October 6 2009

12:00 154 BSRB

Ecolunch: Lauri Green
Department of Ecology and Evolutionary Biology, UCLA

Macroalgal blooms alter food web structure and shorebird foraging in a southern California lagoon.


The potential for catastrophic shifts in trophic function and community structure due to the dominance of ephemeral species has increased due to growing anthropogenic influence on marine ecosystems. Although macroalgal blooms are a growing phenomenon in estuaries worldwide, the role of blooms in changing trophic structure and shorebird vulnerability is not well known. Surveys demonstrated that extensive and persistent macroalgal mats on mudflats supported fewer and less diverse communities of benthic infauna, yet hosted many smaller bodied fauna within mats. Caging experiments controlling abundance of macroalgae were replicated in spring and summer and demonstrated that low abundances of algae had positive effects on sediment infauna. In addition, surface feeding detritivores were favored under thinner mats while deeply burrowing detritivores have higher abundances under thicker mats. To determine the mechanism of this community shift we quantified changes in hydrogen sulfide accumulation and found record concentrations in pore waters of moderate and high abundances of macroalgae. Hydrogen sulfide is toxic to surface feeders but not to highly tolerant, deep burrowing infauna infauna. In another experiment, we determined that that the positive effects of low abundances of macroalgae are biotic, most likely providing trophic support. We used a plastic mimic that provided the structure of macroalgae and compared infaunal responses to plots with low and moderate abundances of macroalgae. All infauna were higher in treatments with macroalgae and lowest in the abiotic mimic showing that macroalgae supported the estuarine food web from the bottom-up. Focal analysis revealed both large (marbled godwits) and small (sandpipers) shorebirds shifted foraging behavior from pecking on bare sediment to probing on macroalgal mats. This may increase energy expenditure for shorebirds due to lower prey availability under mats. While neither marbled godwits nor sandpipers avoided macroalgae, marbled godwits avoided foraging on them especially at high algal abundances. In contrast, sandpipers may benefit from the high abundance of small bodied infauna within the mats which are too small for marbled godwits to consume. Thus, larger shorebirds may be more vulnerable to macroalgal blooms if their prey is eliminated or inaccessible. We demonstrated the potential for macroalgal mats to cause widespread catastrophic shifts in trophic structure in estuaries and the mechanisms driving these changes.















































































































































































































































































































































































































































































































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