Geological Survey of Denmark and Greenland Bulletin 20, 2010, 43–46 43 Nutrient-poor, low-productive (oligotrophic) soft-water lakes in the Atlantic areas of West and North-West Europe – the so-called Lobelia lakes – are of high conservation value as their low nutrient status favours a particular submerged mac- rophyte flora with isoetids, which are becoming increasingly rare or threatened due to nutrient enrichment (eutrophica- tion) associated with landuse changes and urbanisation. Eu- ropean Union member states have a duty of care, under the Habitats Directive, to protect the biodiversity of oligotroph- ic to mesotrophic (moderately productive) standing waters. In Denmark the majority of Lobelia lakes are located on sandy soils in central and western Jylland. These lakes are clear-water ecosystems poor in nutrients and organic carbon and with a unique macrophyte vegetation of predominantly Lobelia dortmanna (Water Lobelia), Littorella uniflora (Shore-weed) and Isoëtes lacustris (Quill-wort). Severe de- terioration of isoetid plant communities is reported from Denmark and many other European countries (e.g. Arts 2002; Pedersen et al. 2006). The isoetids are low and slow growing with relatively poor competitive capabilities. These characteristics make them more sensitive to decreased light levels than other macrophyte groups (Middelboe & Mark- ager 1997) and consequently also particularly vulnerable to eutrophied and turbid waters. In an ongoing project we are investigating the limnologi- cal development of two Lobelia lakes in mid-Jylland during the last 1000 years (Hampen Sø and Rævsø, situated 2.5 km apart). The Hampen Sø investigation is part of a Geo- center Denmark funded project with the title: Lake response to climate change during the last 1000 years. In this paper we present the first results from Hampen Sø with emphasis on changes in the nutrient status of the lake through the last c. 300 years as inferred from diatom and macrofossil analy- ses. Today Hampen Sø is influenced by nutrient enrichment and has a mixture of two vegetation types that normally be- long to two different lake types, namely the lake’s original isoetid vegetation plus species of fast-growing and tall elo- deids, the latter being favoured by increased nutrient levels (Moeslund 2000). Palaeolimnological methods are used to explore the timing and possible causes of the nutrient enrich- ment. These are of significance for understanding the envi- ronmental threat to the lake ecosystem, and for determining its baseline, or reference conditions, defined by the European Water Framework Directive (WFD) as conditions under minimal anthropogenic disturbance. Material and methods Hampen Sø is located in mid-Jylland on sandy soils just west of the Main Stationary Line (Fig. 1). The lake has a surface area of 76 ha, a mean water depth of 4.3 m, a maximum depth of 13.1 m, and a topographic catchment area of 916 ha (Moeslund 2000). The lake is a seepage lake, i.e., a closed lake without natural inlets or outlets. In 2009 a c. 2 m long sediment core with humic, slightly silty and sandy gyttja was retrieved from the lake at a water depth of 9.66 m. The up- permost 1 m of the sediment sequence, which spans approxi- mately the last 1000 years, has been dated by accelerator mass spectrometry (AMS) 14C-age determination (one sample) and the 210Pb dating method (Appleby 2001); here we focus on the period between c. AD 1750 and today. The nutrient status of the lake during this time period is inferred from diatom and macrofossil analyses, and changes in the catch- Recent changes in the nutrient status of a soft-water Lobelia lake, Hampen Sø, Denmark Kaarina Weckström, Peter Rasmussen, Bent Vad Odgaard, Thorbjørn Joest Andersen, Tarmo Virtanen and Jesper Olsen 10°E 14°E 55° 57° 100 km Sweden Denmark Germany Hampen Sø Jylland Main Stationary Line Fig. 1. Map of Denmark with the location of the study site, Hampen Sø. Sandy soils dominate west of the Main Stationary Line. © GEUS, 2010. Geological Survey of Denmark and Greenland Bulletin 20, 43–46. Open Access: www.geus.dk/publications/bull 4444 ment landuse are estimated from land classification on two cadastral maps from 1872–74 and 1984–85. Diatom data are presented as percentages and 300–400 valves (halves of their siliceous cell walls) were counted per sample. Macrofossils are presented as concentrations. Chronology The uppermost 28 cm of the lake sediments were dated by 210Pb and 137Cs assay, estimated from 15 samples spanning the time period 1900–2009. A sample of deciduous leaf frag- ments found at 117 cm depth was AMS 14C-dated with the result cal. AD 910–1020 (range 2 σ). An age–depth curve was constructed for the depth interval 0–117 cm by linear interpolation between the 210Pb- and 137Cs-dated sediments and the 14C date at 117 cm. Landuse change in the catchment area According to the cadastral maps the most significant landuse change in the Hampen Sø catchment area since the end of the 19th century is the virtual disappearance of heathland (Fig. 2). Between c. 1870 and 1980 its area has decreased from c. 24% to less than 1%, while the forested area has increased from c. 36% to 64%. The area of arable land has stayed ap- proximately the same during this time period. The former heathland has been planted with predominantly coniferous trees (pine and spruce) for timber. Development of the nutrient status over the last c. 300 years There are two distinct changes in the Hampen Sø diatom assemblages: in the mid-18th century and around the 1940s (Fig. 3). The first change is defined by a clear decrease in planktonic taxa from c. 28% to 8%. Variations in the abun- dance of planktonic species in temperate lakes are often as- sociated with either changing water levels (Heinsalu et al. 2008; Laird & Cumming 2009) or eutrophication (Sayer et al. 1999; Bennion et al. 2004). Cyclotella comensis, an oligo- trophic species, dominates the planktonic assemblages prior to the decrease, after which benthic taxa belonging particu- larly to the genus Fragilaria (including Staurosira, Stauro- sirella and Pseudostaurosira) increase. Such a change most likely signifies lowering of the water level. The macrophyte vegetation on the other hand does not show distinct changes, although an increase in carophytes (Nitella sp. and Chara sp.) can be observed and might suggest that they have been favoured by better light conditions due to shallower water. As the core was collected in the deepest part of the lake, the concentrations of littoral species such as Lobelia and Isoëtes should be considered with some caution, due to their limited seed and spore dispersal. The first forest plantations in the Hampen Sø catchment area date back to 1805. At first success was limited until the planting methods were changed in the 1830s. An increase in forest cover will increase evapotranspiration from the catch- ment area and hence could affect the water level in the lake due to a decrease in the ground water table. We initially as- sumed that the plantations may have affected the lake levels of Hampen Sø, however, the water-level decrease indicated by the diatom assemblages clearly occurs before the planting of trees. On the other hand, there is evidence from several studies based on a number of proxies that precipitation de- creased in Scandinavia during the latter part of the Little Ice Age (e.g. Linderholm & Chen 2005; De Jong et al. 2009). Hence the diatom-inferred decrease in Hampen Sø water levels in the mid-18th century could reflect this suggested change in precipitation. The second clear change in the diatom assemblages occurs around the 1940s, marked by a pronounced increase in the mesotrophic species Cyclotella pseudostelligera and Fragilaria 1 km 1872–1874 1984–1985 Forest Field Heath Wetland Lakes Landuse Fig. 2. Landuse in the topographic catch- ment of Hampen Sø in 1872–1874 (left) and 1984–1985 (right). 45 crotonensis (up to 23% and 9%, respectively). Their increase is followed by the appearance of the eutrophic Stephanodiscus parvus in the early 1960s. These taxa indicate increased nu- trient concentrations and, with higher phytoplankton pro- ductivity, also decreased transparency of the lake water. The charophytes Nitella sp. and Chara sp., which can occur at wa- ter depths of over 10 m, exhibit a decreasing trend from the 1920s (Fig. 3), whereas according to recent surveys (Moes- lund 2000), tall elodeids, which occur in shallower water (such as Potamogeton and Myriophyllum species), have be- come more abundant. Many elodeids are scarce in sediment records (such as in Hampen Sø) due to lower seed production and limited dispersal compared to other macrophytes. The reasons for these changes are likely to be the increased nutri- ent load from the catchment area (intensified field fertilisa- tion and particularly waste waters from a farm and summer houses near the lake, Moeslund 2000) which, with increasing phytoplankton productivity, also affect water transparency. Concentrations of ephippia (resting eggs) of the water flea genus Daphnia began to increase around the same time as the change in macrophyte vegetation is observed with the high- est concentrations from the 1960s onwards (Fig. 3). Daphnia resting eggs are generally produced when conditions dete- riorate due to e.g. overcrowding, limited food availability, extreme changes in the environment or increased predation (Korhola & Rautio 2001). In Hampen Sø, we interpret the increased numbers of resting eggs as simply an indication of larger Daphnia populations due to a general increase in bio- logical production. In addition, more suitable habitats could be represented by the tall elodeids, which function as refuge from fish predation (Jeppesen et al. 1997). According to surveys made in Hampen Sø from 1971 to 1999, nutrient concentrations were high from the beginning of the survey until the mid-1980s (summer-time total phos- phorus value c. 70–80 μg l-1 and total nitrogen c. 800–1000 μg l-1), after which concentrations decreased to present-day levels of <30μg l-1 total phosphorous and <600 μg l-1 total nitrogen (Moeslund 2000). This decrease in concentrations is attributed to the cessation of waste water effluents (in par- ticular animal manure) from the nearby farm. The decreased nutrient concentrations from the mid-1980s onwards are not reflected in the biota. This could be explained by the sedi- ment nutrient pool which, due to its limited binding capacity (low iron:phosphorus ratio), releases particularly phosphorus back into the water. This is then taken up by phytoplankton and macrophytes (Moeslund 2000). St ep ha no dis cu s p ar vu s Cy clo te lla p se ud os te llig er a Cy clo te lla co m en sis Fr ag ila ria cr ot on en sis Fr ag ila ria e llip tic a ag g. Ac hn an th idi um m inu tis sim um Pl an kt on ic sp ec ie s Dap hn ia sp ., ep hi pp ia Lo be lia d or tm an na , s ee ds N ite lla sp ., o os po re s Ch ar a sp ., o os po re s Iso et es la cu str is 1760 1800 1840 1880 1920 1960 2000 A ge ( ye ar s A D ) m eg as po re s 5 20 20 5 50020 20 20 40 5 50 150 50 15 30 452510000 0 % Concentration (n/100 ml) Fig. 3. Diatom and macrofossil diagram showing selected abundant species. Diatoms (blue) are given as percentages. Macrofossils (Daphnia ephippia (red) and submerged macrophyte remains (green)) are shown as concentrations (remains per 100 ml fresh sediment). Ages based on the constructed age–depth curve are given on the left of the diagram. 4646 It is noteworthy that the abundance of planktonic dia- toms slowly begins to increase before the appearance of the meso- and eutrophic diatom taxa (Fig. 3). This could suggest slightly increasing water levels from the late 19th century on- wards, which may partly have been masked by the marked increase in the planktonic diatom taxa indicating eutrophi- cation. Conclusions and future work Distinct signs of anthropogenic disturbance in Hampen Sø can be seen in the 1920s (reflected in macrophyte vegetation and Daphnia abundance) indicating the onset of nutrient enrichment. Clear indications of eutrophication are evident from the 1960s onwards in all proxies. Although water-col- umn nutrient concentrations have decreased since the mid- 1980s, no change is observed in the biota. This could be at- tributed to increased internal loading of nutrients from the sediments. Compared to the majority of Danish lakes, which have been impacted by anthropogenic activities for centuries (Bradshaw et al. 2005, 2006), the timing of these changes is surprisingly late. It appears that sandy soils of central and western Jylland have been less intensively used for crop culti- vation in the past and hence lakes located in such settings are less affected. According to our results, baseline or reference conditions at Hampen Sø, as defined by the European Water Framework Directive, could be set at the early 1900s. These reference conditions only define the state of the lake before intensified human impact. 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J.O., Centre for Climate, the Environment & Chronology, Archaeology & Palaeoecology Building, Queen’s University Belfast, 42 Fitzwilliam Street, Belfast BT9 6AX, UK.