Diel Variation Related to Thermal Mixing in a Subtropical and in a North-Temperate Shallow Floodplain Lake

Abstract

Shallow floodplain habitats connected to large rivers undergo seasonal flooding making them integral parts of large river ecosystems. In tropical and subtropical regions, water mixing in these habitats is described as polymictic, yet little description exists for other large river floodplain lakes. In this study, the effect of thermal input on die1 mixing was compared between subtropical Gargas Lake (GL)(1.2 m depth) connected to the Parana River, Brazil and temperate Crane Lake (CL)(0.5 m depth) connected to the lllinois River, USA. Physical, chemical, and microbial variables were measured at 3 h intervals on clear, nearly windless (avg. < 1.2 m i') days in summer (GL, 8-9 Jan 02; CL, 8-9 Aug 02). Nocturnal, convective mixing was evident for temperature (range: GL, 28-33 "C; CL, 23-32 "C) and oxygen (range GL: 23-196%; CL: 24-200%) in each system by nightfall (l930h) and correlated with differences in nitrate and ammonia. Although total bacterial density differed between surface and bottom water samples by afternoon, it was similar at both depths by evening (CL, 2230h) or early morning (GL, 430h). Diel mixing occurred in both systems and has important implications in nutrient cycling, coupling of oxic-anoxic processes, and distribution of microbiota. INTRODUCTIONLarge river ecosystems are characterized by many aquatic and aquatic-terrestrial transitional habitats (Junk et al. 1989) and by hydrologic interactions between the main channel of the river and the surrounding floodplain (Ward and Stanford 1995). Historically, intact large river systems flooded annually, providing a pdse of water that moved from the channel over the surrounding floodplain. Periodic flooding was not an ecological disturbance but a natural trait of the system enhancing habitat diversity , species richness (Sparks 1995), and productivity (Junk et al. 1989). Shallow floodplain lakes, typically less than 3 m in depth, have intense sediment-water interaction (Scheffer 1998). These lakes are seasonally connected to rivers, comprise a prominent part of floodplains, and are extremely important components of these systems since they are permanent waterbodies where aquatic organisms find refuge during low water periods. 'Present address:

Migratory benthic fishes may induce regime shifts in a tropical floodplain pond

“…4). Although P release from sediment could partially explain these results, we reject this possibility because the Paraná floodplain's shallow lakes and ponds are highly oxygenated during low water periods (Thomaz et al, 2004;Lemke et al, 2010). Despite the fact that the transient state does not appear to have been clearly detected by the Spearman's analysis, suggesting greater similarity with the turbid state, a transient state may be observed during periods of intermediate water levels.…”

“…June 2007, November 2007and 2008, which can be enhanced by the lack of flushing or mixing with clear water from the river. Although P release from sediment could partially explain these results, we reject this possibility because the Paraná floodplain's shallow lakes and ponds are highly oxygenated during low water periods (Thomaz et al, 2004;Lemke et al, 2010). In addition, owing to the height and density of vegetation sheltering the shoreline of the pond, it is improbable that wind had significant effects on sediment resuspension at high water level.…”