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

Dave Emson,Ian R Patmore,Patrick Rioual,Carl D Sayer,Helen Bennion

doi:10.1007/s10933-017-0008-6

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 long-term 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 (1999–2005). 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 diatom-based investigations of floating plant histories.

Highlights

To date, both ecological and palaeoecological research has focused almost entirely on large, deep and shallow lakes, with minimal attention devoted to the ‘poor cousins’ of shallow lakes, namely ponds
The Rail Pit macrophyte data show a cyclical shift between submerged macrophytes and lemnid dominance (Fig. 2)
The submerged C. submersum and P. natans were abundant in the early 1990s during years of low Lemna abundance (1994–1998), but rapidly

Summary

Introduction

Both ecological and palaeoecological research has focused almost entirely on large, deep and shallow lakes, with minimal attention devoted to the ‘poor cousins’ of shallow lakes, namely ponds. The lack of studies in ponds is acute (Edwards et al 2011; Emson 2015). While a number of palaeo-studies have been undertaken in arctic ponds In recent years, a number of studies have shown the predominantly agricultural pond resource of lowland Europe to be of major importance in terms of aquatic biodiversity conservation (Ruggiero et al 2008; Sayer et al 2012), aquatic—terrestrial energy subsidies (Davies et al 2016) and carbon sequestration (Downing et al 2008), among other roles. Given increasing interest in pond conservation, palaeoecology could have an important role in inferring longterm changes in pond ecology and function in response to environmental impacts

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Discussion

Conclusion