PS 37-21 - Effects of Lemna minor on survival and colonization shape aquatic community assembly

Abstract: Common duckweed (Lemna minor) is a floating aquatic macrophyte capable of rapid growth. Duckweed can quickly grow and cover the surface of lentic water bodies reducing sunlight available to other aquatic macrophytes, alter aquatic macroinvertebrate colonization, and influence dissolved oxygen (DO) and temperature. While lentic organisms are often well suited to a wide range of conditions, lotic organisms are more sensitive to water quality parameters such as temperature and DO. Thus, it is unclear how duckweed may affect these lotic organisms when isolated in lentic systems. Here, we explored the consequences of duckweed colonization for macroinvertebrate assemblages in riverine rock pools, which commonly consist of lotic taxa that colonize pools during flooding and lentic taxa that colonize pools when floodwaters recede. Duckweed may alter colonization and abundance of organisms in this system due to its effects on water quality parameters.

We conducted a mesocosm experiment to identify if duckweed influences the survival of riverine macroinvertebrates and the abundance of aerial colonizers in riverine rock pools. Throughout the experiment, we observed duckweed growth, riverine macroinvertebrate survival, and abundance of colonizing organisms. We predicted that survival of riverine macroinvertebrates would be highest in duckweed pools due to lower daily maximum temperatures via shading effects. However, potential anoxic conditions created by duckweed could affect macroinvertebrates. All mesocosms received an initial assemblage of common rock pool macroinvertebrates consisting of 3 snail species and 1 amphipod species. After 49 days, we measured duckweed growth and counted and identified all aquatic invertebrates.

We found that duckweed percent cover grew quickly and then declined between weeks 4 and 5 of the experiment. Counter to predictions, there was no observed difference in temperature, light availability, or dissolved oxygen. There was lower survival of gilled snails, but not in other taxa the pools were initially inoculated with. None of the initial taxa introduced went extinct over the course of the experiment. Seven organisms colonized pools throughout the experiment. However, only Chironomidae, colonized treatments differently, with higher abundance in our control. Overall species diversity and richness showed no difference among treatments. We found that duckweed has observable effects on percent survival of gilled riverine organisms, but found no difference in oviposition abundance or diversity between treatments. With this understanding, if duckweed does not create anoxic conditions within the pools, then it could potentially serve as a refuge for riverine organisms during isolation events.

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