Temporal dynamics and regulation of lake metabolism

Abstract

We studied temporal dynamics and regulation of oxygen metabolism in the upper mixed layer of a nutrientrich shallow Danish lake by continuous measurements of oxygen, irradiance, wind, and temperature and frequent measurements of algal chlorophyll, organic pools, and inorganic nutrients. Chlorophyll, algal growth rate, and mean irradiance (E mean ) in the mixed surface layer were calculated daily from continuous measurements of irradiance and temperature with depth. There were three to four distinct maxima in gross primary production (GPP) and community respiration (R) during the summer season and minima from fall to spring after broad-scale changes in irradiance, temperature, mixing depth, and biomass and growth rate of the algal community and concentrations of inorganic nutrients. Lake metabolism was annually balanced (mean GPP : R 1. However, GPP : R varied two-to threefold from day to day because lower surface irradiance, higher mixing depth, and thus lower E mean significantly reduced GPP. Normalizing GPP to chlorophyll provided an index of algal growth potential (GPP B ), which followed a hyperbolic relationship to E mean , and both parameters were related to blooms and collapses of algal biomass. Metabolic rates were much more variable from day to day than algal biomass, which integrates growth and loss processes over longer periods. The continuous approach to lake metabolism provides better data and can provide a more accurate picture than averages of a few discrete measurements. Weekly averages reflected the characteristic seasonal peaks and troughs also observed for algal biomass, whereas monthly averages did not. Daily measurements of lake metabolism, therefore, can provide the optimal background for evaluating temporal changes and regulation of algal biomass and organic pools in nutrient-rich shallow lakes.Gross primary production, ecosystem respiration, and the balance between the two are known to vary widely across aquatic ecosystems and to display large temporal variations within ecosystems (D'Avanzo et al. 1996;Smith and Hollibaugh 1997;Cole et al. 2000). Although intersystem differences and long-term changes in annual and seasonal patterns have been studied for several decades (e.g., Odum and Hoskins 1958; Whittaker and Likens 1973), much less is known of the considerable short-term changes in metabolism between days and weeks. Day-today differences in irradiance and temperature obviously affect rates of photosynthesis and community respiration, and weaken statistical relationships to nutrients and biomasses of phytoplankton, bacteria, and zooplankton. Despite the highly dynamic behavior, many previous studies of lake metabolism have been forced to use timeaveraged conditions for years and seasons rather than the dynamic daily values (Kalff 2002), which can now be monitored by new, reliable, and fairly cheap technologies. The improved understanding of temporal dynamics obtained by a continuous monitoring approach therefore has the potential to strengthen predictions of how changes in envi...

Diurnal sampling reveals significant variation in CO2 emission from a tropical productive lake

“…Moreover, although this study was conducted only during the dry season, it is likely that diurnal variation in pCO 2 in Lake Carioca is even larger during the rainy season. The higher mean temperature and greater input of nutrients and dissolved organic carbon from the watershed during the rainy season (summer) might favour both autotrophy and heterotrophy in the lake (Brown et al, 2004;Staehr and Sand-Jensen, 2007;Marotta et al, 2012) likely resulting in higher amplitude of diurnal CO 2 variations (Marotta et al, 2010).…”

Changes in ecosystem resilience detected in automated measures of ecosystem metabolism during a whole-lake manipulation

“…Some regime shifts may involve mechanisms that obscure signals of declining resilience in aggregated system variables like NEP (31), but measurement error or other sources of noise could have a similar effect. In lakes, estimates of GPP, R, and NEP often exhibit high day-to-day variability (32,33) and are often poorly correlated with potential driver variables (34,35). Sensor measurements are subject to measurement errors related to spatial heterogeneity and other processes (32,36,37).…”

Intra- and inter-annual variability in metabolism in an oligotrophic lake

“…To date, most lake metabolism studies using these techniques have focused on temporal variation based on measurements made in the epilimnion and restricted to one year time periods (e.g., Carignan et al 2000;Sadro et al 2011;Klug et al 2012;Solomon et al 2013;Morales-Pineda et al 2014), in part because the deployment of high-frequency sensors is a recent phenomenon (Weathers et al 2013). In the few studies when metabolism was estimated across multiple years in lakes, rivers, or estuaries, metabolic rates were linked to anthropogenic nutrient loading (Uehlinger 2006) and climatic variation (Roberts et al 2007;Staehr and Sand-Jensen 2007;Einola et al 2011;Laas et al 2012;Caffrey et al 2014;Roley et al 2014). Here, we expand upon these earlier studies to intensively investigate the patterns of metabolism over longer-term scales (i.e., [5 years) in a large, oligotrophic lake that has not experienced directional climate variation (Carey et al 2014).…”