Abstract
Diatoms are diverse and widespread freshwater Eukaryotes that make excellent microbial subjects for addressing questions in metacommunity ecology. In the McMurdo Dry Valleys of Antarctica, the simple trophic structure of glacier-fed streams provides an ideal outdoor laboratory where well-described diatom assemblages are found within two cyanobacterial mat types, which occupy different habitats and vary in coverage within and among streams. Specifically, black mats of Nostoc spp. occur in marginal wetted habitats, and orange mats (Oscillatoria spp. and Phormidium spp.) occur in areas of consistent stream flow. Despite their importance as bioindicators for changing environmental conditions, the role of dispersal in structuring dry valley diatom metacommunities remains unclear. Here, we use MCSim, a spatially explicit metacommunity simulation package for R, to test alternative hypotheses about the roles of dispersal and species sorting in maintaining the biodiversity of diatom assemblages residing in black and orange mats. The spatial distribution and patchiness of cyanobacterial mat habitats was characterized by remote imagery of the Lake Fryxell sub-catchment in Taylor Valley. The available species pool for diatom metacommunity simulation scenarios was informed by the Antarctic Freshwater Diatoms Database, maintained by the McMurdo Dry Valleys Long Term Ecological Research program. We used simulation outcomes to test the plausibility of alternative community assembly hypotheses to explain empirically observed patterns of freshwater diatom biodiversity in the long-term record. The most plausible simulation scenarios suggest species sorting by environmental filters, alone, was not sufficient to maintain biodiversity in the Fryxell Basin diatom metacommunity. The most plausible scenarios included either (1) neutral models with different immigration rates for diatoms in orange and black mats or (2) species sorting by a relatively weak environmental filter, such that dispersal dynamics also influenced diatom community assembly, but there was not such a strong disparity in immigration rates between mat types. The results point to the importance of dispersal for understanding current and future biodiversity patterns for diatoms in this ecosystem, and more generally, provide further evidence that metacommunity theory is a useful framework for testing hypotheses about microbial community assembly.
Highlights
Metacommunity theory provides a framework to integrate dispersal dynamics with local community assembly processes, such as environmental filtering, over a patchy and heterogeneous landscape (Leibold et al, 2004; Leibold and Chase, 2017)
Simulations were roughly evenly split across the neutral model (NM), moderate species sorting (MSS), and strong species sorting (SSS) metacommunity types (Table 4)
In comparing the top 5% scenarios across metacommunity types, it is clear that Neutral model (NM) and MSS metacommunity types, both with χ2sim scores < 1, are each capable of providing much more plausible dynamics than SSS scenarios for diatoms in Fryxell Basin (Figure 5)
Summary
Metacommunity theory provides a framework to integrate dispersal dynamics with local community assembly processes, such as environmental filtering, over a patchy and heterogeneous landscape (Leibold et al, 2004; Leibold and Chase, 2017). Simulations can be used to overcome some of the challenges of understanding microbial community assembly by characterizing the likely roles of environmental filters (e.g., species sorting) and dispersal (e.g., source-sink dynamics) in structuring empirically observed metacommunities (Sokol et al, 2015) to compliment field-based experimental observations. In the McMurdo Dry Valleys (MDV) of East Antarctica, “orange” and “black” mat types are common in and around glacial meltwater streams These mats differ based on their appearance, dominant taxa, and location in the stream channel (Alger, 1997; McKnight and Tate, 1997; McKnight et al, 1998; Kohler et al, 2015; Van Horn et al, 2016). The coverage of mat growth varies dramatically within and among streams, where surveys of stream reaches have reported cyanobacterial mat percent coverages ranging from near 0 to near 100% (Alger, 1997; Kohler et al, 2015)
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