Mesotrophic Lake Biotic Responses to Hypolimnetic Oxygenation
Cross, Benjamin Keith
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The distribution, abundance, and diversity of lake biota can be impacted by the duration and extent of low dissolved oxygen levels. To increase dissolved oxygen levels and improve the trout fishery, the hypolimnion of North Twin Lake, Washington was oxygenated during the summers of 2009–2015, while adjacent South Twin Lake provided an un-oxygenated reference. Available trout habitat (i.e., lake volume) was quantified using hydroacoustic surveys with parallel systematic 50 m transect spacing and shoreline coverage. Closer spaced transects provided increasingly accurate estimates of lake volume so no optimal transect spacing was determined, but 50 m spacing on North and South Twin lakes provided sufficient estimates (i.e., within 2% of modeled lake volume). Using Rainbow Trout (Oncorhynchus mykiss) preferred temperature (13–19°C) and dissolved oxygen (>5.0 mg/L) values, the percent of lake volume with preferred habitat under peak stratification was 1.0 % greater on average in South Twin prior oxygenation (2004, 2005, and 2008) and 2.1% greater in South Twin during oxygenation of North Twin (2009–2015). However, oxygenation did increase the amount of suitable trout habitat in North Twin. Oxygenation also altered trout access to prey by increasing Daphnia density in North Twin compared to South Twin (P=0.0011), but Chaoborus water column density was lower in North Twin (P<0.0001). Chaoborus and chironomid densities in the benthos were higher in oxygenated North Twin compared to South Twin (P<0.0001). Increased Daphnia prey was reflected in trout diets during one sampling month, but the growth and condition of trout was not influenced by oxygenation. While the abundance of trout in South Twin generally remained similar or higher than North Twin during 2012–2015, growth rates of Rainbow Trout and Brook Trout were not significantly different between lakes (P=0.8210 and P=0.1474, respectively). Compared to South Twin, Brook Trout relative weight was higher in North Twin before and during oxygenation (P=0.0002), but Rainbow Trout relative weights remained similar (P=0.1873). Hypolimnetic oxygenation may enhance a fishery if preferred habitat or hypolimnetic prey utilization increase for an extended duration, but this was not observed in North Twin Lake.