The coming and going of a marl lake: multi-indicator palaeolimnology reveals abrupt ecological change and alternative views of reference conditions

Abstract

Eutrophication is the most pressing threat to highly calcareous (marl) lakes in Europe. Despite their unique chemistry and biology, comprehensive studies into their unimpacted conditions and eutrophication responses are underrepresented in conservation literature. A multi-indicator palaeolimnological study spanning ca. 1260-2009 was undertaken at Cunswick Tarn (UK), a small, presently eutrophic marl lake, in order to capture centennial timescales of impact. Specific aims were to (1) establish temporal patterns of change (gradual/abrupt) across biological groups, thereby testing theories of resistance of marl lake benthic communities to enrichment, and (2) compare the core record of reference condition with prevailing descriptions of high ecological status. Analyses of sediment calcium (Ca), phosphorus (P), pigments, diatoms, testate amoebae, cladocerans, and macrofossils, revealed three abrupt changes in ecosystem structure. The first (1900s), with biomass increases in charophytes and other benthic nutrient-poor indicators, supported ideas of resistance to eutrophication in Chara lakes. The second transition (1930s), from charophyte to angiosperm dominance, occurred alongside reductions in macrophyte cover, increases in eutrophic indicators, and a breakdown in marling, in support of ideas of threshold responses to enrichment. Core P increased consistently into the 1990s when rapid transitions into pelagic shallow lake ecology occurred and Cunswick Tarn became biologically unidentifiable as a marl lake. The moderate total P at which these changes occurred suggests high sensitivity of marl lakes to eutrophication. Further, the early record challenges ideas of correlation between ecological condition, charophyte biomass and sediment Ca. Instead, low benthic production, macrophyte cover, and Ca sedimentation, was inferred. Management measures must focus on reducing external nutrient and sediment loads at early stages of impact in order to preserve marl lakes.

Refining the palaeoecology of lacustrine testate amoebae: insights from a plant macrofossil record from a eutrophic Scottish lake

“…In contrast, a sustained decline in isoetids and the proliferation of charophytes in zone LLTA-3 is coincident with lower abundances of strains of P. compressa and strains of D. oblonga. A similar trend was noted by Wiik et al (2015), who reported a decreased abundance of D. oblonga ''oblonga'' in a core from Cunswick Tarn (Cumbria, UK) during charophyte-dominated phases. Similarly, dense beds of Chara spp.…”

“…The corresponding testate amoeba assemblage is dominated by D. oblonga ''oblonga'', a common taxon which occurs in a range of lacustrine environments, from oligotrophic marl lakes (Wiik et al 2015) to eutrophic kettle lakes (Roe et al 2010). Strains of Difflugia are known to thrive in sediments rich in organic matter that are thought to sustain high population carrying capacities (Patterson and Kumar 2000a).…”

“…Due to a lack of instrumental monitoring of lake conditions, palaeolimnological approaches are essential for assessing long-term trends, determining pre-disturbance conditions ) and for informing conservation strategies (Kowalewski et al 2013). Multi-proxy approaches are increasingly being used in palaeolimnological studies (Sayer et al 2010b;Wiik et al 2015;Sayer et al 2016) to provide a more comprehensive understanding of communitywide changes resulting from eutrophication and other anthropogenic drivers in lakes, including the sequence and mechanisms of biological change (Sayer et al 2010a;Bennion et al 2015). Typical groups used in multi-proxy work include diatoms, plant macrofossils, cladocerans and algal pigments and invariably the addition of each group enhances understanding.…”

“…In addition, as bottom-dwelling organisms, they occupy a critical habitat for which environmental reconstructions are widely lacking, excepting chironomid-oxygen studies (Quinlan and Smol 2002). Despite an expanding geographical range of lake testate amoeba research that initially centred on Europe (Asioli et al 1996;Wall et al 2010b) and North America (Collins et al 1990;Patterson and Kumar 2002;Reinhardt et al 2005) and more recently Asia (Ju et al 2014) and South America (Caffau et al 2015), UK lake studies are very few (Ellison 1995;Wiik et al 2015). Within the UK, research has largely been confined to peatlands and saltmarshes, with the focus generally on quantitative palaeohydrological reconstructions (Swindles et al 2009) and sea-level change (Gehrels et al 2001;Barnett et al 2017).…”

Sedimentary macrofossil records reveal ecological change in English lakes: implications for conservation

“…The purpose of this work was to attempt to reconstruct the plant community of the tarn prior to any anthropogenic enrichment and hence determine reference conditions and lake type. A subsequent and more detailed multiproxy palaeoecological study was undertaken on Cunswick Tarn revealing community shifts across a range of biological groups, initially in the 1890s associated with drainage of the lake and land improvement in the catchment, followed by abrupt ecosystem shifts in the 1920s-1930s, and more recently in the 1990s when the lake changed into its current condition of low macrophyte diversity and predominance of nutrient-tolerant taxa (Wiik et al 2015). Hornsea Mere, Yorkshire, is a high alkalinity lake and the largest freshwater lake in the county.…”