Considering that most seagrass beds dominated by Thalassia spp. are multi-taxon conglomerates, it is amazing that very few studies have dealt with interactions between the plants that compose the seagrass beds. In particular, interactions between the rooted species have received little attention. Amongst the few experimental studies to examine inter-specific interference between seagrass species is that of Williams (1987), who tested the effect of T. testudinum on S. filiforme in the U.S. Virgin Islands. She found that T. testudinum had a negative effect on the production of S. filiforme and that belowground competition for nutrients was more important than shading by the leaf canopy of T. testudinum. A surprising result was a positive interaction between these two species, in that the leaf canopy of T. testudinum protected S. filiforme leaves from breaking. In Tampa Bay, Rose and Dawes (1999) found that T. testudinum growing in patches intermixed with H. wrightii had reduced growth and biomass in comparison with monospecific patches. Fourqurean et al. (1995) reported that H. wrightii competed successfully with T. testudinum under high-nutrient conditions, and replaced the latter seagrass after 8 years of fertilization of a seagrass meadow in Florida Bay. Under lower-nutrient conditions, H. wrightii often occurs during the early stages of succession to be replaced in time by T. testudinum, which is considered to be the climax species and competitively superior (see section 6.3). Fourqurean et al. (1995) explained the dominance of H. wrightii in their study by the higher nutrient demand of H. wrightii than that of T. testudinum; and during succession T. testudinum replaces H. wrightii due to its capacity to draw nutrients down to concentrations below the requirements of the latter seagrass. Davis and Fourqurean (2001) demonstrated that there is competition for nutrients between co-occurring T. testudinum and the rhizo-phytic calcareous alga Halimeda incrassata in the Florida Keys. The presence of seagrass decreased the size of the algal thalli by 20% but the algae only decreased T. testudinum shoot size by 10%. Tissue nitrogen of T. testudinum decreased in the presence of H. incrassata, suggesting that nitrogen competition was the mechanism of interaction. In Thailand, Nakaoka and Izumi (2000), studied interference at patch level for the seagrasses T. hemprichii and Halophila ovalis. They found that H. ovalis had higher biomass and lower production in the center than at the edges of the patches joining unvegetated areas, whereas these differences between center and edge were not detected when the H. ovalis patches joined T. hemprichii.
Rose and Dawes (1999) studied intra-specific interference in shoots of T. testudinum by comparing their leaf mass and growth in low and high-density monospecific meadows. They reported that in high-density monospecific meadows growth and biomass of the shoots were reduced (compared with lower density meadows), suggesting that the foliar shoots competed for available resources. Van Tussenbroek et al. (2000) reported inhibition of foliar development at high-rhizome density in T. testudinum, and found that the spacing of foliar shoots was regulated just below the critical density at which negative effects on foliar development would occur, which is quite common for clonal plants (Hutchings, 1979). The shoots in which development was inhibited became dormant. Subsequent foliar development of these dormant shoots after experimental fertilization suggests that competition for belowground nutrients played a significant role in the inhibition of the development of foliage leaves. These findings are in apparent contrast with the findings of Rose and Dawes (1999), who found that reduction of light by dense leaf canopies was a likely mechanism of intra-specific competition at high densities. However, it has to be borne in mind that these studies were carried out in contrasting areas, the first an oligotrophic reef lagoon, and the second a mangrove-lined estuary.
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