Periphyton Mat Structure Mediates Trophic Interactions in a Subtropical Marsh

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Freshwater marshes are often subject to severe disturbance from seasonal drying (dry-downs) and frequently have distinct food webs relative to other freshwater systems. Subtropical marshes in the Florida Everglades have a unique trophic structure characterized by low nutrients, high standing stocks of algae in the form of floating and benthic periphyton mats, low standing stocks of primary and secondary consumers (omnivorous small fishes, tadpoles, and large macroinvertebrates), and very low standing stocks of tertiary consumers (large fishes). To account for this trophic structure, two hypotheses have been proposed: 1) high algal standing stocks result from top-down control over omnivores (small fishes, tadpoles, and macroinvertebrates) by large fishes, or 2) that the physical and biotic structure of periphyton mats impedes grazing. We conducted caging experiments before and after the dry season to delineate interactions among species influencing trophic structure in these marshes. Treatments included a refuge cage that was accessible to omnivores but excluded large fishes, an open cage accessible by omnivores and large fishes, and an omnivore exclusion cage designed to exclude fishes, tadpoles, and large macroinvertebrates. The physical and biotic structure of mature periphyton mats mediated direct and indirect interactions of omnivores and large fishes. More omnivores used the refuge treatment compared to the open treatment, likely to avoid large fishes, leading to a trophic cascade where abundance of epiphytic algae was reduced. Reductions in epiphytic algae were especially pronounced after the dry season when neonate sailfin molly were the dominant omnivore. We did not find comparable reductions of periphyton-mat biomass in the refuge treatment, suggesting that edible forms within these mats gain an associative refuge from grazers. Reduced grazing on edible algae in mature periphyton mats may explain the high standing stocks of algae characteristic of Everglades marshes.


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