C.F. Jago

Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter

Abstract Optical instruments have been used effectively in studies of sediment dynamics for several decades. Without accurate instrument calibrations, calculated concentrations of suspended particulate matter (SPM) may be unreliable, with implications for interpretations of sedimentary processes and sediment fluxes. This review aims to quantify the effect of variations in SPM characteristics on the response of optical instruments (optical backscatter sensors OBS and transmissometers) and to note the implications for users of these instruments. A number of factors have a significant impact on instrument response, for example; a change in grain size from medium sands to fine silts may lead to a×100 increase in instrument response; flocculation of fine particles may decrease instrument response by×2; and the presence of plankton in suspension may lead to poor instrument calibrations of SPM concentration. Calibrations carried out in environments either with multi-modal bottom sediments, where flocculation of fine-grained sediments is likely, or where the hydrodynamics or grain type are highly variable must also include a determination of the changing nature of the suspended load in space and time. A more complete understanding of instrument response to SPM and of calibration requirements may enable optical devices to be used to a greater potential as long-term measures of SPM concentration, and also enable improvements in calculations of net sediment fluxes.

OBSERVATION AND MODELLING OF THE DYNAMICS OF BENTHIC FLUFF RESUSPENDED FROM A SANDY BED IN THE SOUTHERN NORTH SEA

The dynamics of suspended particulate matter (SPM) have been investigated at a site in the Central Southern Bight of the North Sea characterised by a well-mixed water column and a non-cohesive sand bed. Measurements of SPM concentration, particle size and quasi in situ settling velocity were made over tidal cycles during three periods (January, May, September) and used to test a 1-D tidal resuspension model. SPM comprised two components: a local tidal resuspension component and a background component in long term suspension. The background component comprised organo-mineral microflocs with a seasonally invariant modal settling velocity of less than 10-6 m s-1 and additional populations of biogenic particles produced during phytoplankton blooms. The tidal resuspension component had a modal settling velocity of ∼5×10-3 m s-1 and comprised benthic fluff and not bed sediment. Superposition of quarter diurnal resuspension and semi-diurnal advection along a horizontal background concentration gradient produced a characteristic ‘twin peak’ concentration time series. The model successfully reproduced the observations and demonstrated that local resuspension was source-limited: carbon-rich fluff, which formed a veneer within ripple troughs at slack water, was rapidly exhausted during tidal resuspension so that SPM concentrations peaked before maximum current velocity. Resuspension potential was therefore governed by the supply of fluff, which was greatest in May after a phytoplankton bloom but was still significant in January.

Dynamics of suspended particles in coastal waters (southern North Sea) during a spring bloom

Measurements of suspended particle concentration, size and settling velocity were made at a shallow site in the southern North Sea during a spring phytoplankton bloom. The site was characterised by strong differences in surface and near-bed residual flows; therefore particle processes in each layer are effectively decoupled as long as the water column is stratified. Four distinct energetic events during the observation period caused variation in the characteristics and behaviour of the particle population: (1) moderate spring tides with low wave activity; (2) strong winds, increased wave activity; (3) strong spring tides; (4) weak neap tides. During Event 1 weak tidal resuspension occurred, median particle diameter was relatively large, but median settling velocities of both chlorophyll and total SPM were low. During the higher energy Events 2 and 3 there was resuspension of relatively small, high-density particles producing high median total SPM settling velocities but low median particle diameter. In addition, a phytodetrital fluff layer, characterised by high chlorophyll settling velocity, was resuspended and dispersed during storm conditions (Event 2). During calm, weak neap tides (Event 4) there was negligible resuspension and enhanced particle settling and deposition, particularly in the phytodetritral component of the particle population, allowing rapid replenishment of the benthic fluff layer. This work indicates the relatively rapid rate at which fluff layers can be formed and dispersed, and highlights the need for high frequency measurements. The range of contrasting physical conditions over which the data-set was collected makes it an ideal candidate for parameterising and validating suspended sediment dynamics models.

Resuspension of benthic fluff by tidal currents in deep stratified waters, northern North Sea

Abstract Suspended particulate matter (SPM) concentration and properties (particle size and settling velocity), water column and boundary layer dynamics were measured during a 60-d period at a site in 110 m water depth in the northern North Sea. The site was in stratified waters and measurements were made during September–November as the seasonal thermocline was progressively weakening. SPM concentration was low, c. 1 mg dm−3 in the surface mixed layer and maximum values of 2 mg dm−3 in the bottom mixed layer. The bottom layer was characterised by larger mean particle size. SPM signals in the two layers were decoupled at the start of the period, when the thermocline was strong, but were increasingly coupled as the thermocline progressively weakened. A spring-neap cycle of resuspension and deposition of SPM was observed in the bottom mixed layer. Bed shear stresses were too small to entrain the bottom sediment (a fine sand) but were competent to resuspend benthic fluff: threshold bed shear stress and threshold current velocity at 10 mab were 0.02–0.03 Pa. and 0.18 m s−1, respectively. Maximum SPM concentration in the bottom layer preceded peak spring tide currents by 3 d. Simulation of fluff resupension by the PROWQM model confirms that this was due to a finite supply of benthic fluff: the fluff layer was stripped from the seabed so that fluff supply was zero by the time of peak spring flow. SPM was redeposited over neap tides. Fluff resuspension must have been enhanced by intermittent inertial currents in the bottom layer but unequivocal evidence for this was not seen. There was some resuspension due to wave activity. Settling velocity spectra were unimodal or bimodal with modal values of 2×10−4–2×10−3 mm s−1 (long-term suspension component) and 0.2–5.7 mm s−1 (resuspension component). The slowest settling particles remained in suspension at peak spring tides after the fluff layer had been exhausted. There was evidence of particle disaggregation during springs and aggregation during neaps.

Determination of settling velocity in the Elbe estuary using quisset tubes

A version of the Owen tube with reduced flow disturbance and wider operating concentration range —UWB QUISSET—is described. Quasi in situ settling velocity distributions of suspended particulate matter (SPM) were determined using these during an intercalibration exercise in the Elbe estuary in June 1993. Systematic variation was observed, in response to tidal resuspension, advection and redeposition. The observed variation in median settling velocity was in good agreement with that obtained by other workers using different settling tubes. Systematic differences in absolute values of up to an order of magnitude between the various techniques may have been caused by differences in tube design, sample handling or raw data interpretation.

Microbial and nutrient pollution of coastal bathing waters in Mauritius

The coastal pollution problem in Mauritius is exacerbated by the hydrogeology of the volcanic substratum. Bacterial contamination of bathing waters and nutrients, water temperature, salinity, and dissolved oxygen (DO) were monitored at three different spatial and temporal scales along the coastline of Mauritius during 1997-1998. Standard techniques for water sample collection and analysis set by the American Public Health Association [APHA. Standard methods for the examination of water and wastewater. 19th ed. Washington, DC: APHA, 1995.] were used at: (a) 16 sites around the island over a period of 7 months; (b) 12 stations along a recreational beach over an 18-month period; and (c) at an underground freshwater seepage point over 1 day. Total coliform (TC), faecal coliform (FC), and faecal streptococci (FS) contamination reported during all surveys varied randomly (e.g., with maximum densities in the ranges of 346-2020 TC, 130-2000 FC, and 180-1040 FS at one site) and at times exceeded the established EEC and Environment Protection Agency (EPA) standards for bathing water (e.g., in >90% of samples) to qualify for beach closure. Computed FC:FS ratios were used to pinpoint human faecal matter as the main source of contamination. Nitrate, phosphate, and silicate concentrations in seepage water were high (3600-9485, 38-105, and 9950-24,775 microg l(-1), respectively) and a cause for concern when compared with levels (5-845, 5-72, and 35-6570 microg l(-1), respectively) in cleaner lagoon water samples. Statistical analysis showed significant correlations (for TC and NO3: r=.75, P<.02; for TC and PO4: r=.779, P<.02; for TC and SiO4: r=.731, P<.05; for FC and NO3: r=.773, P<.02; for FC and SiO4: r=.727, P<.05; for FS and SiO4: r=.801 P<.01) between microbial densities and nutrients recorded, confirming the pathogen-contaminated water to be highly eutrophic. There is an urgency for Mauritius to properly address the issue of sewage treatment and wastewater discharge to safeguard its coastal environment, public health, and tourism expansion.

Sediment composition influences spatial variation in the abundance of human pathogen indicator bacteria within an estuarine environment.

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.

Diagnostic criteria for reconstruction of tidal continental shelf regimes: changing the paradigm

Abstract A biogeochemical flux paradigm is presented which provides a conceptual and numerical framework for reconstructing the dynamical and biogeochemical regimes of ancient tide-driven continental shelves. The paradigm links turbulence, primary production, and suspended particulate matter flux in the water column to microbiological and isotopic proxies in the sediment record and identifies the diagnostic signatures of sediments deposited in stratified, frontal, and mixed dynamic regimes of tidal shelves. The critical governing processes in the water column and at the sediment/water interface are temperature, particulate organic carbon supply, and benthic oxygen consumption, which exhibit strong gradients across tidal mixing fronts. The diagnostic proxies in the sediment record are microplanktonic (e.g. dinoflagellate cysts) and microbenthic (e.g. foraminifera) and their associated stable isotopic properties. Advanced numerical models are available which incorporate biophysical coupling in the water column and benthic boundary layer and which simulate biogeochemical and ecological processes and organic fluxes to the seabed. These models offer potential advances for interpreting microbiological and isotopic proxies of biogeochemical regime preserved in the fine sediment record. Quaternary shelf deposits provide the best potential validation of the biogeochemical flux paradigm since most Quaternary species are living today, but the paradigm is applicable to ancient tidal shelf deposits. The paradigm is particularly applicable to the fine sediment record which potentially preserves the most complete history of shelf evolution.

Measurements and preliminary modelling of current velocity over an intertidal mudflat, Humber estuary, UK

Abstract This paper presents tidal current velocity data from Station D on the LISP transect, Humber estuary, UK, collected during April 1995. Ten impeller-type current meters, aligned along the major flood-ebb tidal axis, were deployed in a vertical array at the site. Five of these measured flood speeds and five measured ebb speeds. First, depth-averaged flow is considered. Observed depth-averaged velocities are compared to results from three sets of modelled velocities: one set in the Hawke channel taken from a 2D model of the Humber estuary, one from a 1D hydrodynamic model of a flooding/drying transect, and one from a uniform elevation, volume-conservation model of a flooding/drying transect. The uniform elevation model is simpler, faster and gives a smoother response than the hydrodynamic model at this scale; and the observed velocities are closest in magnitude and direction to results from this model. The nature of the vertical velocity profile is then considered for both Spring and Neap tidal cycles. It is suggested that the vertical advection of horizontal velocities (due to the large ratio of water level change to water depth) has a significant effect on the velocity profile during the ebb, modifying the expected deviation in profile due to the flow acceleration.

Decay rates of faecal indicator bacteria from sewage and ovine faeces in brackish and freshwater microcosms with contrasting suspended particulate matter concentrations

To safeguard human health, legislative measures require the monitoring of faecal indicator bacteria (FIB) concentrations in recreational and shellfish waters. Consequently, numerous studies have focussed on FIB survival in the water column and more recently in estuarine sediments. However, there is a paucity of information regarding the influence of contrasting suspended particulate matter (SPM) concentrations on the survival of FIB in the water column of estuaries. Here, microcosms containing freshwater or brackish water with low, high and extreme SPM concentrations were inoculated with sewage and ovine faeces and the decay rate of Escherichia coli, coliforms and enterococci were determined by enumeration over five consecutive days. E. coli derived from ovine faeces proliferated and persisted at high levels in both freshwater and brackish microcosms (no decay), whereas ovine enterococci demonstrated a net decay over the duration of the experiment. Furthermore, SPM concentration had a significant effect on the decay rates of both E. coli and enterococci from ovine faeces in brackish microcosms, but decay rate was greater at low SPM concentrations for E. coli, whereas the opposite was observed for enterococci, whose decay rates increased as SPM concentration increased. E. coli, enterococci and coliforms derived from wastewater demonstrated a net decay in both freshwater and brackish microcosms, with contrasting effects of SPM concentration on decay rate. In addition, some FIB groups demonstrated contrasting responses (decay or proliferation) in the first 24h following inoculation into freshwater versus brackish microcosms. Overall, SPM concentrations influenced the proliferation and decay rates of FIB in brackish waters, but had minimal influence in freshwater. These results demonstrate that the survival rates of FIB in aquatic environments are system specific, species and source dependent, and influenced by SPM concentration. This study has important implications for catchment-based risk assessments and source apportionment of FIB pollution in aquatic environments.