Pelagic energy mobilization across crossed gradients of phosphorus and dissolved organic carbon in a chemostat experiment

摘要

Pelagic production depends on biological energy mobilization based on both light energy mobilized by phytoplankton and imported energy bound as allochthonous organic carbon (AOC) and utilized by bacteria. Both autotrophic (phytoplankton) and heterotrophic (bacterioplankton) production form the basis of pelagic energy mobilization (PEM) in lakes (Jansson et al. 2003). The relative importance of these two energy mobilization pathways changes with respect to phosphorus (P) and AOC availability (Karlsson et al. 2002, Jansson et al. 2003). Whereas heterotrophic pelagic energy mobilization (PEM_(het)) increases with AOC (Hessen 1998, Jansson et al. 2000), both autotrophic (PEM_(aut)) and heterotrophic production increase with P (del Giorgio & Peters 1994, Nurnberg & Shaw 1999), although the proportion each contributes to PEM may change with increasing total phosphorus (TP) concentration (Rothhaupt & Gude 1992, Kritzberg et al. 2006).rnJansson et al. (2003) demonstrated from whole lake data from unproductive lakes that nutrient use efficiency (PEM/ TP) is lower in heterotrophic systems than in autotrophic systems. Bacteria can use AOC as a carbon and energy source, thus uncoupling bacterial production from reliance on phytoplankton carbon (Jones 1992). Because bacteria have a higher affinity for P than phytoplankton, they can out compete phytoplankton at low concentrations of P, but would be expected to mobilize less carbon (C) per unit of P because bacteria contain approximately 10 times more P per unit C (by weight) than phytoplankton (Vadstein 2000). Consequently, it would be expected that less energy per unit P would be available for higher trophic levels in pelagic systems dominated by heterotrophy (Jansson et al. 2003).rnA second explanation for PEM/TP being lower in heterotrophic than autotrophic systems is that although a high AOC input rate is correlated with a high TP input rate (Meili 1992), P can be tightly bound to AOC and not always be available for bacterial and phytoplankton production (Jones 1998). An increase in the bioavailability of P may be caused by U V degradation of AOC, or eutrophication (Cotner & Biddanda 2002).rnIn this study we examine how the relative contributions of heterotrophic and autotrophic production change with a range of AOC and P concentrations in a chemostat environment. We hypothesise that the proportion of pelagic energy mobilisation that is heterotrophic (%PEM_(het)) is positively correlated with the input rate of AOC and negatively related to the input rate of inorganic P at a given dissolved organic carbon (DOC) concentration. We also hypothesise that the nutrient use efficiency (PEM/TP) will decrease as heterotrophy increases.