land Article Assessment of River-Sea Interaction in the Danube Nearshore Area (Ukraine) by Bioindicators and Statistical Mapping Anastasiia Snigirova 1 , Yuliya Bogatova 1 and Sophia Barinova 2,* 1 Institute of Marine Biology of the NAS of Ukraine, 65048 Odessa, Ukraine; [email protected] (A.S.); [email protected] (Y.B.) 2 Institute of Evolution, University of Haifa, Haifa 3498838, Israel * Correspondence: [email protected] Abstract: There is a lack of understanding of the main drivers that form the picture of biological communities of transitional waters in deltaic ecosystems under the influence of terrestrial sources. Analysis of hydrochemical parameters in relation to phytoplankton communities in the Ukrainian part of the Danube coastal zone (in August 2018) is the focus of current work. The goal was to identify patterns in the distribution of environmental parameters (salinity and nutrients) in the area of the shipping channel through the Bystry arm, as well as to assess the state of water quality. The ecological bioindicators approach using modern statistical methods, and ecological mapping shows sufficient achievements in interpreting the results. The indicators of salinity (mesohalobes) had better describe the character of the transportation of fresh riverine waters than salinity gradient. The composition of 35 indicator phytoplankton species corresponds to 3 and 4 water quality classes in the coastal zone. High N:P ratios showed an imbalance in the ecosystem as an indicator of production and destruction processes. Statistical maps of the indicator species distribution revealed the river Citation: Snigirova, A.; Bogatova, Y.; current’s influence on the nearshore water mass. Ecological maps of surface and bottom variables Barinova, S. Assessment of River-Sea show various environmental impacts resulting from dredging in the shipping channel and excavated Interaction in the Danube Nearshore soil dumping. Canonical correspondence analysis (CCA) and statistical maps revealed two pools of Area (Ukraine) by Bioindicators and factors with oppositely directed effects on phytoplankton: salinity, on one hand, and nutrients, on Statistical Mapping. Land 2021, 10, the other. Miozoa and Chlorophyta have an opposite interaction with salinity and oxygen and can be 310. https://doi.org/10.3390/ ecosystem change indicators in further analysis land10030310 Keywords: Danube Delta; ecological mapping; nearshore area; nutrients; phytoplankton; transfor- Academic Editors: Antonio Leone mation zones and Angioletta Voghera Received: 15 February 2021 Accepted: 12 March 2021 1. Introduction Published: 18 March 2021 The Danube River plays a great role for the Black Sea by forming the hydrochemical Publisher’s Note: MDPI stays neutral regime in the northwestern shelf, which is the biggest shallow area in the sea with the with regard to jurisdictional claims in high stocks of biological production. The Danube, together with the Dnieper and Dniester published maps and institutional affil- rivers, make up to 70% of the input in the northwestern part of the Black Sea [1–3]. Under- iations. standing the processes of interaction of one of the largest rivers of Europe with the sea is of great importance. The Danube nearshore area represents a transitional river-sea contact zone. It is formed due to dynamic interaction, mixing, and transformation of the river’s water masses and the sea. The Ukrainian part of the nearshore area covers the region from the edge of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the Kiliya Arm (Kiliya Delta of the Danube River) to the seawaters’ border with a salinity This article is an open access article of 17‰. The nearshore outer border is conditional because its location depends on the distributed under the terms and runoff rate of the Danube River and the wind regime [1,2]. The vertical distribution of conditions of the Creative Commons waters at the nearshore area is characterized by sharp salinity changes and temperature, Attribution (CC BY) license (https:// especially in spring and early autumn. It is noted a wide variability and fractal in the creativecommons.org/licenses/by/ distribution of hydrochemical and biological parameters. They are associated with the 4.0/). seasonal variability of river runoff in the nearshore, which is the primary source of nutrients Land 2021, 10, 310. https://doi.org/10.3390/land10030310 https://www.mdpi.com/journal/land Land 2021, 10, 310 2 of 18 that ensure the seasonal development of production processes [2]: the special hydrological- hydrochemical and hydrobiological regimes in the transition waters of Danube nearshore results in increased biological productivity. The Danube water flowing to the seaside occupies its upper layer up to 5 m. As a rule, these are seaside areas located directly near the mouths of large delta’s arms. With a distance from the mouth, the river water gradually transforms into seawater, passing through three successive zones: the principal (salinity up to 10‰), frontal (10–12‰), and final (12–17‰) [3]. There is only one navigable arm in the Ukrainian part of the Kiliya Danube Delta—the Bystryi Arm. To maintain the Danube–Black Sea channel’s working depths through the Bystryi Arm, periodically dredging operations are carried out. These operations’ impact is overlapped with the river-sea interactions that form the main background in these transition waters. The salinity of the nearshore water masses at a depth of more than 5 m makes 16–18‰. The dumping area is located at a distance of 8 km from the delta edge, in the final zone. Since 2004, the Institute of marine biology of the NAS of Ukraine organizes environmental monitoring to determine the impact of these manipulations on the ecotone marine-river ecosystem [4,5]. However, there is still a lack of a general description of the ecosystem functioning and understanding of how river-sea interactions influence biological communities. The studies of 2004–2011 describe phytoplankton’s biodiversity in the Ukrainian part of the Danube nearshore area with 409 taxa [6,7]. The most remarkable species richness is typical for diatoms (155 species) and green (122) algae. They are followed by dinophytes (70) and cyanobacteria (34). A small number of taxa represents golden, euglenoid, and cryptophyte algae. Compared to the Romanian part of the Danube Delta, 957 species of microalgae are indicated, excluding the section of green ones, which include another 526 species [8]. The monitoring of water bodies, especially assessing the anthropogenic pressure, implies following the European directives’ requirements (Water Framework Directive and Marine Strategy Framework Directive) [9,10], using different approaches to assessing the quality of the aquatic environment [11,12]. One of the most effective methods for water quality assessment is bioindication based on species composition shifts as a response of biota to the environmental changes and influence from land base sources [13–15]. In the Black Sea and many other marine water areas, species indicators’ elaboration suitable for environmental monitoring is in the initial stage [16–18]. The level of nutrients and chlorophyll concentrations is widely used as an indicator of the eutrophication assess- ment [19]. In the northwestern part of the Black Sea and western part of the Azov Sea, the bioindication method was started for benthic species [20–23]. Colleagues from Ukraine, working within the framework of the development of a national marine strategy, are actively introducing a morphological and functional approach to assess the state of the en- vironment using indices of the specific surface area of macrophytes and microalgae [24–26]. Integrated indices and comparative statistics, using various structural indicators of marine and estuarine ecosystems, is widespread [27,28]. Modern statistical programs and mapping have already been tested for many freshwa- ter objects in different climatic zones [13–15,29–31]. However, applying these approaches to assessing the state of marine ecosystems has not yet received widespread use due to the limited number of indicator species [20,21,32]. The combined approach of studying the species composition of algae and statistics in the coastal area has good examples of its application [28]. We believe it rational to apply this complex of methods for the Danube nearshore area—a dynamically changing ecosystem, using phytoplankton’s structural indicators and the content of nutrients. In addition, we think it is essential to contribute to the development of the database on marine species indicators that could be the base for assessing the ecological class of the water areas. We hypothesize that it is possible to reveal the Danube Delta waters’ influence on the coastal ecosystems of the Black Sea using bioindication of phytoplankton communities and statistical methods. Land 2021, 10, 310 3 of 18 The work aimed to assess the hydrochemical state and phytoplankton community of the Ukrainian part of the Danube nearshore area in the summer employing statistical analysis and ecological mapping. This work will be the base for the long-term analysis to identify the patterns in the distribution of environmental parameters and phytoplankton in the studied region. Land 2021, 10, x FOR PEER REVIEW 4 of 18 2. Materials and Methods The materials used in the study were obtained during environmental monitoring at the Ukrainian part of the nearshore area of the Danube Delta in August 2018 (Figure1). Water samples forIV the determination