Importance of nutrients, organic matter and light availability on epilimnetic metabolic rates in a mesotrophic tropical lake

Abstract

Abstract Global changes, such as climate and land use alterations, are expected to affect freshwater ecosystems, modifying environmental conditions with ultimate effects on carbon and nutrient cycling. While effects of these changes have been the focus of several studies in temperate lakes, remarkably little effort has been dedicated to understanding the behaviour and responses of tropical lakes despite their importance in tropical environments. In a factorial mesocosm study conducted in a tropical lake, we investigated how changes in environmental conditions, such as additions of inorganic nutrients and allochthonous organic matter, and shading, affect gross primary production (GPP), community respiration (R) and net community production (NCP) rates, and the occurrence of GPP light saturation and photoinhibition. GPP rates in the mesocosms were enhanced by additions of inorganic nutrients (+NP) and organic matter (+C) and decreased with increasing mean light available (Ī). Apparent photoinhibition was pronounced in full light treatments (+L) without additions of nutrients (−NP) and organic matter (−C), occurring in 91% of the experimental days. This supports previous evidence of the importance of photoinhibition in the upper mixed layer of the lake, especially during nutrient depletion and high light availability in the warmer rainy season. The main factor affecting metabolic rates was nutrients (NP), followed by organic matter (C). Effect of light (L) and the interaction between C and L were also significant for GPP and R rates. Addition of allochthonous organic matter (+C) with a high content of coloured dissolved organic matter (CDOM) stimulated GPP rates and reduced the occurrence of apparent photoinhibition and light saturation. This also increased R rates in the mesocosms. However, R was strongly coupled to GPP, indicating that changes in respiration were more related to autotrophic activity than to a direct and independent stimulation of heterotrophic communities. Our results strongly support recent evidence of the interactions between nutrients, organic matter and light conditions, which under a scenario of changes in the frequency and intensity of precipitation events and human‐mediated changes in land use, may affect the magnitude, variability and balance of metabolic processes in the productive upper mixed layers of tropical lakes.