Year

2012

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

The rocky intertidal zone represents the interface between the marine and terrestrial environment and is considered one of the most thermally complex ecosystems on earth. Biologists have long understood that life within the intertidal zone is considerably influenced by a steep thermal gradient produced by the rising and falling tide. In recent years, however, studies have discovered that numerous small-scale processes, other than elevation in tidal height, can produce mosaic patterns of thermal stress along rocky intertidal shores. Linking this local temperature variability to the physiological and demographic patterns of intertidal invertebrates is an essential first step to understanding how climate change might be expected to influence these communities.

The early life history stages of many rocky intertidal invertebrates are particularly vulnerable to heat and desiccation stress and play a key role in structuring adult populations. Therefore, temperature variability may considerably influence important demographic processes within intertidal populations. Nevertheless, temperature measurements are often taken at large spatial scales (10 to 1000m2) while early life history processes are typically measured at much smaller scales, such as at the quadrat-level (10 to 100cm2). Processes that operate at these different spatial scales may produce different patterns of temperature variability and consequently limit our ability to relate an organism’s physiological response to its environment and ultimately the demography of the population.

Differences between body temperatures of sessile rocky intertidal invertebrates distributed across local spatial scales (100m) may be substantially influenced by the density and species composition of the surrounding biotic community. By contrast, differences between body temperatures of sessile rocky intertidal invertebrates distributed across large spatial scales (100 to 1000km) most likely reflect geographical variation in temperature. Yet, the characteristics of biotic communities have been shown to vary across both regional and large spatial scales. Thus, local-scale differences in the biotic community may counteract or negate temperature variability caused by larger-scale processes. Such might be the case along the south east coast of Australia where densities of dominant space occupiers vary with latitude.

My thesis aimed to examine how large- and small-scale temperature variability influences the early life history processes of the habitat forming rocky intertidal barnacle Tesseropora rosea and to characterise the thermal environment at a range of scales. For the large-scale aspect of this study I investigated whether early life history processes could explain the decline in adult abundances along a latitudinal temperature gradient. I then assessed the common use of satellites and terrestrial weather stations as proxies of in situ rocky intertidal water and air temperatures, respectively. This was in order to determine the most appropriate method for measuring rocky intertidal temperature variability. Due to the high variability detected by loggers within a single shore, I next used infrared (IR) imagery to characterise fine-scale temperature variability relevant to recently settled larvae. By undertaking a series of manipulative experiments I investigated how this fine-scale temperature variability influences the settlement and early post-settlement growth and survival of T. rosea.

The large-scale component of this study initially revealed that abundances of adult T. rosea decline with increasing latitude, suggesting that important demographic processes, such as settlement and early mortality (i.e. recruitment), may be particularly vulnerable to changes in sea surface temperatures. However, sampling across 11 rocky shores spanning 450km over a two year period did not reveal any latitudinal gradients in either the production, settlement or early post-settlement mortality of larvae even though in situ temperature measurements confirmed the existence of a latitudinal temperature gradient. Indeed, settlement and adult mortality were highly variable among locations and the original decline in adult abundance observed during February 2007 was no longer present in December 2008. These results indicate that local variation in early life-history processes and adult mortality dictate regional variability and observed latitudinal patterns of adult T. rosea abundance. Such local variation in early life-history processes and adult mortality may reflect high temperature variability at these localscales. Therefore, local temperature variability may be an important factor governing biogeographic patterns of abundance.

Large-scale settlement and recruitment studies commonly use remote sensing to characterise the thermal environment of recently settled larvae. The use of these temperature measurements have yet to be validated as useful surrogates of in situ temperature variability. Although I found that daily and monthly average temperatures derived from satellites and terrestrial weather stations were significantly correlated, the temperatures reported were considerably different from temperatures derived from in situ data loggers. Daily satellite sea surface temperatures (SSTs) were up to 6.7°C, and on average 1°C, higher than in situ water temperatures, while daily maximum air temperatures measured by weather stations were up to 23.2°C, and on average 4.2°C, lower than in situ air temperatures over a 14-month period. The frequency, duration and number of days greater than 30°C, as well as rates of temperature change, were all significantly lower when measured by weather stations. These differences suggest that satellite SSTs and weather stations are ineffective at capturing extremes in intertidal water and air temperature variability, which considerably influence biological processes. Therefore, to understand the impacts of temperature variability on populations at scales relevant to demographic processes we need to use emerging data logger and infrared technology.

Despite the fact that experimental manipulations are essential for determining causation, there are few studies that successfully manipulate temperatures on rocky intertidal shores to test for effects on early life history processes. I used different coloured settlement plates deployed within the mid shore region to experimentally alter the substratum temperatures experienced by newly settled T. rosea larvae. I found that maximum mean surface temperatures of black and grey plates were 5.8°C and 4.8°C warmer than white plates, respectively. Black and grey plates over the entire sampling period were on average 2.2°C and 1.6°C warmer than white plates, respectively. Importantly, cooler white plates had significantly greater settlement and early postsettlement growth of T. rosea than warmer black plates. However, temperature differences between black and white plates did not influence early survival and recruitment.

Substratum temperatures of unmodified areas within mid intertidal regions increased with increasing free space (r2=0.75, pT. rosea. Therefore, I undertook a manipulative thermal experiment that altered mid intertidal rock temperatures (which considerably influence body temperatures of recently settled T. rosea) by manipulating the amount of free space, a limiting resource for benthic invertebrates. Quadrats withT. rosea. Here, I found that natural temperature variability between quadrats with 100% free space (i.e. across areas that were unmanipulated) significantly influenced the early post-settlement growth and survival of T. rosea. Such among-quadrat variation at the same tidal height on shore is both underappreciated and a potentially confounding factor in studies of recruitment and population dynamics.

Finally, I used IR imagery to test the hypothesis that in situ rocky substrates exhibit repeatable ultra fine-scale (1mm) temperature variation during aerial exposure and that this variability significantly influences early life history processes of T. rosea. Here, larval settlement did not vary with ultra fine-scale variation in rock temperature, but early post-settlement growth and survival were both inversely related to temperature variability at this scale. Furthermore, I found that rock temperatures decreased significantly with increasing proximity to adult T. rosea and that larvae that settled within 15mm of adults survived better than those that settled within 16-30mm. This is partially explained by conspecific adults shading rock and reducing rock temperatures. These results demonstrate, for the first time, that fine-scale variation in thermal stress impacts the early-life history stages of a benthic marine invertebrate.

The results of my research have broad ranging implications for understanding how rocky intertidal invertebrates will respond to increasing temperatures and extreme events associated with climate change. Firstly, the geographic distribution of T. rosea does not appear to be limited by a reduction in air and seawater temperatures towards its southern range limit and therefore increasing temperatures may not cause T. rosea to extend its southern distribution. Secondly, broad-scale temperature measurements derived from weather stations and satellites are not the most appropriate estimates of rocky intertidal temperature variability. Therefore, climate change models and predictions should preferentially use local-scale, or better yet, estimates of temperature variability relevant to individuals. Thirdly, the thermal manipulations developed in this study may be particularly useful for simulating the effects of future temperature variability or extreme temperature events on intertidal communities because they can directly elevate body temperatures to reflect future scenarios and do not require longterm monitoring or extreme weather to simulate extreme events. Finally, the high within site temperature variability revealed by IR imaging may override the effect of largescale temperature variability on intertidal populations and reduce our ability to predict the effects of climate change on species distributions.

In summary, numerous factors need to be considered when assessing the effect of temperature on the early life history processes of intertidal invertebrates, including consistent small-scale temperature variability. The results of this study show that for a single species, the effects of temperature not only vary during different life history stages, but also, across different spatial scales. These findings contribute to, and advance, the growing body of work that highlights the importance of small-scale temperature variability in influencing the biogeographic and physiological responses of rocky intertidal communities in the face of climate change.

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