Rhizophytic green algae (e.g., Caulerpa, Halimeda, Penicillus, Rhipocephalus, and Udotea) are abundant macrophytes throughout tropical and subtropical regions that anchor with a net of rhizoids that binds sediments to form holdfasts in unconsolidated sediments. We examined
both the above- and belowground components of these understudied algal communities with transect sampling at three tropical sites with carbonate sediments in the Lower Florida Keys and one subtropical site (Sunset Beach) with quartz sediments on the central west coast of Florida, an area previously
unstudied. Species composition differed among sites, with Caulerpa spp. comprising >50% of organic biomass at Sunset Beach, and a greater diversity of calcified species present in the Keys. The mean density (365 vs 68–143 thalli m–2) and mean volume of holdfasts
(1034 vs 504–199 ml m–2) of rhizophytic algae were greater at Sunset Beach than at Keys sites. For species found at both regions, analyses of morphometric measurements revealed taller thalli at Sunset Beach, larger holdfast volumes at sites with larger sediment grain
size, a positive correlation between the wet weight of the aboveground portion of the thallus and the volume of the holdfast, and an anchorage depth generally shallower than the rooting depth of co-occurring seagrasses. Differences among species in contributions to above- and belowground components,
even within a genus, indicate that species identity is important for predicting a rhizophytic alga's role in ecosystem function. These data are some of the first on both the above- and belowground components of natural mixed rhizophytic algal communities.
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