COS 162-5 - Building from the bottom up: Effects of macrophyte colonization and identity shape aquatic community assembly

Abstract: Plant colonization and identity can shape subsequent formation of upper trophic levels, altering biodiversity and ecosystem function. Exploring mechanisms of community assembly driven by plant colonization and identity can be difficult in natural settings. Riverine rock pools are habitat for a variety of macrophytes, including both free-floating and submerged growth forms, that can affect water chemistry, temperature, and habitat complexity, which may affect macroinvertebrate communities from the bottom-up. To investigate mechanisms driving these patterns, we performed a mesocosm experiment. We hypothesized that the colonizing macrophyte identity would lead to differences in diversity through habitat alterations. We also explored how simple and complex life cycle macroinvertebrates responded to macrophyte colonization.

Utilizing a 2 x 2 factorial design with 10 replicates, mesocosms were inoculated with the same biomass of Hydrilla and the native duckweed, L. minor. Mesocosms mimicked mean rock pool volumes within our study system, were open to aerial colonizers and were inoculated with an initial community of common rock pool invertebrates consisting of 3 species of snails and 1 species of amphipod. The experiment ran for 49 days after which we collected, identified, and enumerated all macroinvertebrates and macrophytes.

We found that duckweed was cooler than Hydrilla treatments. Dissolved oxygen decreased over time in all treatments but never reached anoxic conditions. Daily means, minimum, maximum, and change in DO showed no significant difference between treatments. Chlorophyll concentrations were significantly higher in treatments that received no macrophyte additions, indicating that algae was the dominant primary producer in those treatments. Macroinvertebrate abundance did not differ between the treatments, however, Hydrilla treatments had greater diversity. Plant identity did not impact the survival of the initial macroinvertebrate community, but Hydrilla treatments had a higher abundance and diversity of colonizers (e.g., odonates and tadpoles).

Our study supports our hypothesis that colonizing plant identity leads to differences in diversity, specifically, Hydrilla having the highest diversity. These differences could lead to differences in community structure and could be caused by increased habitat complexity and/or oviposition selection. Our experimental results are consistent with patterns of diversity in natural rock pools, where Hydrilla pools had higher diversity of macroinvertebrates, as well as higher abundances of odonates and tadpoles. These results show more evidence for how colonizing plants can have subsequent impacts on the community assemblage of upper trophic levels.