How Do You Measure a Year in the Life? Examining Ecosystem-Wide Responses to Senescence of Invasive Hydrilla in Upland Streams of the Cumberland Plateau, Tennessee
Date
2019
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Publisher
Austin Peay State University
Abstract
Aquatic macrophytes provide structural complexity and regulate nutrient availability between periods of growth and senescence. Invasive macrophytes disrupt this process by altering both biotic and abiotic dynamics within streams. Hydrilla ("Hydrilla verticillata") is an invasive submerged aquatic macrophyte whose quiescent propagules, low-light requirements, and fast growth rates allow it to dominate in any body of freshwater where it establishes. In 2004, monoecious hydrilla, which experiences seasonal dieback, was discovered in the Emory River Watershed (ERW), a high-elevation drainage characterized by high quality waters and notable
diversity. Considering its high rate of biomass production, hydrilla has the potential to influence normal nutrient processes. Water chemistry, periphyton, and macroinvertebrates samples were collected from eight stream reaches, four with and four without hydrilla. Water samples were analyzed for concentrations of orthophosphate and nitrate/nitrite nitrogen, and abiotic parameters were measured via YSI. Periphyton were sampled from cobble, and chlorophyll a concentration was used as a proxy for algal biomass. Macroinvertebrates were subsampled and identified to allow analysis of diversity, function feeding groups, and similarity metrics. Orthophosphate and conductivity were higher during the growing season, while nitrate/nitrite was higher during the senescent season. Periphyton growth and proportions of grazer/scrapers and collector-filterers were greater at hydrilla sites during the growing season. Macroinvertebrate analyses showed no significant effects of hydrilla on diversity metrics. Analyses of community similarity indicated high similarity within hydrilla sites than within non-hydrilla sites during the growing season, suggesting that hydrilla may have a homogenizing effect on macroinvertebrate communities. I hypothesize that water level fluctuations may be causing hydrilla senescence during the growing season when stream discharge declines, exposing and desiccating plants growing too close to the bank; nutrients from these decomposing plants are then returned to the water when water levels rise following rain events. Suggestions for future studies regarding hydrilla in the ERW would be to determine the physiological responses of hydrilla to water level fluctuations as well as evaluate effects on macroinvertebrates via secondary productivity rather than diversity and community metrics, as biomass data may be able to give greater insight into the metabolic outcomes of seasonal dieback.