Pamela Geddes

Differential heterotrophic utilization of organic compounds by diatoms and bacteria under light and dark conditions

The heterotrophic utilization of organic substrates by diatoms is likely an important survival strategy when light levels are too low for photosynthesis. The objectives of this study were: (1) to determine if heterotrophic utilization of a large array of organic compounds by eight common freshwater benthic diatom taxa was light-dependent, and (2) to determine if organic substrate utilization patterns differed between darkgrown diatoms and bacteria as a possible means of reducing competition by niche separation. Eight lightand dark-grown diatom taxa and five bacterial species were incubated in 96-well Biolog® Microtiter plates with each well containing 1 of 95 different organic substrates. Oxidation rates of each organic substrate were measured through time. There was a substantial increase in the number of organic substrates oxidized by diatoms grown in the dark compared to their light-grown counterparts, indicating that the transport systems for these molecules may be light activated. Therefore, diatoms likely only utilize these metabolically expensive uptake mechanisms when they are necessary for survival, or when substrates are plentiful. A principal components analysis indicated discernible differences in the types of organic-C substrates utilized by dark-grown diatoms and bacteria. Although bacteria were able to oxidize a more diverse array of organic substrates including carboxylic acids and large polymers, diatoms appeared to more readily utilize the complex carbohydrates. By oxidizing different organic substrates than bacteria, heterotrophically metabolizing diatoms may be reducing direct competition and enhancing coexistence with bacteria.

Uncoupling of omnivore-mediated positive and negative effects on periphyton mats

The riverine grass shrimp (Palaemonetes paludosus) and eastern mosquitofish (Gambusia holbrooki) consume periphyton and small invertebrates, potentially affecting periphyton through negative effects (i.e., consumption) and/or positive effects such as nutrient regeneration, physical stimulation, and trophic cascades. We performed field experiments in the Everglades in which omnivores and periphyton were maintained in cages, with a fraction of the periphyton held in omnivore-exclusion bags that allowed passage of nutrients but prevented its consumption or physical disturbance. In some instances, periphyton growth rate increased with increasing omnivore biomass. Omnivores probably stimulated periphyton growth through nutrient regeneration, possibly subsidizing periphyton with nutrients derived from ingested animal prey. The net balance of omnivore-mediated negative and positive effects varied among experiments because of seasonal and spatial differences in periphyton characteristics. Consumption of periphyton mats might have been reduced by the arrangement of palatable algae (green algae and diatoms) within a matrix of unpalatable ones (CaCO3-encrusting filamentous cyanobacteria). In a laboratory feeding experiment, mosquitofish consumed more green algae and diatoms in treatments with disrupted mat structure than in those with intact mats. No difference in diet was observed for shrimp. Our study underscores the complexity of consumer-periphyton interactions in which periphyton edibility affects herbivory and consumers influence periphyton through multiple routes that cannot be fully appreciated in experiments that only investigate net effects.

Periphyton mat structure mediates trophic interactions in a subtropical marsh

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.

Decoupling carbon effects and UV protection from terrestrial subsidies on pond zooplankton

Studies on spatial subsidies have overwhelmingly addressed their trophic nature. However, certain subsidies might affect recipient communities beyond the feeding pathway. Terrestrial colored dissolved organic matter (DOM) is such a subsidy providing a carbon source for bacteria and heterotrophic nanoflagellates which zooplankton can consume (trophic pathway), but also protecting zooplankton against damaging UV radiation through its color (non-trophic pathway). These mechanisms have been quantified separately, but few studies have attempted to decouple them and evaluate their effects in the context of subsidies. In this study, I experimentally isolated the trophic and non-trophic pathways by which DOM (as maple leaf leachate) affects zooplankton, and also addressed how local food resources (i.e., phytoplankton) could mediate the response of zooplankton to DOM subsidies. Terrestrial subsidies (DOM) effectively shielded zooplankton against damaging UV radiation. Local resources (i.e., phytoplankton), however, did not seem to mediate the fitness response of zooplankton to UV radiation. This study also suggested that the carbon in DOM might be slightly detrimental to zooplankton independent of the UV protection effect. High levels of local resources combined with carbon subsidies from DOM did not translate into higher zooplankton survival, presumably because of the detrimental effects of DOM on zooplankton. This study provides further support for the importance of terrestrial subsidies in affecting communities through non-trophic pathways (UV attenuation) and documents that the trophic pathway might be strongly dependent on subsidy quality. Further studies on the role of subsidies that include the less documented non-trophic pathways are needed to improve our mechanistic understanding of how communities and ecosystems respond to spatial subsidies.

Experimental evidence that subsidy quality affects the temporal variability of recipient zooplankton communities

Theory suggests spatial subsidies affect the stability of recipient food webs. Furthermore, context dependencies such as subsidy quality might influence how stabilizing/destabilizing a subsidy may be. Previous studies have shown that high-quality resources destabilize consumers, yet this prediction has rarely been tested empirically within a subsidy framework. I compared the effects of a low-quality, terrestrial carbon subsidy (cDOM derived from maple leaves) and a high-quality one (sucrose) on zooplankton communities. Subsidy quality strongly affected temporal variability of zooplankton taxa, while their mean abundance was less affected. Variability of several taxa was higher in the sucrose than in control and cDOM treatments. In addition, multivariate analyses of zooplankton assemblages indicated that community-level response to subsidy quality was consistent with responses of individual taxa. Zooplankton communities in the sucrose treatment exhibited higher temporal dispersion (greater community variability) relative to controls and cDOM additions. Although the higher biomass of edible phytoplankton in the sucrose treatment could induce zooplankton destabilization, this mechanism failed to explain the observed patterns. Instead, indirect evidence suggested that the increased temporal variability in zooplankton was likely mediated by the effect of subsidy quality on microbial communities, an alternative food source for several zooplankton species. This study thus provides experimental evidence indicating that the more labile carbon subsidy destabilized zooplankton consumers, and implies that subsidy quality may mediate the concordance between theoretical predictions and observations regarding how consumers respond to subsidies.