Diel turbidity cycles in a headwater stream: evidence of nocturnal bioturbation?

Purpose: A small number of recent studies have linked daily cycles in stream turbidity to nocturnal bioturbation by aquatic fauna, principally crayfish, and demonstrated this process can significantly impact upon water quality under baseflow conditions. Adding to this limited body of research, we use high-resolution water quality monitoring data to investigate evidence of diel turbidity cycles in a lowland, headwater stream with a known signal crayfish (Pacifastacus leniusculus) population and explore a range of potential causal mechanisms. Materials and methods: Automatic bankside monitoring stations measured turbidity and other water quality parameters at 30-min resolution at three locations on the River Blackwater, Norfolk, UK during 2013. Specifically, we focused on two 20-day periods of baseflow conditions during January and April 2013 which displayed turbidity trends typical of winter and spring seasons, respectively. The turbidity time-series, which were smoothed with 6.5 hour Savitzky-Golay filters to highlight diel trends, were correlated against temperature, stage, dissolved oxygen and pH to assess the importance of abiotic influences on turbidity. Turbidity was also calibrated against suspended particulate matter (SPM) over a wide range of values via linear regression. Results and discussion: Pronounced diel turbidity cycles were found at two of the three sites under baseflow conditions during April. Spring night-time turbidity values consistently peaked between 21:00 and 04:00 with values increasing by ~10 nephelometric turbidity units (NTU) compared with the lowest recorded daytime values which occurred between 10:00 and 14:00. This translated into statistically significant increases in median midnight SPM concentration of up to 76% compared with midday, with night-time (18:00 – 05:30) SPM loads also up to 30% higher than that recorded during the daytime (06:00 – 17:30). Relating turbidity to other water quality parameters exhibiting diel cycles revealed there to be neither any correlation that might indicate a causal link, nor any obvious mechanistic connections to explain the temporal turbidity trends. Diel turbidity cycles were less prominent at all sites during the winter. Conclusions: Considering the seasonality and timing of elevated turbidity, visual observations of crayfish activity, and an absence of mechanistic connections with other water quality parameters, the results presented here are consistent with the hypothesis that nocturnal bioturbation is responsible for generating diel turbidity cycles under baseflow conditions in headwater streams. However, further research in a variety of fluvial environments is required to better assess the spatial extent, importance and causal mechanisms of this phenomenon

First results on the genetic diversity of the invasive signal crayfish Pacifastacus leniusculus (Dana, 1852) in Europe using novel microsatellite loci

The introduction of non-native crayfish in aquatic ecosystems is very common due to human activities (e.g. aquaculture, recreational and commercial fisheries). The signal crayfish, Pacifastacus leniusculus (Dana, 1852), is one of the most widespread invasive species in Europe. Although several important ecological and economic impacts of this species have been reported, its European population genetic characterisation has never been undertaken using nuclear markers. Thus, the aim of this study was to develop and characterise new microsatellite markers for signal crayfish that can be useful in future studies in its invaded range, since only five are available so far. In total, 93 individuals from four geographically distinct European populations (Portugal, Great Britain, Finland and Sweden) were scored for the new markers and for those previously described, with the Bayesian analysis revealing a clear distinction among populations. These markers are suitable for future studies of the population genetic structure of this important invasive species, by increasing information about the possible pathways of introduction and dispersal, and by giving insights about the most important vectors of introduction.Financial support was provided by the Portuguese Foundation for Science and Technology (FCT) project PTDC/AACAMB/117688/2010. LF was supported by Office National de l’Eau et des Milieux Aquatiques (ONEMA), AP by the Czech Science Foundation (project no. P505/12/0545) and LE by the Swedish Research Council FORMAS and SwAM.info:eu-repo/semantics/publishedVersio