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 detri...在全球范围内，生态补贴（ecological subsidies）可提升生态系统生产力，进而为各类群生物多样性提供营养支撑。尽管针对陆地与水生生态系统的研究表明，输入至生态系统的补贴量差异悬殊，但关于补贴从产生正向效应转向负向效应的临界阈值，目前仍缺乏深入认知。在河口（estuary）生态系统中，富营养化（eutrophication）会催生漂浮大型藻类，这些藻类沉降至底栖生境（benthos）、覆盖潮间带滩涂达数月之久，并作为可供底栖消费者利用的压实型资源补贴。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由正向转为负向。我们开展了为期8周的控制性野外实验，设置0.5 cm、1.5 cm及4 cm三种大型藻垫厚度梯度，并量化了加州泻湖泥滩生境中大型底栖动物（macrofauna）所受的影响。我们观测到，藻垫厚度为0.5 cm与1.5 cm的样地物种多样性更高，其生境可同时支撑表层取食与穴居食腐类（detri...）