Understanding how biotic indicators respond to natural variability is essential for the health state evaluation of the benthic ecosystems. This study analysed the composition and structure of the benthic communities in the coastal lagoon Stagno Longu of Posada (Sardinia) to investigate the seasonal response of the biological indicator M-AMBI. Multivariate analysis and biodiversity descriptors showed temporal (winter–summer) and spatial variability (across the three basins of the lagoon).

The ecological status (ES) of Stagno Longu was assessed using M-AMBI index: results mainly showed the lagoon to be in “good” and “moderate” status across seasons and basins, while “poor” status was recorded in the North basin during summer.

Despite the absence of direct human pressures, sediment organic content (OC) varied seasonally and spatially and M-AMBI revealed winter to summer shifts on the ES of 3 out of 6 sampling stations. Overall, this study reveals negative significative correlation on the variations on M-AMBI with the increase of sediment OC across the studied sites, none the less discrepancies on the ES of certain stations were detected causing over/underestimation.

Results suggested that natural seasonal variability is a relevant factor to be considered while performing ES assessments and implementing monitoring programmes in Mediterranean transitional ecosystems when applying indicators such as M-AMBI.

对于底栖生态系统的健康状况评估，了解生物指标如何响应自然变异是至关重要的。这项研究分析了波萨达（撒丁岛）沿海泻湖Stagno Longu底栖动物群落的组成和结构，以调查生物指标M-AMBI的季节性响应。多变量分析和生物多样性描述符显示了时间（冬季至夏季）和空间的可变性（在泻湖的三个盆地之间）。

利用M-AMBI指数评估了Stagno Longu的生态状况（ES）：结果主要显示泻湖在整个季节和盆地中处于“良好”和“中度”状态，而北部盆地在2006年期间记录为“较差”状态夏天。

尽管没有直接的人为压力，沉积物有机物含量（OC）随季节和空间变化，M-AMBI在6个采样站中的3个的ES上揭示了冬季到夏季的变化。总体而言，这项研究揭示了在整个研究地点，M-AMBI的变化与沉积物OC的增加呈负显着相关，尽管如此，某些站点的ES差异仍导致高估/低估。

结果表明，当应用M-AMBI等指标进行地中海ES评估和在地中海过渡生态系统中实施监测计划时，应考虑自然季节性变化。