Understanding the effects of predictability, duration, and spatial pattern of drying on benthic invertebrate assemblages in two contrasting intermittent streams

In the present study, we examined the effects of different drying conditions on the composition, structure and function of benthic invertebrate assemblages. We approached this objective by comparing invertebrate assemblages in perennial and intermittent sites along two intermittent Mediterranean streams with contrasting predictability, duration, and spatial patterns of drying: Fuirosos (high predictability, short duration, downstream drying pattern) and Rogativa (low predictability, long duration, patchy drying pattern). Specifically, we quantified the contribution of individual taxa to those differences, the degree of nestedness, and shifts in the composition, structure and function of benthic invertebrate assemblages along flow intermittence gradients. We observed greater effects of drying on the benthic invertebrate composition in Fuirosos than in Rogativa, resulting in a higher dissimilarity of assemblages between perennial and intermittent sites, as well as a lower degree of nestedness. Furthermore, a higher number of biotic metrics related to richness, abundance and biological traits were significantly different between perennial and intermittent sites in Fuirosos, despite a shorter dry period compared to Rogativa. At the same time, slightly different responses were detected during post-drying (autumn) than pre-drying (spring) conditions in this stream. In Rogativa, shifts in benthic invertebrate assemblages along increasing gradients of flow intermittence were found for three metrics (Ephemeroptera, Plecoptera and Trichoptera (EPT) and Odonata, Coleoptera and Heteroptera (OCH) abundances and aerial active dispersal. Furthermore, we demonstrated that combined gradients of dry period duration and distance to nearest perennial reach can generate complex, and different, responses of benthic invertebrate assemblages, depending on the flow intermittence metric. Our study advances the notion that special attention should be paid to the predictability, duration and spatial patterns of drying in intermittent streams in order to disentangle the effects of drying on benthic invertebrate assemblages, in particular in areas subject to high spatial heterogeneity and temporal variability in drying conditions.本研究探讨了不同干涸条件对底栖无脊椎动物群落（benthic invertebrate assemblages）的组成、结构与功能的影响。为达成该研究目标，我们对比了两条地中海间歇性河流的常年性与间歇性河段的无脊椎动物群落——这两条河流分别为富罗索斯河（Fuirosos，高可预测性、短干涸时长、下游型干涸格局）与罗加蒂瓦河（Rogativa，低可预测性、长干涸时长、斑块状干涸格局），二者在干涸的可预测性、持续时长与空间格局上存在显著差异。具体而言，我们量化了单个类群对群落间差异的贡献程度、嵌套性（nestedness）程度，以及沿水流间歇性（flow intermittence）梯度变化的底栖无脊椎动物群落的组成、结构与功能转变。 研究发现，相较于罗加蒂瓦河，富罗索斯河的干涸对底栖无脊椎动物群落组成的影响更为显著，导致常年性与间歇性河段的群落相异度更高，且嵌套性程度更低。此外，尽管富罗索斯河的干涸时长较罗加蒂瓦河更短，但其常年性与间歇性河段间在物种丰富度、个体丰度及生物学性状相关的生物指标（biotic metrics）上存在显著差异的数量更多。同时，该河流在干涸后（秋季）与干涸前（春季）的群落响应存在细微差异。 在罗加蒂瓦河中，随水流间歇性梯度升高，底栖无脊椎动物群落在3项指标上出现显著变化：蜉蝣目、襀翅目与毛翅目（Ephemeroptera, Plecoptera and Trichoptera, EPT）的丰度，蜻蛉目、鞘翅目与异翅亚目（Odonata, Coleoptera and Heteroptera, OCH）的丰度，以及主动空中扩散（aerial active dispersal）能力。此外，我们证实，干涸时长与到最近常年性河段的距离这两类复合梯度，会根据所选用的水流间歇性指标的不同，对底栖无脊椎动物群落产生复杂且各异的响应。 本研究进一步明确，在间歇性河流中应特别关注干涸的可预测性、持续时长与空间格局，以厘清干涸对底栖无脊椎动物群落的影响，尤其在干涸条件存在高度空间异质性与时间变异性的区域。