Low shifts in salinity determined assembly processes and network stability of microeukaryotic plankton communities in a subtropical urban reservoir

Abstract

Background: Freshwater salinization may result in significant changes of microbial community composition and diversity, with implications for ecosystem processes and function. Earlier research has revealed the importance of large shifts in salinity on microbial physiology and ecology, whereas studies on the effects of smaller or narrower shifts in salinity on the microeukaryotic community in inland waters are scarce. Our aim was to unveil community assembly mechanisms and the stability of microeukaryotic plankton networks at low shifts in salinity.

Results: Here, we analyzed a high-resolution time series of plankton data from an urban reservoir in subtropical China over 13 consecutive months following one periodic salinity change ranging from 0 to 6.1‰. We found that (1) salinity increase altered the community composition and led to a significant decrease of plankton diversity, (2) salinity change influenced microeukaryotic plankton community assembly primarily by regulating the deterministic-stochastic balance, with deterministic processes becoming more important with increased salinity, and (3) core plankton subnetwork robustness was higher at low-salinity levels, while the satellite subnetworks had greater robustness at the medium-/high-salinity levels. Our results suggest that the influence of salinity, rather than successional time, is an important driving force for shaping microeukaryotic plankton community dynamics.

Conclusions: Our findings demonstrate that at low salinities, even small increases in salinity are sufficient to exert a selective pressure to reduce the microeukaryotic plankton diversity and alter community assembly mechanism and network stability. Our results provide new insights into plankton ecology of inland urban waters and the impacts of salinity change in the assembly of microbiotas and network architecture. Video abstract.

Keywords: Community ecology; Core taxa; Deterministic processes; Microeukaryotic plankton; Network stability; Salinity; Satellite taxa; Stochastic processes; Subtropical reservoir.

Fig. 1

Sampling sites and principal coordinates analysis of the microeukaryotic plankton community. a Location of the three sampling sites in Xinglinwan Reservoir, Xiamen City, Southeast China. Water samples were taken from stations C, L, and G. b Principal coordinates analysis (PCoA) of microeukaryotic plankton community composition at station G in the Xinglinwan Reservoir. Each circle represents one sample and is color-coded according to time (month) and sized according to salinity

Fig. 2

Abiotic and biotic drivers of microeukaryotic plankton community composition. Pairwise comparisons of environmental and biotic factors are shown at the upper-right, with a color gradient representing Spearman’s correlation coefficients. Microeukaryotic plankton community composition was correlated to each environmental or biotic factor by partial Mantel tests. The line width represents the partial Mantel’s r statistic for the corresponding correlation, and line color means that significances are tested based on 999 permutations. WT, water temperature; DO, dissolved oxygen; Chl-a, chlorophyll-a; EC, electrical conductivity; ORP, oxidation-reduction potential; TC, total carbon; TOC, total organic carbon; TN, total nitrogen; NH₄-N, ammonium nitrogen; NO₃-N, nitrate nitrogen; NO₂-N, nitrite nitrogen; TP, total phosphorus; PO₄-P, phosphate phosphorus; Note that the precipitation data are the 7-day accumulation before the sampling day, and the wind represents daily average wind speed. B_richness, bacterial OTU number; B_SW, bacterial Shannon-Wiener index; B_NMDS1, bacterial NMDS ordination axis 1; B_NMDS2, bacterial NMDS ordination axis 2. Note that only significant correlations are shown for simplicity

Fig. 3

Community structuring of microeukaryotic plankton across the salinity gradient at station G in Xinglinwan Reservoir. a Non-metric multidimensional scaling (NMDS) ordination based on Bray-Curtis dissimilarity showing the variation of microeukaryotic plankton communities across three salinity levels. Significant level of all, core, and satellite taxa is P = 0.001. b Venn diagram showing the numbers of unique and shared OTUs between three different salinity levels. c Shannon-Wiener index along the salinity level at station G. Different letters indicate significant difference at P < 0.05 according to Tukey’s post-hoc test. All, all microeukaryotic plankton communities at station G; Core, core microeukaryotic plankton subcommunities at station G; Satellite, satellite microeukaryotic plankton subcommunities at station G

Fig. 4

Ecological processes shaping the microeukaryotic plankton community assembly at station G in Xinglinwan Reservoir. a The predicted occurrence frequencies for low, medium, and high salinity representing microeukaryotic plankton communities from low, medium, and high salinity periods in Xinglinwan Reservoir. The solid blue line is the best fit to the neutral community model (NCM), and the dashed blue line indicates 95% confidence intervals around the NCM prediction. OTUs that occur more or less frequently than predicted by the NCM are shown in green and red, respectively. R² represents the fit to this model. b Comparison of mean habitat niche breadth for all taxa among low, medium, and high salinity levels (different letters indicate significant difference at the P < 0.05 level using Tukey’s post hoc test). c C-score metric using null models. The values of observed C-score (C-score_obs) > simulated C-score (C-score_sim) indicate non-random co-occurrence patterns. Standardized effect size <− 2 and > 2 represent aggregation and segregation, respectively

Fig. 5

Network modules and stability for microeukaryotic plankton OTUs at station G. a Network revealing the modular associations among microeukaryotic plankton OTUs (left). Relative abundance of microeukaryotic OTUs from major modules along the three different salinity levels (right). A connection indicates a strong (SparCC |r| > 0.6) and significant (P < 0.01) correlation. The size of each microeukaryotic OTU (node) is proportional to the number of connections (i.e., degree). b Network stability for all, core and satellite taxa at different salinity levels (i.e., low, medium, and high), respectively. The robustness of all, core, and satellite networks under different salinity conditions in Xinglinwan Reservoir. Insert: overview of pairwise community dissimilarity of microeukaryotic plankton communities at three different salinity levels. Statistical analysis is non-parametric Mann-Whitney U test. ***P < 0.001. All, all microeukaryotic plankton communities; Core, core microeukaryotic plankton subcommunities; Satellite, satellite microeukaryotic plankton subcommunities

Fig. 6

Conceptual models of microeukaryotic plankton diversity, community assembly processes, and network stability driven by low shifts in salinity