Seasonal Changes in Metabolic Rates of Two Tropical Lakes in the Atlantic Forest of Brazil

Abstract

We studied the importance of environmental drivers for the seasonal dynamics of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production in surface waters of two tropical lakes in southeast Brazil (Carioca-CA; and Dom Helvé cio-DH), 2011 and 2012, using high frequency measurements of dissolved oxygen. Metabolic rates were approximately twice as high during the fully mixed winter periods, compared to the summer periods. For both lakes, GPP was approximately 30% lower during the warmer and dryer spring of 2012 compared to 2011. Seasonal changes in GPP and R were negatively correlated to water column stability (GPP r = -0.82, p < 0.001; R r = -0.80, p < 0.001). Periods with high stability coincided with warm waters which reduced mixing and internal inputs of nutrients from hypolimnetic waters. GPP was accordingly suppressed in both lakes during summer due to a combination of nutrient depletion and photo-inhibition, which was more pronounced during summer. These conditions were more prevailing during the warm and dry year of 2012, indicating ecosystem responses in carbon cycling to the ongoing regional climate changes.

Seasonal changes in optical properties of two contrasting tropical freshwater systems

“…As the presence of chromophoric carbon in the water protects the aquatic organisms from harmful ultraviolet Optical properties in tropical systems radiation and from the excessive incidence of solar radiation (Zhang et al, 2007a), our results suggest that the photoinhibition of phytoplankton primary production observed in DH during summer by Brighenti et al (2015) is directly linked with the seasonal pattern of photobleaching of CDOM and nutrients availability, where both dependent of the sources and quality of DOM. Thereby, we demonstrated that the main source of DOM in DH come from the terrestrial input during the rainy period, reinforcing the importance of preserving the original forest surrounding the lakes to the global carbon cycle.…”

“…Positive correlations between S 350-400 and nutrients (TP and TN) only during the rainy season suggest that microbial degradation in the epilimnion might be limited by nutrients during thermal stratification. Nutrient limitation of primary production during the rainy season in DH Lake was observed by Brighenti et al (2015) in a two year study of lake metabolism. In agreement with this Chla concentrations were lower in the rainy period in DH.…”

“…In agreement with this Chla concentrations were lower in the rainy period in DH. Furthermore, nutrient limitation of phytoplankton growth raises light transparency and is expected to explain elevated levels of photoinhibition during summer (Brighenti et al, 2015). While our results showed that the increase of aromatic DOM originates from surface runoff during rainy period, DOC and nutrients concentrations were lower in the epilimnion in this period, indicating that much of the newly introduced DOM remained in the hypolimnion until later mixing of the water column in the dry and cooler winter period (Reynolds, 2009), when their concentrations increased in the water surface (Barbosa and Tundisi, 1980;Petrucio and Barbosa, 2004).…”

“…The input of DOM and nutrients to DH occurs via runoff during the rainy season, when the environment is thermally stratified (Petrucio et al, 2005). However, most of the DOM remains below the thermocline due to temperature differences (Reynolds, 2009) and becomes dispersed into the entire water column during the mixing of water in the dry season, increasing the DOC and CDOM concentrations in surface waters (Brighenti et al, 2015). In contrast, Pampulha (Belo Horizonte, MG, Brazil) is a shallower (max 17 m) and less dendritic eutrophic reservoir with reoccurring phytoplankton blooms throughout the year, especially in the summers during thermal stratification.…”

Partitioning of the diffuse attenuation coefficient for photosynthetically available irradiance in a deep dendritic tropical lake

“…Although there is a greater input during rainy season, the water column stability in DH is large in this period (Brighenti et al 2015) and the allochthonous matter coming with runoff from rains remains retained below the thermocline due to differences in water temperature (Reynolds 2009). Redistribution of particulate matter into the upper mixed layers accordingly occurs during the dryer and colder season which therefore is characterized by less transparent water.…”