Lakes are evolving too quickly

Posted on July 25, 2012

Floodplain lakes are formed as oxbows that become separated from meandering rivers.  After separation, such lakes gradually lose depth as flood sediments and organic matter accumulate, and eventually, often after only a few hundred years, they are transformed into wetlands.  This process is accelerated by sediment runoff from agricultural land.  The lakes are fertilised by imported nutrient-rich sediments, and the organic matter produced settles and increases the rate of infilling.  Loss of lake depth has marked effects on physical, chemical and biological conditions, including temperature and oxygen regimes.  Shallower lakes are more susceptible to nutrient resuspension and turbidity caused by wind action and bottom-feeding fish, and their higher rates of production and respiration reduce oxygen availability.  Sediment build-up creates more homogenous benthic environments and reduces the scope for the vertical differentiation of habitats.  These changes have prompted the prediction that lakes of different maximum depth should vary in terms of the types of aquatic communities that they support.  To test this idea a range of variables, including fish species composition, were measured in 42 study lakes in the YazooRiver basin in Mississippi, U.S.A.  Maximum lake depths were 0.5 – 6.6 m.  A total of 54 fish species were collected.   Multivariate analyses were used to model variation in suites of water quality and fish assemblage variables relative to depth.  The main findings were that shallow lakes were strongly dominated by small-bodied opportunistic fish species that mature early and have extended breeding seasons.  Such species were the most likely to recolonise after stressful events such as oxygen depletion, freezing and desiccation, which are more common in shallow water bodies.  By comparison, the environments of deep lakes were relatively stable and supported more species with large bodies, low environmental tolerances and predatory life styles.  There are worries that the accelerated infilling of lakes could locally eliminate significant numbers of fish species, a concern heightened by the fact that human interventions such as bank stabilisation, channelisation and dredging prevent the natural formation of new floodplain lakes

Reference:  Miranda, L.E.  2011.  Depth as an organizer of fish assemblages in floodplain lakes.  Aquatic Sciences 73, 211–221.