High rates and close diel coupling of primary production and ecosystem respiration in small, oligotrophic lakes

Studies on small, shallow lakes are few and have traditionally focused on humic lakes, whereas transparent, oligotrophic lakes dominated by submerged macrophytes have been overlooked. This may have given rise to a skewed perception of shallow lakes as being well mixed, turbid and dominated by ecosystem respiration relative to primary production. Mixing patterns and ecosystem metabolism in five oligotrophic shallow lakes dominated by charophytes were investigated in order to determine gross primary production, ecosystem respiration, their regulation and mutual coupling in this very common lake type. Although lakes were very shallow (< 0.5 m), high charophyte biomass caused strong daytime stratification followed by nocturnal mixing. Despite the nutrient-poor water, volumetric rates of production and respiration during spring-summer were high compared to most medium to large lakes. This intensive metabolism is likely a result of the high charophyte biomass and the shallow mixed surface layer. Areal rates of production and respiration were also high compared to values from other aquatic systems. Strong coupling between daily rates of production and respiration suggested that the majority of organic substrates for ecosystem respiration were produced within the lakes. Net ecosystem production was slightly positive during the growth season. This study highlights the role of submerged macrophytes as primary drivers of temperature dynamics, stratification-mixing as well as high metabolism in small, shallow lakes with dense vegetation.