Long-term patterns in pH and colour in small acidic boreal lakes of varying hydrological and landscape settings

Abstract

Changes in water pH and colour since the late 1980s were studied in 35 small boreal lakes of varying hydrological and landscape settings but similar climate and acid deposition. The data was collected during the autumnal overturn on the annual basis except in lake with weekly sampling during the ice-free period. In addition to the deposition data information about catchment soil types as well as local meteorological and hydrological conditions were used for the long-term data interpretation. The lakes are situated in a small area in southern Finland, 130 km north from Helsinki, where sulphate deposition declined by [60% in one decade since the mid1980s. The results showed that water colour increased in most lakes while pH did not. In lakes dominated by surface runoff there was a distinct upward shift in colour, with an initial increase after the mid-1990s and a second increase in 2004. The first shift appeared when the sulphate deposition reached a level ca. 25% of that in 1988. However, the upward shift in colour also coincided with a change in hydrological conditions after several dry summers. In contrast, the second shift in colour clearly coincided with a switch in hydrology due to the abnormally wet summer of 2004 after severe drought in 2002-2003. Although the hydrological conditions indisputably had a key role in determining the annual variability in colour, a distinct negative relationship between acid deposition and water colour in 90% of the lakes strongly suggested that reduction in sulphate deposition fostered the leaching of coloured organic substances from the catchment soils. Increase in colour, in turn, strongly influenced lake water pH, and the present day higher organic matter concentrations seemingly depress pH values more than in the 1980s, before the reduction in acid deposition.

Iron as a source of color in river waters

“…It is also a critical factor when natural water is purified to produce good-quality water for domestic and industrial purposes (Chow et al, 2007). During the past two decades over the northern hemisphere, color has increased along with the concentration of iron (Fe) and total organic carbon (TOC) (Roulet and Moore, 2006;Arvola et al, 2010;Haaland et al, 2010;Kritzberg and Ekström, 2012;Köhler et al, 2013;Sarkkola et al, 2013;Weyhenmeyer et al, 2014). The organic chromophores of TOC (referred to here as colored organic carbon, COC; see Table 1 for acronyms used; Coble, 2007;Fichot and Benner, 2011;Carter et al, 2012) and chromophores of Fe (Shapiro, 1964;Kritzberg and Ekström, 2012;Xiao et al, 2013) are important sources of color (Weyhenmeyer et al, 2014).…”

Relationships between lake transparency, thermocline depth, and sediment oxygen demand in Arctic lakes

“…Many of the world's surface waters are becoming less transparent as a result of increased inputs of DOC from the watershed and recovery from acidification (Forsberg 1992, Gunn et al 2001, Arvola et al 2010. Evans et al (2006) showed DOC concentration of lakes in the United Kingdom has increased by 63% between the periods of 1988-1993-2003.…”

Food Web Responses to Artificial Mixing in a Small Boreal Lake

“…Because of increased runoff the lakes received high load of inorganic nutrients and organic matter from their catchment areas [28,31]. Therefore, epilimnetic DOC, colour, TP, TN, NO 3 , NH 4 and Fe concentrations increased 2-to 4-fold until the middle of August compared to the early June [21]. As a result of the increase in colour and DOC, light penetration into the lake radically diminished.…”

“…Seasonal and inter-annual changes in the weather therefore have a major impact on the thermal characteristics of lakes. However, lakes with contrasting morphometry may respond differently to changes in weather [3,4]. For example, deep lakes with large water volume integrate the effects of meteorological forcing over longer time periods than shallow lakes.…”

“…More information about the physical, chemical and biological characteristics of the reference lake, Valkea-Kotinen (Table 1 and Figure 1), including biogeochemical processes and long-term changes, are given in the following publications [28][29][30][31]. Both lakes have high dissolved organic carbon content due to high load of allochthonous humic substances, a characteristic feature for small lakes in the region [4,7,32]. Both lakes are influenced by similar meteorological drivers, because they are close to each other (distance is 4.5 km; for more information on the climate and deposition of the area) [33,34].…”