Abstract
Chlorophyll is frequently used as a proxy for autochthonous production in lakes. This use of chlorophyll concentrations in sediments to infer historical changes in lake primary production relies heavily on the assumption that preservation is sufficient to reflect the productivity in a meaningful way. In this study, we use a series of freeze cores from a lake with annually laminated sediments to assess how reliable down-core trends in chlorophyll are, and to what extent chlorophyll is degraded in the sediment. A striking consistency in the down-core chlorophyll trends in four sediment cores collected in different years (1983, 1992, 2002 and 2010) shows that the sediment preserves a consistent chlorophyll signal over longer timescales. However, there are also clear signs that diagenetic processes within the sediment affect the chlorophyll concentration in sediment layers younger than 10–15 years. This implies that care is needed when interpreting chlorophyll trends in recent sediments. Further, our data show that high-performance liquid chromatography (HPLC) and visible reflectance spectroscopy (VRS) detect similar chlorophyll concentrations for recently dried samples. A third analytical technique, pyrolysis–gas chromatography/mass spectrometry, which provides semi-quantitative values for chlorophyll, also produce a temporal trend that is highly correlated with data from the two quantitative techniques. For samples that have been stored dried at room temperature for several years there is, however, a large discrepancy between the two quantitative techniques. The VRS method is more robust with regards to degradation during storage, while HPLC results demonstrate clear storage effects.
Highlights
Chlorophylls (Chl) and other photosynthetic pigments are used extensively in paleolimnology and limnology as proxies for autochthonous production and algal community composition (Ady and Patoine 2016; Leavitt and Hodgson 2001; Michelutti et al 2005; Reuss et al 2010; Stow and Cha 2013)
We report chlorophyll concentrations determined using visible reflectance spectroscopy (VRS) (Michelutti et al 2005; Wolfe et al 2006), high-performance liquid chromatography (HPLC) (Vinebrooke and Leavitt 1999) and Py–GC/MS (Tolu et al 2015) from a unique temporal series of sediment cores from Nylandssjon
There is a clear effect of diagenesis on the chlorophyll concentrations during the initial years after the formation of a specific varve. This diagenetic effect is less pronounced when taking into account chlorophyll degradation products, either by using the VRS method or by including the chlorophyll degradation products from the HPLC or Py–GC/MS measurements
Summary
Chlorophylls (Chl) and other photosynthetic pigments are used extensively in paleolimnology and limnology as proxies for autochthonous production and algal community composition (Ady and Patoine 2016; Leavitt and Hodgson 2001; Michelutti et al 2005; Reuss et al 2010; Stow and Cha 2013). Short-term (i.e., hours to days) experimental studies have shown that chlorophyll is subjected to degradation both in the water column and in surface sediments (Carpenter et al 1986; Cuddington and Leavitt 1999; Chen et al 2005). This raises the question regarding how well preserved chlorophyll is in a sediment record over timescales of years to decades, i.e., timescales that are known to have a profound effect on the concentrations of, e.g., carbon and nitrogen in the sediment (Galman et al 2008). The conventional analytical approach for the quantification of chlorophyll (as well as a range of its degradation products) is to analyze sediment
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