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.
There are several categories of marine pollution. In this chapter, six categories of pollution encountered by macroalgae are discussed: metals, such as mercury, lead, cadmium, zinc, and copper; oil; synthetic organic chemicals, such as pesticides, industrial chemicals, and antifouling compounds; eutrophication (excessive nutrients, such as nitrogen or phosphorus); radioactivity, and thermal pollution. In the 1970s and ’80s, marine pollution was a hot topic and therefore some of these important early references have been retained. Over the last two decades, research on metals and eutrophication has been particularly active, followed by oil, antifouling paints, and organic wastes. There has been very little research on thermal pollution, even though it could be a good surrogate at local sites for assessing the potential long-term effects of global warming/climate change. Emerging anthropogenic issues are ocean acidification (see sec. 7.7 and Essay 4) and nanoparticles such as titanium dioxide (TiO2) (Miller et al. 2010, 2012).
General aspects of pollution
Several general considerations apply to studies on pollutants. Among these are the choice of test organisms, whether to study chronic or acute effects, the level of the effect such as lethal or sub-lethal, the complexities at various levels of organization from physiology to communities, and the issue of what is the biologically available quantity and form or species of the pollutant. Overall effects of a compound are assessed by acute or chronic exposure. Acute effects are the result of short-term exposure (e.g. 48–96 h) and are determined from the percent survival of an organism over a range of toxin concentrations. Chronic effects are the result of exposure for a relatively long time (e.g. 10% of the organism’s life span or longer (Walker et al. 2006)).
There have been very significant advances in many areas of phycology since the last edition nearly 20 years ago. In particular, the advances in our understanding of the endosymbiotic origin of algal plastids, and molecular aspects and genetics, stand out. The wealth of new literature alone in all the areas has warranted adding two new co-authors, Catriona Hurd, who focuses on water motion and seaweed physiological ecology, especially in the southern hemisphere, and Kai Bischof, who is well known for his research on photobiology and stress physiology. Hence, the previous edition’s chapter on “Temperature and salinity” has been expanded to include other environmental stressors such as UV radiation, ocean acidification, oxidative stress responses and the interactions between stressors.
Seaweed Ecology and Physiology is a textbook for senior undergraduates and a reference book for researchers. The rapid growth of knowledge in this field is both exciting and daunting. Our goal was to select papers that help put together a coherent story on a wide variety of ecological and physiological aspects. This book provides an entry to the literature, not a systematic literature review. With two of our co-authors having experience in the tropics and the temperate southern hemisphere, we have tried to avoid the typical temperate northern hemisphere bias.
As seaweed consumption has increased in the last several decades, seaweed mariculture has filled the gap between wild stock harvest and the present demand. Ancient records show that people collected seaweeds for food starting in about 2500 BP in China (Tseng 1981), and 1500 in Europe (Critchley and Ohno 1998). Presently, the wild harvest of seaweeds is about 1.8 m tonnes y-1, mainly brown seaweeds used for alginates (FAO 2009). In Japan, China, and other Asian countries, where seaweeds have long composed an important part of the human diet, seaweed farming is a major business and over 90% of the seaweed production is from farming for human consumption. Since 1970, the culture of seaweeds has increased at ~8% per year (FAO 2009). Seaweed production from farming nearly doubled from 8.8 to 15.9 million tonnes from 1999 to 2008, with a value of US$7.4 billion (FAO 2010). Most of the world seaweed supply comes from aquaculture and seaweeds were the first to pass the 50% farmed/wild harvest threshold in 1971, compared to fish aquaculture that will exceed the 50% threshold by 2012 (Chopin 2012). About 99% of the farmed production is in Asia and over 70% of the production (10.9 million tonnes) is in China, followed by Indonesia, the Philippines, South Korea, and Japan. Chile is the most important producer outside of Asia with a production of 90 000 tonnes y-1 of wild harvested seaweeds. Table 10.1 illustrates the production, value, price and the three main producing countries for the six most important seaweed genera that are grown in aquaculture systems. Brown seaweeds compose about 64% of the production (67% of the value), reds about 36% (33% of the value), and greens, with ~99% being produced by Asian countries, 0.2% of the production and value (Chopin and Sawhney 2009). There has been a rapid increase in production in the last decade, especially of reds and browns (Fig. 10.1). The largest production (4.6 million tonnes; Table 10.1) is from Saccharina japonica (previously Laminaria japonica; or kombu in Japan or haidai in China), mainly in China. Korea grows mainly Undaria pinnatifida (wakame) with 1.8 million tonnes annually and Pyropia (previously Porphyra, or nori), while Japan focuses mainly on Pyropia.
Seaweeds exist as individuals, but they also live together in communities with other seaweeds and animals – communities that affect and are affected by the environment. Ecologists and physiologists alike are drawn to coastal marine ecosystems because of the easy access to strong environmental gradients over short spatial scales. Marine organisms grow in often distinctive vertical or horizontal “zones” or “bands” along these gradients, thereby providing “natural laboratories” in which to study environmental (abiotic) and biological processes shaping the communities. Zones of vegetation are also found in terrestrial habitats, but here the spatial scales are typically much greater. On a mountain, for example, vegetation is zoned with altitude, but the vertical distance over which changes occur can be in the order of 1000 m rather than several meters in the intertidal zone (Raffaelli and Hawkins 1996). Vertical gradients in the intertidal are easily observed at low tide, but also extend underwater where the surface irradiance can be reduced to 1% at 15 m depth in many coastal waters (Lüning and Dring 1979; sec. 5.2.2). Horizontal gradients include the salinity gradients of estuaries and salt marshes, and wave exposure (Raffaelli and Hawkins 1996).
In Chapters 1 and 2, we reviewed the morphologies, life histories, and developmental processes of seaweeds as species. In this chapter we consider the patterns and processes in marine benthic communities as a starting point for later factor-by-factor dissection of the environment. We open with an overview of zonation patterns seen in the intertidal and subtidal environments.
The environment of an organism includes both biotic and abiotic (physiochemical) factors. Communities of marine organisms encompass not only the seaweed communities but also the animal communities, of which the benthic grazers and their predators are most important to seaweed ecology. Thus, the biotic interactions of seaweeds include not only competition with other seaweeds (both within and between species) and with sessile animals but also predator–prey relations at several trophic levels, and facilitation; the mix of such interactions will change as the individual changes with age and environmental history.
Biotic interactions are complex, and their study often requires large-scale and long-term observations and manipulations in the laboratory, as well as in the field. Interactions can be positive (e.g. facilitation, mutualism, and commensalism), negative (e.g. competitive exclusion, consumption) or neutral, where there is no effect of one species on another. Studies on biotic interactions in the marine environment have traditionally focused on competition but more recently facilitation has been recognized as an important way in which biota interact. The minireviews of Olson and Lubchenco (1990), Carpenter (1990), Paine (1990), and Maggs and Cheney (1990) remain useful frameworks, as are the more recent syntheses found within Marine Community Ecology (Bertness et al. 2001) and Marine Ecology (Connell and Gillanders 2007).
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