The Response of Aquatic Macrophytes to Lake Management Practices and the Role of Light in the Germination of Macrophyte Propagules

Macrophytes are a vital component to functioning aquatic ecosystems. Specifically, macrophytes promote good water clarity by stabilizing sediments, sequestering nutrients, and reducing the abundance of phytoplankton in the water column. Also, macrophytes provide habitat for other aquatic organisms. Healthy, robust aquatic macrophyte communities are indicated by diverse, abundant stands in the littoral zones of lakes. Poor water clarity and invasive species are primary limiting factors that cause diminished aquatic plant communities. Poor water clarity reduces the light quantity, impeding the growth of macrophytes. Invasive fish, such as common carp (Cyprinus carpio), damage macrophyte communities by uprooting plants and suspending sediment and nutrients in the water column. Invasive plants, such as curlyleaf pondweed (Potamogeton crispus) and Eurasian watermilfoil (Myriophyllum spicatum), often outcompete native species creating dense monoculture stands. To improve the growing conditions for macrophyte communities, several management actions can be pursued to limit the damage by invasive species and improve the water clarity. Common management practices in Midwestern lakes include invasive species control and nutrient sequestration. These practices have been documented to enhance native macrophyte communities. Lake management often requires several years of consistent, adaptive management to effectively restore the ecosystem. Adaptive management is the systematic process of learning from past management outcomes and subsequently incorporating that knowledge into current management decisions. I evaluated the change in the macrophyte community in Lake Riley, Chanhassen, MN over the course of 6 years of lake management actions using aquatic plant point-intercept surveys from 2011 to 2016. The results of the surveys found that after a carp removal in 2010, curlyleaf pondweed dominated the littoral zone and water clarity did not greatly improve. Once invasive macrophytes were managed starting in 2013, incremental increases in the species richness of the macrophyte community occurred. However, native macrophyte expansion was limited because water clarity was still poor during the summer growing season. In 2016, after an alum treatment, water clarity improved and the macrophyte community abundance and richness further increased. Species richness increased from 9 observed species in 2011 to 15 in 2016. During peak growth in August, the native species frequency of occurrence was 50% through 2013 and then increased up to 80% of sites in 2016. The August native macrophyte biomass increased from 30g/m2 in 2011 to 600g/m2 in 2016 (p<0.05). Prior to 2016, the average maximum depth of rooted native plant growth was 3.1m and in 2016 it increased to 4.1m. Overall, the density, coverage, and richness of the macrophyte community increased throughout the study period demonstrating that the macrophyte community had a positive response to the multi-year management practices on Lake Riley. The specific mechanism of macrophyte recruitment following improved growing conditions, such as in Lake Riley, is an understudied area of macrophyte restoration. Macrophytes typically propagate through clonal growth and fragmentation. However, when macrophyte populations are reduced, the lake seed bank may contribute to the reestablishment of the population. In previous studies on temperate lake seed banks, seeds from vascular aquatic plants and spores from macroalgae have been found in varying densities and viability levels suggesting that recruitment from the seed bank is possible in some systems. I conducted a controlled laboratory experiment using sediment from Lakes Ann and Riley located in Chanhassen, MN, to 1) evaluate the response of the seed banks to different treatments and 2) compare the observed taxa sprouting from the seed banks to the taxa observed growing in the lakes. The treatments included a maximum germination treatment using a germination promoter to evaluate the full extent of the viable seed bank, a treatment representative of a lake with good water clarity, and a treatment representative of a lake with poor water clarity. The good and low clarity treatments were designed to evaluate the response of seeds to two different light levels that were observed in lakes with high turbidity (low-light intensity) and low turbidity (high light intensity). It was hypothesized that the maximum germination treatment would have the highest amount of germination, the high clarity treatment would have the second highest amount, and the low clarity treatment would have the lowest amount of germination due to the low-light quantity. The seed banks of both Lakes Riley and Ann were similar to the macrophyte community observed growing in the lake. In Lake Ann, 16 species were observed sprouting and every species observed in the experiment grew in the lake. In Lake Riley, 17 species were observed sprouting and all but two species were observed both in the lake and in the seed bank. The seed banks did not show any significant difference in response to the germination treatments. Chara, curlyleaf pondweed, and wild celery were the most frequent species observed. Under maximum germination conditions, Lake Riley had a viable vascular seed density of 2,916 ± 1,828 seeds/m2 and a viable chara spore density of 1,033 ± 698 spores/m2. Lake Ann had a viable vascular seed density of 1,100 ± 440 seeds/m2 and viable chara spore density of 13,833 ± 2,825 spores/m2. The study demonstrated that germinating propagules from a lake seed bank can be a valuable tool for managers to evaluate the viable macrophyte taxa present and better understand the potential for recruitment from the seed bank. Overall, to restore native macrophyte communities, it requires several multi-year management actions and will likely include multiple forms of propagule recruitment.
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University of Minnesota (Minneapolis, Minnesota)
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Dunne, Melaney
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