High Turbidity Levels Research Articles

Investigating shellfish deposition and landscape history at the Natia Beach site, Fiji

The relationship between environmental variation and subsistence practices is a central point of discussion in much Oceanic archaeology. While human predation can significantly reduce prey populations, environmental variation also contributes to reductions in prey abundance, possibly leading to increased human competition and resource scarcity. At the Natia Beach site, Nacula Island, Fiji, geoarchaeological evidence suggests that coastal progradation began soon after initial occupation of the coastal plain. Additionally, at approximately 650 BP a marked increase in clay and silt deposition occurred. Changes in coastal geomorphology may be explained by landscape response to regional Mid-Holocene sea level fall combined with human induced soil erosion due to upland settlement. Smaller scale environmental changes associated with climate variability may have also played a role. Additionally, landscape change appears to have had a measurable impact on local nearshore mollusks that are sensitive to high levels of water turbidity. Minor evidence of human exploitation is observable in this shellfish assemblage, although changes in predation pressure may have allowed shellfish populations to recover. Increased ceramic diversity and fortified settlements also appear at approximately 650 BP on Nacula and other parts of Fiji. The suite of changes at Natia may be explained by processes of regional and local environmental changes, and human adaptation in terms of subsistence, spatial organization, and competition.

Energy Dissipation and Chemical Treatment to Improve Stilling Basin Performance

Surface water pumped from construction sites frequently contains high levels of turbidity and suspended sediment, which are not effectively removed using gravity-based systems. This study assessed the effects of modifying a permanent pool stilling basin with energy dissipaters and with the addition of polyacrylamide (PAM) on turbidity and suspended sediments. Turbidity was generated by injecting soil into flowing water at a fixed rate for 30 min in a source basin. Turbid water from this basin was pumped from the surface to the stilling basin with physical and chemical treatments. Three energy dissipater treatments were tested: bottom inlet level spreader (BILS; silt fence fabric installed with 40 mm opening from the basin bottom), coir baffles (900 g m-2 coir fabric with 0.45 open space fraction (OSF), and Pyramat baffles (synthetic fabric with 0.10 OSF). The tests were run either with or without PAM dosing by passing the flow over a solid PAM block at the stilling basin inlet. The physical treatments (i.e., energy dissipation) did not significantly affect the turbidity and total suspended solids (TSS) of the water exiting the basin, which were reduced by up to 29% and 36%, respectively. The chemical treatment was much more effective regardless of the physical treatment, either in combination or alone, reducing turbidity and TSS up to 88% and 84%, respectively. The baffle materials collected much more suspended sediment when PAM was added, with twice as much sticking to the coir than the Pyramat, although overall the latter may be more effective in settling the flocs. The patterns of turbidity and TSS within the basin suggest that only one porous baffle is adequate for PAM-treated water, and that the reduction observed near the outlet was likely floc interception by the sloped wall of the basin outlet. This study provides a relatively simple, inexpensive approach to improving the function of stilling basins for treating turbid water.

Efficiency of top-down control depends on nutrient concentration in a Mediterranean shallow lake: A mesocosm study

In shallow lakes, the influence of top-down control is considered to be less important under a Mediterranean bioclimate, although little experimental data support this hypothesis. The present study aimed to experimentally assess the importance of top-down forces in the trophic status of Mediterranean shallow lakes. A bifactorial mesocosm experiment was carried out in a shallow lake using a range of nutrient concentrations and fish population densities. The results demonstrated an expected increase in turbidity levels as nutrients and fish density increased, as a result of both sediment resuspension and chlorophyll a concentrations. Nevertheless, at higher nutrient additions, turbidity decreased in the mesocosms in response to cladoceran proliferation because the fishes were unable to control zooplankton growth under such conditions. Fish diet content analysis showed a generalist diet with an important contribution of non-planktonic components; the proportion of zooplankton in the diet contents increased with nutrient additions. Fishes preferred cladocerans over copepods and rotifers, these latter taxa dominated the resulting zooplankton community. Some fish-added mesocosms retained high transparency levels, most likely because of physiological changes in response to high ammonium concentrations. In summary, the dietary spectrum of Chondrostoma arcasii [Steindachner], despite some electivity patterns, depends largely on available resources. The trophic ecology of cyprinids, acting both through direct and indirect effects on food webs, is a determinant factor explaining, in part, stable high turbidity levels in many shallow lakes.

Comment on: Acinetobacter spp. and time-kill studies

Sir, We are in agreement with Dr Lopardo on various issues that affect antibiotic synergy testing, and in particular, on some of the intrinsic limitations of time–kill methodology. We also agree that the MIC is not a static parameter, and that one of the limitations of our study (as mentioned in the Discussion section) was that we were unable to measure antibiotic concentrations within the testing medium. It is worth noting that MIC endpoints are read after 18 h of incubation, while the combination testing was performed over 24 h. It is also a point of interest that at least two other studies using similar time–killing assays against Acinetobacter baumannii have demonstrated bacterial regrowth at concentrations of 1 MIC of colistin. It remains to be determined if this is a true phenomenon or a limitation of the testing methodology. With regard to the points raised regarding bacterial counts, we would like to point out that a turbidity equivalent to that of a 0.5 McFarland standard (barely detectable turbidity) equates to a bacterial count of 1.5 10 cfu/mL, and Acinetobacter species will achieve substantially higher turbidity levels in Mueller–Hinton broth over a 24 h growth period. Finally, in our study, synergy was defined as a 2 log10 decrease in cfu/mL for the antibiotic combination compared with its more active constituent. We were unable to ascribe the presence of synergy to one isolate in our study because the difference between the colony count in the combined antibiotic testing ( 20 cfu/mL) and the most active single antibiotic tested (colistin;1200 cfu/mL) did not fulfil the study definition.

A comparison of natural and created depressional wetlands in central Oklahoma using metrics from indices of biological integrity

Created wetlands and water bodies that have wetland characteristics (old farm ponds) appear to provide many of the habitat attributes of natural systems. To compare the biological and physical characteristics of natural and created wetlands, we evaluated water chemistry and a suite of metrics associated with the plant, macroinvertebrate, and avian assemblages at 12 natural and six created systems in central Oklahoma. The natural wetlands had significantly shallower depths and higher turbidity levels than the created wetlands. Of 43 metrics across the three biotic assemblages, seven were significantly different between the two wetland types. The proportions of hemipteran insects from the family Corixidae and insectivorous bird species were both greater in natural than created wetlands. The proportion of perennial plant species, the proportion of invertebrates in the shredder feeding guild, the number of Ephemeroptera, Trichoptera, Sphaeridae, and dragonfly (ETSD) taxa, the proportion of individuals in the dominant bird taxa, and the proportion of avian edge species were all greater in created wetlands than in natural wetlands. The community similarity (based on Jaccard’s similarity index) in the two wetland types was 38% for plants, 56% for macroinvertebrates, and 65% for birds. For some individual metrics, assemblage members had similar attributes (e.g., proportion of omnivorous taxa) although the specific taxa often differed between natural and created wetlands. These differences may influence the performance of certain assessment methods when they are applied to created wetlands.

Can giant clam (Tridacna squamosa) populations be restored on Singapore's heavily impacted coral reefs?

1. Giant clams have been a sustainable resource for millennia, but unregulated harvesting has led to local extinctions within the Indo-Pacific region. Giant clam mariculture can produce large numbers of juveniles for restocking wild populations where natural recruitment is low or absent. 2. Singapore is surrounded by more than 60 small islands, many with fringing reefs. These reefs, however, experience increased turbidity and sedimentation resulting from massive coastal development projects and regular dredging of shipping lanes. 3. Seven reefs off Singapore's southern islands were surveyed (9670 m2) for giant clams. Also, an experiment was conducted to determine the growth of Tridacna squamosa reared in aquaria under three light treatments: ∼50% ambient photosynthetically active radiation (PAR); ∼25% ambient PAR; and ∼12% ambient PAR. Finally, 144 clams (T. squamosa) were transplanted to four reefs around Singapore to study survival and growth in a heavily impacted environment. 4. A total of 23 adult clams from three species were found during the survey, representing a mean density of 0.24 per 100 m2. Most clams were found at Raffles Lighthouse, Singapore's ‘best’ reef. No juvenile clams were encountered. In the aquarium experiment, clam growth was significantly different among the three light treatments, with growth greatest in the ∼50% ambient PAR treatment. Of the 144 transplanted clams, 116 (80.6%) were recovered after 7 months. All specimens had increased in size, with growth rates among reefs ranging from 3.3 mm month−1 (SD=1.3 mm) to 4.8 mm month−1 (SD=1.6 mm). 5. Results suggest that, despite high levels of sedimentation and turbidity on Singapore's reefs, giant clams can survive and grow well. Restocking efforts using maricultured clams may be effective in enhancing the dwindling local populations. It is not clear, however, whether a self-sustaining community can be established as high sedimentation may hinder larval settlement and survival. Copyright © 2007 John Wiley & Sons, Ltd.

The Use of Lime as A Coagulant or A Coagulant Aid With Alum in Turbidity Removal

This study revealed the efficiency of using lime as a coagulant and coagulant aid with alum in turbidity removal when it used with medium and high initial turbidity levels (100, 500) ntu with dosage of (10, 40) mg/l respectively with alum dosage of 20 mg/l, It not recommended to use lime with low turbidity limits of (25) ntu and lower. Also the used of lime with alum improved the settling time especially in high limits of turbidity. A mathematical relationship was developed variables including in the study to determine the residual turbidity by using stepwise regression analysis.

Water quality effects on clogging rates during reclaimed water ASR in a carbonate aquifer

Summary Well-clogging remains a major operational issue, particularly in light of the increased attention being given to the reuse of reclaimed water through the method of aquifer storage and recovery (ASR) in seasonally arid regions. At a reclaimed water ASR trial site in a limestone aquifer at Bolivar in South Australia, rates of clogging were related to the quality of the injectant over a 4 year study period, during which time water quality generally improved due to advancements in pretreatment. Rates of clogging within a 4 m radius of the ASR well, defined by changes in the relative intrinsic permeability of the aquifer, were enhanced at higher levels of turbidity, total nitrogen (NTOT) and pH due to their resultant effects on particle filtration, microbial growth, and calcite dissolution, respectively. Water quality changes due to the movement of the injectant to an observation well at a radius of 4 m verified these processes. At this site acceptably low rates of short- and long-term clogging were achieved for turbidity

Microbiological performance of a water treatment unit designed for household use in developing countries

Interventions to improve water quality, particularly when deployed at the household level, are an effective means of preventing endemic diarrhoeal disease, a leading cause of mortality and morbidity. Following a national survey indicating widespread faecal contamination of drinking water in Indian cities, a point-of-use water treatment unit was developed, which combines filtration and disinfection, does not require power or water pressure and has an operating cost of Rs. 0.25 (US$0.0057) per litre. We assessed the microbiological performance of the unit in the laboratory over the full 1500 l design life of the unit's consumable components. Geometric mean reductions for the units were more than 6 logs (99.9999%) for bacteria, more than 7 logs (99.99999%) for viruses and more than 3 logs (99.9%) for the test surrogate for protozoan cysts. Geometric mean reductions exceeded levels established for microbial water purifiers. The product water was free of detectable chlorine. If these results are validated in field trials, the deployment of the unit on a wide scale among vulnerable populations may make an important contribution to public health efforts to control intractable waterborne diseases.

The Use of Bentonite clay as a Coagulant or a Coagulant Aid With Alum in Turbidity Removal

The main goal of this study is to use bentonite clay as a coagulant and coagulant aid in removing different water turbidities levels in order to improve drinking water quality at economical costs. The study showed that this material as coagulant has been proven to be efficient especially at low turbidity levels (i.e. not more than 25 ntu) with a dose of not more than 25 mg/l. As a coagulant aid with alum bentonite has shown to very effective with dose as much as 15 mg/l for low and intermediate initial turbidity level (not more than 100 ntu) and with a dose not more than 30 mg/l for high initial turbidity level 500 ntu, this efficiency is be better for low initial turbidity level and with low alum dose.

Effects of clay turbidity and density of pikeperch (Sander lucioperca) larvae on predation by perch (Perca fluviatilis)

Increased turbidity reduces visibility in the water column, which can negatively affect vision-oriented fish and their ability to detect prey. Young fish could consequently benefit from high turbidity levels that can provide a protective cover, reducing predation pressure. Perch (Perca fluviatilis) are commonly found in littoral zones of temperate lakes and coastal areas of the Baltic Sea. Pikeperch (Sander lucioperca) spawn in these areas, so perch is a potential predator for pikeperch larvae. We conducted laboratory experiments to test the predation of perch on pikeperch larvae at different turbidity levels (5-85 nephelometric turbidity units), densities of pikeperch larvae (2-21 individuals l(-1)) and volumes of water (10-45 l). The logistic regression showed that the probability of larvae eaten depended significantly on turbidity and volume of water in the bags, while density of larvae was not significant. However, because container size is known to affect predation, the data was divided into two groups based on water volume (10-20 and 25-45 l) to reduce the effects of container size. In either group, probability of predation did not significantly depend on volume, whereas turbidity was significant in both groups, while density was significant in larger water volumes. Thus, high turbidity impaired perch predation and protected pikeperch larvae from perch predation. Because density of larvae was also a significant factor affecting predation of perch, the dispersal of pikeperch larvae from spawning areas should also increase the survival of larvae.

Larval Chironomid Communities (Diptera: Chironomidae) Associated with Establishing Rice Crops in southern New South Wales, Australia

Chironomid larvae were sampled sequentially from small experimental rice fields in southern New South Wales, Australia. Sampling was undertaken during the 1995, 1997, 1998 and 1999 crop establishment periods, starting 10 days after flooding (DAF) and continuing at 5 day intervals until 35 DAF. No agrochemical treatments were applied to the fields at any stage. Seventeen species were identified from 3429 recovered larvae. The most abundant species were Chironomus tepperi Skuse (0–91% of 35 day totals), Procladius paludicola Skuse (7–78%) and Polypedilum nubiferum (Skuse) (2–12%). No other taxon accounted for more than 8% of recovered larvae in any year. C. tepperi colonised fields more rapidly than other taxa and, when present, had only a single generation. The abundance of C. tepperi varied dramatically from year to year: from total absence in 1997 to a peak density exceeding 13 000 larvae m−2 in 1998. Significant differences between post-flood sampling times (R = 0.407, p= 0.009) and between years (R = 0.495, p = 0.001) were found using ANOSIM2 analysis. C. tepperi had a greater impact on community variability at 10 and 15 DAF (42–58%) than any other species, with P. paludicola exerting the most influence from 20 DAF onwards. Exceptionally high densities of C. tepperi in 1998 led to high levels of turbidity, which apparently prolonged the duration of the single C. tepperi cohort by suppressing water temperatures. Comparison of plant densities in the monitored bays to those in nearby bays receiving effective chemical protection shows that rice plant loss increased in response to higher C. tepperi densities. No plant loss occurred in 1997 when C. tepperi was absent, suggesting the other chironomid species present in the fields are not routinely involved in causing significant crop damage.

Spatial and temporal variation in algal-assemblage structure in isolated dryland river waterholes, Cooper Creek and Warrego River, Australia

The scale at which algal biodiversity is partitioned across the landscape, and the biophysical processes and biotic interactions which shape these communities in dryland river refugia was studied on two occasions from 30 sites in two Australian dryland rivers. Despite the waterholes studied having characteristically high levels of abiogenic turbidity, a total of 186 planktonic microalgae, 253 benthic diatom and 62 macroalgal species were recorded. The phytoplankton communities were dominated by flagellated cryptophytes, euglenophytes and chlorophytes, the diatom communities by cosmopolitan taxa known to tolerate wide environmental conditions, and the macroalgal communities by filamentous cyanobacteria. All algal communities showed significant differences between catchments and sampling times, with a suite of between 5 and 12 taxa responsible for ~50% of the observed change. In general, algal assemblage patterns were poorly correlated with the measured environmental variables. Phytoplankton and diatom assemblage patterns were weakly correlated with several waterhole geomorphic measures, whereas macroalgal assemblage patterns showed some association with variability in ionic concentration.

NUMERICAL ANALYSIS OF TURBIDITY AND ITS OCCURANCE IN SMALL MOUNTAINOUS BASIN

It is well known that turbidity increases during snowmelt season and storm rainfall of summer season in river flow. Although the reason for the highest degree level of turbidity during snowmelt season is not clear. M.Yamanashi et al. (1995) suggested that the high degree level of turbidity during snowmelt season depends on surface flow on a mountainous basin and the turbidity during summer season is mainly caused by a shearing force between river bed and river flow. There is a possibility to analyze runoff mechanism through the observation of turbidity according his suggestion. The maximum snowmelt which is equivalent to rainfall intensity is about 3 to 4 mm/hr. On the other hand, such rainfall intensity, 3 to 4 mm/hr during summer season does not cause so high turbidity in river flow.This paper studies the mechanisms of the turbidity increase in mountainous river flow through field observations and theoretical analysis. The conditions of turbidity variation by using turbidity occurrence and also inverse estimation method of turbidity occurrence are clarified under the assumption that soil-water content plays an important role.

Variación ambiental en el río Grande de Térraba y algunos de sus afluentes, Pacífico sur de Costa Rica

Environmental variation in the Grande de Terraba river and some of its tributaries, south Pacific coast of Costa Rica. Rio Grande de Terraba drains one of the largest basins in Costa Rica, with a grow- ing human population. It has been deforested in its lower region for the development of agriculture, but has attracted little attention on its environmental challenges. The river was studied once in the dry and wet season for a period of four years (1999-2003). A total of 13 sites were studied along its course, from the upper middle reach to near a mangrove forest. We measured temperature, dissolved oxygen, conductivity, total hardness, turbidity, suspended solids, nitrate and phosphate, macrobenthic taxa richness and a water quality index (BMWP) in order to assess river deterioration. The level of oxygenation was high, with the exception of the lowest point (Palmar Norte), which means that organic pollution is restricted to localized areas near pollution sources. However, erosion in the agricultural lands and the loss of forest cover in the basin, produced high levels of turbidity and suspended solids at the lower reaches of the river, mainly during the rainy season. This was also reflected by a decrease in the species richness and the BMWP index value at the most affected sites. Rev. Biol. Trop. 54 (Suppl. 1): 265-272. Epub 2006 Sept. 30.

Hydrologic characteristics of a large rockfill dam: Implications for water leakage

The Unmun Dam, a rockfill dam with a clay core, filter zone, and sandy gravel shell, is located in the Nakdong river basin near Cheongdo-gun in Korea. In the dam area, the Cretaceous Gyeongsang supergroup andesite lies over intrusive granite. These form the intrusive contact near the main dam and cofferdam areas. The stratigraphic units in upward sequence consist of Cretaceous granite and andesite, weathered soft rock, and a soil layer of silty clay with gravel. The reservoir was first filled in August 1994 and was full by April 1998. Right after complete filling, three sinkholes were found in the dam crest near stations 2, 12, and 10 when the reservoir level was at around 150 m. The amount of leakage increased with rise of the reservoir level mainly caused by rainfall events. As a countermeasure for the abnormal leakage and dam rehabilitation, comprehensive surveys were performed on the dam including observational drillings, grouting, geophysical surveys, and tracer tests by Korea Water Resources Corporation (KOWACO). There were also further investigations for hydrologic characterization of the dam to identify water leakages. The groundwater levels at the 26 observation wells installed on the crest and dam toe were measured to evaluate the relationship between the reservoir level and groundwater level, and the influences on the dam leakage. Groundwater level at the left side of the embankment was high compared with a normal phreatic line; it was only 4 m lower than the reservoir level while 6.65 m lower than that at its right side. Fluctuation of the groundwater levels in the crest wells coincided well with the reservoir level without a significant lag time, which reflected a good hydraulic connection between the reservoir and the dam interior. Hydraulic conductivity estimated from 149 hydraulic tests at 9 monitoring wells constructed in the dam core showed a wide range between 1.42 × 10 −4 and 4.35 × 10 −8 cm/s with a geometric mean of 3.13 × 10 −6 cm/s. Furthermore, water quality at the monitoring wells were much different from that of the reservoir. High levels of turbidity and electrical conductivity were observed at the monitoring wells in the left side of the bank. All these hydrologic characteristics reflected possible occurrence of the leakage on the dam, especially in the left embankment. This paper presents a summary of several investigation studies on the hydrologic characteristics of the rockfill dam, which implied abnormal water leakage.

Macrobenthic community responses to long-term environmental change in an east Australian sub-tropical estuary

Long-term temporal and spatial differences in benthic community structure in Port Curtis (a semi-enclosed estuary in Queensland, Australia) were examined from quantitative grab samples. A total of 2660 grabs were collected from 30 fixed sites over the 6-year study period, and 35 421 individuals and 409 species identified. Few species were highly abundant and nearly 90% were apparently undescribed. Species abundance data revealed strong ecological gradients that were principally driven by depth and sediment grain size. Depth related differences in benthos were most pronounced between the subtidal and intertidal zones, and total abundance and richness were both significantly lower in the intertidal. Total abundance and richness were also found to be significantly lower in sediments that were either extremely coarse or extremely fine. Over the 6-year period of this study, mean species richness and abundance progressively declined 72% and subsequently recovered by 68%. Similar temporal trends were displayed by all common dietary groups (filter feeders, deposit feeders, scavengers and predators), and it appears that drivers underpinning observed changes have a consistent influence at most trophic levels. Explanations for long-term trends in abundance and richness were determined through correlation analyses with key environmental variables. Both species richness and abundance were highly correlated (positively) with turbidity measurements observed 4 months previously. This finding suggests that high levels of turbidity promote recruitment and growth of benthic organisms in Port Curtis. Strong correlations between regional rainfall, freshwater inflow, nutrient and chlorophyll a concentrations, further support the hypothesis that recent changes in benthic productivity (as defined by total infaunal abundance) within the estuary are principally the result of long-term climatic cycles including El Niño events.

The U.S. EPA Lake Erie Indicators Monitoring Program 1983–2002: Trends in Phosphorus, Silica, and Chlorophyll a in the Central Basin

During the past 20 years, Lake Erie has exhibited a series of complex chemical changes resulting from changing anthropogenic influences and introductions of exotic species. Since 1990, some apparent trends in nutrient concentrations have been inconsistent with the predictions of models originally used to guide Lake Erie water quality management. We performed time trend analysis on total phosphorus (TP), chlorophyll a (Chl a), and dissolved reactive silica (DRS) measurements collected during spring and summer in the central basin of Lake Erie between 1983 and 2002. Three distinct time trends in basin-specific, station-averaged open-water TP concentrations were observed over the 20-year period: 1983–1989 decreases, 1990–1997 increases, and 1997–2001 decreases. Exceptionally high levels of turbidity and TP were observed in spring 2002, possibly reflecting increasing frequency of winter storm events. Open-water concentrations of TP declined during the 1980s as annual TP loadings to Lake Erie declined below the 11,000 metric ton target level that had been expected to reduce central basin eutrophication. This was accompanied by a significant increase in available silica and in Chl a concentrations. During the period 1990–2002, when dreissenid mussels were abundant, spring TP concentrations increased but summer TP concentrations declined. Unexpected increases in spring central basin concentrations of DRS (exceeding 1 mg/L) were observed. During the same period, Chl a concentrations declined in the spring and rose slightly during summer.

Structure and development of detrital reef deposits in turbid nearshore environments, Inhaca Island, Mozambique

Coral communities are intermittently developed along the seaward margins of wide (up to 500 m) intertidal flats that fringe the western margins of Inhaca Island, southern Mozambique. The coral communities occur in an environment that can be considered as marginal for coral growth in the sense that they are subject to high turbidity levels (which results in rapid light attenuation) and to the effects of periodic smothering and burial associated with a mobile, siliciclastic-dominated sediment substrate. These constraints limit coral community development both spatially and bathymetrically. In addition, historical evidence suggests that the main sites of coral growth shift over decadal timescales due to periodic sediment remobilisation. Such rapid rates of turnover restrict the development of rigid framework deposits and, in contrast, the main depositional product of coral growth is an abundance of branched (mainly Acropora sp.) coral rubble. Rubble production is aided both by intense internal bioerosion (sponge, bivalve and worm traces) and by periodic storm damage. Much of this rubble accumulates along the channel margins and provides a substrate for subsequent coral recruitment. The deposits can be distinguished from those associated with clear water reef environments by (1) the lack of a rigid reef framework either comprising of in-situ coral or encrusted coral rubble, (2) the distinct lack of rubble encrustation/syn-depositional cementation, and (3) the predominance of a siliciclastic-rich sediment matrix. The deposits indicate, however, a long-term history of coral community occurrence in sites dominated by siliciclastic sedimentation. Cores recovered from a transect across the tidal flats identify a coral rubble floatstone facies extensively developed along the western margins of the island. It comprises mainly branched coral rubble (often heavily bioeroded, but sparsely encrusted), along with spheroidal rhodoliths and molluscan fragments set within a poorly sorted, mixed siliciclastic: carbonate sediment matrix. 14C dating indicates that most of the rubble deposits date from the period 6500–4700 years BP, which corresponds to a period of regional sea level highstand. A detailed growth chronology cannot be established due to the lack of in-situ coral, but rapid rubble deposition and reef flat progradation (around 350 m) can be inferred for this period (equivalent to an average progradation rate of around 0.2 m year −1). Over the subsequent 4700 years, progradation of only around 25 m has occurred. This may be attributed to reworking and erosion of the seaward margin of the deposits during periods of fluctuating sea level between 4700 and 900 years BP, or to a lack of seaward accommodation space.

The stability of tidal flats in Venice Lagoon—the results of in-situ measurements using two benthic, annular flumes

Seabed properties in Venice Lagoon were examined in situ in two multidisciplinary field campaigns. The purpose of this study was to understand the mechanisms controlling the stability of bed types. Two benthic annular flumes (Sea Carousel and Mini Flume) were deployed simultaneously from a floating pontoon at 24 sites during summer (1998), which were considered representative of the range in bed/habitat types. As well, bottom sampling and coring, water-column monitoring and benthic habitat analyses were carried out. All but three sites were on cohesive sediments. Bed types included bare shelly mudflats and regions colonised by the seagrasses Cymodocea nodosa and Zostera noltii, by filamentous cyanobacteria, and by patches of the macrophytes Ulva rigida and Chaetomorpha sp. A subset (13) of these sites was visited during the subsequent winter to evaluate seasonal changes. Six of the sites were intertidal, the remainder were in the sublittoral zone. Water temperature varied between 5 and 30 °C, and salinity varied between 20 and 38 psu. In the absence of waves, turbidity was generally low (<10 mg/l) and was composed of high amounts of organic matter (25–50%). This indicates that the tidal flows were not competent to support estuarine sediment. Higher levels of turbidity were measured during wind events or boat passage as a result of resuspension from the bed. Bed (saturated) density was, on average, 1770 kg/m 3, which was extremely high for estuarine sediments. Sea Carousel and Mini Flume provided comparable results, despite large differences in instrument footprints. Trends from the two instruments were similar and showed that summertime bed strength exceeded the winter by up to five times. Mean summer erosion thresholds for Sea Carousel and Mini Flume were 1.10 and 0.82 Pa, respectively, whereas during winter, they were 0.69 and 0.74 Pa. The northern lagoon had the most resistant tidal flats due to the stabilizing effect of filamentous cyanobacteria (Biostabilization Index: BI=244%), microphytobenthos (BI=153%) and Z. noltii (BI=206%). The stabilizing effects of C. nodosa (BI=74%), U. rigida (BI=115%) and shell debris (BI=115%) were intermediate, while bare sublittoral mud beds were the least resistant (BI=58%). Summer erosion rates (as a function of applied stress) were lower than winter ones, probably due to water temperature changes. The algorithm E= χτ s β , yielded good results and indicated that erosion rates in Venice Lagoon were high, notwithstanding the high erosion thresholds. The mean summertime friction coefficient was φ=62° and was highest in the central lagoon. The wintertime φ=69° showed that there was no seasonal fluctuation in bed stability. Mass settling in Venice Lagoon was a strong function of suspended sediment concentration ( S) and a decay constant ( k) of the exponential function S( t)= S o e − kt ; it was found to be in continuity with examples from other locations worldwide (and therefore normal).