Spatial and temporal occurrence of hypoxia influences fish habitat quality in dense Hydrilla verticillata

The abundance and architecture of some macrophyte species can alter water chemistry and impact fish habitat quality and quantity. Hydrilla (Hydrilla verticillata) is an invasive aquatic plant that exhibits rapid growth and may contribute to low dissolved oxygen (DO) concentrations (hypoxia, DO < 2.0 mg/L) during warm summer months. DO dynamics were evaluated at a range of spatial and temporal scales in three habitat types: open water, edge of hydrilla beds, and the dense interior of a hydrilla bed at two Florida lakes. Results showed that habitat type, month, and depth all significantly influenced DO, and 100% of the water column was hypoxic in late summer in the small lake. However, hydrilla biomass (kg/m(2)) was much less and no hypoxia was detected in the larger lake, suggesting that lake morphology and size may influence the abundance and impacts of hydrilla on DO. Additionally, hypoxia in edge habitat indicates that increasing ecotone may not greatly influence DO concentrations unless substantial open water area adjacent to dense beds is maintained for adequate water cycling. Our results indicated that complete water column hypoxia occurred in one of our two sample lakes and only during late summer; thus, investigations of hypoxia should consider small spatial and temporal scales.