What is stress?

Any given environmental factor can become a “stress” to a given seaweed species, if it exceeds the upper or lower threshold values of tolerance. Seaweed communities are shaped by the complex interplay of a multitude of external biotic and abiotic factors and the intrinsic responses of the individual seaweed species. These factors are not stable over space or time, requiring frequent metabolic adjustments, termed acclimation (sec. 1.1.3). The genetic frame setting the limitations of acclimation is termed adaptation. Species-specific adaptation and the effectiveness of acclimation to change determine the competitive success of each species in the interaction with other species and thus shape the complex composition of a seaweed community in the field. For example, it is now well established that many biotic interactions of macroalgae (e.g. competition, predation, etc.) are mediated by the environmental stress, and the ways in which they manage it (Menge et al. 2003; Essay 2, Chapter 3). Major changes in abiotic factors occur along spatial and temporal gradients: spatially, on a global scale along latitudinal gradients, large changes in temperature, light availability, and seasonality are observed; along the coastline steep gradients in abiotic factors exist stretching from the intertidal to the subtidal zone; but even on very small scales the abiotic environment of seaweeds may change dramatically, e.g. within algal mats (Bischof et al. 2006b). Temporally, there are natural fluctuations of abiotic factors due to seasonal events, daily or tidal cycles, and climate variability such as El Niño Southern Oscillation (ENSO) events. The extent of natural change in the physico-chemical environment to which a seaweed species is exposed can be summarized by the term “habitat stability” and is often tightly linked to vertical zonation patterns along the phytal zone, with intertidal species populating the most demanding, least stable habitat (Davison and Pearson 1996; sec. 3.1). Apparently, the magnitude of the environmental stress along different spatial scales is important to explain the distribution patterns of macroalgae as was reported for rocky intertidal assemblages from Helgoland Island (Valdivia et al. 2011). Valdivia et al. (2011) indicated that vertical variation in community structure was significantly higher than patch- and site-scale horizontal variation but lower than shore-scale horizontal variation. Most concern and research effort is now directed towards the additional anthropogenic sources of variation in the abiotic environment, from the local to the global scale.