Anthropogenic eutrophication shapes the past and present taxonomic composition of hybridizing Daphnia in unproductive lakes

Abstract

It has been proposed that anthropogenic eutrophication of lakes facilitated the establishment of populations of the cladoceran Daphnia galeata into the originally oligotrophic lakes north of the European Alps in the 1960s. This hypothesis lacks the support of studies on unproductive lakes, in which the past eutrophication is assumed to have never been on the level necessary for D. galeata to reach high abundances and to establish permanently. In order to investigate if such species shifts also happened in unproductive systems, we studied the past and present taxonomic composition of three ultra-oligotrophic Swiss lakes that were only marginally affected by eutrophication using molecular methods on diapausing eggs sampled from sediment cores. D. galeata temporarily established in unproductive lakes, but its colonization success seemed to depend on the general trophic state of the lake and the magnitude of eutrophication. In two of the studied lakes, D. galeata could establish a significant population size, whereas it was not successful in the most unproductive lake with the weakest eutrophication. Even in unproductive lakes, eutrophication led to partly irreversible species changes, providing evidence that this anthropogenic disturbance is responsible for species shifts in many pre-alpine lakes in Central Europe.

Modes and mechanisms of aDaphniainvasion

“…As a result, not only is D. galeata able to invade Swiss lakes after they are eutrophied, this taxon is unable to persist after a lake has experienced reoligotrophication, whereas D. longispina survives under both conditions. This pattern provides a mechanistic underpinning for the palaeogenetic patterns observed in several lakes north of the Alps: the appearance of either D. galeata clones or its genes in D. galeata  D. longispina hybrids as primary productivity increased and their subsequent decline following nutrient reduction owing to governmental regulation of wastewater inputs [14,15].…”

“…Among the ecologically most important freshwater taxa responding to environmental change are members of the genus Daphnia [11,12], which in many lakes are critical species in the pelagic food chain [13]. Recently, Brede et al [14] and Rellstab et al [15] proposed that the changes during the past half century in the geographical distribution of Daphnia galeata in central Europe represent invasion success as Lake Constance and other unproductive lakes in Switzerland became eutrophied by nutrient pollution that accompanied population and economic growth following World War II [16]. Using molecular genetic analyses of diapausing eggs isolated from dated lake sediments, these authors showed that genes diagnostic of D. galeata first appeared in the 1950s when phosphorus (P) concentrations began to increase.…”

Daphnia galeata and D. dentifera are geographically and ecologically separated whereas their hybrids occur in intermediate habitats: A survey of 44 Chinese lakes

“…Successful establishment of hybrids in environments novel to one of their parental species has been documented in zooplankton cladocerans of the genus Daphnia , specifically in the Daphnia longispina complex (Brede et al, ; Rellstab, Keller, Girardclos, Anselmetti, & Spaak, ). In this complex, which includes D. cucullata , D. dentifera , D. galeata and D. longispina (revised by Petrusek, Hobaek et al, ), long periods of parthenogenetic reproduction alternate with short periods of sexual reproduction, the latter offering an opportunity for the production of interspecific hybrids (Schwenk & Spaak, ).…”

Range limits in sympatric cryptic species: a case study in Tetramorium pavement ants (Hymenoptera: Formicidae) across a biogeographical boundary

“…Nevertheless, identification results based only on mitochondrial DNA barcodes might sometimes be biased (e.g. involving cryptic species or inter-specific genetic exchanges (Rellstab et al, 2011;Dantas-Torres et al, 2013;Schmidt-Roach et al, 2014;Wagner et al, 2017). Wilson-Wilde et al (2010) also suggested that using COI for species identification is insufficient when dealing with closely related species.…”