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.

在全球范围内，生态补贴（ecological subsidies）可提升生态系统生产力，进而为各类群的更高生物多样性提供营养支撑。尽管陆地与水生生态系统的相关研究表明，输入生态系统的补贴强度差异悬殊，但补贴从产生正效应转向负效应的临界阈值仍鲜为人知。在河口生态系统中，富营养化作用催生漂浮大型藻类，这些藻类沉降至底栖环境，覆盖潮间带滩涂长达数月，成为底栖消费者的资源补贴来源。我们提出假说：大型藻类生物量存在临界阈值，当超过该阈值时，生态系统层面的效应将从正向转为负向。我们在加利福尼亚州的一处泻湖泥滩开展了为期8周的操控性野外实验，通过调整大型藻类藻垫厚度（0.5、1.5和4厘米），量化其对大型底栖动物的影响。研究结果显示，藻垫厚度为0.5厘米和1.5厘米的样地，由于同时支持表层取食和穴居食腐动物的生存，其物种多样性更高。非度量多维标度（Non-metric Multidimensional Scaling）分析结果显示，实验过程中底栖群落组成随藻垫厚度出现分化。实验开展8周后，表层沉积食性动物主要与0.5厘米厚度的大型藻类补贴相关联，而穴居沉积食性的蛰龙介科（Capitellidae）动物则与4厘米厚度的藻垫紧密相关。从4厘米厚度藻垫中采集的孔隙水硫化物浓度剖面均值为7622±5294 μM（均值±标准误，为各剖面均值的平均）；而0.5厘米处理组的硫化物浓度仅为4厘米组的0.25%。这表明，过量大型藻类补贴产生负效应的机制是缺氧环境的形成，进而促进了硫化物的积累。综上，本研究首次发现了临界阈值（或称生态临界点），当人为增强的补贴输入河口泥滩群落超过该阈值时，其效应将从正向转为负向；同时本研究还阐明了过量补贴改变非生物环境、并可能降低生态系统功能的作用机制。