Transient physicochemical microhabitats facilitate fish survival in inhospitable aquatic plant stands

1. Aquatic plants add structural complexity to aquatic environments, provid e resources for the food web and generally promote diversity and stability of aquatic fauna. However, plants also alter the abiotic properties of the water and at high densities can produce physical and chemical conditions in tolerable to fish. 2. We identified substantial horizontal and vertical, as well as spatial an d temporal, variability in oxygen and temperature over micro-scales within macrophyte stands. Areas with suitable dissolved oxygen and temperature occ urred close to areas with hypoxic and hyperthermic levels regarded as highl y stressful to lethal for fish. Fish densities within the plant stands did not change between dusk and dawn, suggesting that fish did not leave the ve getation when conditions became adverse. 3. Variability in oxygen and temperature within the plant bed conceivably p rovided refugia for fish. Portions of the macrophyte bed, particularly the surface, provided suitable conditions during times of the day when oxygen t ensions in deeper water were low, but not during times when surface conditi ons were hyperoxic and hyperthermic. During periods of low oxygen, fish may rise to the surface and either remain there, or use the surface as a corri dor to travel until oxygenated pockets are located. 4. Aquatic plant stands may be thought of as a mosaic landscape composed of patches of transient physicochemical microhabitats. Within this landscape, physicochemical conditions can be precarious and fish may have to continuo usly change microhabitats to avoid being trapped in unsuitable locations du ring the daily reshuffling of physical and chemical conditions.