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Article Excerpt
Introduction
In many estuaries of North America, Europe, China, Australia, and likely other parts of the world, abundances of macroalgae have increased sharply during the latter half of the 1900's and into the 2000's. The macroalgae have increased so much that they often form huge thick mats (biomasses) on wide shoreline flats. The principal mat-forming types are sea lettuce, Ulva lactuca (Fig. 1), Enteromorpha spp., Gracilaria spp., and Cladophora spp. Few macroinvertebrates grow on the surfaces of Ulva spp. sheets (MacKenzie, 2000; Harder et al., 2004), and few can live beneath them (Soulsby et al., 1978; Nicholls et al., 1981; Olafsson, 1988; Bonsdorff, 1992; Norkko and Bonsdorff, 1996; MacKenzie, 2000; Rafaelli, 2000; Sfriso et al., 2001; Osterling and Pihl, 2001; Jones and Pinn, 2006). The shoreline flats once provided good habitats for large numbers of macroinvertebrates that were the prey of small fishes, crabs, and shrimps, which were in turn, food for wading birds and other predators (Breber, 1985; Norkko and Bonsdorff, 1996; Thiel et al., 1998; MacKenzie and McLaughlin, 2000; Sfriso et al., 2001).
Where the algal or Ulva mats are present, they have covered and eliminated the areas as sources of food. In doing so, they have altered the estuaries' trophic food webs within the shallow zones and also within the entire estuaries, in part, because large commercial and sport fish in the deeper waters of estuaries had fed on some of the macroinvertebrate predators, especially the small fish (Valiela et al., 1992; Hartog, 1994; Isaksson et al., 1994; Peterson and Turner, 1994; Short et al., 1995; Norkko and Bornsdorff, 1996; Short and Burdick, 1996; Rafaelli et al., 1998; Hauxwell et al., 2001; Sfriso et al., 2001; Deegan et al., 2002; Cummins et al., 2004).
[FIGURE 1 OMITTED]
Large influxes of nitrates and also phosphates, carried by freshwater to the estuaries, have led to eutrophication of the waters and fueled the algal growth. The influxes are the result of increased urbanization and industrialization of the estuaries' watersheds (Wilkinson, 1963; Sawyer, 1965; Buttermore, 1977; Soulsby et al., 1978, 1982; Montgomery and Soulsby, 1980; Nicholls et al., 1981; Rosenberg, 1985; Valiela et al., 1997; Deegan et al., 2002; DeJonge et al., 2002).
Sea lettuce, Ulva lactuca, has a bright green color and imparts an apparent healthy appearance (Fig. 2). Along the east coast of the United States, its presence has not been regarded by the public as pollution-related, and its overabundance has not been particularly noticed except by boaters and swimmers who regard it as a nuisance. Many people even consider that large amounts of a green plant, such as sea lettuce, in the water denotes a healthy environment. The name lettuce also connotes to some a positive impression (like lettuce as a human food). But in Europe, sea lettuce has become regarded as a "green tide," a term somewhat analogous with brown tides and red tides that cause harm to other marine life and are also caused by eutrophication (Vasserot, 1990).
Autecology of Sea Lettuce
Sea lettuce, Ulva spp., is present in the estuaries of eastern and western North America, South America, western and southern Europe from Norway southward into the Black Sea. Sea lettuce is also found in the western Pacific from Japan, Korea, and China to Australia and New Zealand, and also in India and Pakistan (Taylor, 1957; Tseng, 1983; Lavery et al., 1991; Tagliapietra et al., 1998; Sfriso et al., 2001; Harder et al., 2004). It grows in polyhaline areas (Pirou et al., 1991; Raffaelli et al., 1998; Brush and Nixon, 2003), and the mats occur on wide gently sloping sand and mud flats in low energy areas where tidal circulations and wind-driven waves are weak. Any flats that are exposed to moderate winds commonly have scattered sea lettuce thalli (leaf fronds). In exposed areas, winds may drive scattered thalli and small mats into piles against and onto shorelines.
The thallus of sea lettuce begins as a thin, undifferentiated vegetal frond that grows quickly and takes up nutrients rapidly (Littler and Littler, 1980). Its growth is nutrient limited rather than light limited (Valiela et al., 1997). Ulva spp. and Enteromorpha spp. can take up nutrients 4-6 times faster than slower growing perennial plants (Pederse and Borum, 1997). The thalli that grow in sewage-polluted waters contain more nitrogen than those in unpolluted waters (Fritsch, 1956; Wilkinson, 1963). The thallus is two cells thick and is laminate to rounded, often somewhat lobed and undulate (Taylor, 1957), and is usually about 30 cm long and nearly as wide, but may be twice this size (Lee, 1977). The cells are uninucleate and have a single cup-shaped chloroplast. Cell division may occur anywhere in thalli, but all divisions are in a plane perpendicular to the thalli surfaces (Smith, 1955). The stalk is thin and inconspicuous or absent (Taylor, 1957).
[FIGURE 2 OMITTED]
Toxin production in marine macroalgae may be common. Harder et al. (2004) observed that the thallus surfaces of Ulva reticula in China are free of macroinvertebrates. Upon investigation, they discovered that the surface boundary layer of the thalli produces antifouling agents, waterborne toxic macromolecular substances of at least two types, one of which originates from the thalli and the other from an epibiotic bacterium. Magre (1974) and Johnson and Welsh (1985) showed that, in finger bowls, fragments of sea lettuce are toxic to estuarine invertebrates. Also, Aneer (1987) found that in a natural situation in Europe, the eggs of the Atlantic herring, Clupea harengus, were killed in large numbers by exudates released by filamentous brown algae, predominantly Pilayella littoralis.
The published literature reports on the sizes of sea lettuce mats in two areas....
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