Macroalgal mats control trophic structure and shorebird foraging behavior in a southern California estuary

Macroalgal blooms that cover extensive sections of shorebird foraging grounds influenced prey availability, skewing it towards generalist foragers (sandpipers and willets) and away from specialist foragers (marbled godwits). Additionally, sandpipers and godwits switched from pecking to probing on mats demonstrating that macroalgae obscured visual hunting cues. Willets did not avoid blooms nor change foraging technique, likely because they consume epifauna not affected by macroalgae. This study demonstrated that shorebirds with limited diet diversities and prey search techniques may suffer in eutrophic estuaries with extensive and persistent macroalgal blooms.;Field experiments demonstrated that macroalgae influenced shorebird prey availability biotically from the bottom up. Higher abundances of detritivorous macrofauna were found under live mats not macroalgal mimics. Sediment pigment analysis indicated in situ macroalgal decomposition that may stimulate the detrital pathway. As mat thickness and duration of macroalgal treatments increased, subsurface cletritivores, which burrow out of the reach of many predators, replaced surface deposit feeders. Thicker mats produced low sediment oxygen and high sulfide concentrations eliminating surface feeders, which may have negative effects on estuarine food webs including reduced prey availability for shorebirds.;Macroalgae affected microphytobenthic communities but the effect was seasonal. A spring caging experiment indicated that cyanobacteria, sporeling macroalgae benthic diatoms increased under moderate and thick mats. Increased MPB could result in higher abundances of shorebird prey. However, if this shift is primarily towards cyanobacteria, a poor quality food for macrofauna, prey availability could decline. Effects of macroalgae may be temporal. In summer I recorded no response of the MPB by any macroalgal treatment. Sulfide may have slowed microphytobenthic growth in summer and eliminated effects of mats. Interactions between mats and the MPB indicated a positive biotic response in spring but more research is needed to identify effects in summer.;This study identified that macroalgal blooms can shift benthic communities toward those that reduce trophic support to shorebirds in less than eight weeks and significantly altered shorebird foraging behavior. My work showed that monitoring and management of macroalgal blooms may need to become a critical component part of conservation efforts, especially in key shorebird foraging habitats.