Daphniid zooplankton assemblage shifts in response to eutrophication and metal contamination during the Anthropocene

Abstract

Human activities during the Anthropocene result in habitat degradation that has been associated with biodiversity loss and taxonomic homogenization of ecological communities. Here we estimated effects of eutrophication and heavy metal contamination, separately and in combination, in explaining zooplankton species composition during the past 125-145 years using analysis of daphniid diapausing egg banks from four lakes in the northeastern USA. We then examined how these community shifts influenced patterns of diversity and homogenization. Analysis of past lake production (via subfossil pigments) and metal contamination (via sedimentary metals) demonstrated that eutrophication alone (19-39%) and in combination with metal pollution (17-54%) explained 36-79% of historical variation in daphniid species relative abundances in heavily fertilized lakes. In contrast, metal pollution alone explained the majority (72%) of historical variation in daphniid assemblages at the oligotrophic site. Several species colonization events in eutrophying lakes resulted in increased species richness and gamma diversity through time. At the same time, daphniid assemblages in three eutrophied lakes became more similar to each other (homogenized), but this pattern was only seen when accounting for species presence/absence. We did not observe consistent patterns of divergence between the assemblages in the eutrophying lakes and the low-nutrient reference site. Given the pervasive nature of fertilization and metal pollution, and the sensitivity of cladocerans to these factors, we suggest that many inhabited lake districts may already exhibit similar patterns of daphniid assemblage shifts.

Does salinization impact long‐term Daphnia assemblage dynamics? Evidence from the sediment egg bank in a small hard‐water lake

“…Each size fraction was surveyed for the isolation of Daphnia ephippia under a stereomicroscope (Olympus SZ61). All ephippia were collected and enumerated from the sediment and grouped by species-specific morphological characteristics consistent with previous studies (Mergeay et al 2004;Rogalski et al 2017). For each layer, the flux (#ephippia g sediment À1 yr À1 ) of each species' ephippia to the sediment was calculated using a dating and sedimentation rate model (see Supporting Information Data S1).…”

“…Evidence from sediment egg banks must be considered critically. First and foremost, there is no empirical conversion from egg bank abundances to active animal abundance in the water column (Rogalski et al 2017). However, it is generally thought that an individual should produce relatively fewer ephippia than other commonly assayed subfossil remains (e.g., shed carapaces) over a lifetime; thus our abundance estimate would be more conservative (Arnott et al 2020).…”

The power of numbers: dynamics of hatching and dormant egg production in two populations of the water flea Daphnia magna

“…It might be that in ponds with lower productivity or higher predator densities, where Daphnia densities are lower, the ratio of egg production on the number of hatchlings is closer to one. Nevertheless, large egg banks as observed in our study systems have important consequences, amongst others because they ensure population persistence in the face of environmental change by buffering against occasional catastrophes (Rogalski et al 2017). If, following unfavourable conditions, the habitat becomes favourable again, the population may rebound from the dormant egg bank even if the likelihood of an individual egg to hatch is very low.…”

Speciation inDaphnia

“…Furthermore, habitat choice and reciprocal transplant experiments have not been utilized to directly study the impact of habitat isolation in Daphnia speciation. However, indirect evidence for the role of habitat isolation in maintaining the integrity of species comes from studies that show how environmental stressors that impact the quality of the habitat (pollutants, metals, contaminants, temperature, etc) can facilitate hybridization and introgression between closely related species and alter species distributions (Brede et al, 2009; Millette et al, 2020; Rogalski et al, 2017). Of particular interest are the genes that may be associated with habitat preference.…”