Dissolved organic matter concentration, optical parameters and attenuation of solar radiation in high-latitude lakes across three vegetation zones

Abstract

19High-latitude lakes usually have a high penetration of light, due to their low productivity 20 and low concentration of dissolved organic matter (DOM), but large variations in lake optical 21 properties can be found within and between regions. We investigated the underwater light shallow lakes that are already illuminated to the bottom even in the darkest systems. 40Keywords: dissolved organic matter, high-latitude lakes, lake optical properties warming (Williamson et al., 1999). They regulate the transmission of both photosynthetically 47 active radiation (PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) (Scully & 48 Lean, 1994; Morris et al., 1995; Laurion, Vincent & Lean, 1997; Huovinen, Penttilä & 49 Soimasuo, 2003; Bracchini et al., 2006) and therefore contribute to defining the species 50 composition in lakes (Rautio & Korhola, 2002), the ratio between auto-and heterotrophic 51producers (Jansson et al., 2000, Forsström, Roiha & Rautio, 2013, and the overall benthic and 52 pelagic productivity (Karlsson et al., 2009).The vegetation in the catchment, catchment to lake ratio, and the productivity of the lake have 55 a prominent impact on the concentration and composition of DOM. In small oligotrophic lakes 56 with low chlorophyll-a concentration and large catchment areas a high proportion of carbon is 57 derived from terrestrial and wetland sources dominated by higher terrestrial plant productivity 58 (Bade et al., 2007). The organic carbon leaching from forests and wetlands constitute mainly 59 of slow-degrading and nutrient poor material dominated with humic and fulvic constitutes 60 (McKnight & Aiken, 1998; McKnight, Aiken & Smith, 1991; McKnight et al., 1994) With the exception of Lake Kilpisjärvi (lake area 3710 ha, max depth 57 m), the MBW (Table I). Due to their shallowness, the majority of lakes (11) were isothermal during the sampling. 214Lake water temperature varied between 5.9 and 11.9 ºC being highest at lowest altitudes. 215Conductivity had highest range in ST, between 0.7 and 4.3 mS m -1 . The pH of three lakes, two (inorganic nutrients mainly below the detection limit, total P 3-13 µg l -1 and total N 71-410 µg 219 l -1 ) and low turbidity (< 1.0 FNU), with highest values generally measured from MBW. 220Chlorophyll a concentration varied between 0.2 µg l -1 and 2.4 µg l -1 and phytoplankton biomass 221 was low (less than 0.5 mg l -1 ) in all the study lakes. Only in the deepest lake (Kilpisjärvi) of the 222 three that were sampled from two different water layers, was chlorophyll a markedly lower in 223 the deeper water layer compared to the 1m depth (Table I). Most lakes were dominated by 224 chrysophytes (Chrysophyceae), but in a few lakes the dominating algal group was green algae 225(Chlorophyceae), cryptophytes (Cryptophyceae) or dinoflagellates (Dinophyceae). 226Dinoflagellates were most common in MBW lakes with high DOC and color (Korsajärvi, 227Koddojärvi and Ristijärvi) (L. Forsström, unpublished data). 228ANOSIM identified two groups separating ...

Role of terrestrial carbon in aquatic UV exposure and photoprotective pigmentation of meiofauna in subarctic lakes

“…Increasing SUVA is an indicator for increasing terrestrial contribution for the aquatic DOC pool (Hood, Williams & McKnight, ). It does not necessarily depend on DOC concentrations (Jaffe et al ., ; Forsström et al ., ), although in the current lake set there was a significant correlation between SUVA and DOC (Table ). Highest SUVA values in the current study occurred in lakes with widely paludified catchments (#2, 9, 14, 18–19), suggesting that humic substances from the surrounding mires are major components of the allochthonous carbon transported into the lakes.…”

“…A central component in the underwater UV environment and UV attenuation of lake water is organic carbon of terrestrial origin (allochthonous carbon) that arrives to the lakes from surrounding soils and vegetation (Tranvik et al, 2009;Jansen, Kalbitz & McDowell, 2014). It impacts optical properties of lake waters and accordingly also underwater UV exposure (Vincent & Pienitz, 1996;Vincent et al, 2007;Forsstr€ om et al, 2015). Climate warming has increased the risk of this carbon becoming mobile (Davidson & Janssens, 2006;Friedlingstein et al, 2006), which can cause significant changes in aquatic systems and their underwater UV regimes opposing stress to organisms and changing ecosystem functions (Schindler et al, 1996;Vincent et al, 2007).…”

Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems

“…2.2), indicating that photodegradation may decline in some slump-impacted systems due to adsorption of CDOM to basic cations and clay particles. A strong response in UVB attenuation to small changes in CDOM was observed in lakes of NW Finnish Lapland, suggesting that even minor shifts in CDOM may greatly change the UV radiation exposure of high-latitude lakes, with likely consequences on the photochemistry and biota (Forsström et al, 2015). To understand the role of sunlight in DOM processing in thermokarst waters, future work must quantify UV irradiance in the water column, residence time of DOM in the UV-exposed portion of the water, and identify the factors that control vertical losses of DOM and the lability of permafrost DOM to photodegradation.…”

Climate and productivity shape fish and invertebrate community structure in subarctic lakes

“…We used lake bathymetry, oxygen and light profiles (see Hayden et al, 2014;Hayden, Harrod, Sonninen, & Kahilainen, 2015 for details) to identify littoral (shore to compensation depth), profundal (compensation depth to deepest point) and pelagic (area above the profundal) zones. The theoretical compensation point in these lakes was calculated from water column light attenuation curves (Forsstr€ om et al, 2015). lakes 4, 5, 9, 10, 19) were defined as entirely littoral systems and were not subdivided.…”

“…lakes 4, 5, 9, 10, 19) were defined as entirely littoral systems and were not subdivided. The theoretical compensation point in these lakes was calculated from water column light attenuation curves (Forsstr€ om et al, 2015). Lake fish communities were assessed using gillnets set in littoral, profundal and pelagic (floating nets; 0-2 m depth) habitats.…”