Drivers of metabolism and net heterotrophy in contrasting lakes

Abstract

We investigated the influence of light, nutrients, and organic matter on gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = GPP ‐ R) in a dystrophic forest lake and an open eutrophic lake. Forest vegetation reduced incoming irradiance (20%) and wind speed (34%) in dystrophic Gribsø, having thermal stratification 1 month longer than in eutrophic Slotssø. While Gribsø had nutrient‐limited phytoplankton during most of the year, Slotssø only experienced nutrient depletion during algal blooms. Colored dissolved organic matter (CDOM) absorbed most light (average 82%) in dystrophic Gribsø, while phytoplankton and other particles absorbed most light (45%) in eutrophic Slotssø. GPP and NEP were positively related to irradiance in both lakes. However, because of higher CDOM absorbance, three times more light was needed to attain autotrophy in Gribsø, being net heterotrophic (NEP < 0) for 79% of all days, compared to 59% in Slotssø. This difference vanished when NEP was scaled to light absorption by pigments, although the eutrophic lake maintained a higher photon yield. Metabolic rates varied much more in Slotssø, where higher light and nutrient availability facilitated occasional phytoplankton blooms, while low light and nutrient availability in Gribsø dampened temporal variability. Both lakes were annually net heterotrophic with similar annual areal rates (NEP, ‐14 mol C m−2). Net heterotrophy in dystrophic Gribsø derives from high import of organic carbon‐rich water, while heterotrophy in eutrophic Slotssø is fueled by degradation of sediment pools of organic matter accumulated under previous hypereutrophic conditions, emphasizing the importance of lake history on the contemporary metabolic state.

Year-round measures of planktonic metabolism reveal net autotrophy in surface waters of a Great Lakes estuary

“…The environmental variables identified as drivers of planktonic metabolism in the present study (T, irradiance, phosphorus, ammonium, DOC and DO), are in general agreement with those reported in the literature for other freshwater systems (del Giorgio & Peters 1994, Cole et al 2000, Kalff 2002, Smith & Prairie 2004, Staehr et al 2010. Our study also stresses the importance of obtaining winter metabolism measurements to account for the full annual cycle, and sheds light on the complex interaction between physical and biological processes taking place in highly seasonal temperate lakes (Mortazavi et al 2001).…”

“…The greatest GPP:R ratios were observed during breakdown of summer stratification. This result is contrary to 2 earlier studies that concluded stratification breakdown results in reoxygenation of the hypolimnion, and the accumulated reduced substances (including CDOM) are reoxidized, resulting in increased R and reduced GPP:R values for a short period of time (Staehr & Sand-Jensen 2007, Staehr et al 2010. Other studies have reported that mixing reduces GPP due to increased turbidity through resuspended sediments and decreased light penetration (Reynolds 2010).…”

“…In contrast, Site MD is deeper (20 m) and slightly more sheltered by sand dunes from prevailing Lake Michigan winds. Staehr et al (2010) attributed wind exposure and depth to differences in stratification in 2 Danish lakes.…”

Short-term variation in thermal stratification complicates estimation of lake metabolism

“…Despite the importance of Z mix , previous studies have often used discrete weekly temperature profiles to assess mixing depth (Lauster et al 2006; Van de Bogert et al 2007;Coloso et al 2008), although the use of high-frequency temperature profiles is increasing (Staehr and SandJensen 2007;Hanson et al 2008;Staehr et al 2010). Weekly estimates of Z mix are interpolated to daily or subdaily time series for use in a model to estimate daily rates of metabolism.…”

Dynamics of Ecosystem Metabolism and Flocculent Detritus Transport in Estuarine Taylor River

“…Correcting for autocorrelation in regression models has been used in the statistics and economics fields for decades (e.g., Durbin 1960, Beach andMacKinnon 1978), but has gained increasing use in ecological studies in recent years (e.g., Carpenter et al 1998, Lichstein et al 2002, Staehr et al 2010b …”