whether such a latitudinal gradient in sea temperatures man-</p><p>ifests on rocky intertidal seashores. Indeed, preliminary</p><p>research within this region reveals that rocky intertidal shores</p><p>separated by approximately 400 km can experience equiva-</p><p>lent maximum temperatures during low tide (Lathlean et al.,</p><p>2011). Such geographical differences in extreme heat stress</p><p>may be a result of variability in the number of midday low</p><p>tides and solar radiation (Mislan et al., 2009). If so, the</p><p>future response of rocky intertidal communities in this</p><p>region to climate change might be substantially different</p><p>from that of their subtidal counterparts.</p><p>Data from meteorological stations located close to rocky</p><p>shores are often used to predict the thermal variability experi-</p><p>enced by rocky intertidal organisms (Helmuth et al., 2011; Mi-</p><p>slan &amp; Wethey, 2011; Wethey et al., 2011a; Denny &amp; Dowd,</p><p>2012). However, our research in south-eastern Australia has</p><p>shown that simply using air temperatures recorded by coastal</p><p>weather stations is a poor predictor of extreme thermal stress</p><p>on rocky intertidal shores (Lathlean et al., 2011). More accurate</p><p>predictions generally incorporate several additional parameters</p><p>such as relative humidity, wind speed, precipitation and solar</p><p>radiation (see Wethey et al., 2011a). Such meteorological data</p><p>are often readily available because they are used for weather</p><p>forecasting, and this is particularly true along the south-eastern</p><p>coast of Australia where hundreds of weather stations are situ-</p><p>ated within 5 km of the coast and have been in operation for</p><p>more than 20 years (Australian Bureau of Meteorology, 2012).</p><p>If shown to be an effective surrogate, such meteorological data</p><p>could be used to estimate large-scale thermal variability of rocky</p><p>intertidal shores along south-eastern Australia and identify</p><p>rocky shore communities vulnerable to climate change.</p><p>The aim of this study was to use a combination of empirical</p><p>and modelled temperature data, along with estimates of midday</p><p>exposure and solar radiation, to quantify latitudinal variability in</p><p>heat stress on rocky intertidal shores of south-eastern Australia.</p><p>First, we analysed in situ temperature measurements for five</p><p>locations spanning more than 1200 km of coastline, in order to</p><p>test the hypothesis that rocky intertidal heat stress decreases with</p><p>increasing latitude. Second, we quantified temporal and latitudi-</p><p>nal variability in midday exposure and solar radiation to test</p><p>whether patterns of heat stress reflect the coincidence of high lev-</p><p>els of solar radiation during midday low tides. We then tested</p><p>the capacity of a meteorological model to predict rocky intertidal</p><p>heat stress and extrapolate to other sites along the shore.</p><p>MATERIALS AND METHODS</p><p>Quantifying latitudinal patterns of heat stress</p><p>In situ temperature measurements</p><p>To assess latitudinal variability in rocky intertidal heat stress,</p><p>two TidbiT v2 Temp data loggers (Onset Stowaway logger,</p><p>Journal of Biogeography 41, 14781491 2014 John Wiley &amp; Sons Ltd</p><p>1479</p><p>Latitudinal variability in heat stress</p></li><li><p>model UTBI-001, accuracy 0.2 C) were deployed 10 to15 m apart within the mid-intertidal zone (0.81 m above</p><p>mean low water mark) across five locations spanning</p><p>1200 km and 9 of latitude: Cape Byron (2838012 S,15338023 E) (site no. 10), Port Macquarie (3127055 S,15256018 E) (site no. 20), Garie Beach (3411019 S,15103059 E) (site no. 27), Bermagui (3625022 S,15004056 E) (site no. 36), and Mallacoota (3734026 S,14945056 E) (site no. 42; Fig. 1). These five locations arelinearly arrayed along the east coast of Australia and were</p><p>selected as a test for latitudinal variation. Loggers were also</p><p>deployed across two additional locations in southern Victo-</p><p>ria: Cape Paterson (3840028 S, 14538000 E) (site no. 51)and Kilcunda (3833003 S, 14528008 E) (site no. 52).These were included to examine differences in heat stress</p><p>along a different coastline at approximately the same latitude</p><p>(Fig. 1). From November 2009 to March 2012 each logger</p><p>continuously recorded ambient temperatures at 10-min</p><p>intervals and was attached to horizontal emergent rocky sub-</p><p>strata to reduce temperature variability caused by small-scale</p><p>topographic heterogeneity (Harley, 2008; Denny et al., 2011).</p><p>Consequently, temperatures recorded by these data loggers</p><p>most likely reflect that of the underlying rocky substratum</p><p>(hereafter referred to as substratum temperature). Whilst</p><p>these substratum temperatures may not directly represent the</p><p>internal body temperatures of any specific species, they can</p><p>broadly be seen as an approximate estimate for numerous</p><p>species that are usually in direct contact with the primary</p><p>substrata (e.g. barnacles, limpets, chitons, whelks). Our pre-</p><p>vious comparisons using both biomimetic loggers and infra-</p><p>red imagery confirm that substratum temperatures, measured</p><p>by unmodified Tidbit loggers, significantly influence the</p><p>body temperatures of a range of both adult invertebrates and</p><p>early settlers (see Lathlean et al., 2012, 2013). Substratum</p><p>temperatures recorded by the two loggers at each location</p><p>were shown to typically vary by less than 1 C and weretherefore averaged to produce a single value for each 10-min</p><p>interval.</p><p>We focused our analyses on summer months (December</p><p>to February) because they constitute the period with the</p><p>highest probability of rocky intertidal shores in this region</p><p>experiencing extreme heat events (Lathlean et al., 2011).</p><p>Therefore, characterization of latitudinal substratum temper-</p><p>ature variability consisted of equivalent sampling intervals</p><p>over 3 years: (1) 1 December 2009 to 28 February 2010; (2)</p><p>1 December 2010 to 28 February 2011; and (3) 1 December</p><p>2011 to 28 February 2012. The 29 February 2012 was</p><p>excluded from analyses to standardize the number of days</p><p>across the three sampling intervals. For each of these inter-</p><p>vals, we compared acute heat stress (i.e. the absolute maxi-</p><p>mum substratum temperature), chronic heat stress (i.e. mean</p><p>daily maximum substratum temperature) and cumulative</p><p>heat stress at each location. Cumulative heat stress was</p><p>defined as the number of hours spent above 30 C or 35 C.The selection of these temperature thresholds was somewhat</p><p>arbitrary because no information exists on the physiological</p><p>tolerances of rocky intertidal organisms in south-eastern</p><p>Australia. Nevertheless, previous studies undertaken at simi-</p><p>lar latitudes in the Northern Hemisphere have shown such</p><p>temperatures to represent thermally stressful conditions for a</p><p>range of intertidal species (Helmuth, 1998; Denny et al.,</p><p>2006). Owing to damaged or malfunctioning loggers no data</p><p>could be acquired for Bermagui during the summer of 2009</p><p>and 2010 or for Mallacoota during the summer of 2011 and</p><p>2012. For each of the three separate sampling intervals Spear-</p><p>man rank correlations were used to test the hypothesis that</p><p>rocky intertidal heat stress decreases with increasing latitude.</p><p>Cumulative aerial exposure and solar radiation</p><p>With the seven locations being spread across a broad geo-</p><p>graphical region (&gt; 1500 km), we expected temperature dif-ferences among rocky intertidal shores to reflect underlying</p><p>latitudinal patterns in the tidal cycle and or levels of solar</p><p>radiation (Helmuth et al., 2002). To separately test whether</p><p>aerial exposure of low intertidal regions and solar radiation</p><p>vary with latitude we used tidal data from 15 tidal gauges</p><p>and daily solar radiation data from 17 weather stations rang-</p><p>Figure 1 Map of the study region in south-eastern Australia,illustrating the location of numerous temperature data loggers,</p><p>weather stations and tidal gauges used to characterizebiogeographical variability in rocky intertidal temperatures. See</p><p>Appendix S1 for the name of the location associated with eachnumber.</p><p>Journal of Biogeography 41, 14781491 2014 John Wiley &amp; Sons Ltd</p><p>1480</p><p>J. A. Lathlean et al.</p></li><li><p>ing from Noosa Heads in southern Queensland to Point</p><p>Lonsdale in southern Victoria (Fig. 1, and see Appendix S1</p><p>in Supporting Information). Previous studies have primarily</p><p>focused on characterizing midday exposure (Helmuth et al.,</p><p>2002; Finke et al., 2007; Mislan et al., 2009) as this is consid-</p><p>ered the most thermally stressful time of day. Whilst preli-</p><p>minary analyses confirmed in situ temperatures were</p><p>typically greater during the middle of the day we character-</p><p>ized latitudinal variability in aerial exposure for three distinc...</p></li></ul>

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