Over the past 50 years, many Florida springs and their associated rivers have experienced shifts in vegetative community structure and declines in ecosystem health. Once dominated by rooted macrophytes, a large number of spring ecosystems are now populated by nuisance macroalgae such as Lyngbya wollei and Vaucheria spp. . Such changes have been linked to alterations in food web structure and key ecosystem processes. Here we use stable isotope analysis (SIA) coupled with dietary analyses to address the following questions regarding the role of nuisance algae in the spring-river food webs of Florida: What are the major pathways of energy flow and material transport in Florida spring-river ecosystems? Which consumers forage on nuisance algae and to what degree? Which predators consume algal grazers?
We performed this study within a 1st magnitude spring-river system, Silver River, in northcentral Florida. SIA was performed on dominant vegetation types, herbivores, and organisms occupying higher-order trophic levels. Dietary analysis was performed on multiple species of fish and turtles as well as American alligators. We used isotopic mixing model analyses (SIAR) to estimate the proportional contribution of identifiable resource groups to consumers.
We found δ13C values of nuisance filamentous algae (mean ± SD = -42.6 ‰ ± 2.4) to be distinct from submersed (-34.7 ‰ ± 3.7) and emergent macrophytes (-30.3 ‰ ± 2.0) as well as epiphytic (-34.2 ‰ ± 6.5) and other unattached algae (-37.9 ‰ ± 3.4). Findings suggest that submersed macrophytes and their epiphytes fuel much of the secondary production that, in turn, supports higher trophic levels. SIAR results suggest only a few groups of emergent insects, i.e. chironomids and trichopterans, and certain herbivorous fish and emydid turtles rely to a significant degree on nuisance algae for their energetic needs. Median estimates for the proportional contribution of nuisance algae to consumer diets ranged from nearly 0% to as high as 74% (trichopterans). These findings suggest that the energy and nutrients sequestered by nuisance algae in this system and likely other Florida spring-river systems circumvents the lotic food web. It is likely that emergent insects transport much of the nutrients and energy to the surrounding terrestrial landscape while long-lived turtles and fish retain assimilated materials as biomass. Essentially, nuisance algae production in Florida spring-rivers may be largely decoupled from the lotic food web.