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

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 differs widely, the thresholds where subsidies
may switch from exerting positive to negative effects are poorly
understood. In estuaries, eutrophication promotes drift macroalgae that
deposit on the benthos, cover intertidal flats 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
effects would turn from positive to negative. We used manipulative field
experiments varying macroalgal mat thickness (0.5, 1.5 and 4 cm) over
eight weeks and quantified effects on macrofauna on a lagoon mudflat 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 profiles of pore water sulfide
concentration collected from 4 cm mats were 7622 ± 5294 μM, mean ± s.e.,
(mean of means across depth profiles), whereas 0.5 cm treatments resulted
in sulfide concentrations that were 0.25% of the 4 cm treatments. This
suggests that the mechanism of negative effects for elevated macroalgal
subsidies was development of anoxic conditions promoting sulfide
accumulation. Thus, our study was the first to find a critical threshold,
or ecological tipping point, beyond which the effects of anthropogenically
enhanced subsidies to estuarine mudflat communities switched from positive
to negative and to describe the mechanism by which elevated subsides
altered the abiotic environment and likely reduced ecosystem functioning.