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Faecal contamination of estuarine and coastal waters can pose a risk to human health, particularly in areas used for shellfish production or recreation. Routine microbiological water quality testing highlights areas of faecal indicator bacteria (FIB) contamination within the water column, but fails to consider the abundance of FIB in sediments, which under certain hydrodynamic conditions can become resuspended. Sediments can enhance the survival of FIB in estuarine environments, but the influence of sediment composition on the ecology and abundance of FIB is poorly understood. To determine the relationship between sediment composition (grain size and organic matter) and the abundance of pathogen indicator bacteria (PIB), sediments were collected from four transverse transects of the Conwy estuary, UK. The abundance of culturable Escherichia coli, total coliforms, enterococci, Campylobacter, Salmonella and Vibrio spp. in sediments was determined in relation to sediment grain size, organic matter content, salinity, depth and temperature. Sediments that contained higher proportions of silt and/or clay and associated organic matter content showed significant positive correlations with the abundance of PIB. Furthermore, the abundance of each bacterial group was positively correlated with the presence of all other groups enumerated. Campylobacter spp. were not isolated from estuarine sediments. Comparisons of the number of culturable E. coli, total coliforms and Vibrio spp. in sediments and the water column revealed that their abundance was 281, 433 and 58-fold greater in sediments (colony forming units (CFU)/100g) when compared with the water column (CFU/100ml), respectively. These data provide important insights into sediment compositions that promote the abundance of PIB in estuarine environments, with important implications for the modelling and prediction of public health risk based on sediment resuspension and transport.

In pycnoclines, the density differences can cause light scattering -schlieren -even though only few particulate scatterers may be present. This may pose problems for the interpretation of results obtained with instruments relying on light scattering and transmission, for example the LISST (Laser In Situ Scattering and Transmissometry) particle sizer, and various cameras. Here, the influence of schlieren on in situ forward light scattering, beam attenuation and image analysis is evaluated using a LISST-100 and a digital floc camera. Automated image analysis routines detect schlieren as particles, causing an apparent increase in particle size and volume. Re-analysis omitting schlieren-affected parts of the images reveals no increase. LISST beam attenuation and Volume Scattering Function (VSF) measurements indicate that schlieren can cause increases in beam attenuation due to a marked increase in the VSF at angles smaller than ~1.5°-2°, and falsely indicate accumulation of suspended particles in the pycnocline. Light scattering caused by density differences can also cause multiple scattering, which produces an apparent decrease in particle size derived from the LISST. Schlieren is visible in images when the buoyancy frequency exceeds ~0.12 s -1