The good, the bad and the Ulva: the density dependent role of macroalgal subsidies in influencing diversity and trophic structure of an estuarine community

Abstract

Worldwide, ecological subsidies enhance ecosystem productivity and therefore trophic support for greater biodiversity of taxa. While studies in terrestrial and aquatic ecosystems demonstrate that the magnitude of subsidies into ecosystems diff ers widely, the thresholds where subsidies may switch from exerting positive to negative eff ects are poorly understood. In estuaries, eutrophication promotes drift macroalgae that deposit on the benthos, cover intertidal fl ats for months and serve as pressed resource subsidies for benthic consumers. We hypothesized there would be a critical threshold of macroalgal biomass where ecosystem-level eff ects would turn from positive to negative. We used manipulative fi eld experiments varying macroalgal mat thickness (0.5, 1.5 and 4 cm) over eight weeks and quantifi ed eff ects on macrofauna on a lagoon mudfl at in California. We documented that plots with mat depths of 0.5 and 1.5 cm had higher diversity by supporting both surface feeding and burrowing detritivores. Non-metric multidimensional scaling showed that the benthic community diverged with mat depth over the course of the experiment. After eight weeks, surface deposit feeders were associated mainly with 0.5 cm macroalgal subsidies, whereas subsurface deposit feeding capitellids were closely linked with 4 cm mats. Depth profi les of pore water sulfi de concentration collected from 4 cm mats were 7622 Ϯ 5294 μ M, mean Ϯ SE, (mean of means across depth profi les), whereas 0.5 cm treatments resulted in sulfi de concentrations that were 0.25% of the 4 cm treatments. Th is suggests that the mechanism of negative eff ects for elevated macroalgal subsidies was development of anoxic conditions promoting sulfi de accumulation. Th us, our study was the fi rst to fi nd a critical threshold, or ecological tipping point, beyond which the eff ects of anthropogenically enhanced subsidies to estuarine mudfl at communities switched from positive to negative and to describe the mechanism by which elevated subsides altered the abiotic environment and likely reduced ecosystem functioning.

Impact of exotic macroalga on shorebirds varies with foraging specialization and spatial scale

“…Subsurface-deposit feeders are less likely to be affected; however, these deeper prey are less important for many short-billed shorebirds that are limited to feeding on shallow, surface-deposit-feeders and epifauna. The impact of A. vermiculophyllum for foraging shorebirds depends on their dietary flexibility, and the magnitude of the changes to the abundance and distribution of the prey community [4,5,31,32]. Observing shorebird distribution and behavior may provide a clearer, more integrated picture of how A. vermiculophyllum affects prey resources than monitoring invertebrates directly.…”

“…Therefore, when considering the effects of A. vermiculophyllum on shorebirds, the most important traits to consider would be foraging mode, rather than diet. Macroalgae change the structure of substrate [19], may obscure visual foraging cues [31], and alter prey distributions [4,32]. It is possible shorebirds with specialized foraging modes for soft muds would be unable to optimally forage from the vegetated microhabitat created by A. vermiculophyllum.…”

Context dependency in the effect of Ulva‐induced loss of seagrass cover on estuarine macrobenthic abundance and biodiversity

“…In preparation for the 2008 Beijing Olympic sailing events, 1 × 10 6 t were removed from Qingdao in the Yellow Sea at a cost of >US$85 million (Liu, Keesing, Xing, & Shi, ; Smetacek & Zingone, ). The magnitude of the effect of such blooms may vary, however, dependent on functional group (Green, Sutula, & Fong, ), their severity (Green & Fong, ), their timing (Olyarnik & Stachowicz, ) and their persistence (Arroyo & Bonsdorff, ).…”

Tipping points and multiple drivers in changing aquatic ecosystems: A review of experimental studies

“…The authors showed that DIN additions change algal group diversity and the abundance of different groups, suggesting that ambient DIN concentrations should not exceed breakpoint concentrations (25 and 50 μ mol L −1 ). On the other hand, Green and Fong (2015) focused on the effect of eutrophication on macrofauna and microphytobenthos diversity (community) as well as sediment iron and sulfide concentrations (ecosystem functioning). The authors identified a point beyond which macroalgal mat thickness (used to represent eutrophication) reduced macrofauna biodiversity (> 1.5 cm).…”