The survey conducted in the studied wetlands showed that they presented a great heterogeneity in size, environmental features, and community structure. The dimensions of sampled wetlands (length, wide and depth) were highly variable and included very small puddles up to large semi-temporary wetlands. The chemical features such as the nutrients concentrations, DOC, conductivity, DO and Chl a were also highly variable. Despite the variable human impact and geographical distance of these set of wetlands, none of these factors appeared to explain the pattern of abundance, diversity, and assemblage of aquatic organisms. For the contrary the high level of heterogeneity probably explained better the observed patterns.
In accordance to other studies, wetlands bear high diversity of aquatic organisms (Modenutti et al. 1998a; Stein et al. 2014). The assemblages of the studied wetlands had a higher diversity of zooplankton, with Rotifera as the dominant group with high species richness and abundance, followed by Cladocera and Copepoda. In the case of caddisfly and amphibian larvae the assemblages had a lower diversity, with 8 species of caddisflies, including some rare species, and only two common and abundant species of amphibians, B. taeniata and P. thaul.
In our study, the zooplankton abundance and distribution were assessed by means of a CCA analysis (53%) and explained by variables including depth, pH, SRP, S275 − 295, DO, conductivity and chl a. In this line, the zooplankton can exploit the whole water column, and therefore DO has a great impact on the abundances of this group (Banerjee et al., 2019). Also chemical factors like primary production, nitrates, conductivity, pH, phosphates, total dissolved solids, among others, are mostly responsible for control of DO and zooplankton variation (Walz 1995; Bukvić-Ternjej et al. 2001; Banerjee et al. 2019). Although no clear pattern was observed in the CCA, some copepods such as Boeckella brevicaudata and P. sarsi have been known to have a wider distribution, e.g. these copepods species are common dwellers of Fantasma pond (Laspoumaderes et al. 2010; Garcia et al. 2013), a possible explanation for our results can be related to the extremely unusual short hydroperiod during 2021 in the region. The combination of low precipitations and high temperatures during austral spring reduced the aquatic phase in most of these wetlands. When a pond dries up, zooplankton species may survive as dormant eggs, and since not all eggs hatch in the next season, then abundances could vary annually (Olmo et al. 2012). It is well known that in temporary wetlands such as ponds and pools, the zooplankton community’s reestablishment highly relates on recruitment from the egg bank (García-Roger et al. 2006), and a successful settlement of newly arrived species depends on the interaction with local abiotic and biotic factors (Olmo et al. 2012; Fu et al. 2021). This strategy to survival in temporary waters could be especially profitable in the small forested wetlands due to the low physical disturbances in the wetland’s sediments.
Rotifers distribution in Patagonia is characterized by species that typically are cold water adapted (Modenutti et al. 1998b). Rotifers usually inhabit with cladocerans and copepods. Their interaction includes an interference competition that occurs with cladocerans (Modenutti et al. 1998; Diéguez and Gilbert 2011), or the predation that rotifers assemblages suffer by copepods (Diéguez and Balseiro 1998).
In contrast to zooplankton communities, a less clear pattern was found in the caddisfly larvae and anurans tadpoles’ abundances and the explanatory environmental variables. Only three variables (pH, conductivity and SRP) explained the 38% of the observed variation. A possible explanation is the fact that we found several species of caddisflies that only were recorded in one wetland. On the other hand, tadpole abundances probably responded to other factors such as risk of predation by insects, seasonal temperatures, and amount of precipitation during the breeding season, as have been observed in another studies (Griffith 1997; Baber et al. 2004).
Interestingly, both types of communities, zooplankton and benthos, shared one explanatory variable, the SRP. This parameter represents the “available” phosphorous, which is commonly related to the density and/or biomass in primary producers, which in turn can translate upper in the food chain (Diaz et al. 2007). This finding suggests the importance of primary producers to support a high species richness in small forested wetlands. In ultra and oligotrophic lakes of the region the phytoplankton is nitrogen-regulated (Diaz et al. 2007), therefore the “phosphorous” seems not be limiting for the primary producers biomass, but can in turn influence the consumers (zooplankton) distribution.
In wetlands, richness, density and composition of macroinvertebrates is determined mainly by the hydroperiod and density of aquatic vegetation (Tarr et al. 2005; Jara et al. 2013; Moraes et al. 2014). Species with long larval period cannot survive in intermittent ponds or short duration water bodies. In some wetlands in the area, trichopterans species have a relatively long larval period (up two 9 months) and are obligate overwintering as larvae (Sganga et al. in press). The absence of trichopterans in three wetlands, although they had a profuse vegetation and did not have predatory fish, could be related to other sources of variation in the aquatic communities’ structure resulting from an interplay of interspecific interactions, such as competition (Tilman 2004), predation (Kurle et al. 2008), physical disturbances, and environmental variables (Cloern et al. 2007; Thibault and Brown 2008). Previous studies reveal that the thermal properties of the small wetlands in the studied area follow a common pattern: cold and stable temperature during the winter and temperature increase during spring and summer with high diurnal thermal amplitude (Jara 2021). The understanding of the relationships between communities and environmental variables has become important during the last decades since many abiotic parameters changed dramatically, e.g., temperature (IPCC 2007) and nutrient concentrations (Smith et al. 1999). It is therefore of crucial importance to understand these relationships in the view of future environmental changes that could have consequences for community and ecosystem processes.
The impacts in local wetlands
Currently we have several concerns about the future of wetlands in the region. As indicated before, human impact is one of the main factors that put at risk the future of wetlands. Deforestation, and also using the wetland for cattle raising, changes the natural conditions of the wetlands, incrementing the solar radiation and therefore the thermal properties. Also, cattle raising reduces the aquatic vegetation and removes the bottom of wetlands. Climate change in the region produces a decrease in precipitation during the autumn and winter, and also increases the maximum temperatures during spring and summer, reducing the hydroperiod of wetlands (Garreaud et al. 2013; Barros et al. 2015). These exclude several species that have prolonged life cycles and need longer hydroperiods to complete one generation, for example odonates, trichopterans, and some anuran species (Jara 2018; Jara et al. 2021). However, these are not the only impacts, for example, a recent study in Patagonia showed that wetlands invaded by wild boar had ~ 24% less vegetation cover than non-invaded wetlands, and the vegetation was 72% shorter in height (Motta et al. 2020). This has a direct impact on the reproductive success of several endemic amphibian and insect species that lay their eggs in the vegetation (Jara 2019; Jara et al. 2021; Sganga et al. in press). Also the introduction of exotic plants such as Potentilla anserina, changes the chemical properties of the water that could impact the native fauna in unexpected way (Cuassolo et al. 2012).
One important concept taken into account for future studies that include small wetlands and biodiversity patterns, is the environmental heterogeneity. As we know environmental heterogeneity is one of the most important factors driving species diversity and community composition (Stein et al. 2014). Habitat structure generated by the forest and herbaceous vegetation bring different microhabitats for more species, therefore more heterogeneous habitats will contain more diversity of plants and animals (Stain et al. 2014). The heterogeneity found in the studied wetlands located in the Andean forest, reveals that they serve as refugees for invertebrates and vertebrate’s diversity. Our results showed that in a small spatial scale, and despite their geographical proximity, all the studied wetlands were highly heterogeneous and different, even small puddles such as Poza and Pitranto. Some wetlands as Llao Llao have more than 20 different species of herbaceous plants that generate a great complexity (Jara 2010; Cuassolo and Diaz-Villanueva 2019), then providing a high environmental heterogeneity. In this sense, in order to understand the patterns of diversity and community assemblage’s we need to measure habitat complexity and also to increase the monitoring frequency to catch successional process of species and the wetland's dynamic in their variables such as nutrients and DOC, but also to accurately assess the threats that small wetlands can confront. For future researches, we highly recommend to capture the highest and lowest hydrological connectivity with the terrestrial environments, nutrients, and carbon export or retention and shifts in the aquatic communities within the wetlands.