Mission possible: diatoms can be used to infer past duckweed (lemnoid Araceae) dominance in ponds

Abstract

Compared to larger lakes, ponds have rarely been the focus of palaeoecological studies. A common feature of ponds, especially those subject to eutrophication, is mass surface coverings of lemnoid Araceae (duckweed) which have severe implications for ecological processes in small waterbodies, in particular lowered oxygen content. To help understand the implications of duckweed dominance for the longterm ecology of ponds, and to determine the potential for palaeoecological studies in ponds more generally, we develop a new diatom-based Lemna-indicator metric. Recent studies of diatom host-plant relationships have shown significant associations between duckweed and the epiphytes Lemnicola hungarica and Sellaphora saugerresii (formally known as Sellaphora seminulum). To determine the potential of these species as palaeo-indicators of long-term duckweed dynamics in ponds, we investigated the diatom composition of surface sediment assemblages in sets of duckweed and non-duckweed-dominated ponds in Norfolk, eastern England. In addition, we undertook diatom analysis of two cores from a small farmland pond (Bodham Rail Pit) subject to a known duckweed dominance event (1999)(2000)(2001)(2002)(2003)(2004)(2005). Both L. hungarica and S. saugerresii were significant predictors of past Lemna dominance in the surface sediments. Further, in the core study, both diatom species accurately and closely tracked the documented ''on-off'' duckweed cycle. Our study suggests huge potential for using ponds in palaeoecological studies and for diatombased investigations of floating plant histories.

Prospects and challenges of environmental DNA (eDNA) monitoring in freshwater ponds

“…Anoxic conditions were shown to slow marine eDNA decay (Weltz et al, 2017) but impacts of anoxia on pond eDNA have not been investigated. Slow decay may affect inferences made from eDNA regarding contemporary species presence; however, anoxic conditions dramatically enhance preservation of pond sediments and the communities that live there, providing information on historical pond biodiversity (Alderton et al, 2017;Emson et al, 2017).…”

Once a pond in time: employing palaeoecology to inform farmland pond restoration

“…→ P. natans → fine‐leaved Potamogeton spp. suggest that aquatic vegetation composition was dynamic, as observed in long‐term pond monitoring data locally (Emson et al 2018). Woody marginal vegetation, including C. monogyna , Salix spp., and the shrub R. fruticosus agg., was likely present during the first century of the pond's history, but, given the clear dominance of submerged and floating‐leaved plants, it seems likely that scrub encroachment, and hence shading, was minimal.…”

“…M. spicatum is found locally in lakes and ponds (Sayer et al 2012; Clarke et al 2014), but has generally declined in agricultural landscapes due to eutrophication (Sayer et al 2010; Sayer et al 2016). Indeed this may explain its early loss from Mystery Pit, with its disappearance immediately followed by appearance of the duckweed L. minor (which may have shaded out M. spicatum ), a plant known to be indicative of advancing eutrophication (Sayer et al 2010; Emson 2015; Emson et al 2018). Meanwhile O. fistulosa has declined to localized patches in the study region during the last 50–100 years due to changes in farming practices and an associated loss of fen habitat (Southam & Wigginton 2002).…”

“…Paleoecology has hitherto rarely been used to establish the past ecological dynamics of small farmland ponds (see Emson et al 2018). Nonetheless, extensive paleoecological studies of shallow lakes suggest tremendous potential for detecting long‐term changes in both aquatic and surrounding terrestrial vegetation (Birks 1973; Madgwick et al 2011), and for demonstrating the impact of changing agricultural practices (Brush & Hilgartner 2000; Bradshaw et al 2005) and other land use changes (Tolonen 1978; Riera et al 2006) on aquatic ecosystems.…”

Invasive alien plant species, fragmentation and scale effects on urban forest community composition in Durban, South Africa

“…The strata were categorised according to height as follows: lower trees (> 6 m), tall shrubs (> 2-6 m), low shrubs (0.5-2 m) and all climbers (regarded as a separate stratum because they grow across all layers). The total cover from all species within a plot was assessed according to Tansley's DAFOR scale (Emson et al 2018): D = Dominant (5); A = Abundant (4), F = Frequent (3), O = Occasional (2) and R = Rare (1). The overall species cover values were obtained by determining the average cover from all plots and used as a dependent variable with the environmental variables treated as independent.…”