estimating latitudinal variability in extreme heat stress on rocky intertidal shores

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    Estimating latitudinal variabilityin extreme heat stress on rockyintertidal shoresJustin A. Lathlean*, David J. Ayre and Todd E. Minchinton

    Institute for Conservation Biology and

    Environmental Management & School of

    Biological Sciences, University of Wollongong,

    Wollongong NSW 2522, Australia

    *Correspondence: Justin A. Lathlean, Institute

    for Conservation Biology and Environmental

    Management & School of Biological Sciences,

    University of Wollongong, NSW 2522,




    Aim Broad-scale patterns of heat stress play an important role in shaping the

    geographical distributions of many species and may differ from large-scale

    changes in average temperatures. For species living on rocky intertidal shores

    extreme heat stress occurs when hot dry aerial conditions coincide with midday

    low tides. We used empirical and modelled temperature data, and estimates of

    cumulative aerial exposure and solar radiation, in order to test the hypothesis

    that heat stress on Australian rocky intertidal shores decreases with increasing


    Location Rocky intertidal shores of south-eastern Australia spanning

    > 1500 km and 13 of latitude (2624023 S to 3907047 S).

    Methods In situ temperature measurements, hourly tidal elevations and daily

    solar radiation taken over three consecutive summers (December 2009Febru-

    ary 2012) were used to quantify latitudinal variability in extreme heat stress,

    cumulative aerial exposure and solar radiation, respectively. Comparisons

    between hourly in situ temperatures and meteorological data were used to pro-

    duce a large-scale statistical model capable of estimating intertidal substratum

    temperatures during daytime low-tides, which was then extrapolated across 22


    Results Heat stress estimated using in situ loggers deployed across five east

    coast locations typically did not decline with increasing latitude and neither

    did midday exposure or solar radiation. The meteorological model proved to

    be a successful method for estimating rocky shore heat stress and in contrast

    to the empirical data displayed strong latitudinal trends in mean daily maxima

    and cumulative heat stress. Modelled acute heat stress (i.e. summer maxima),

    however, did not decline with increasing latitude, as there was greater thermal

    variability at higher latitudes.

    Main conclusions The meteorological model developed in this study repre-

    sents a useful approach for estimating broad-scale patterns of heat stress on

    rocky intertidal shores. Results also indicate that latitudinal patterns of acute

    and chronic heat stress may differ from average temperatures, which are com-

    monly assumed to decline with increasing latitude. Such broad-scale patterns

    of thermal stress as described in this study will significantly contribute to our

    ability to understand the impact of climate change on vulnerable rocky inter-

    tidal communities.


    Biogeography, climate change, climatology, ecological forecasting, heat stress,

    intertidal zone, south-eastern Australia, temperature.

    1478 2014 John Wiley & Sons Ltddoi:10.1111/jbi.12311

    Journal of Biogeography (J. Biogeogr.) (2014) 41, 14781491


    Temperature has long been recognized as a major factor influ-

    encing broad-scale patterns of distribution and abundance

    (Shelford, 1911; Grinnell, 1917; Orton, 1920) because it affects

    virtually all physiological processes (Helmuth, 2009).

    Although ecologists have typically been concerned with quan-

    tifying large-scale changes in average temperatures and the

    associated effects on biological communities, more recent

    attention has been given to understanding whether infrequent

    extreme heat events produce longer lasting impacts than con-

    tinual gradual change (Gaines & Denny, 1993; Denny et al.,

    2009; Harley & Paine, 2009; Wethey et al., 2011b). This ques-

    tion has become increasingly important, as global climate

    change is expected to increase the number and frequency of

    extreme weather events, reducing the time ecosystems will

    have to recover from such disturbances (Bertness et al., 2002).

    Despite this, very few studies have attempted to quantify

    broad-scale patterns of extreme heat stress to determine the

    relative importance of local and regional influences in struc-

    turing geographical patterns of temperature variation.

    Rocky intertidal ecosystems have emerged as excellent

    study systems in which to investigate the effects of extreme

    heat events on biological populations and communities (Hel-

    muth et al., 2006b; Denny et al., 2009; Harley & Paine, 2009;

    Wethey et al., 2011b; Lathlean et al., 2012). Living at the

    interface between the marine and terrestrial environment,

    rocky intertidal organisms experience dramatic fluctuations

    in daily temperature, at times in excess of 30 C in less than12 h (Firth & Williams, 2009; Lathlean et al., 2011). These

    daily fluctuations in temperature are primarily driven by the

    tidal cycle (Helmuth et al., 2006b). During aerial exposure

    an organisms body temperature will be the product of sev-

    eral climatic and non-climatic factors, including most nota-

    bly solar elevation and intensity, air temperature, wind

    speed, wave height and humidity (Helmuth et al., 2011).

    These factors can vary over biogeographical scales to produce

    complex spatial and temporal patterns in thermal variability.

    For example, Helmuth et al. (2002, 2006a) demonstrated

    along the west coast of the USA that, as a result of spatial

    variability in the number of midday low tides in summer,

    body temperatures of the mussel Mytilus californianus were

    unexpectedly higher in cooler northern locations than in war-

    mer southern locations. Likewise, using only variability in tidal

    patterns and solar elevation, Mislan et al. (2009) produced a

    simple model which predicted that thermal stress on rocky

    intertidal shores along the west coast of the USA would vary

    independently of latitude in the coming decade. Such studies

    contradict the widely reported inverse relationship between tem-

    perature and latitude (Sorte & Hofmann, 2004; Schoch et al.,

    2006; Jones et al., 2010). They also challenge the common bio-

    geographical assumption that across a species distribution indi-

    viduals situated at higher latitudes will experience lower

    temperatures and reduced heat stress (Sagarin & Gaines, 2002).

    Sea-surface temperatures along the east coast of Australia

    typically decline with increasing latitude due to the weaken-

    ing East Australia Current (EAC) as it flows from the Coral

    Sea in Queensland to the Tasman Sea in New South Wales

    and Victoria (Lough, 2009). It remains unclear, however,

    whether such a latitudinal gradient in sea temperatures man-

    ifests on rocky intertidal seashores. Indeed, preliminary

    research within this region reveals that rocky intertidal shores

    separated by approximately 400 km can experience equiva-

    lent maximum temperatures during low tide (Lathlean et al.,

    2011). Such geographical differences in extreme heat stress

    may be a result of variability in the number of midday low

    tides and solar radiation (Mislan et al., 2009). If so, the

    future response of rocky intertidal communities in this

    region to climate change might be substantially different

    from that of their subtidal counterparts.

    Data from meteorological stations located close to rocky

    shores are often used to predict the thermal variability experi-

    enced by rocky intertidal organisms (Helmuth et al., 2011; Mi-

    slan & Wethey, 2011; Wethey et al., 2011a; Denny & Dowd,

    2012). However, our research in south-eastern Australia has

    shown that simply using air temperatures recorded by coastal

    weather stations is a poor predictor of extreme thermal stress

    on rocky intertidal shores (Lathlean et al., 2011). More accurate

    predictions generally incorporate several additional parameters

    such as relative humidity, wind speed, precipitation and solar

    radiation (see Wethey et al., 2011a). Such meteorological data

    are often readily available because they are used for weather

    forecasting, and this is particularly true along the south-eastern

    coast of Australia where hundreds of weather stations are situ-

    ated within 5 km of the coast and have been in operation for

    more than 20 years (Australian Bureau of Meteorology, 2012).

    If shown to be an effective surrogate, such meteorological data

    could be used to estimate large-scale thermal variability of rocky

    intertidal shores along south-eastern Australia and identify

    rocky shore communities vulnerable to climate change.

    The aim of this study was to use a combination of empirical

    and modelled temperature data, along with estimates of midday

    exposure and solar radiation, to quantify latitudinal variability in

    heat stress on rocky intertidal shores of south-eastern Australia.

    First, we analysed in situ temperature measurements for five

    locations spanning more than 1200 km of coastline, in order to

    test the hypothesis that rocky intertidal heat stress decreases with

    increasing latitude. Second, we quantified temporal and latitudi-

    nal vari


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