Aquatic Plant Dynamics in Lowland River Networks: Connectivity, Management and Climate Change

Abstract

Abstract:The spatial structure and evolution of river networks offer tremendous opportunities to study the processes underlying metacommunity patterns in the wild. Here we explore several fundamental aspects of aquatic plant biogeography. How stable is plant composition over time? How similar is it along rivers? How fast is the species turnover? How does that and spatial structure affect our species richness estimates across scales? How do climate change, river management practices and connectivity affect species composition and community structure? We answer these questions by testing twelve hypotheses and combining two spatial surveys across entire networks, a long term temporal survey (21 consecutive years), a trait database, and a selection of environmental variables. From our river reach scale survey in lowland rivers, hydrophytes and marginal plants (helophytes) showed contrasting patterns in species abundance, richness and autocorrelation both in time and space. Since patterns in marginal plants reflect at least partly a sampling artefact (edge effect), the rest of the study focused on hydrophytes. Seasonal variability over two years and positive temporal autocorrelation at short time lags confirmed the relatively high regeneration abilities of aquatic plants in lowland rivers. Yet, OPEN ACCESSWater 2014, 6 869 from 1978 to 1998, plant composition changed quite dramatically and diversity decreased substantially. The annual species turnover was relatively high (20%-40%) and cumulated species richness was on average 23% and 34% higher over three and five years respectively, than annual survey. The long term changes were correlated to changes in climate (decreasing winter ice scouring, increasing summer low flows) and management (riparian shading). Over 21 years, there was a general erosion of species attributes over time attributed to a decrease in winter ice scouring, increase in shading and summer low flows, as well as a remaining effect of time which may be due to an erosion of the regional species pool. Temporal and spatial autocorrelation analyses indicated that long term hydrophyte biomonitoring, for the Water Framework Directive in lowland rivers, may be carried out at 4-6 years intervals for every 10 km of rivers. From multi-scale and abundance-range size analyses evidence of spatial isolation and longitudinal connectivity was detected, with no evidence of stronger longitudinal connectivity (fish and water current propagules dispersal) than spatial isolation (bird, wind and human dispersal) contrary to previous studies. The evidence for longitudinal connectivity was rather weak, perhaps resulting from the effect of small weirs. Further studies will need to integrate other aquatic habitats along rivers (regional species pool) and larger scales to increase the number of species and integrate phylogeny to build a more eco-evolutionary approach. More mechanistic approaches will be necessary to make predictions against our changing climate and management practices.

Natural variation of macrophyte vegetation of lowland streams at the regional level

“…Still, the degree of change at the scale of the region is low, compared to the drastic changes which occurred locally during the observation period (Wiegleb et al, 2014a). At the reach level, temporal autocorrelation for most properties of the vegetation (species composition, dominance, growth form spectrum) is lost after 3-5 years (Wiegleb et al, 2014a;Demars et al, 2014). After 7-10 years autocorrelation may even be negative indicating a different system state.…”

“…An analysis of more than 300 sites in our study area showed that none of the species found in northwest Germany has any significant relation to phosphorus concentrations of the water (Steffen et al, 2014). Nor did changes in nutrient concentrations have any influence on the stochastic changes in cover, richness, species composition, dominance and growth form over 21 years (Wiegleb et al, 2014a;Demars et al, 2014).…”

“…As species frequency and average abundance are positively correlated, in particular for hydrophytes (Riis and SandJensen, 2002;Demars et al, 2014), we can assume that frequent species contribute more to the standing biomass. The presence of six core species and 15 satellite species (H1) indicates that colonization is somehow limited but frequent species do not exclude each other, they only use the resources or space differently.…”

“…Measures of abundance and cover are strongly dependent of the applied sampling method and short-term change in the vegetation (Staniszewski et al, 2006;Pentecost et al, 2009). Still, the spatio-temporal variation of reach-based data was broadly underestimated (Wiegleb et al, 2014a;Demars et al, 2014). Recognizing in addition the natural variability of macrophyte vegetation at the regional level requires that species frequencies of the wider geographical area in which a physiographic unit is embedded must be known.…”

“…At the reach level macrophyte growth can be quantified using a variety of metrics such as cover, abundance, richness, diversity or evenness of taxa, growth forms, or attribute types (Willby et al, 2000;Birk and Willby, 2010;Wiegleb et al, 2014a). Along a river course or in the course of time turnover and autocorrelation measures can be related either to hydrochemical, morphological, sedimentological, hydrological and spatial variables, or discontinuous variables such as disturbance events including unusual weather conditions (Wiegleb et al, 2014b;Demars et al, 2014). At the level of the catchment, frequency of species (Wiegleb, , 1983bKohler et al 2003), frequency and distribution of communities (Holmes, 1983;Paal et al, 2007) as well as spatial variables (connectivity, isolation) can be studied (Demars and Harper, 2005).…”