The influences of historic lake trophy and mixing regime changes on long-term phosphorus fraction retention in sediments of deep eutrophic lakes: a case study from Lake Burgäschi, Switzerland

Abstract

Abstract. Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones.

Shallow Sediment as a Phosphorus Reservoir in an Oligotrophic Lake

“…P storage in lake sediments and subsequent release to the water column is potentially controlled by several processes including reduction‐oxidation (redox) conditions, mineralization of organic matter, sediment resuspension, and nutrient loading. Oxygenation and redox at the sediment‐water interface is found to be an important factor influencing P retention in surface sediments (Tu et al., 2020). Net retention of P in the sediment ultimately depends on the difference between two overarching processes: (a) the flux of P to the lake floor sediment mostly from the settling of particulate matter continuously entering or produced in the water column; and (b) the release of P from sediment, which is influenced by sediment texture and composition, decomposition of organic matter, and redox conditions (Søndergaard et al., 2003).…”

Scanning Hyperspectral Imaging for In Situ Biogeochemical Analysis of Lake Sediment Cores: Review of Recent Developments

“…RABD 670 (or similar) has been widely used as an index of green pigments and as a proxy for aquatic productivity in a wide variety of environments, including marine [ 9 , 23 ] and lake sediments [ 27 , 36 , 37 , 39 , 63 , 64 ], and over timespans greater than 100,000 years [ 65 ]. A modified version of the RABD Formula (2) flexibly utilizes the wavelength associated with maximal absorbance, which is preferable for reconstructions of total algal production [ 14 , 53 , 66 ]: where R λ is the reflectance at the wavelength (λ), R 655–680min is the trough minimum (i.e., lowest reflectance value measured between 655 and 680 nm), X is the number of spectral bands between R 730 and the trough minimum, and Y is the number of spectral bands between the trough minimum and R 590 . Endpoint wavelengths (590 and 730 nm in Equation (2)) are selected by examining spectral endmembers to identify the edges of the absorption trough and should be selected from wavelengths that show minimal variability throughout the sample material.…”

“…HSI has been applied to lake sediments from across the globe, with the majority of sites being in Europe. The most common application has been the reconstruction of past aquatic productivity based on the absorbance of chloropigments [ 12 , 13 , 14 , 43 , 44 , 45 , 47 , 49 , 50 , 51 , 53 , 66 , 73 , 74 , 75 ]. Other sedimentary variables interpreted from HSI include bacteriopheophytin a [ 6 , 12 , 13 , 43 , 44 , 45 , 47 , 49 , 50 , 51 , 53 , 75 ], phycocyanin [ 44 ], bulk organic matter [ 11 , 55 ], aromatic organic matter [ 44 ], charcoal [ 12 , 62 ], tephra [ 54 ], flood layers [ 17 , 18 ], particle size [ 15 , 16 ], calcite [ 12 , 47 ], and lithogenic minerals [ 12 , 45 , 46 ].…”

Theoretical Principles and Perspectives of Hyperspectral Imaging Applied to Sediment Core Analysis

“…Four main types of hyperspectral cameras from different manufacturers are available on the hyperspectral core-logging market. The visible and near-infrared (VNIR) camera covers the range of 400-1000 nm, allowing the detection of photosynthetic pigments (chlorophylls) and bacterial pigments (bacteriochlorophylls) [52][53][54][55][56], iron oxides, and altered organic matter [57]. The short wave-infrared (SWIR) sensor, which acquires a spectrum between 1000-1700 nm or 1000-2500 nm, allows the study of organic compounds (aromatic, aliphatic) [58][59][60] and types of deposits (clays, carbonates) [61,62].…”

“…Currently, two spectral areas are widely used with the VNIR camera data. The first is specific to chlorophyll a (Chl a), pheophytin a (Phe a) and their derivatives, which can be identified by integrating the absorption area of these compounds in the range from 584 nm to 730 nm (Area584-730) or measuring their absorption peak height [52][53][54][55][56]. Chl a allows the reconstruction of aquatic productivity within the lake.…”