22 Phytoplankton Ecology
Macroalgal and Periphyton Ecology
Macroalgae, also known as seaweeds, and periphyton algae (smaller unicellular, colonial, or filamentous algae) typically grow attached to substrata, in contrast to the floating or swimming phytoplankton discussed in the previous chapter. Substrata for macroalgae and periphyton include rocks, coral, carbonate or silica sands, other algae, and aquatic animals and plants. Most seaweeds occur along ocean coastal regions (Figure 22.1), but a few, such as Cladophora, Ulothrix, and Bangia, have colonized freshwaters. Periphytic algae occur in both the oceans and freshwaters. Both macroalgae and the periphyton algae are immensely important, as they provide the community structure and primary productivity that supports a wide array of other organisms. An understanding of the composition and function of periphyton and macroalgal communities is essential to effective management, protection, and restoration of freshwater wetlands, lakes and streams, coastal salt marshes and mangrove forests (mangals), coral reefs (see Text Box 22.1), and rocky ocean coastlines.
On a global basis, coastal ocean waters inhabited by macroalgae are sites of intense primary productivity that support coastal ecosystems, including fisheries. Primary productivity in seaweed communities is equal to or greater than that of the most productive terrestrial plant communities. Productivity of the Postelsia community, which inhabits high-energy, wave-exposed sites along the Pacific U.S. coast (Figure 22.2), is estimated to be 14.6 kg m–2 yr–1, in contrast to less than 2 kg m–2 yr–1 for rain forests (Leigh et al. 1987). Kelp-dominated seaweed communities along the eastern United States coast have an estimated primary productivity of 1.75 kg m–2 yr–1. An estimated 10% of seaweed biomass is directly consumed by herbivores, whereas 90% enters detrital food webs, thereby contributing indirectly to animal biomass (Branch and Griffiths 1988) (Figure 22.3). Fossil evidence suggests that coastal marine seaweed communities have been in existence for the past 500 million years and have been supporting coastal ecosystems with intense primary productivity throughout this time (Xiao et al. 1998). Freshwater periphyton communities also contribute significantly to primary productivity, particularly in streams or lakes having a relatively large proportion of nearshore substrate to open water. In many bodies of water, periphyton algae contribute more primary productivity than do the phytoplankton (Burkholder and Wetzel 1989).
As a result of human population pressures, coastal ecosystems and inland freshwaters around the world are being seriously affected by inputs of nutrients and contaminants, exposure to increased levels of ultraviolet radiation, overexploitation of resources, loss of species diversity, and colonization by nonnative species. Most studies of nearshore communities have been performed on very small scales, on local levels, and for short time periods. Many ecologists believe that longer-term, regional- and global-scale analyses must be undertaken that include as many of the significant biotic and abiotic ecosystem components as possible. The U.S. National Science Foundation’s Long-Term Ecological Research (LTER) program includes several intensively studied aquatic sites, while the Global Ocean Observing System (National Research Council) is an example of an international effort to increase the range of scientific information available to coastal zone planners and policymakers. Remote-sensing techniques such as satellite-based scanners are being used to survey seaweeds and freshwater algae.
Macroalgal and periphyton ecologists are interested in defining the factors that influence algal occurrence and distribution patterns. Coastal marine communities have proven especially valuable as ecological models for determining the relative importance of physical factors, algal morphological and physiological adaptations, and biotic factors such as herbivory and interspecific competition in structuring macroalgal communities. Studies suggest that in both seaweed and periphyton communities, there is a hierarchy of controlling factors, with the limits of algal growth set primarily by the interplay between algal physiological constraints and physical environmental factors. Within these limits, biotic factors—specifically grazing by herbivores and the activities of pathogens—can modify algal distribution patterns. The concept of the keystone species was developed in the marine intertidal zone (Paine 1969), and trophic dynamics are major factors in rocky intertidal, kelp forest, and coral reef communities. The habitats of seaweeds and periphytic algae are highly variable and have a significant level of disturbance. In such conditions, according to ecological theory (Chapter 21), interspecific competition cannot go to equilibrium, and species diversity is high. In view of this apparent hierarchy of controlling factors, physical factors and associated algal adaptations are discussed here first, followed by consideration of the roles of herbivory and competition.
Many species of macroalgae occur in marine coastal waters, but in freshwaters the branched, filamentous green alga Cladophora is the largest, most conspicuous alga. Attached diatoms, cyanobacteria, and filamentous eukaryotic algae dominate the microscopic periphyton of both freshwaters and marine waters. Although there are some circumstances in which growth of freshwater periphyton resembles that of marine turf-forming seaweeds, the habitats of seaweeds and marine periphyton differ significantly in wave energy, tidal exposure to desiccation and solar radiation, variation in temperature and salinity, and types of herbivores from the habitats of freshwater periphyton. The first part of this chapter focuses on marine macroalgae, the second section is concerned with turf-forming periphytic algae of marine waters, and the third part discusses freshwater periphyton algae.
