Influence of habitat connectivity and seasonality on the ichthyofauna structure of a riverine knickzone

ABSTRACT Knickzones are originated from basaltic outcroppings, present runs, riffles and pools and are highly influenced by flood pulses, which maintain their natural dynamic. However, the construction of hydroelectric power plants alters or eliminate the dynamism of this area and can affect the resident fauna that may be dependent on it. The aim of this study was to evaluate the organization of a knickzone’s ichthyofauna considering the influence of seasonality and connectivity of habitats. The study was performed in a knickzone located in the Sapucaí-Mirim River, Southeast Brazil. We sampled four rocky pools connected to the river and three isolated pools, during rainy and dry conditions. The analysis of the two factors (connectivity and seasonality) and of their interaction showed a significant influence only for seasonality on ichthyofauna structure, with higher values of abundance in the rainy season. The species that most contributed to the high dissimilarity between seasons were Knodus moenkhausii (50% of contribution) and Astyanax bockmanni (21%). The former is the most abundant species in the rainy season and the later in the dry season. The alteration between low and high water level occurs frequently in knickzones, as it is a rocky shallow platform in the middle of a river, with floods occurring seasonally or in stochastic short-term periods. This hydrological seasonal dynamic, high limnological variability and complex interactions of different habitats (pools, runs and rapids) explain the particular ichthyofauna structure in such small area. Our results also indicate the potential importance of basaltic knickzones for regional fish diversity conservation, especially due to the imminent threat by intensive hydropower reservoir construction.摘要 裂点带（knickzone）源于玄武岩露头，区域内分布有平流段、浅滩与深潭，且受洪水脉冲影响显著，这一过程维系着其天然动态平衡。然而，水力发电厂的建设会改变乃至消除该区域的动态特征，并可能对依赖于此的本土鱼类区系（ichthyofauna）造成不利影响。 本研究旨在探究季节变化与栖息地连通性的双重影响下，裂点带鱼类区系的群落结构。研究在巴西东南部的萨普卡伊-米林河（Sapucaí-Mirim River）中的一处裂点带开展。我们于雨季与旱季期间，对4处与河流连通的岩质深潭及3处孤立深潭进行了采样。 针对连通性、季节变化及其交互作用这三类因素的分析结果显示，仅季节变化对鱼类区系结构存在显著影响，且雨季的鱼类丰度更高。 对季节间物种组成差异贡献最大的两个物种分别为莫氏克氏脂鲤（Knodus moenkhausii，贡献占比50%）与博克曼丽脂鲤（Astyanax bockmanni，贡献占比21%）。前者为雨季优势物种，后者为旱季优势物种。 裂点带作为分布于河流中段的岩质浅滩平台，水位高低交替频繁，洪水可季节性发生或随机短期出现。这种水文季节动态、较高的湖沼学变异性，以及不同栖息地（深潭、平流段与急流段）间的复杂交互作用，共同塑造了该小型区域内独特的鱼类区系结构。 本研究结果同时表明，玄武岩裂点带对区域鱼类多样性保护具有潜在重要意义，尤其是考虑到其正面临集约化水力发电水库建设的迫在眉睫的威胁。