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S . Macia J. Exp. Mar. Biol. Ecol. 246 2000 53 –67
1. Introduction
Seagrass beds are complex ecosystems composed of many interacting species of plants, invertebrates, and fishes. Tropical and subtropical seagrass beds in particular have
an added level of complexity resulting from the direct grazing of seagrasses, which is more prevalent than in temperate areas Ogden, 1976, 1980; Zieman, 1982. The
seasonal occurrence of large blooms of drift algae also contributes to the complexity of subtropical seagrass meadows Holmquist, 1994, 1997; Bell and Hall, 1997. These two
factors can have major effects on the structure and function of warm-water seagrass beds, yet they have rarely been studied in concert.
Several studies have documented blooms of drift algae in subtropical seagrass beds Gore et al., 1981; Virnstein and Carbonara, 1985; Holmquist, 1994, 1997; Bell and Hall,
1997. These blooms can result in very large mats of algae that compete with the seagrass for light and form bare patches void of seagrass Hull, 1987; Short et al., 1995;
Holmquist, 1997. Dense accumulations of algae can also decrease the flux of oxygen to the sediment, thereby lowering the amount of oxygen available to animals living in or
beneath the algae Hull, 1987; Hansen and Kristensen, 1997. Conversely, drift algal mats greatly increase the structural complexity of the seagrass community and can
provide superior habitat for invertebrates Gore et al., 1981; Holmquist, 1994, 1997. Thus, the effects of drift algae on the floral and faunal components of seagrass beds can
be very complex, and may depend on the physical and biological characteristics specific to the particular seagrass bed in question Hull, 1987.
Sea urchins are the main invertebrate grazers of live seagrass material Greenway, 1976; Zieman, 1982; Valentine and Heck, 1991. The urchin Lytechinus variegatus
Lamarck is commonly found in seagrass beds throughout the western Atlantic and Gulf of Mexico. Much work has been devoted to the ecology of this important grazer,
¨ especially in turtlegrass Thalassia testudinum Banks ex Konig beds, where L
. variegatus most commonly occurs at population densities less than 10 individuals per
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m Moore et al., 1963; Engstrom, 1982; Keller, 1983; Oliver, 1987; Montague et al.,
1988; McGlathery, 1995. Larger urchin populations, however, have been reported and can have serious impacts on the seagrass community in which they reside. The most
dramatic example of this is the formation of grazing fronts — extremely dense
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aggregations up to 600 individuals m of urchins capable of completely denuding
´ areas of seagrass as large as thousands of square meters Camp et al., 1973; Macia and
Lirman, 1999. However, even populations of urchins far smaller than those of the
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grazing fronts 20–40 urchins m can remove a large proportion of the seagrass
standing crop Greenway, 1976, 1995; Valentine and Heck, 1991. The presence of either drift algae or urchin grazers can have important effects on the
seagrass community. The potentially synergistic effect of these factors, however, has not received much attention. This study uses caging experiments in the field to investi-
gate the effects of both algae when present and urchin grazing on a subtropical sea- grass bed. Both the plant and animal components of the seagrass community are con-
sidered.
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2. Methods and materials
