The spatial and temporal dynamics of sediment phosphorus attenuation and release in impacted stream catchments.

Sediments can attenuate phosphorus (P) from overlying water and reduce trophic status in zero and first order ditches and streams. These features can be considered as intermediate mitigation features between P mobilised from land, and onward delivery to river systems, if the risk of chemical P release from sediments is minimal. However, risk assessments are rarely based on temporal scale dynamics and especially at fine scale in both sediment and water column environments. In this study, in eutrophic stream catchments, bed sediments were tested fortnightly and spatially over one year for EPC0 (to derive phosphate exchange potential-PEP) and for P across a spectrum from labile to recalcitrant fractions. At the same time stream discharge and P concentrations were measured synchronously at high frequency and resolved to 1-hour intervals and indicated high water quality pressures at all flow rates. PEP indicated spatial and temporal changes most likely caused by periods of source disconnection/reconnection and sediment mobilisation during storm events, moving from periods of high attenuation potential to near saturation. Despite these spatial and temporal changes, PEP did not indicate much potential for chemical P release from the sediments (distributing mostly below or close to zero). However, this may be a misleading risk assessment by itself as physical P release, especially of the labile bicarbonate-dithionite (B-D) P fraction of sediments, was a more dominant process mobilised during storm events reducing by up to 84 % during a succession of summer storm events. The total P and total reactive P loads monitored leaving the catchments were coincident with these changes. The specific downstream trophic effects of this episodic P release will need to be assessed in terms of its bioavailability, in combination with other more noted diffuse and point P source processes.

Effects of freshwater sponge Ephydatia fluviatilis on conjugative transfer of antimicrobial resistance in Enterococcus faecalis strains in aquatic environments.

Filter feeding is a biotic process that brings waterborne bacteria in close contact with each other and may thus support the horizontal transfer of their antimicrobial resistance genes. This laboratory study investigated whether the freshwater sponge Ephydatia fluviatilis supported the transfer of vancomycin resistance between two Enterococcus faecalis strains that we previously demonstrated to exhibit pheromone responsive plasmid conjugation. Microcosm experiments exposed live and dead colonies of laboratory-grown sponges to a vancomycin-resistant donor strain and a rifampicin-resistant recipient strain of Ent. faecalis. Enterococci with both resistance phenotypes were detected on double selection plates. In comparison to controls, abundance of these presumed transconjugants increased significantly in water from sponge microcosms. Homogenized suspensions of sponge cells also yielded presumed transconjugants; however, there was no significant difference between samples from live or dead sponges. Fluorescent in situ hybridization analysis of the sponge cell matrix using species-specific probes revealed the presence of enterococci clusters with cells adjacent to each other. The results demonstrated that sponge colonies can support the horizontal transfer of antimicrobial resistance although the mechanism underlying this process, such as binding of the bacteria to the sponge collagen matrix, has yet to be fully elucidated.

Low flow water quality in rivers; septic tank systems and high-resolution phosphorus signals.

Rural point sources of phosphorus (P), including septic tank systems, provide a small part of the overall phosphorus budget to surface waters in agricultural catchments but can have a disproportionate impact on the low flow P concentration of receiving rivers. This has particular importance as the discharges are approximately constant into receiving waters and these have restricted dilution capacity during ecologically sensitive summer periods. In this study, a number of identified high impact septic systems were replaced with modern sequential batch reactors in three rural catchments during a monitoring period of 4 years. Sub-hourly P monitoring was conducted using bankside-analysers. Results show that strategic replacement of defective septic tank systems with modern systems and polishing filters decreased the low flow P concentration of one catchment stream by 0.032 mg TPL(-1) (0.018 mg TRPL(-1)) over the 4 years. However two of the catchment mitigation efforts were offset by continued new-builds that increased the density of septic systems from 3.4 km(-2) to 4.6 km(-2) and 13.8 km(-2) to 17.2 km(-2) and subsequently increased low flow P concentrations. Future considerations for septic system mitigation should include catchment carrying capacity as well as technology changes.

Defining the sources of low-flow phosphorus transfers in complex catchments.

Nutrient transfers from the land to rivers have the potential to cause persistent eutrophic impacts at low flows even though the transfers may constitute a minor percentage of total annual fluxes. In rural catchments, the contribution from agricultural soils during storm events can be particularly large and untangling the relative contributions from multiple sources that vary in time and space is especially problematic. In this study, the potential for domestic septic tank system pollution during low flows was investigated in 3 small catchments (3 to 5 km(2)) using an integrated series of methods. These included septic system surveys, continuous (10 min) total phosphorus (TP) monitoring at the outlet of each catchment, repeated low-flow water quality surveys in sub-catchments upstream of the catchment outlets and single day river-walk water quality surveys. A series of faecal matter and grey-water fingerprinting techniques were also employed. These included determining sterol ratios in stream sediments, monitoring the presence of proteins, E. coli and enterococci bacterial signatures and boron. The total density and density of poorly maintained septic systems mirrored the magnitude of frequent TP concentrations in the catchments although this relationship was less apparent in the nested sub-catchments. The exception was possibly related to the simple hydraulics in one particular catchment and indicated temporary effluent attenuation in the other catchments. Repeated low-flow and river-walk water quality surveys highlighted discrete areas and reaches where stepped changes in nutrient concentration occurred. Bio-chemical fingerprinting showed that between 7% and 27% of sediments were contaminated with human faecal material and correlation matrices indicated that, at least during low flows, P fractions were positively correlated with some markers of faecal and grey-water contamination.