Decade‐long time delays in nutrient and plant species dynamics during eutrophication and re‐oligotrophication of Lake Fure 1900–2015

Abstract

1.A central topic in nature conservation and ecological restoration is the potential of ecosystems to recover after the reduction of negative anthropogenic impacts. Often, protracted delays in biotic response to abiotic change have been observed. 2. We analysed a unique long-term data series in Lake Fure, Denmark, spanning the transformation from pristine environmental conditions in the early 1900s through a period of eutrophicationfrom accelerating sewage input of phosphorus (P)and subsequent re-oligotrophication after sewage cleaning . We examine time delays between P inputs, in-lake P concentrations and the richness and composition of submerged macrophyte communities. 3. Lake P concentration exhibited decade-long delays in response to periods of increasing or decreasing P inputs. It took 40 years and a 25-fold increase in P input before P concentrations suddenly took off in the 1960s, reflecting profound sediment accumulation. Following reduced P input from c. 1970, it took 5 years before P concentrations showed the first signs of a decline. In 2014, water P concentration was still markedly higher than in 1931, despite much lower P inputs, because of elevated sediment release. 4. Fifty years of eutrophication led to a reduction in aquatic macrophyte richness from 36 species to 12. Species' responses were closely related to their growth strategy and depth distribution. Deepgrowing mosses, charophytes and short angiosperms disappeared, while tall angiosperms survived and pollution-tolerant macroalgae colonized and spread. Subsequently, 45 years of oligotrophication led to clearer waters, macrophyte richness recovering to 28 species and some charophytes and short angiosperms reappearing. Dominance of pollution-tolerant macroalgae persisted, however. Change in species dominance takes longer than colonization by new species. 5. Synthesis. Time delays of P concentrations, water clarity and macrophyte richness and composition were long and complex. Neglecting growth strategies of species makes application of extinction debt and colonization credit concepts dubious, because numbers of oligotrophic species decrease and eutrophic species increase concomitantly during eutrophication and vice versa during oligotrophication. Although the original high species richness may be attained, it is unlikely that the original species composition is restored because many oligotrophic species have vanished from the regional species pool.

Recovery of lake vegetation following reduced eutrophication and acidification

“…There has been an overall increase in species richness and a substantial change in species composition of macrophytes in Danish lakes from 1990 to 2010. Earlier comprehensive investigations of oligotrophication of lakes have not, to our knowledge, found strong indications of increased species richness (but see case studies such as Sand‐Jensen et al., ; Hilt, Kohler, Adrian, Monaghan, & Sayer, ; Sand‐Jensen et al., ) despite clear evidence of higher macrophyte coverage (Jeppesen et al., ).…”

“…Recruitment of new species following improved environmental conditions can be substantially delayed compared to changes in the environmental conditions (Sand‐Jensen et al., ). Slow recruitment can be due to absence of macrophyte‐rich habitats upstream or near the focal sites (Baastrup‐Spohr et al., ; Brederveld, Jahnig, Lorenz, Brunzel, & Soons, ; Dahlgren & Ehrlén, ).…”

“…When our expectations concerning the consequences of oligotrophication are drawn from studies of eutrophication, it is important to note that the reverse process can follow alternative routes and rates of recovery (Scheffer, Carpenter, Foley, Folke, & Walker, ). Studies of grasslands (Isbell, Tilman, Polasky, Binder, & Hawthorne, ), coastal marine waters (Duarte, Conley, Carstensen, & Sánchez‐Camacho, ; Middelboe & Sand‐Jensen, ; Valdemarsen, Quintana, Kristensen, & Flindt, ) and lakes (Sand‐Jensen, Bruun, & Baastrup‐Spohr, ; Sand‐Jensen et al., , ) show that vegetation responses to changes in nutrient loading can be slow and may not be fully reversible.…”

Temporal development of biodiversity of macrophytes in newly established lakes

“…This shift can be explained by the inability of short species to compete with tall, canopy-forming species, which has repeatedly been shown during increasing lake eutrophication (Murphy et al, 2018;Sand-Jensen et al, 2017). This shift can be explained by the inability of short species to compete with tall, canopy-forming species, which has repeatedly been shown during increasing lake eutrophication (Murphy et al, 2018;Sand-Jensen et al, 2017).…”

Indicator Values of Emergent Vegetation in Overgrowing Lakes in Relation to Water and Sediment Chemistry

“…On the other hand, assessment of the lake overgrowth process is only possible through many years of research [17][18][19]. Field tests of this type are conducted mostly in relation to selected water bodies [10,20].…”