The impact of summer mortality of invasive zebra mussels on nutrient cycling in a Texas reservoir
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Large mortality events can cause nutrient pulses that affect nutrient cycling within a system and ecosystem functioning. Invasive zebra mussels (Dreissena polymorpha) in Canyon Lake, Texas occur at the southern edge of their North American distribution and hot temperatures during summer can lead to high mortality. The goal of this study was to examine nutrient release in decaying mussels in the laboratory and to combine this with field observations of zebra mussel density and mortality to estimate the amount of nutrients released during summer mortality events. Zebra mussels were collected from Canyon Lake and decayed at 30°C in lab to determine mass loss and nutrient release rates. Dive surveys along several transects in July and October 2019 and 2020 were used to estimate population size of zebra mussels at different depths throughout the lake. Cages with smaller (<15mm) and larger (> 15mm) zebra mussels were placed at three marinas and monitored bimonthly to determine mortality rates. The decline of zebra mussels in summer 2019 was larger compared to 2020, which was associated with a longer period of high water temperatures (27 vs. 17 days over 30°C respectively). Mortality in the cages varied with mussel size, depth and location. Temperature was likely the most important driver, but other factors such as total suspended solids and dissolved oxygen also played a role. Nitrogen and carbon were lost more quickly from the decaying tissue than phosphorus. Estimated nutrient releases for the lake with low to high summer mortality over a month ranged between 1.2 and 47t for nitrogen and 0.04 to 14t of phosphorus. This would mean zebra mussel mortality could release nitrogen into Canyon Lake by 52 to 2, 000% and phosphorus by 6 to 240% compared to inputs from the Guadalupe River. The potential impacts on ecosystem processes remain to be studied.