water Article The Species Diversity of Tropical Freshwater Rotifers (Rotifera: Monogononta) in Relation to Environmental Factors Nhat-Truong Phan 1, Quang Hung Duong 1, Quynh Anh Tran-Nguyen 1,* and Mau Trinh-Dang 2,* 1 DN-EBR, The University of Da Nang—University of Science and Education, Da Nang City 550000, Vietnam; [email protected] (N.-T.P.); [email protected] (Q.H.D.) 2 Faculty of Biology and Environmental Sciences, The University of Da Nang—University of Science and Education, 459 Ton Duc Thang St., Danang City 550000, Vietnam * Correspondence: [email protected] (Q.A.T.-N.); [email protected] (M.T.-D.); Tel.: +84-948765483 (Q.A.T.-N.); +84-905436189(M.T.-D.) Abstract: This study aims to evaluate the diversity of rotifers in various freshwater habitats in Da Nang City, Vietnam, and to investigate the relationship between community structure and environmental conditions. A total of 75 rotifer species belonging to 25 genera and 17 families were recorded, out of which seven species are new to Vietnam. Species from the families Brachionidae and Lecanidae are abundant (containing about 50% and 69.44%, respectively) in the total rotifer species in both lacustrine and riverine habitats. The number of rotifers recorded in the lakes (64 taxa) was much higher than those in rivers (36 taxa), and this is possibly due to the conditions in lakes being more favorable for the development of small zooplanktons. The significant influences of environmental factors (temperature, pH, turbidity, and trophic state) on the rotifer community were well-reflected in the distribution of commonly found species in lakes (p-value < 0.05). Our study results enrich the Citation: Phan, N.-T.; Duong, Q.H.; data on the diversity and the ecology of rotifers in Vietnam. Tran-Nguyen, Q.A.; Trinh-Dang, M. The Species Diversity of Tropical Keywords: Rotifera; rotifer community; community structure; freshwater habitats; species diversity Freshwater Rotifers (Rotifera: Monogononta) in Relation to Environmental Factors. Water 2021, 13, 1156. https://doi.org/10.3390/ 1. Introduction w13091156 Rotifers represent an important zooplankton group since they can contribute more than 50% of zooplankton productivity [1,2]. The rotifer species can be found in all kinds of Academic Editor: Artem Y. Sinev water bodies worldwide due to their high adaptability to a wide range of environmental conditions and their ability to occupy open niches quickly. In aquatic systems, rotifers play Received: 15 March 2021 a crucial role in the energy flows of that system. This group of organisms is responsible Accepted: 15 April 2021 for transferring carbon from picoplankton and nanoplankton to macro-zooplankton; they Published: 22 April 2021 act as links between microbial loops and the larger aquatic food web [3–5]. Additionally, rotifers are considered effective indicators for environmental conditions because of their Publisher’s Note: MDPI stays neutral sensitivity to the changes in that particular environment [6–9]. It was well-documented with regard to jurisdictional claims in published maps and institutional affil- that the rotifer community structure is influenced by factors such as water temperature, iations. salinity, transparency, trophic status, and predators [10,11]. The rotifer species composition, its total abundance, and its diversity indices have been widely applied in order to assess water quality [12–14]. Numerous studies have focused on seasonal or periodic changes in the rotifer commu- nity in relation to water characteristics in a single type of water body. However, very few Copyright: © 2021 by the authors. studies compared their structures in different water bodies within the same area. Liang Licensee MDPI, Basel, Switzerland. et al. (2019) [15] revealed that the rotifer community structure differs significantly among This article is an open access article three types of water bodies: river downstream, reservoir, and tidal creek. These differences distributed under the terms and conditions of the Creative Commons could be attributed to the heterogeneity in the trophic state, the salinity, the turbidity, and Attribution (CC BY) license (https:// the hydrological regime of the water body. creativecommons.org/licenses/by/ Despite several efforts made recently, the data on rotifer species distribution in tropical 4.0/). freshwater, whether in different countries in general or in Vietnam specifically, are still Water 2021, 13, 1156. https://doi.org/10.3390/w13091156 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 13 Water 2021, 13, 1156 2 of 13 Despite several efforts made recently, the data on rotifer species distribution in trop- ical freshwater, whether in different countries in general or in Vietnam specifically, are still relatively limited.relatively Moreover limited., Moreover,less attention less has attention been paid has to been the relationship paid to the relationship between between the the rotifer communityrotifer community structure and structure the environmental and the environmental conditions. The conditions. aims of this The study aims of this study were to examinewere and to investigate examine and the investigatefollowing: (1) the the following: diversity (1) of therotifers diversity in freshwater of rotifers in freshwater bodies in Da Nangbodies city, in Vietnam Da Nang, and city, (2) Vietnam, the influences and (2) of the some influences environmental of some factors environmental on factors on the distributionthe of distributionrotifers in different of rotifers aquatic in different systems aquatic in this systemsparticular in area. this particular area. 2. Materials and2. MaterialsMethods and Methods 2.1. Study Area2.1. Study Area The sampling Thesites samplingconsisted sitesof eight consisted freshwater of eight lakes freshwater (8 sites) and lakes two (8 rivers sites) (Han and two rivers (Han River and Cu DeRiver river) and (4 Cu sites) De in river) both (4 rural sites) and in bothurban rural areas and of Da urban Nang areas city of. The Da Nangsites city. The sites represent a varietyrepresent of lotic a varietyand lentic of loticfreshwater and lentic habitats freshwater with different habitats natural with different conditions natural conditions and anthropogenicand anthropogenicimpacts (Figure impacts 1). The (Figurewater quality1). The characteristics water quality characteristicsof the sampling of the sampling sites during thesites sampling during campaign the sampling were campaign identified were by the identified authors by (Table the authors 1). These (Table habi-1). These habitats tats had a trophichad state a trophic ranging state from ranging oligotrophic from oligotrophic to eutrophic, to eutrophic, their water their temperature water temperaturess ranged ◦ ◦ ranged from 24.74from °C 24.74 to 27.32C to °C 27.32, andC, their and turbidity their turbidity ranged ranged from 0.9 from to 26.5 0.9 to NTU. 26.5 NTU.The The electrical electrical conductiviconductivityty values values were weregenerally generally low (≤ low1), which (≤1), whichindicates indicates that all that of the all sam- of the sampling sites pling sites werewere freshwater. freshwater. The The pH pH of ofwater water fluctuated fluctuated from from around around neutral neutral values to weakly alkaline weakly alkalinevalues, values, specifically, specifically, from from 6.61 6.61 to to 7.78. 7.78. Figure 1. The study area. Figure 1. The study area. Table 1. Water qualityTable 1.characteristicsWater quality at characteristics 12 sampling sites at 12. sampling sites. ◦ Site Code Type Region Trophic StateTemp TRIX TempEC ( C) EC (mS/cm)Tur. pH Tur. (NTU) Site Code Type Region Trophic State TRIX pH Xanh Lake LHX Lake Rural Mesotrophic(°C 5.04) (mS/cm) 27.17 0.166(NTU)7.78 0.9 Han River RSH River Urban Mesotrophic 5.02 25.73 1.001 6.69 26.5 Xanh Lake Ham NghiLHX Lake Lake LHNRural LakeMesotrophic Urban Eutrophic5.04 27.17 6.32 0.166 26.14 7.78 0.815 0.9 6.92 9.3 Cong Vien Lake LCV Lake Urban Eutrophic 6.47 26.47 0.728 6.92 24.3 Han River Sen LakeRSH River LHSUrban LakeMesotrophic Urban Eutrophic5.02 25.73 6.58 1.001 26.27 6.69 0.469 26.5 6.86 24.6 Bau Trang Lake LBT Lake Urban Mesotrophic 5.63 26.37 0.556 7.20 13.1 Ham Nghi Lake Truoc DongLHN Lake Lake LTDUrban LakeEutrophic Rural Mesotrophic6.32 26.14 5.09 0.815 26.9 6.92 0.090 9.3 7.47 1.4 Hoi Khe Lake LHK Lake Rural Oligotrophic 4.82 27.32 0.056 6.61 25.6 Cong Vien Lake Hoi PhuocLCV River Lake RHPUrban RiverEutrophic Rural Oligotrophic6.47 26.47 4.75 0.728 25.35 6.92 0.067 24.3 7.02 1.2 Luong Dong River RLD River Rural Oligotrophic 4.13 24.74 0.069 6.80 1.0 Dong Xanh—Dong Nghe Lake LDN Lake Rural Oligotrophic 4.6 26.39 0.050 6.72 1.7 Cu De River RCD River Rural Oligotrophic 4.22 25.9 0.079 7.41 2.2 Water 2021, 13, 1156 3 of 13 2.2. Sampling The sampling campaign took place during the dry season in August 2018. Qualitative samples of rotifers were collected by dragging a 50 µm mesh size cast-net at 15–20 cm depth and were then preserved with 4% formaldehyde. Rotifer specimens were then sorted and examined under a Hund Wetzlar H600 microscope. The trophi of rotifers were observed by adding a drop of sodium–hypochlorite (NaOCl) to dissolve and isolate the hard trophic parts. Rotifer taxa were identified in the following major work: Koste and Shiel (1987, 1989a, 1989b, 1990a, and 1990b) [16–20], Shiel and Koste (1992) [21], and Segers (1995) [22]. The taxonomic classification and nomenclatures of the rotifers followed the work of Segers (2007) [23]. Quantitative samples of the rotifers were collected by filtering 25 L of water through a 50 µm mesh size cast-net and preserved with 4% formaldehyde.