Introduction

High elevation shrublands occur above the tradewind inversion (c. 2000 m), on the geologically young (< 1 000 000 years old) volcanoes of Maui (Haleakala volcano) and Hawaiian (Mauna Kea, Mauna Loa and Hualalai volcanoes). Except for Mauna Loa, which recently erupted at the 3000 m level in 1984, these volcanoes are quiescent. Vegetation of the high elevation Hawaiian volcanoes consists of shrubs (Styphelia, Coprosma, Vaccinium, Dubautia, Dodonaea, Geranium), small trees (Sophora, Myoporum, Santalum), perennial graminoids (Deschampsia, Agrostis, Trisetum, Luzula, Carex), and other perennial herbaceous species (Pteridium aquilinum, Asplenium, Silene, Sanicula, etc.). With increasingly severe climatic conditions at higher elevations and with proximity to a mountain's summit, vegetation becomes more sparse and smaller in stature. Vegetation at the uppermost limit consists of prostrate shrubs, grasses, and ferns (Hartt & Neal 1940; Fosberg 1959; Mueller-Dombois & Krajina 1968; Whiteaker 1983). The fresh lava substrates of Mauna Loa (elevation 4170 m) result in much less development of soil and vegetation there than on older Mauna Kea (4207 m) and Haleakala (3056 m), both of which have extensive outcrops of cinder and ash deposits (Fosberg 1959). Hualalai Volcano (2522 m) barely reaches into the high elevation zone.

Mean temperature in the Hawaiian Islands decreases upward on the volcanoes at a rate of about 0.53 °C per 100 m.

Introduction

The tropical alpine environment is surprising. Although it is predictable (Troll 1961; Humboldt, in Botting 1973), it is still remarkable to experience cool, misty mountains that rise out of the lowland tropics. These alpine places bear surprising resemblance in vegetation to the arctic, and to the alpine zones of temperate mountains. They also bear climatic and vegetational resemblance to remote islands of the Southern Ocean. The similarities of vegetation derive primarily from the changes in physical climate that occur as altitude increases on humid mountains. As temperatures decline, different plant species replace those adapted to growth in warmer conditions. The conspicuous changes to vegetation are a transition from tall, tree-dominated communities to low, shrub or herb dominated communities. These transitions occur on a grand scale that has been expressed as a three-dimensional model of climate and vegetation of the earth (Troll 1961; Humboldt, in Botting 1973).

There is controversy about the terms that should be applied to the regions. The argument turns on the degree of recognition given to thermal seasonality, which is weak or absent on the high tropical mountains and profound in the temperate alpine and near polar regions. There are many regional names: alpine, páramos, pahonales, mountain grassland, orosubantarctic, gras maunten, ais, etc. Each has its utility for its own zone – none is without misleading connotations when applied outside its region of origin. In this chapter, the phrase ‘tropical alpine’ is being used to conform to the usage of this book.

Introduction

A notable feature of tropical alpine floras in many parts of the world is the presence of rosette plants with monocarpic growth habits. These long-lived perennials flower only once in their lives, producing a giant inflorescence with large numbers of flowers before the parent plant dies. In the South American páramos of Venezuela, species of Espeletia (Asteraceae) provide classic examples of this life-form (Cuatrecasas 1986; Berry & Calvo, Chapter 13)). Giant species of Afroalpine Lobelia (Campanulaceae) reach to 5–6 m in height when flowering before dying, and form one of the most spectacular elements of the flora of the tropical African highlands (Mabberley 1974; Young, Chapter 14). On Tenerife in the Canary Islands, two alpine species of Echium (Boraginaceae) have also evolved rosette growth forms and a monocarpic habit (Carlquist 1974). In each of these groups, the monocarpic rosette plants have evolved from polycarpic ancestors with a shrubby growth form.

Argyroxiphium sandwicense (Asteraceae) provides another well-known example of the evolution of a monocarpic rosette plant in a tropical alpine environment. The genus Argyroxiphium, with five species, is one of three genera of Hawaiian tarweeds that have evolved from a monophyletic North American origin (Carr 1985; Witter & Carr 1988; Baldwin et al. 1991). This group displays a remarkable diversity of growth forms and adaptive morphologies. Monocarpic rosette plants have evolved not only in species of Argyroxiphium in bog and in alpine habitats, but also in Wilkesia gymnoxiphium, a stalked rosette plant in scrub and open forest habitats.

Introduction

Draba is one of several genera that are distributed throughout the north temperate latitudes and at high elevations in Central and South America from Mexico to Tierra del Fuego (Good 1974). Individuals of Draba typically are among the last vascular plants to drop out at the upper elevational limit to plant growth in North and South America. In the northern Andes, species of Draba occur in continuous páramo vegetation at 3500 m and are typical of rocky habitats and discontinuous vegetation up to 4800 m elevation (Cuatrecasas & Cleef 1978; Monasterio 1981a).

The basic vegetative form of the genus Draba is a rosette. The ‘typical’ form of north-temperate alpine drabas is a perennial, loose mat to tight cushion of small-leaved rosettes (Hitchcock 1941). The variety of morphologies present within the Draba in the highest elevations of the Venezuelan Andes is remarkable in contrast to their morphologically uniform north temperate alpine congeners, although in both areas species of Draba occur in what might be regarded as similar habitats: protected rocky cliffs to exposed rock outcrops, ridges and scree slopes at the limit of plant growth. In the tropical Andes two quite distinct growth forms occur: upright branching shrubs (Sections Calodraba and Dolichostylis), and thick stemmed rosettes with relatively large leaves (Section Chamaegonagyle: Schulz 1927) (Figure 8.1).

The year-round growing season of the páramo may have represented a release from the strong selective limitations of the temperate alpine environment, consequently allowing the expression of a diversity of relatively unusual forms.

Seaweeds grow in circumstances that feature exceptionally diverse and dynamic lighting climates. The water clarity and the continual ebb and flood of tides have profound effects on the quantity and quality of the light that reaches seaweeds, adding greatly to the variation already present in the irradiance at the earth's surface. The primary importance of light to seaweeds is in providing the energy for photosynthesis, energy that ultimately is passed on to other organisms. Light also has many photoperiodic and photomorphogenetic effects, as we saw in Chapter 1. Thus light is the most important abiotic factor affecting plants, and also one of the most complex.

The principles of photosynthesis are similar in algae and higher plants, and indeed some principles (e.g., the Calvin cycle) were worked out using algae. Most of the catalytic proteins involved in the thylakoid reactions of red algae, for instance, are homologous with those in all other photosynthetic plants, but some are analogous (Raven et al. 1990). There are, moreover, several important features of seaweeds and their habitats that stand in sharp contrast to those in higher plants, the land plants, and it is on these that we shall focus. Such features include the diversity of pigmentation among marine algae and the diversity of the light climate in the oceans, the nature of the carbon supply in the sea, and the diversity of photosynthetic products in different algal classes.

Natural ranges of temperature and salinity

Open coastal waters

The surface temperatures of the oceans vary in two primary ways. First, they decrease toward higher latitudes, from about 28°C in the tropics to 0°C toward the poles, although this trend is markedly affected by ocean currents. Because of the California Current, for example, fairly uniform cool temperatures prevail in the seawater along much of the west coast of North America, even though land temperatures change considerably. Second, the seasonal changes in ocean temperatures are larger at midlatitudes. In the tropics and at the poles the annual temperature range is often less than 2°C (Kinne 1970), whereas in midlatitudes 5–10°C is common.

The salinity of open-ocean surface water is generally 34–37 parts per thousand (‰), though lower off the coasts of areas with great rainfall (e.g., the northwest coast of North America), and higher in subtropical areas with high rates of evaporation and low rainfall (Groen 1980). Certain seas have markedly higher or lower salinities: The Mediterranean, because there is a high rate of evaporation and little freshwater influx, has salinities of 38.4–39.0‰; the Baltic, essentially a gigantic estuary, is notably brackish, particularly at the surface, ranging from 10‰ near its mouth to 3‰ or less at the northern extreme. In coastal waters, especially those that are partially cut off from the ocean or are subject to heavy runoff, salinity is characteristically 28–30‰ or lower, even quite far along a coast from the mouth of a major river.

Introduction: the plants and their environments

Seaweeds

The term “seaweeds” traditionally includes only macroscopic, multicellular marine red, green, and brown algae. However, each of these groups has microscopic, if not unicellular, representatives. All seaweeds at some stage in their life cycles are unicellular, as spores or zygotes, and may be temporarily planktonic (Amsler & Searles 1980). Some remain small, forming sparse but productive turfs on coral reefs (Hackney et al. 1989). The blue-green algae are widespread on temperate rocky and sandy shores (Whitton & Potts 1982) and have occasionally been acknowledged in “seaweed” floras (e.g., Setchell & Gardner 1919; Newton 1931). They are particularly important in the tropics, where large macroscopic tufts of Oscillatoriaceae and smaller but abundant nitrogen-fixing Nostocaceae are major components of the reef flora (Hackney et al. 1989). Again, there are many unicellular blue-green algae. On the other hand, some benthic diatoms – normally not considered seaweeds – form large and sometimes-abundant tube-dwelling colonies that resemble seaweeds and presumably respond to the environment in much the same way (Lobban 1989). A deep-water green, Palmoclathrus, forms a morphologically complex thallus built from an apparently amorphous matrix with a nearly uniform distribution of cells (Womersley 1971; O'Kelly 1988), and a tropical chrysophyte, Chrysonephos lewisii, forms large, Ectocarpus-like thalli (Taylor 1960). On a smaller scale are the colonial filaments of some simple red algae, such as Goniotrichum. In this book we shall consider macroscopic and microscopic benthic environments and how algae respond to those environments.

Introduction

Mariculture, or marine agronomy (Doty 1977), distinct from simple harvesting of wild stocks, is the cultivation of the sea. It involves large-scale cultivation of commercially useful organisms, including seaweeds. 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 (Table 9.1). In other regions of the world, where the primary uses of seaweeds are as animal fodder, fertilizers, or sources of phycocolloids, wild stocks usually are harvested (Hoppe & Schmid 1969) and managed (e.g., some habitat improvement). In recent years, seaweeds have also been considered as potential solar-energy converters, to provide biomass as a source of nutrients and energy for methane-producing bacteria.

Mariculture depends on improving the conditions found in the sea, improving the plant material, or creating artificial environments, which can provide optimum conditions for growth of the plant. Thus, just as agriculture depends on vascular-plant ecology and physiology for a basic understanding of the crops, successful mariculture depends on an extensive basic knowledge of the biology and physiology of the seaweeds under cultivation and how factors important to seaweed growth can be manipulated to improve yields.

Ancient records show that people collected seaweeds for food as long ago as 2500 B.P. in China (Tseng 1981) and 1500 B.P. in Europe (Levring 1977). In the past 300 years, and particularly in the past 50 years, the practice has grown and changed, first in Japan and then in China, from the process of simply harvesting the wild stands to the processes of selecting, breeding, and cultivating certain species.

Introduction

Public concern over marine pollution has developed only relatively recently, because of several important events, such as the world's first major oil spill (106,000 metric tonnes) by the supertanker Torrey Canyon, which accelerated public concern in the early 1960s. This concern was renewed recently by the 11.2-million-gallon Exxon Valdez oil spill in Alaska (Leschine 1990; Maki 1991) and the largest spill in history in the Persian Gulf.

There is no precise definition of the term “pollution,” but one possible general definition is a stress on the natural environment caused by human activities, resulting in unfavorable alteration of an ecosystem. Other definitions, referring to the introduction of a substance into the environment by humans, are more restrictive because they do not include thermal pollution. The term “unfavorable” in the definition involves human value judgments, and therefore it is common to see disagreement among scientists and politicians on whether or not certain events are examples of pollution (Rosenberg et al. 1981). The disagreement stems in part from the complexities of measuring pollutant effects over time scales ranging from minutes to decades and over at least five levels of biological organization, involving biochemical, physiological, population, community, and ecosystem structural changes (Hood et al. 1989) (Fig. 8.1).

The biochemical and physiological effects of a contaminant can result in reduced phenotypic fitness, as shown in Figure 8.2 (Bayne 1989). Pollution studies at the population level for many benthic invertebrates have focused on recruitment, mortality, size and age structure, and biomass and population production. It is likely that these parameters apply to seaweeds as well.

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; 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; see this series of minireviews: Olson and Lubchenco (1990), Carpenter (1990), Paine (1990), Maggs and Cheney (1990). Two quotations from reviews that are both methodological and philosophical will serve to introduce two major topics of this chapter – competition and herbivory: “The objectives of studies of competition include, first, a demonstration that competition occurs, second, identification of the mechanism by which it occurs, and, third, determination of the importance of competition to the ecology of species or communities” (Denley & Dayton 1985). “Approaches and solutions to the problems and measurement of herbivory are strongly influenced by the nature of the questions being asked. Many algal–herbivore interactions have significance in an ecological context only when examined by a wholecommunity approach, often involving experimental manipulation of seemingly-unrelated parameters” (Vadas 1985).

Seaweed communities

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. In Chapter 1 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 overviews of three major habitats and the seaweeds in them: rocky intertidal zone, tropical reefs, and kelp forests. We hope that these personal essays by some noted algal ecologists will also give the reader a glimpse of the phycologist at work and a sense of the excitement of physiological ecology. Near the end of the chapter, three more ecologists tell about some less well known habitats: salt marshes, seagrasses, and the Arctic.

Essay: The rocky intertidal zone

Few habitats are so frequently visited by ecologists as the rocky intertidal zone, for it offers intermittent access to a fascinating variety of organisms. It must be unique, however, in that it is invariably examined when most of its inhabitants are out of their element. The number of ecologists who study the shore at high tide when its residents are active and operational could, I suspect, be counted on the arms of a starfish. This is a pity, for it is the shore when underwater that is the shore in action (Fig. 2.1).

The field of experimental phycology continues to grow, feeding on advances in other fields and sometimes, as in the past, contributing to them. The wealth of new literature alone would have warranted a revision of our original book, The Physiological Ecology of Seaweeds. However, the reasons for this revision – and its changes – go even deeper. In fact, the original book has been so thoroughly reworked and rewritten that we have given it a new title.

Seaweed Ecology and Physiology, like its predecessor, is intended primarily as a textbook. The rapid growth of knowledge in this field is at once exciting and daunting. Even more than in the first book, our method has been to select papers that help put together a coherent (if reticulate!) story. This book provides an entry to the literature, not a systematic literature review.

Our recent experiences in the tropics and an increasing literature on tropical algae have allowed us to redress the temperate bias of our earlier writing. Austral countries such as Australia, Chile, and South Africa have also been active in seaweed physiological ecology and have provided additional perspectives on seaweed biology.

Our teaching experiences suggested that the sequence of chapters could be improved. Chapters on communities and morphogenesis, which formerly served to review and tie together earlier themes, are now introductions to the organisms and their interactions. We have included an encapsulation of algal structure and life histories, but still expect that students using this book will have learned these subjects in more detail or will be learning about them concurrently.

The waters of the oceans are in constant motion. The causes of that motion are many, beginning with the great ocean currents, tidal currents, waves, and other forces, and ranging down to the small-scale circulation patterns caused by local density changes (Vogel 1981; Thurman 1988). Hydrodynamic force is a direct environmental factor, but water motion also affects other factors, including nutrient availability, light penetration, and temperature and salinity changes. The forces embodied in waves are difficult to comprehend, unless one has been dangerously close to them; because of the density of water, a wave or current exerts much more force than do the winds. “Imagine a human foraging for food and searching for a mate in a hurricane and you will have only an inkling of the physical constraints imposed on wave-swept life” (Patterson 1989, p. 1374). The energy amassed from a great expanse of air–ocean interactions is expended on the shoreline as waves break (Leigh et al. 1987). Equally difficult to visualize and measure are the microscopic layers of water next to plant surfaces where the plants' cells interact with water. Too much water motion imposes drag stresses on seaweeds; too little imposes diffusion stresses and impairs nutrient uptake (Wheeler 1988). Biomechanical studies of seaweed form and function are beginning to give some insight into this trade-off; see the reviews by Koehl (1984, 1986). Denny (1988) has provided a solid foundation in fluid mechanics for the study of marine organisms.