Figure 3.3 Map of the Benguela Current region bordering Namibia and South Africa, showing the 500 m depth contour (dashed line) and the approximate locations of the Lüderitz upwelling cell (which separates the northern and southern Benguela Current subsystems) and the Agulhas Current.
Source: From Roux et al. (2013). Reproduced with permission from the Bulletin of Marine Science.
There is increasing modelling and experimental evidence that pronounced alterations to oceanographic features (dominant currents and upwelling systems) due to climatic change are rearranging species’ distributions globally (Lourenço et al. 2017). Researchers at the University of Miami found that the Indian Ocean’s warm Agulhas Current is getting wider rather than strengthening (Beal & Elipot 2016). Their findings, which have important implications for global climate change, suggest that intensifying winds in the region may be increasing the turbulence of the current, rather than increasing its flow rate. In the case of the Benguela Current, global climate change has led to a rise in the temperature of this cold current. Not only has there been a rise in water temperature, but the water has become increasingly saline (Global Climate 2019). An El Niño5 effect has already been detected. The outcome of these changes from the Benguela El Niño is expected to have a dramatic effect on marine life on the southwest coast of Africa. How exactly it will impact global distributions of marine mussels remains to be seen.
Factors Affecting Local Distribution
Marine mussels on a local scale dominate the intertidal to subtidal regions of rocky shores (see earlier). Mussels extend from high intertidal to shallow subtidal (1–5 m) and even deeper waters, from estuarine to fully marine conditions and from sheltered to extremely wave‐exposed shores. On rocky shores of open coasts, mussels attach to the rock surface and crevices, and in sheltered harbours and estuaries they attach on rocks and piers, often occurring as dense masses of one or more layers, with individuals bound together by byssus threads. Young mussels colonise spaces within the bed, increasing spatial complexity and providing numerous niches for other organisms. It comes as no surprise that temperature and salinity, which play a key role in geographic distribution, are also important in determining species distribution on a local scale. For subtidal species, additional factors such as water depth, substrate type, currents, turbidity and predation and competition play an important role. Upper distributional limits in the intertidal zone are believed to be determined primarily by desiccation and temperature stress, while lower limits are determined by biological factors, particularly predators. Anthropogenic factors such as waterborne pollutants, introduced species and disease can also be significant in determining local distribution patterns.
Rocky shore ecosystems are governed by the tidal movement of water, which creates zonation patterns from high to low tide. The area above the spring high‐tide mark is called the supralittoral (splash) zone, which is regularly splashed but not submerged by seawater, except during storms with high tides (Purcell 2018). The area around the high‐tide mark is known as the intertidal fringe; organisms in this sparse habitat include anemones, barnacles, crabs, lichens, limpets, mussels, periwinkles and whelks, with very little vegetation. Below this zone is the intertidal, with three zones: high, mid and low, based on the average exposure. Each of these zones has its own characteristic animal and plant community. The high intertidal area is only flooded during high tides; common organisms are anemones, barnacles, chitons, crabs, isopods, mussels, sea stars and snails. This zone can also contain rock pools inhabited by small fish and larger seaweeds. The mid intertidal zone is covered and uncovered twice a day with saltwater from the tides. Therefore, temperatures are less extreme due to shorter direct exposure to the sun, and salinity is only marginally higher than ocean levels. However, wave action in the mid intertidal zone is generally more extreme than in the high tide and splash zones. Typical organisms are snails, sponges, sea stars, barnacles, mussels, crabs and brown and green algae. The low intertidal is only exposed on unusually low tides. Common organisms in this region are brown and green seaweed, crabs, hydroids, mussels, limpets, sea cucumber, sea urchins, sea stars, shrimp and snails. These organisms are not well adapted to long periods of dryness or to extreme temperatures. Below the low‐tide mark is the littoral zone, which can again be divided into three zones based on areas of tidal action, from shallow to deep: the supralittoral zone, the eulittoral zone and the sublittoral zone. The littoral zones are much more stable than the intertidal zones. Different types of oysters, star fish, sea urchins, coral, crabs and anemones live in the littoral zones, some of which are significant predators of marine mussels (see later).
Physical Factors
Temperature
Organisms in the rocky intertidal zone have to cope with being out of water at regular intervals. For those in the high intertidal, emersion times are longest, and consequently these individuals are often subjected to temperature extremes and desiccation. Upper distribution limits for a species are set by its ability to tolerate such extremes via a well‐coordinated set of physiological, behavioural, biochemical and molecular adaptive responses (McQuaid et al. 2015).
Considering the enormous amount known about the ecology and physiology of mussels, the main focus in this section will be on Mytilus species and the impacts of thermal stress on them in the rocky intertidal zone – a habitat that has been described as among the most physically harsh environments on Earth (Tomanek & Helmuth 2002), but which has emerged as a model system for investigating the ecological impacts of global climate change (Zhou et al. 2018; see later). When mussels are submerged during high tide, their body temperature is close to that of the surrounding water. However, during aerial exposure at low tide, body temperatures are driven by the interactions of climatic factors such as solar radiation, cloud cover, wind speed, relative humidity and air and ground temperatures, which drive the flux of heat into and out of the mussels’ bodies (Helmuth & Hofmann 2001; Helmuth 2002 and references therein). Consequently, body temperatures can be considerably higher (≥20 °C) than those experienced during submersion and can vary substantially from surrounding air and substrate temperatures (Helmuth 1999). It is important to note that the rate of heat transfer between an organism and its environment is determined to some extent by the size and morphology of the organism, and can be strongly affected by characteristics such as colour and material properties (Helmuth 2002). Also, changes in shell coloration, presence or absence of the periostracum and algal growth on the shell are all likely to modify a mussel's reflectivity and hence its body temperature (Helmuth 1999). Consequently, organisms exposed to identical environmental conditions can experience quite different body temperatures (Helmuth 2002). For example, barnacles keep a relatively large proportion of their total surface area in contact with the underlying substrate and display body temperatures that are tightly coupled with ground temperature. In contrast, mussels are predicted to have body temperatures that are largely decoupled from the temperature of the underlying substrate, at least while living in beds of conspecifics, and are often several degrees warmer than the surrounding air. Daily variations in temperature may also be extremely different between individuals that are just a few metres apart but located on surfaces facing different directions, or in or out of crevices or other shaded areas (Helmuth et al. 2010).
Until recently, measuring body temperatures in the intertidal zone was inefficient or unfeasible at large spatial or temporal scales, as temperature loggers were large and expensive and required frequent servicing (Lima et al. 2011). In addition, the intertidal zone, with its rapidly varying temperatures, tremendous wave forces and hard substrata, often caused loss of sensors and data loggers. In the early 2000s, however, advances in technology made it possible to
