Response of small motile epifauna to complexity of epiphytic algae on seagrass blades

Field collections and experiments were performed to examine the relationship between the biomass of epiphytic algae (= habitat complexity) on Thalassia testudinum blades and the density of associated motile epifauna. Samples collected at Egmont Key, Florida indicated a significant positive association between the density of harpacticoid copepods, the most numerous taxon on seagrass blades, and biomass of the dominant epiphyte, Giffordia mitchelliae. Similar results were noted for nematodes, amphipods, and crustacean nauplii. Seagrass blades with large amounts of epiphytic algae are often older than blades with lesser amounts of algae, and have had a longer time to be colonized by epifauna. A recolonization experiment was conducted in a T. testudinum bed in the Indian River lagoon, Florida examining associations between the density of epifauna on blades of the “same” age (blades defaunated and replanted in the field) with various amounts of epiphytes. After 2 days a significant positive relationship between epiphyte biomass and density of dominant epifauna (copepods, crustacean nauplii, and nematodes) was recorded, suggesting that results were attributable to differential amounts of epiphytic algae since length of colonization time did not vary among blades. Recolonization experiments in a T. testudinum bed at Ft. Desoto, Florida using artificial blades (polypropylene ribbon) and 3 densities (no, intermediate, and high) of artificial epiphytes (clumps of cotton-polyester thread) produced results comparable to those above within 3 days. Highest densities of dominant epifauna (copepods, nematodes, and polychaetes) were found on blades in the high epiphyte treatment. Mean numbers of copepods and also polychaetes on high and intermediate epiphyte treatments were not significantly different from each other, but were significantly higher than on blades with no epiphytes. Nematode densities declined significantly with decreasing amounts of artificial epiphytes. These results suggest that much of the relationship observed between motile epifauna and epiphytic algae on seagrass blades may be attributed to the physical structure of the algae, given the similar responses of most fauna to both living and artificial plant material. The pattern observed for harpacticoid copepods and epiphytic algae on seagrass blades in field collections and experiments varied among species. Laophontid sp., Harpacticus sp., and Diosaccus sp. were strongly positively associated with the amount of epiphytic algae, however, Amphiascus sp., Ectinosomatid spp., and Dactylopodia tisboides showed little association. The results of this study demonstrate that relationships which have been shown for larger plant structure and fauna also hold for small motile epifauna and fine scale vegetative architecture