Aquatic macrophyte response to carp removal and the success of transplanting aquatic macrophytes to restore the littoral community

Abstract

Aquatic macrophytes play a significant role in maintaining water quality and ecosystem health. They provide refuge for algae-consuming zooplankton, stabilize sediment, consume nutrients, and provide habitat for aquatic vertebrates. However, high densities of non-native invasive macrophytes can cause harm to the lake ecosystem. A healthy aquatic macrophyte community consists of a diversity of native taxa. Common carp, an invasive species, can disrupt native macrophyte communities. High densities of common carp uproot aquatic macrophytes and stir up sediments, encouraging a turbid water state and low density of macrophytes. Water clarity and aquatic macrophytes will likely increase after the removal of common carp. However, it can take several years for native macrophytes to return after a large-scale fish manipulation. There is also concern that non-native invasive macrophytes will quickly dominate the littoral zone after carp removal.I evaluated the response of the aquatic macrophyte community, between 2009 and 2012, after the large scale removal of common carp from Lake Susan, Carver County, MN (MN DNR DOW ID 10-001300). To assess plant frequency of occurrence, plant communities were assessed using a point intercept survey method in the spring and mid-summer. To quantify changes in plant biomass, approximately 40 random biomass samples were taken during each survey. To further understand response of Eurasian watermilfoil (Myriophyllum spicatum), a non-native invasive plant, milfoil herbivore surveys were also conducted throughout each summer. To evaluate recruitment potential of the non-native curlyleaf pondweed (Potamogeton crispus) turions were sampled each October. Comparisons of the plant community were done between the years to understand the succession of natural plant recruitment.To promote the recruitment of native plants, I transplanted six native taxa from nearby Lake Ann into Lake Susan to increase the species richness and distribution macrophytes within the lake. Four separate transplant experiments were conducted, each assessing a different variable. The primary focus of experiment one was to assess the survival of selected taxa and compare growth between open locations and those protected from herbivore access. The primary focus of experiment two was to increase the number of transplant locations geographically around the littoral zone, and correlate environmental growth factors (light, sediment bound nitrogen, sediment bulk density, sediment organic matter, and wave protection behind lily beds), to the expansion of the transplanted taxa. Experiment three tested transplant survival and expansion at greater depths. Experiment four evaluated if earlier planting of transplants improved survival of deeper transplants.There was an increase in the diversity and overall abundance of submersed aquatic macrophytes in Lake Susan after common carp population was reduced. Species richness increased naturally from 13 taxa in 2009 to 14 in 2012. The number of sites with moderate diversity (>4 taxa per site) increased from 8 sites to 2009 to 12 sites in 2012. There was also an increase in the frequency of occurrence of most taxa. Total dry shoot biomass of both native and invasive taxa also showed a statistically significant increase (p <0.001). The non-native curlyleaf pondweed increased significantly from 17% of sites in 2009 to 41% of sites by 2011. Eurasian watermilfoil decreased in frequency and biomass between 2009 and 2012. This decline, with the persistent high density of milfoil weevils suggests the weevils provided effective biocontrol of Eurasian watermilfoil. Transplanting whole adult plants in shallow water (<1 meter) was generally successful for the duration of this study (four years). Most transplanted taxa showed survival during the initial growing season. Overwintering success was a better predictor of long term success than initial survival and protection from by lily beds. Wild celery (Vallisnaria americana), and water-stargrass (Zosterella dubia) had the highest survival, whereas bushy pondweed (Najas flexilis), and muskgrass (Chara spp.), had inconsistent survival, and northern watermilfoil (Myriophyllum sibiricum), largely failed to survive. The most important environmental factor in the success and expansion of transplants was depth, which affected light availability. Whole plants transplanted into deeper zones (approximately 1.4m depth) failed due to low light (summer Secchi depths were often <1.0m). Both native and non-native aquatic macrophytes responded favorably to the removal of high carp densities, and transplanting aquatic macrophytes can help to restore the littoral community.