Abstract
AbstractChanges in the abundance of a taxon can have large effects on communities, particularly if that taxon is a strong interactor. These changes may arise as a consequence of environmental change, recruitment from dormant stages, or quirks of population dynamics, and have effects that ripple through a community interaction network. We hypothesized that cyanobacteria, which are increasing in many freshwater lakes globally, may be strong interactors because they can exert large and persistent effects on the biomass and composition of other phytoplankton. To test this hypothesis, we evaluated how the phytoplankton community responded to different densities of Gloeotrichia echinulata, a large colonial cyanobacterium increasingly observed in low‐nutrient lakes in northeastern North America, in an in situ mesocosm experiment. We observed that many phytoplankton taxa, especially diatoms and green algae, responded primarily to increased nutrient availability (a result of Gloeotrichia's nitrogen fixation and translocation of phosphorus from the sediments), while a few taxa (two euglenophytes, one dinoflagellate, and one cyanobacterium) responded to both the direct and indirect effects of Gloeotrichia. Surprisingly, Gloeotrichia reduced the compositional variability of the phytoplankton community relative to the non‐Gloeotrichia control treatment; there was no effect on the aggregate temporal variability of total non‐Gloeotrichia biovolume. Moreover, experimentally increased densities of Gloeotrichia coincided with increasing complexity of the phytoplankton community in network analyses of taxon co‐occurrences, as indicated by significantly greater network density and transitivity and shorter path lengths. Taken together, these findings suggest that Gloeotrichia may be a strongly interacting species in low‐nutrient lakes, with the potential to increase the resilience of phytoplankton communities to future disturbance by increasing compositional stability and network complexity.
Highlights
Ecological communities can be described and visualized as networks of interacting populations (Dunne et al 2002)
Increases in strong interactors may occur as a result of environmental change, recruitment from dormant stages, or changes in population dynamics, and have effects that ripple through the community network
We show that there are microscopic strong interactors: This study demonstrates that the cyanobacterium Gloeotrichia echinulata can be a strong interactor with the ability to increase biomass, stabilize community composition, and increase network complexity of the phytoplankton community
Summary
Ecological communities can be described and visualized as networks of interacting populations (Dunne et al 2002). Fluctuations in the relative abundance of certain taxa within communities can trigger rapid changes in population interactions and alter network structure, relative abundance, and aggregate properties such as total biomass (Micheli et al 1999). If the addition of a new strongly interacting species (sensu Paine 1980), or an increase in the total or relative abundance of a previously rare one, has a large impact on resource availability, it may change which other species can persist in the community, and their. Less is known about the consequences of increases in potential strong interactors for microscopic communities, especially those that have the potential to become macroscopic problems from the perspective of the public, such as large surface aggregations of cyanobacteria in freshwater ecosystems (blooms). Cyanobacterial blooms have the potential to substantially affect the community structure of, and interactions among, other phytoplankton (Cottingham et al 2015), and could be of increasing importance in the future, since their duration, magnitude, and geographic extent appear to be increasing globally (Paerl and Huisman 2008, Brookes and Carey 2011)
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