Macroalgal blooms can trigger adverse biogeochemical conditions at the sediment-water interface of shallow coastal areas, hence threatening critical habitats such as seagrasses meadows. The direction and magnitude of macroalgal blooms impacts on the aquatic ecosystem can be context-dependent, varying according to the local hydrodynamic conditions. Thus, studies investigating the impacts of stagnant algal depositions on the benthos may fail to address realistic situations and interactions which are common in well-flushed systems. This is especially true for the South America coast, where no study has investigated the effects of macroalgal blooms on seagrasses meadows. To fully understand the impacts of macroalgal blooms on sediment biogeochemistry and seagrass habitats across distinct environmental conditions and biogeographical regions, two independent, complementary field experiments replicated the natural temporal patterns of drift macroalgal mats depositions on unvegetated and vegetated (Ruppia maritima meadows) shoals of the Patos Lagoon estuary (PLE), a subtropical, high hydrodynamic system in southern Brazil. Transitory depositions of algal mats alleviated deleterious biogeochemical conditions in the sediment-water interface of unvegetated bottoms. Nevertheless, these unstable algal depositions promoted significant reductions in R. maritima biomass, by reducing their shoot height and density, and rhizome length. That plant biomass reductions were followed by a decrease in the abundance of the dominant infaunal tanaidacean Monokalliapseudes schubarti, indicating that algal impacts on seagrasses were transferred to higher trophic levels. Our results suggest that, although unstable deposition of drift algal mats can attenuate potential adverse impacts at the sediment-water interface, the physical stress during mats advection can still trigger small seagrass losses. This process may diminish the resilience of R. maritima meadows in the PLE, with impacts on estuarine nutrient cycling and secondary production. We conclude that, although harmful drift macroalgal blooms area global phenomenon, the mechanisms through which macroalgae impair seagrass habitats may vary according to the environmental context. Therefore, further studies are necessary to identify the underlying mechanisms of drift macroalgae-seagrass-macrofauna interactions in high hydrodynamic systems and their generality across distinct biogeographical areas.

大型藻华会在浅海沿海地区的沉积物-水界面触发不利的生物地球化学条件，从而威胁到诸如海草草甸等重要栖息地。大型藻华对水生生态系统的影响的方向和大小可能取决于环境，并根据当地的水动力条件而变化。因此，研究停滞的藻类沉积物对底栖生物的影响的研究可能无法解决现实情况和相互作用，这在井井有条的系统中很常见。对于南美海岸尤为如此，在南美海岸，尚无任何研究调查藻类大量繁殖对海草草甸的影响。为了充分了解大型藻华对不同环境条件和生物地理区域内沉积物生物地球化学和海草栖息地的影响，川蔓藻草地）的帕托泻湖河口（PLE），在巴西南部属亚热带，高水动力系统的滩涂。藻类垫层的短暂沉积减轻了无植被底部沉积物-水界面中有害的生物地球化学条件。然而，这些不稳定的藻类沉积促进了显著减少R.袍生物质，通过减少它们的拍摄高度和密度，以及根茎长。在植物生物量减少之后，占主导地位的臭名昭著的tanaidacean Monokalliapseudes schubarti的丰度降低了。，表明藻类对海草的影响已转移到较高的营养水平。我们的结果表明，尽管漂流藻席的不稳定沉积可以减弱沉积物-水界面的潜在不利影响，但席席平流过程中的物理应力仍然可以触发少量海草损失。此过程可能会降低滨海罗勒（R. maritima）的适应力PLE中的草地，对河口养分循环和次级生产产生影响。我们得出的结论是，尽管有害漂移的大型藻类开花区域是全球性现象，但大型藻类损害海草栖息地的机制可能会根据环境而有所不同。因此，有必要进行进一步的研究来确定高水动力系统中大型藻类-海草-大型动物之间的相互作用的潜在机理及其​​在不同生物地理区域中的普遍性。