Lower Respiration in the Littoral Zone of a Subtropical Shallow Lake

Abstract

Macrophytes are important sources of dissolved organic carbon (DOC) to littoral zones of lakes, but this DOC is believed to be mostly refractory to bacteria, leading to the hypothesis that bacterial metabolism is different in littoral and pelagic zones of a large subtropical shallow lake. We tested this hypothesis by three approaches: (I) dissolved inorganic carbon (DIC) accumulation in littoral and pelagic water; (II) O2 consumption estimate for a cloud of points (n = 47) covering the entire lake; (III) measurement of O2 consumption and CO2 accumulation in dark bottles, pCO2 in the water, lake-atmosphere fluxes of CO2 (fCO2) and a large set of limnological variables at 19 sampling points (littoral and pelagic zones) during seven extensive campaigns. For the first two approaches, DIC and O2 consumption were consistently lower in the littoral zone, and O2 consumption increased marginally with the distance to the nearest shore. For the third approach, we found in the littoral zone consistently lower DOC, total phosphorus (TP), and chlorophyll a, and a higher proportion of low-molecular-weight substances. Regression trees confirmed that high respiration (O2 consumption and CO2 production) was associated to lower concentration of low-molecular-weight substances, while pCO2 was associated to DOC and TP, confirming that CO2 supersaturation occurs in an attempt to balance phosphorus deficiency of macrophyte substrates. Littoral zone fCO2 showed a tendency to be a CO2 sink, whereas the pelagic zone showed a tendency to act as CO2 source to the atmosphere. The high proportion of low-molecular-weight, unreactive substances, together with lower DOC and TP may impose lower rates of respiration in littoral zones. This effect of perennial stands of macrophytes may therefore have important, but not yet quantified implications for the global carbon metabolism of these lakes, but other issues still need to be carefully addressed before rejecting the general belief that macrophytes are always beneficial to bacteria.

Determining the high variability of pCO2 and pO2 in the littoral zone of a subtropical coastal lake

“…We are aware that lakes generally present differences between pelagic and littoral zones, and that shallow areas are more influenced by benthic processes than pelagic zones They et al, 2013). However, our aim here was the temporal variability specifically in the littoral zone, since no spatial variation of pCO 2 and pO 2 in nutrients and chlorophyll a was recorded in Peri Lake (Hennemann and Petrucio, 2011).…”

Physical conditions driving the spatial and temporal variability in aquatic metabolism of a subtropical coastal lake

“…In a study during 48 h in a subtropical lake, Tonetta et al (2015a) have recorded that the daily CO 2 dynamics was highly variable and related to organic carbon availability and bacterioplankton, but no spatial variation was studied by the authors. In another shallow subtropical lake, however, They et al (2013) found spatial variability between pelagic and littoral areas, where lower respiration rates were recorded at littoral areas, related to the macrophytes presence. In this way, more studies in subtropical areas are needed to fill the gaps of this knowledge and to understand how both spatial and temporal variation are related to physical conditions in shallow subtropical lakes.…”

“…Several factors may have added the observed higher temporal variability in GPP and NEP at the littoral site, such as macrophytes presence (Obrador and Petrus, 2013;They et al, 2013); sediments heterogeneity (Lauster et al, 2006); wind speed creating spatial patchiness of nutrients and organisms (Van de Bogert et al, 2012), and rainfall (Marotta et al, 2010) introducing allochthonous organic matter. Our methodology, however, did not allow us to access the direct importance of these variables for the observed temporal variability observed.…”

“…http Many variables have been related to the spatial and temporal variability of the metabolic processes ( Van de Bogert et al, 2012). Oxygen dynamics in lakes are under the influence of different benthic substrate types, wind speed and direction, presence of macrophytes, inputs from streams, and the surrounding terrestrial vegetation, which may affect the distribution of nutrients and organisms in the water column, consequently influencing the spatial heterogeneity (Laas et al, 2012;Muduli et al, 2012;They et al, 2013). Wind, light and temperature are associated to the vertical distribution of nutrients and organisms in water column (Serra et al, 2007).…”

Can chlorophyll-a in meso-oligotrophic shallow waters be estimated using statistical approaches and empirical models from MODIS imagery?

“…We used multiple linear regression, artificial neural network, nonparametric multiplicative regression, and four models (Appel, Kahru,FAI and O14a) to retrieve the Chl-a concentration in the Lake Mangueira, a large shallow subtropical lake located in Southern Brazil. This approach is the first attempt to assess the application of remote sensing techniques in such a system, which the oligotrophic conditions are common most of the time (CROSSETTI et al, 2013;LIMA et al, 2016), and the spatial heterogeneity has large differences between the littoral and pelagic zones (CARDOSO et al, 2012;CROSSETTI et al, 2013;THEY;SOUZA, 2012).…”