Eutrophication homogenizes shallow lake macrophyte assemblages over space and time

Abstract

Eutrophication is commonly implicated in the reduction in macrophyte species richness in shallow lakes. However, the extent to which other more nuanced measures of macrophyte diversity, such as assemblage heterogeneity, are impacted concurrently by eutrophication over space and time and the joint influences of other factors (e.g., species invasions and connectivity) remains relatively poorly documented. Using a combination of contemporary and paleoecological data, we examine how eutrophication influences macrophyte assemblage heterogeneity and how nutrient enrichment interacts with watercourse connectivity, lake surface area, and relative zebra mussel abundance over space (within and among lakes) and time (decades to centuries) at the landscape scale. The study system is the Upper Lough Erne, Northern Ireland, UK, which is composed of a large main lake and several smaller satellite lakes that vary in their hydrological connectivity to the main lake. By applying homogeneity analysis of multivariate dispersions and partial redundancy analysis, we demonstrate that contemporary lake macrophyte heterogeneity and species richness are reduced in lakes with intensified eutrophication but are increased in lakes with greater zebra mussel abundance and lake surface area. Watercourse connectivity positively influenced assemblage heterogeneity and explained larger proportions of the variation in assemblage heterogeneity than local environmental factors, whereas variation in species richness was better related to local abiotic factors. Macrophyte fossil data revealed within‐ and among‐lake assemblage homogenization post‐1960, with the main lake and connected sites showing the highest rates of homogenization due to progressive eutrophication. The long‐term and contemporary data collectively indicate that eutrophication reduces assemblage heterogeneity over time by overriding the importance of regional processes (e.g., connectivity) and exerts stronger pressure on isolated lakes. Our results suggest further that in connected lake systems, assemblage heterogeneity may be impacted more rapidly by eutrophication than species richness. This means that early effects of eutrophication in many systems may be underestimated by monitoring that focuses solely on species richness and is not performed at adequate landscape scales.

Connectivity and zebra mussel invasion offer short‐term buffering of eutrophication impacts on floodplain lake landscape biodiversity

“…Sites hydrologically connected to the central lake ( Group 1 ) had the highest species richness and abundances of macrophytes regarded as conservation priorities (Table ; Table S5.1). A companion study by Salgado, Sayer, Brooks, Davidson, Goldsmith, et al () showed that macrophyte communities in this group of connected lakes were also more heterogeneous in species relative abundances and composition than in the less specious and eutrophic isolated lakes. Subsidiary natural water flows from the associated satellite lakes with variable nutrient enrichment conditions may therefore introduce a range of different plant propagules to the central lake while recurrent floods likely also transport plant propagules in the opposite direction.…”

“…A further methodological limitation is the lack of data on historical climatic or environmental variables (other than eutrophication and changing water levels and connectivity) that may have played a role in structuring the community over space and time. Nonetheless, as indicated by both macrophyte and invertebrate compositional shifts, it is strongly suspected that these factors were the key drivers of change in the ULE system (Battarbee, ; Salgado, Sayer, Brooks, Davidson, Goldsmith, et al, ; Salgado, Sayer, Brooks, Davidson, & Okamura, ), as is typically the case for Northern Hemisphere lakes.…”

Invasive species and biotic homogenization in temperate aquatic plant communities

“…Such conditions may promote invasive species as ‘passengers’ of change ( sensu MacDougall & Turkington, ) and are likely to favour common native species that employ a more generalist or environmentally tolerant strategy versus rarer species with narrower environmental niches (Devictor, Julliard, & Jiguet, ). There is also extensive evidence of environmental changes driving shifts in macrophyte communities (Jeppesen, Peder Jensen, Søndergaard, Lauridsen, & Landkildehus, ; Sand‐Jensen et al, ) even leading to BH (Salgado et al, ). These mechanisms are not mutually exclusive and multiple processes could be driving the overall patterns we observed.…”

No biotic homogenisation across decades but consistent effects of landscape position and pH on macrophyte communities in boreal lakes

“…Studies that have found clear signs of biotic homogenisation for aquatic macrophytes are either considering other biodiversity measurements or beta diversity measures, or are based on paleoecological methods (Salgado et al 2018). There are only a few studies of temporal changes in spatial beta diversity (Winegardner et al 2017, Larsen et al 2018, Wengrat et al 2018) and even fewer considering aquatic macrophytes.…”