Zooplankton Diversity of a Sub-Tropical Small Urban Wetland of Meghalaya, Northeast India with Remarks on Spatial Variations

Limnological survey of a small urban wetland of Meghalaya state of northeast India (NEI), undertaken at the littoral and semi-limnetic stations, reveals one of the biodiverse zooplankton assemblages (148 species belonging to 72 genera and 30 families) known from any lentic environ of the Indian sub-region. The speciose nature, peak constellation/sample of 83 zooplankton species, and diverse Rotifera (90 species) are hypothesized to environmental heterogeneity of this urban wetland in contrast to the general pattern of reduced richness expected in highly modified urban aquatic environments. The soft and de-mineralized waters are characterized by low zooplankton abundance. This study records high species diversity and evenness, and low dominance attributed to low and equitable abundance depicts ‘generalist’ nature of all species. Rotifera > Cladocera and Chydoridae > Lecanidae > Lepadellidae > Daphniidae are important at both stations, and Testudinellidae, Trichocercidae, and Macrothricidae are notable at the littoral station. Individual abiotic factors exert limited and differential spatial influence on various taxa, while the CCA registers a high cumulative influence of 10 abiotic factors on the littoral (87.37%) and semi-limnetic (75.81%) zooplankton assemblages. The spatial variations of composition, richness, similarities, abundance, diversity indices, and of the influence of individual abiotic factors are hypothesized to habitat heterogeneity amongst the sampled stations.


Introduction
The smaller water bodies (wetlands and ponds < 10 ha in surface area) form over 90% of the standing waters of our biosphere and ~ 30% of the global lentic biotopes by surface area (Downing et al. 2006). Seekell et al. (2013) and Verpoorter et al. (2014) suggested even higher conservative estimates as high-resolution satellite imagery inventories omit water bodies smaller than 0.2 ha in size, although even the very smallest of these have important roles in ecosystem processes (Holgerson and Raymond 2016). The small urban biotopes are recognized as keystone systems for the conservation of biodiversity (Céréghino et al. 2008(Céréghino et al. , 2014Vad et al. 2017;Oertli 2018) despite vulnerability to severe threats of extinction and habitat degradation (Moss et al. 2011). Mullins and Doyle (2019) remarked that limnology will benefit from a renewed focus on these water bodies as they provide valuable ecological services and are likely important hot spots of carbon transformations and carbon sequestration (Downing et al. 2008;Céréghino et al. 2014). The small wetlands are important for maintaining regional biodiversity in urban landscapes that have been highly modified but may not follow the general pattern of reduced taxonomic richness expected in highly modified urban terrestrial or aquatic environments (Hill et al. 2017).
The small wetlands and ponds form an integral part of the rural and urban landscape of India. These environs have attracted relatively more attention since the inception of the Indian limnology and have resulted in the proliferation of 'ad-hoc' and 'routine' reports on zooplankton assemblages due to incomplete species lists, unidentified species, limited sampling, and inadequate data analysis Sharma 2019a, 2019b). The selected Indian studies with variable and limited extant of useful information are from small wetlands of Bihar (Kumar et al. 2011;Pandey et al. 2013), Jammu & Kashmir (Jyoti et al. 2009), Haryana (Tyor et al. 2014;Chopra and Jakhar 2016), Karnataka (Majagi 2014), Telangana (Karuthapandi et al. 2016), Uttarakhand (Kumar et al. 2012) and West Bengal (Datta 2011;Halder Mallick and Chakraborty 2015;Patra et al. 2015;Adhikari et al. 2017;Saha et al. 2017;Midya et al. 2018). Amongst other wetlands, some useful studies are from the floodplains of Bihar (Sanjer and Sharma 1995), Kashmir (Khan 1987;Ahangar et al. 2012;Slathia and Dutta 2013) and West Bengal (Khan 2002(Khan , 2003Ganesan and Khan 2008), while the notable zooplankton diversity works are restricted to the floodplain wetlands of Assam (Sharma and Hussain 2001;Sharma 2008, 2012;Sharma 2011a;Sharma and Hatimuria 2017;Sharma and Noroh 2020) and Manipur (Sharma 2011b;Sharma and Sharma 2011) states of NEI. On the other hand, some notable studies from small wetlands of NEI are yet limited to surveys from Meghalaya (Sharma and Wanswet 2006), Arunachal Pradesh (Saikia et al. 2017), and Assam (Deka and Goswami 2015). Also, Sharma et al. (2016), Sharma and Kensibo (2017), and Sharma and Sharma (2019a, 2019b, 2019c highlighted small wetlands of NEI to be one of the Rotifera biodiversity hot-spots of the Indian sub-region. The literature from India thus reflects the overall paucity of extensive investigations till date on various aspects of zooplankton diversity of small wetlands of India and NEI in general and urban wetlands in particular.
The present study, a follow-up of our survey of the rotifer assemblage , endeavors to undertake detailed analyses of zooplankton diversity of a small urban wetland of Meghalaya state of NEI; it assumes biodiversity and limnology interest in light of the stated lacunae. Our study deals with the spatio-temporal variations of richness, species composition, community similarities, abundance, species diversity, equitability and dominance, and individual and cumulative influence of abiotic factors on zooplankton assemblages. The results are discussed vis-a-vis zooplankton diversity of India and of small freshwater environs in particular.
Water samples collected from the two stations were examined for 13 abiotic parameters each. Water temperature was recorded using a centigrade thermometer; pH and specific conductivity were recorded with the field probes; dissolved oxygen was estimated by the modified Winkler's method, and other abiotic factors namely total alkalinity, total hardness, calcium, magnesium, chloride, dissolved organic matter, total dissolved solids, phosphate, and nitrate were analyzed following APHA (1992). The qualitative plankton and semi-plankton samples, collected from each sampling station by towing nylobolt plankton net (mesh size: 40 µm), were preserved in 5% formalin. All samples were screened with a Wild stereoscopic binocular microscope, zooplankton taxa were isolated and mounted in polyvinyl alcohol-lactophenol mixture, and were observed with Leica (DM 1000) stereoscopic microscope. Zooplankton species were identified following Michael and Sharma (1988), Sharma (1998Sharma ( , 2016, Sharma and Sharma (1999a, 1999b, 2000, 2008, and Sharma et al. (2016). The community similarities were calculated vide Sørensen's index and the hierarchical cluster analysis was done using SPSS (version 20). Monthly quantitative samples were obtained from the two stations by filtering 25 L of water each through nylobolt plankton net and were preserved in 5% formalin. Quantitative enumeration of zooplankton and constituent taxa was done by using a Sedgewick-Rafter counting cell. The abundance of various taxa was expressed as n/l as well as ranges and mean ± S.D. Species diversity (Shannon Weiner's index), dominance (Berger-Parker's index) and evenness (E1 index) were calculated vide Ludwig and Reynolds (1988) and Magurran (1988). Two-way analysis of variance (ANOVA) was used to ascertain significance of spatio-temporal variations of abiotic and biotic parameters between the sampled stations and months. Pearson's correlation coefficients for the two stations (r1 and r2, respectively) were calculated between abiotic and biotic factors; p values were calculated vide http://vassarstats.net/tabs.html and their significance was ascertained after applying Bonferroni corrections. The canonical correspondence analysis was done using XLSTAT (version 2015) to record the cumulative influence of 10 abiotic factors (for logistic limitations of the study period): water temperature, pH, specific conductivity, dissolved oxygen, total alkalinity, total hardness, chloride, dissolved organic matter, total dissolved solids and phosphate on zooplankton assemblages.

Discussion
The sub-tropical NEHU wetland is characterized by soft, slightly acidic-circum neutral, calcium poor waters with low nutrients, chloride indicating a certain influence of human impact, and total alkalinity attributed to bicarbonate ions. The low specific conductivity is attributed to the leached nature of the soil and weathered condition of rocks due to heavy rainfall in NEI (Sharma 1995; and lowered buffering capacity of de-mineralized waters (Steinitz-Kannan et al. 1983). ANOVA depicts significant variations of pH, specific conductivity, dissolved oxygen, total alkalinity, and total hardness between stations and months; water temperature and dissolved oxygen indicated significant variations between stations; and calcium, magnesium, chloride, dissolved organic matter, total dissolved solids, phosphate, and nitrate recorded significant monthly variations between the sampling stations. The spatial variations of abiotic factors are hypothesized to the habitat heterogeneity of the two stations.
Our report of 148 species, belonging to 72 genera and 30 families, reveals one of the biodiverse zooplankton assemblages known from any individual lentic environ of the Indian sub-region. This salient feature is hypothesized to the overall environmental heterogeneity of NEHU urban wetland. The biodiversity significance is in contrast to the general pattern of reduced taxonomic richness hypothesized to be expected in highly modified urban aquatic environs (Hill et al. 2017). We thus categorize this urban wetland as ' The diverse Rotifera (90 species belonging to 29 genera and 15 families) merit biodiversity interest as ~56.0, ~38.0, and ~23.0% of the species of this taxon known till date from Meghalaya and NEI (Sharma and Sharma 2019c) and India (Sharma and Sharma 2017a), respectively. The salient features of poor Brachionidae richness and lack of Brachionus spp. noted in our collections are attributed to slightly acidic -circum neutral waters concurrent with the reports of Sharma and Pachuau (2013), Sharma et al. (2016), and Sharma and Sharma (2020), while Brachionidae paucity concurs with the report from Arunachal Pradesh (Saikia et al. 2017). Lecanidae > Lepadellidae collectively comprise ~57.0% of the rotifer richness and affirm the littoral-periphytic character of the taxon vides Sharma (2017a, 2019c). Additional details of Rotifera diversity vis-à-vis new reports (Sharma 2016), species composition, and elements of biogeographic interest are dealt with separately by Sharma et al. (2016). Cladocera, the second speciose group, indicated 36 species belonging to 28 genera and 6 families; it depicts diverse nature in comparison with our conservative estimate of the occurrence of 60-65 species from the tropical and subtropical waters of the Indian subcontinent (Sharma and Michael 1987;Sharma and Sharma 2017b). High richness (~ 69%) of the Chydoridae affirms the littoral periphytic nature of cladoceran assemblages (Sharma and Sharma 2017b).
Higher monthly zooplankton richness at the semi-limnetic > the littoral station is hypothesized to greater habitat diversity of the former region, while ANOVA indicates significant richness variations between stations and months. The zooplankton richness follows oscillating monthly variations at the two stations, while peak consortium/sample of 83 species in monsoon (July 2015) and high assemblage of 81 (August 2014) species, noted at the semilimnetic station, are attributed to the possibility of coexistence of many species due to high amount of niche overlap as hypothesized by MacArthur (1965). We categorize these assemblages as 'Zooplankton paradox' vs. small urban wetland; such consortia are hypothesized to the intriguing possibility of the coexistence of a number of species in a relatively unstructured environment of small wetlands (Sharma and Sharma 2019a). Peak monsoon richness at the two stations is affirmed by a positive correlation with water temperature; the latter concurs with the results of Thakur et al. (2013). Further, monsoon peaks concur with the reports from Holmari beel of Assam (Sharma and Hatimuria 2017), Karnataka (Majagi 2014), Meghalaya (Sharma and Wanswett 2006), North Bengal (Datta 2011), and Telangana (Karuthapandi et al. 2016) but deviate from the reports of the post-monsoon peak by Ganesan and Khan (2008) and winter peak by . Zooplankton register 47.2-74.5 % and 55.7-84.0 % community similarities at the littoral and semi-limnetic stations, respectively; ~ 92% instances record 51-70% similarities at the former station, while similarity matrix record 61-80% similarities in ~84% instances at semi-limnetic station. The hierarchical cluster analysis indicates closer affinities between September-October, March-April, and January-February assemblages while August collection indicates peak species divergence at the littoral station. High affinity is noted only between July-August and maximum divergence is noted during November > February > December collections at the semi-limnetic station. The heterogeneity in monthly richness, affirmed both by community similarities and the differential cluster groupings, is hypothesized to habitat heterogeneity of the two stations. The rotifers contribute to zooplankton richness (r1=0.908, p=0.0003; r2=0.915, p =0.0002) at the two stations, respectively, and register significant variations between stations and months (vide ANOVA). The report of 46 species in September 2014 (semi-limnetic station) vs. this urban wetland is categorized as 'Rotifera paradox' analogous to the reports of Sharma (2019a, 2019b). The community similarities reflect greater rotifer heterogeneity at the former station. Cladocera contributed to zooplankton richness (r2=0.915, p =0.0002) at the semi-limnetic station and higher similarities affirm lower heterogeneity of Cladocera at the two stations.
Our results highlight low zooplankton abundance with the littoral > semi-limnetic station except in June 2015; ANOVA registers significant variations of abundance between stations and months. The low densities are attributed to soft and de-mineralized waters of NEHU wetland in particular; these remarks correspond with the reports from identical aquatic environs of NEI (Sharma 1995(Sharma , 2011aSharma and Wanswett 2006;Sharma 2012, 2020;Sharma and Pachuau 2013;Sharma and Noroh 2020) as well as from Bhutan (Sharma and Bhattarai 2005). The zooplankton follow oscillating monthly variations with higher abundance during warmer months and peak densities during monsoon (August 2014) at both the stations, these are affirmed by positive correlation with water temperature (r1 = 0.789, p = 0.0067; r2 = 0.837, p = 0.0025). The latter concurs with the results of Patra et al (2011), Thakur et al. (2013), and Singh and Sharma (2020) but differ from the inverse correlation indicated by Pandey et al. (2013) and Slathia and Dutta (2013). Further, monsoon peaks concur with the reports from Arunachal Pradesh (Saikia et al. 2017), Assam (Deka and Goswami 2015), Uttarakhand , and Myanmar (Twin and Aung 2019) but differ summer peaks listed from wetlands of Bihar (Pandey et al. 2013 Patra et al. 2015). Low and equitable abundance categorizes 'generalist' nature of zooplankton species concurrent with the reports from Himachal Pradesh (Jindal and Prajapat 2005;Jindal and Thakur 2014), NEI (Sharma 1995;Sharma and Lyngskor 2003;Sharma 2011a, 2011b, Sharma and Sharma 2011Sharma and Noroh 2020) and Uttarakhand (Malik and Panwar 2016;Singh and Sharma 2020). Rotifera (42.9±11.3, 44.2±4.6%) ≥ Cladocera (40.0±5.2, 38.4±4.3%) contribute to zooplankton abundance at the two stations, respectively but with different spatio-temporal patterns.
Copepoda and Rhizopoda, two sub-dominant groups, indicate low abundance and limited spatial variations at the two stations. Copepoda > Rhizopoda abundance pattern is noted during September 2014-January 2015 and again in July 2015 at the littoral station, while Rhizopoda > Copepoda abundance is recorded throughout the study at the limnetic station, except January 2015. Copepoda indicates low abundance at the two stations with significant variations between stations (vide ANOVA). The subdominance of copepods concur with the reports from Assam (Sharma and Sharma 2012;Deka and Goswami 2015;Sharma and Noroh 2020), Himachal Pradesh (Jindal and Prajapat 2005), Jammu & Kashmir (Jyoti et al. 2009;, Manipur (Sharma 2011a) and Uttarakhand (Malik and Panwar 2016;Singh and Sharma 2020). Monsoon maxima of this group at the two stations concur with the findings of Jindal and Thakur (2014) but deviate from pre-monsoon peaks vides the reports of Ganesan and Khan (2008) and . Copepoda abundance, largely influenced by Cyclopidae, is attributed to the prevalence of stable environmental conditions for these 'k-strategists' as suggested by Allen (1976). Rhizopoda abundance broadly concurs with the report of Sharma and Noroh (2020), it is higher than the results of Sharma and Pachuau (2013) and Sharma and Hatimuria (2017) but differs from the poor abundance reported by . The rhizopods record maxima during monsoon at the two stations. Ostracoda forms an insignificant fraction of zooplankton.
Zooplankton are characterized by high species diversity at the semi-limnetic station > the littoral station; the former station recorded higher diversity throughout the study, except in November 2014; ANOVA depicts significant diversity variations between stations. The limnetic station recorded H´ values > 4.0 during 9 months, while the semi-limnetic station indicated H´ values > 4.0 during 7 months. High zooplankton species diversity of this urban wetland, coupled with low densities of individual species, is hypothesized to fine niche portioning in combination with micro-and macro-habitat heterogeneity as hypothesized by (Segers 2008). The diversity is directly influenced by richness of zooplankton (r2 = 0.964, p < 0.0001), Rotifera (r2 = 0.875, p = 0.0009) and Cladocera (r2=0.700, p=0.024), and abundance of zooplankton (r2 = 0.962, p < 0.0001), Rotifera (r2 = 0.827, p = 0.003) and Lecanidae (r2 = 0.722, p = 0.018) at the semi-limnetic station, while so such relationship is noted at the littoral station. Shannon Weiner diversity index is a suitable option for assessing the health of aquatic biotopes (Wilhm and Dorris 1968). Mean annual diversity values indicate a relatively more clean water nature of the semi-limnetic region while in general NEHU wetland is characterized by a very cleanclean water nature. Low dominance noted at the two sampling stations, shared by a large number of 'generalist' species (Osborne et al. 1976), is hypothesized to the fact that the habitat of this urban wetland has resources for utilization by all species providing a low amount of niche overlap as hypothesized by MacArthur (1965). Low densities and equitable abundance of zooplankton species affirm higher evenness and reiterate that zooplankton are 'generalist' vis-à-vis the general environment of this wetland.
This study depicts the differential spatial influence of individual abiotic parameters on the richness and abundance of zooplankton. Our results affirm the importance of water temperature with a positive influence on zooplankton and Rotifera richness and abundance of zooplankton, Rotifera, and Lecanidae at the two stations, respectively; positive influence on the abundance of Trichocercidae density at the littoral station and Rhizopoda at the semilimnetic station. Besides, dissolved organic matter indicates importance with inverse correlations on zooplankton and Rotifera richness, and abundance of zooplankton, Rotifera and Lecanidae, and zooplankton richness and abundance is inversely correlated with total dissolved solids; Lecanidae abundance is positively correlated with specific conductivity at the former station. Rotifera density is inversely correlated with dissolved organic matter at the semi-limnetic station. The limited and differential influence of individual abiotic factors affirms the reports of Sharma and Hussain (2001), Sharma (2011a), Sharma (2011, 2012), Sharma and Noroh (2020), and .
The canonical correspondence analysis depicts high but the differential cumulative influence of 10 abiotic factors on zooplankton assemblages at the littoral (87.37%) and semi-limnetic (75.81%) stations, respectively; it broadly concurs with the report of . Also, the former broadly concurs with 84.8% cumulative variance reported from a subtropical reservoir of Mizoram (Sharma and Pachuau 2013), while our results record higher cumulative influence in contrast to the reports from the floodplain wetlands of NEI (Sharma 2011a;Sharma and Sharma 2012;Sharma and Hatimuria,2017;Sharma and Noroh 2020). CCA coordination biplot at the littoral station indicates ~ 75% and ~12% influence of abiotic factors along axis 1 and 2, respectively with the influence of water temperature on Rotifera abundance; dissolved oxygen on richness and abundance of Cladocera, and abundance of Chydoridae; dissolved oxygen and chloride on zooplankton richness; conductivity on zooplankton abundance; dissolved organic matter and total dissolved solids on Rhizopoda density; and total hardness on Copeopoda abundance. The semilimnetic station indicates ~ 54% and ~21% influence of abiotic factors along axis 1 and 2, respectively with the influence of water temperature on Rotifera richness and abundance; DOM and TDS on Cladocera and Rhizopoda abundance; dissolved oxygen on zooplankton and Cladocera richness; total alkalinity on zooplankton abundance. Our results thus highlight the importance of cumulative influence over the individual influence of abiotic factors, while the impact of biotic factors vs. zooplanktonmacrophytic associations in this urban wetland needs to be assessed.
To sum up, the biodiversity significance of zooplankton of NEHU wetland and its importance as 'keystone' system of NEI vs. lentic environs of the Indian sub-region is hypothesized to overall environmental heterogeneity of this urban wetland. Peak consortia indicating 'zooplankton' and 'Rotifera paradox' hypothesized to the possibility of co-existence of several species due to high amount of niche overlap in the relatively unstructured environment of the small wetland; low zooplankton abundance attributed to 'soft' and de-mineralized waters; and the differential spatial patterns of richness, abundance and diversity indices hypothesized to habitat heterogeneity of the sampled stations are noteworthy. High species diversity, and low dominance, and high equitability attributed to 'generalist' species are notable features; the former depicts the 'very clean -clean water nature' of this wetland. The importance of high cumulative influence vis-à-vis limited spatial influence of individual abiotic factors is noteworthy. This study is an important contribution to zooplankton diversity of small water bodies of the Indian sub-region, and highlights need for the future-focused studies on zooplankton diversity to avoid the proliferation of 'ad-hoc' and 'routine' reports.