The maximum number of centric diatom taxa found in one station was 21, the minimum 10. islandica and is the first confirmed record for the studied area since the 1930s. Among them, Aulacoseira nivalis is the first report for Ukraine, A. Twenty-four centric diatom taxa belonging to 11 genera were identified, analysed, and documented by scanning electron microscopy (SEM) and light microscopy (LM). Benthic samples of the following sites were investigated: north of Mykolaiv City (approximately 5 km), in Mykolaiv City (near Varvarivskyi Bridge), and 5 km south of the city. The diversity of centric diatoms is documented for the transitional zone of the lower part of the Southern Bug River (Ukraine) just before entering the Dnipro-Bug Estuary and compared to earlier results from the upstream sampling sites of the same river system. Hence, this work contributes to the understanding of the macrotidal estuaries functioning, reinforcing the importance of phytoplankton monitoring for the identification of ecological changes. and Bellerochea horologicalis in the transitional period, and Protoperidinium sp., Thalassiosira subtilis, Cyclotella striata and Thalassiosira rotula in the dry period. Thus, the key species selected by IndVal method was Discostella stelligera, Coscinodiscus oculus-iridis and Trieres sinensis in the rainy period, Nitzschia reversa, Thalassiosira sp. The highest salinity and light availability (low turbidity and total particulate matter) in the dry season favored a significant growth of phytoplankton (cell abundance and biomass). During the rainy and transitional seasons, it was observed a spatial gradient of salinity, characterized by the gradual increase in phytoplankton abundance and biomass from the freshwater zone towards the coastal zone, following an opposite nutrient gradient. Thus, three scenarios related to seasonality and salinity gradient were identified: tidal river zone, mixing zone and coastal zone. The phytoplankton structure was well-defined by a salinity gradient governed by rainfall and river discharge. From 171 identified taxa, Bacillariophyta was the most representative division (128 taxa). The ecological patterns of phytoplankton were explored by Cluster Analysis and key species were selected by the indicator value method (IndVal).

To understand the regulation mechanisms driving the phytoplankton dynamics and the key indicator species for monitoring ecological change, the study considered seasonal and spatial analysis of environmental and biological variables in thirteen sampling points during the three seasonal periods in 2017. The structure of phytoplankton community was studied in a tropical macrotidal estuary (Paciência river estuary), which is socially and ecologically important because is surrounded by one of the most extensive mangrove forests of the world and plays a key role in the local economy, through artisanal fishing and bivalve extraction activities in natural environments. Such two-ecocline models may also exist in other marine systems, such as rocky shores. This allows a redefinition of the Remane diagram, with estuarine species removed, and supports previous arguments that brackish-water species do not exist.

We propose, therefore, that the estuary represents a two-ecocline model, with fauna inhabiting the mid-estuary being either freshwater or marine species at the edge of their range, rather than ‘ true estuarine organisms ’. However, the estuary differs from previously defined ecoclines in having two overlapping gradients in the major stressor: from river to mid-estuary for freshwater species and from sea to mid-estuary for marine species. This pattern closely fits an ecocline model. Data for periods of high and low freshwater flow allowed the impact of large-scale fluctuations implicit in both models to be determined.A continuum of assemblages existed along the salinity gradient from freshwater river to the North Sea, with shifts in the ranges of organisms apparent in response to changes in freshwater flow. An extensive data set from the Thames estuary was analysed using multivariate techniques and species-range analysis, in order to investigate whether the ecocline or the ecotone model was most relevant to this estuary. At this scale, the estuary represents a boundary between rivers and the sea, but there has been no attempt to fit empirical data for estuaries to these boundary models. Two main ecological boundaries, ecotone and ecocline, have been defined in landscape ecology.

Ecotone vs continuum