Effects of Temperature Increases

Temperature is probably the most dominant rate-determining factor in biology; ranging from subcellular to community-level processes, with direct and indirect effects on organisms' physiology, ontogeny, trophic interactions, biodiversity, phenology, and biogeography. Increases in temperature due to climate change have the potential to impact most marine ecosystems, directly through the impact on species physiology (growth, reproduction, etc.) or indirectly through impacts on ocean dynamics (currents) or species interactions. The magnitude of ecological effects of rising temperatures would inherently vary among and even within species, as different species and even different ontogenetic stages may be unequally susceptible to thermal stress or steep fluctuations in temperature.

The most obvious and direct biological effect of global warming is attributed to the fundamental relationship between temperature and physiology. A wide range of physiological processes are influenced by temperature, among them are protein structure and function, membrane fluidity, organ function (Hochachka and Somero, 2002; Harley et al., 2006), heart function, and mitochondrial respiration (Somero, 2002). For some of these thermally sensitive traits, the acclimation of marine species to a given environment has resulted in the creation of narrow thermal optima and limits. In addition, many marine species live near their thermal tolerance limits, and small temperature increases could negatively impact their performance and survival. This was demonstrated in heat-shock response patterns of different coastal Tegula snails that were shown to have limited thermal tolerance, which depended on the region and habitat of the species studied (Tomanek and Somero, 1999), and again in the thermal tolerance of rocky intertidal porcelain crab species (Stillman, 2002). In the Caribbean, McWilliams et al. (2005) demonstrated that a shift of only +0.1°C resulted in 35% and 42% increases in geographic extent and intensity of coral bleaching, respectively.

Indeed, coral bleaching is one of the most well-known and studied phenomenon related to temperature stress in the marine environment. During thermal stress, corals expel most of their pigmented microalgal endosymbionts, called zooxanthellae, to become pale or white (i.e., bleached). The link between climate change and bleaching of corals is now indisputable, as episodes of coral bleaching have already increased greatly in frequency and magnitude over the past 25 years (Glynn, 1993; Hughes et al., 2003; Hoegh-Guldberg et al., 2007), strongly associated in many cases with recurrent ENSO (El Niño - Southern Oscillation) events (Baker et al., 2008). Bleaching episodes have occurred almost annually in one or more of the world's tropical or subtropical seas, resulting in catastrophic loss of coral cover in some cases, and coral community structure shift in many others. Prolonged and severe events of bleaching may result in massive mortality of overheated corals (Hughes et al., 2003). Biochemical and physiological mechanisms of symbiosis breakdown was attributed to temperature or irradiance damage to the symbionts' photosynthetic machinery, resulting in the overproduction of oxygen radicals and cellular damage to hosts and/or symbionts (Lesser, 2006). Another somewhat controversial approach addresses bleaching episodes as an important ecological process that can ultimately help reef corals to survive future warming events in which corals get rid of suboptimal algae and acquire new symbionts. This point of view defines bleaching as a strategy that sacrifices short-term benefits of symbiosis for long-term advantage (Baker, 2001).

Temperature is also a key factor in ontogenetic development, and is known to affect different ontogenetic stages distinctively (Foster, 1971; Pechenik, 1989). Hence, increased temperature can affect the timing of ontogenic transitions, sometimes resulting in a temporal mismatch between larval development and key control factors like food supply or predation intensity. An example of this is the earlier spawning of the clam Macoma balthica in the Wadden Sea (northwestern Europe), but not to earlier spring phytoplankton blooms (Philippart et al., 2003).

Therefore, the period between spawning and maximum food supply was extended, and food availability during the pelagic phase reduced. Furthermore, predation intensity by juvenile shrimps on juvenile Macoma has also increased because of earlier recruitment of juvenile shrimp to the mud flats (Philippart et al., 2003). Trophic mismatch events are a potential severe consequence of temperature rise. A phenological study across three trophic levels using five functional groups in the North Sea showed different responses to temperature changes over the years 1958-2002 (Edwards and Richardson, 2004). Using this long-term data set of 66 plankton taxa, the authors demonstrated shifts in the timing and size of seasonal peaks of different populations, related to physiological (e.g., respiration, reproduction, mortality) or environmental (e.g., stratification) conditions. Such shifts can have profound consequences to community structure and stability, like in the case of the of North Sea cod stock declines implicated to worsen by key planktonic prey declines and shifts in their seasonality (Beaugrand et al., 2003, 2008), or in the case of the northern shrimp, Pandalus Borealis, and its temperature-dependant timing of egg-hatching, intended to match spring phytoplankton blooms (Greene et al., 2009). On rocky intertidal shores where upwelling prevails, mussel growth responds strongly to changes in water temperature associated with ENSO and PDO (Pacific Decadal Oscillation) cycles, suggesting potential community-level effects of climate change, as mussels have important ecological roles, serving as both food and habitat for a multitude of species on the shore (Menge et al., 2008).

Rising temperatures can potentially alter significant community-controlling interactors such as predators, competitors, ecosystem engineers, mutualists, or pathogens. The behavior of a keystone predator, the sea star Pisaster ochraceus, in the upwelling system off the US West Coast was followed by Sanford (1999) at different water temperatures and was shown to exhibit higher mid-intertidal abundance and increased consumption rates when exposed to slightly warmer waters. The author suggested that if water temperatures rise due to climate change, more intense predation might alter the vertical extent of the prey (habitat-forming mussels) and various species inhabiting its matrix and thus affect the community as a whole (Sanford, 1999). Global warming may also reduce predation, for example in the case of the Humboldt squid, Dusidicus gigas, a top predator in the eastern Pacific that exhibited lower metabolic rates and activity levels when exposed to high CO2 concentrations and temperatures, thus affecting growth, reproduction, and survival of the squid and possibly impairing predator-prey interactions in the pelagic system (Rosa and Seibel, 2008).

Another important illustration of warming water effects is the change in benthic community structure near the thermal outfall of a power-generating station on the rocky coast of California. There, communities were greatly altered in apparently cascading responses to reduced abundances of habitat-forming species like subtidal kelps and intertidal red algae (Schiel et al., 2004). In contrast, grazers showed positive response to temperature, attributed by the authors to physiological tolerances, trophic responses, space availability, and recruitment dynamics (Schiel et al., 2004).

An example of what rapid ocean warming can do on regional and community scales can be seen in the mass mortality event of 25 rocky benthic macro-invertebrate species (mainly gorgonians and sponges) in the entire Northwestern Mediterranean region that followed a heat wave in Europe in 2003 (Garrabou et al., 2009). The heat wave caused an anomalous warming of seawater, which reached the highest temperatures ever recorded in the studied regions, between 1°C and 3°C above the climatic values (both mean and maximum). Such increases are certainly within the range of expected long-term global warming of the oceans, and the authors also suggest that heat waves may become more common in the future possibly driving a major biodiversity crisis in the Mediterranean Sea.

Local or regional mortality of species is but one aspect of global climate change. Water temperature rise has already shown to drive extensive biogeo-graphical shifts, expressed mostly as poleward movement of species. Significant shifts were seen, for example, in marine fish populations in the North Sea, where nearly two thirds of the species shifted in latitude or depth or both over 25 years in correlation with warming waters (Perry et al., 2005). Another example is shift in the population dynamics of the sea urchin Centrostephanus rodgersii along the eastern Tasmanian coastline (Ling et al., 2009). Ling et al. (2009) revealed range extension through poleward larval dispersal via atmospheric-forced ocean warming and intensification and poleward advance of the East Australian Current (EAC). Shifts are also seen in the intertidal zone, which represents a unique situation as it is situated at the interface between the land and the sea and therefore species living there are expected to be influenced by changes in both water and air temperature. On the shore, species geographic distributions are expected to shrink or shift due to changes in thermal stress and ocean circulation either directly or indirectly through species interactions. Some species could be purged from the intertidal zone by alterations in water temperature, upwelling regime (Leslie et al., 2005), or oxygen levels (Grantham et al., 2003; Chan et al., 2008). Others may be squeezed out of the system when their upper limit is reduced to the upper limit of their consumers (Harley et al., 2003). Alternatively, some species may find that environmental conditions become physiologically tolerable at regions that were previously uninhabitable, or ocean circulation changes may bring distant species to new locations, resulting in range expansion. Indeed, long-term monitoring shows that the poleward-range edges of intertidal biota have shifted by as much as 50 km per decade in some regions (Helmuth et al., 2006b). Poleward range extension was documented in various intertidal species of invertebrates and algae (Herrlinger, 1981; Weslawski et al., 1997; Lohnhart and Tupen, 2001; Zacherl et al., 2003; Helmuth et al., 2006b; Mieszkowska et al., 2006). However, change in distribution due to thermal stress may not be a simple linear/longitudinal process. Helmuth et al. (2002, 2006a) have demonstrated that thermal stress on the rocky shore exhibit a mosaic of localized hotspots that do not necessarily follow latitudes. Thermal-stress hotspots are determined mainly by the timing of low-tide during summer spring tides. These low tides on the US West coast frequently occur at the hottest time of the day at the higher latitudes (Washington and Oregon) while they happen at night time further south (California). This means that increasing water temperature may facilitate the establishment of species invading from warmer waters in complex patterns along the shore, potentially affecting community structure and function in mosaic patterns.

The link between global warming and invasion of alien species is an obvious one, as warming can allow warm water species to extend to or invade previously nonhospitable regions (Occhipinti-Ambrogi, 2007). For example, the establishment of three abundant introduced ascidians on the shores of New England was explained by the strong positive correlation between their recruitment rates and rising winter sea temperatures in the region (Stachowicz et al., 2002). In the Mediterranean, one of the hottest hotspots of marine bioinvasions, warming events, and change in circulation patterns due to climate shifts (e.g., the Eastern Mediterranean Transient) in the past century have been suggested to facilitate invasions of tropical species (Rilov and Galil, 2009).

Climate change thermal effects are not just bound to coastal or sea-surface environments, but they were also shown to impact deep sea ecosystems. For example, decadal nematode community surveys conducted in the Eastern Mediterranean revealed a significant increase in nematode abundance and diversity, which was related in this case to temperature decrease of 0.4°C (Dennavoro et al., 2004).

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