The delta of the Parnaíba River has a set of highly dynamic, productive ecosystems located in the Amazon–semiarid interface environment on the coast of Brazil. Phytoplankton is a key element in the evaluation of aquatic environments, as it is extremely sensitive to the broad spatial and seasonal variability of masses of freshwater and the physicochemical conditions of the water. To evaluate spatial–seasonal variation in the hydrological characteristics and the phytoplankton community in relation to the saline gradient, three sampling campaigns were performed at eight points considering seasonal periods (dry, rainy and transition) and spatial distribution (Nearshore Turbid Zone [NTZ], Mixing Zone [MZ] and Tidal River Zone [TRZ]) during neap tides. Variability in the environmental characteristics and phytoplankton was tested using permutational multivariate analysis of variance, dissimilarities and correlations. The data revealed a strong spatial–seasonal​ influence throughout the entire length of the estuary, with a clear spatial gradient related to salinity. Thalassiosira subtilis was the bioindicator for the NTZ, Cyclotella meneghiniana for the MZ and Oocystis parva for the TRZ. Concentrations of chlorophyll a, suspended particulate matter and nitrite were the main variables affected by the two factors combined (seasonality and spatial distribution). Moreover, total dissolved solids, nitrate and nitrite were the variables that promoted favorable ecological conditions for the development of the phytoplankton community associated with seasonality, especially the growth of Cyclotella meneghiniana, which proved to be better adapted to these conditions and was considered a key species in the area studied.

帕尔纳伊巴河三角洲位于巴西海岸的亚马逊河与半干旱河流域的交界处，有一组高度动态，高产的生态系统。浮游植物是评估水生环境的关键要素，因为它对淡水质量的广泛空间和季节变化以及水的理化条件极为敏感。为了评估与盐梯度有关的水文特征和浮游植物群落的时空变化，在八个点上进行了三个采样活动，其中考虑了季节（干旱，多雨和过渡）和空间分布（近岸混浊带[NTZ]），潮汐期间的混合区[MZ]和潮汐河区[TRZ]）。使用方差，相异性和相关性的置换多元分析对环境特征和浮游植物的变异性进行了测试。数据显示，在整个河口整个长度上都有强烈的时空影响，并且与盐度有关的空间梯度明显。枯草藻（Thalassiosira subtilis）是NTZ的生物指示剂，MZ的米氏小环藻和TRZ的卵囊小卵菌是生物指示剂。叶绿素a，悬浮颗粒物和亚硝酸盐的浓度是受两个因素（季节和空间分布）共同影响的主要变量。此外，总溶解固体，硝酸盐和亚硝酸盐是促进与季节相关的有利于浮游植物群落发展的生态条件的变量，尤其是环戊小环藻的生长，事实证明其更适合于这些条件，被认为是关键物种在所研究的领域。