Sediment Granulometry Research Articles

Habitat structure and biological characteristics of a maerl bed off the northeastern coast of the Maltese Islands (central Mediterranean)

Forty stations within a 20 km2 Maltese maerl bed were sampled by grab to gather data on sediment granulometry and the percentage mass, sphericity, and morphotype of rhodoliths. Two stations were monitored between July 1996 and April 1998 to study temporal variation in species diversity and abundance of the epi- and endo-benthos. Maerl was commonest at 51–90 m depth with 20–39% live rhodoliths in central parts of the maerl bed, while the peripheral parts had less than 20% live rhodoliths. The most abundant rhodolith morphotypes were branching forms and those with a rugged surface. The maerl bed proved to have high species diversity with 244 animal and 87 algal taxa recorded; molluscs, crustaceans, and annelids were the dominant taxa in the endobenthos, and bryozoans and sponges in the epibenthos. Community composition, rhodolith morphology and sediment characteristics at the two sites were related to differences in the hydrodynamic regime resulting from seabed topographical heterogeneity.

Detrital sediment supply in the southern Okinawa Trough and its relation to sea-level and Kuroshio dynamics during the late Quaternary

During Leg 195 of the Ocean Drilling Program, Site 1202 was drilled in the subtropical northwestern Pacific Ocean beneath the Kuroshio (“Black Current“) between northern Taiwan and the Ryukyu Island Arc on the northern flank of the I-Lan Ridge at 1274 m water depth. The upper 110 m of the Site 1202 section, composed of dark grey calcareous silty clay, provide an expanded record of environmental changes during the last 28 kyr. The sediments were deposited at high sedimentation rates between 3.0 and 5.0 m/kyr and peak values of 9.0 m/kyr between 15.1 and 11.2 ka BP. Variations in the modes and sources of detrital sediment input, as inferred from sediment granulometry, mineralogy, and elemental XRF-scanner data, reflect changes in environmental boundary conditions related to sea-level changes, Kuroshio variability, and the climate-driven modes of fluvial runoff. The provenance data point to increased sediment supply from northwestern Taiwan between 28 and 19.5 ka BP and from East China sources between 19.5 and 11.2 ka BP. The change in provenance at 19.5 ka BP reflects increased fluvial runoff from the Yangtze River and strong sediment reworking from the East China Sea shelf in the course of increased humidity and postglacial sea-level rise, particularly after 15.1 ka BP. The Holocene was dominated by sediments that originated from rivers in northeastern Taiwan. For the pre-Holocene period prior to 11.2 ka BP, low portions of sortable silt (63–10 μm) show that the Kuroshio did not enter the Okinawa Trough, because of low sea-level. In turn, high proportions of sortable silt and sediment provenance from northeastern Taiwan point to strong ocean circulation under the direct and persistent influence of the Kuroshio during the Holocene. The reentrance of the Kuroshio to the Okinawa Trough was heralded by two pulses in relative current strengthening at 11.2 and 9.5 ka BP, as documented by stepwise increases in sortable silt in the lower Holocene section. From a global perspective, environmental changes in the southern Okinawa Trough show affinities to climate change in the western Pacific warm pool with little influence of climate teleconnections from the North Atlantic realm, otherwise seen in many other marine and terrestrial palaeoclimate records from southeastern Asia.

Numerical study of circulation on the inner Amazon Shelf

We studied the circulation on the coastal domain of the Amazon Shelf by applying the hydrodynamic module of the estuarine and coastal ocean model and sediment transport. The first barotropic experiment aimed to explain the major bathymetric effects on tides and those generated by anisotropy in sediment distribution. We analyzed the continental shelf response of barotropic tides under realistic bottom stress parametrization (C d ), considering sediment granulometry obtained from a faciologic map, where river mud deposits and reworked sediments areas are well distinguished, among others classes of sediments. Very low C d values were set in the fluid mud regions off the Amapa coast (1.0 10 − 4), in contrast to values around 3.5 10 − 3 for coarser sediment regions off the Para coast. Three-dimensional experiments represented the Amazon River discharge and trade winds, combined to barotropic tide influences and induced vertical mixing. The quasiresonant response of the Amazon Shelf to the M2 tide acts on the local hydrodynamics by increasing tidal admittance, along with tidal forcing at the shelf break and extensive fluid mud regions. Harmonic analysis of modeled currents agreed well with the analysis of the AMASSEDS observational data set. Tidal-induced vertical shear provided strong homogenization of threshold waters, which are subject to a kind of hydraulic control due to the topographic steepness. Ahead of the hydraulic jump, the low-salinity plume is disconnected from the bottom and acquires negative vorticity, turning southeastward. Tides act as a generator mechanism and topography, via hydraulic control, acts as a maintainer mechanism for the low-salinity frontal zone positioning. Tidally induced southeastward plume fate is overwhelmed by northwestward trade winds so that they, along with background circulation, probably play the most important role on the plume fate and variability over the Amazon Shelf.

Community structure and microhabitat preferences of harpacticoid copepods in a tropical reef lagoon (Zanzibar Island, Tanzania)

Three microhabitat types (dead coral fragments, coral gravel and coral sand) were distinguished and sampled at two locations (Matemwe and Makunduchi) in a tropical lagoon (Zanzibar Island, Tanzania), and the community structure, habitat preferences and biodiversity of the associated harpacticoid copepod fauna was investigated. The harpacticoid fauna is affected by sediment granulometry and by the structural differences between coral and both gravel and sediment. The coral fragments contained a specific assemblage composed of typical ‘phytal’ taxa(Tisbe,ParadactylopodiaandDactylopusia)along with other eurytopic and sediment-dwelling forms (Ameira,EctinosomaandAmphiascus), which may be attracted by the sediment retained between the coral branches. The assemblages of coral gravel and upper sediment layer did not differ significantly from each other and had mostly the same dominant genera. The sediment from Matemwe was dominated by the interstitial Paramesochridae and the sediment from Makunduchi by Tetragonicipitidae. The coral fragments from Makunduchi sustained a more diverse assemblage than gravel and the different sediment layers. It was assumed that coral form and complexity, with implications for habitable space, nutritional resources and level of predation, are important in structuring diversity of the associated assemblage.

Assemblages of peracarid crustaceans in subtidal sediments from the Ría de Aldán (Galicia, NW Spain)

Peracarid crustaceans inhabit many marine benthic habitats and are good indicators of environmental conditions. There is, however, a lack of information about diversity and distribution of peracarid crustaceans on the shallow subtidal sediments of the Galician rias. In the summer of 1997, 27 subtidal stations were sampled in the Ria de Aldan, a ria on the southern margin of the mouth of the Ria de Pontevedra (Galicia, NW Spain). A total of 16,191 peracarid individuals were collected, comprising 125 species belonging to five orders. Amphipods were dominant in number of species and individuals, followed by isopods and cumaceans. Multivariate analyses of these data indicated that depth and sediment granulometry were major determinants of distribution and composition of peracarid assemblages in the ria.

Meiofauna in closed coastal saline lagoons in the United Kingdom: Structure and biodiversity of the nematode assemblage

Coastal saline lagoons are rare in North West Europe and tend to be closed, brackish water systems. In the United Kingdom they are small and isolated, each exhibiting extremely variable habitat conditions, both spatially and temporally. This paper represents the first species-level study of lagoonal nematode assemblages in the UK. Samples were taken from seven ponds in a saline lagoon system on the south coast of England in order to describe the nematode assemblage in relation to habitat type and to assess the possibility of lagoonal specialisation. At each site samples were also taken, or data were already available, for salinity, sediment granulometry, sediment organic carbon content, photosynthetic pigments and lagoon topography. A lagoonal specialist nematode fauna was not identified, but it was found that the nematode communities strongly reflected the differences between environmental conditions in the lagoons. Sample and site diversity were found to be relatively similar, but the importance of habitat networks was indicated by the relatively high system diversity, species turnover being highest between lagoons with different salinity and/or granulometry regimes. Salinity was the principal factor correlated with assemblage structure and species diversity was highest at the higher salinity sites. Median salinity (averaged from weekly records over the previous 4 months) correlated more clearly with nematode assemblage structure than salinity at the time of sampling. This shows the importance of considering historical as well as contemporaneous data when undertaking ecological studies: Contemporary conditions may influence species fecundity, and therefore relative abundances, whilst historic conditions may influence species occurrence through the effects of recruitment and survival.

The marine soft‐sediment benthic communities of Hong Kong: a comparison of submarine cave and open habitats

In October 2002, staff from the Swire Institute of Marine Science (SWIMS), the University of Hong Kong, and the Natural History Museum (NHM), London, mounted a joint expedition to study submarine caves (zawns) off the eastern coast of Hong Kong, to test hypotheses that they may support either endemic or unusual communities, or they may act as refugia for such taxa. The present paper analyses the sedimentary macroinfaunal communities from that survey. Sampling was by hand‐coring undertaken by SCUBA divers. Muddy‐sand habitats within two caves were sampled in comparison with four control sites outside the caves, one of heterogeneous muddy sand, one of clean sand and two of fine mud; the samples were sieved across a 0.5 mm mesh. Sedimentary granulometry, water temperature, pH and dissolved oxygen were also measured. The benthic community within the caves showed a higher density and diversity than most of those described from non‐cave sediments in Hong Kong waters. The community was distinct from that found at the control sites, partly owing to sediment granulometry differences. Thirty‐three species were found only in the caves, some of which are known from elsewhere in non‐cave habitats; six species are, to date, only known from these caves. It is concluded that the cave habitat conditions offer a stable and sheltered habitat which allows the development of a distinct community, although not representing an endemic fauna, as well as being a refuge for species under threat from anthropogenic or natural stresses outside the caves.

Response of infaunal macrobenthos to the sediment granulometry in a tropical continental margin–southwest coast of India

Surficial sediment samples, collected from the continental margin of the southwest coast of India in July 2004, were examined for the grain size and soft-bottom macrobenthic fauna, to understand the sediment granulometry and its effect on the faunal distribution. Samples were collected using Smith-McIntyre Grab, from 20 to 200 m depth range, consisting of mid-shelf, outer shelf and slope. Fine-grained sediment located in the mid shelf and supported low faunal abundance. Polychaetes constituted the bulk of the fauna. Feeding guild changed with depth and sediment granulometry. Coexistence of deposit feeders and carnivores in outer shelf and deposit feeders and filter feeders in the slope region indicated the effective utilization of different food resources. In general, richness and diversity were high in the southern region. Depth wise, the diversity and abundance were relatively high in the 50–75 m depth range. Correlation and BIO-ENV analysis showed that combination of different factors such as sediment texture, sediment sorting and depth were found to influence the distribution of macrobenthos. Hence, spatial variations observed in benthic community were presumably linked to the variations in sediment granulometry and the energy level conditions prevailing in the area.

Influence of drought on algal biofilms and meiofaunal assemblages of temperate reservoirs and rivers

The role of hydrological droughts in shaping meiofauna abundance through alterations in biofilm biomass and composition was investigated. In January 2005, continental Portugal was under a moderate to severe drought resulting from a 40% to 60% decrease in rainfall during the previous 12 months relative to the long-term average (1961–1990). Reservoir capacity was reduced by 30–50% relative to average values and the width of streams was reduced by 20–80% in the Zezere River Basin (central Portugal). Algal biomass and algal class composition of biofilms was assessed through quantification of algal pigments in three reservoir and six river locations. During drought, habitat alterations are expected to be sharp in rivers while, in the absence of water quality deterioration, the habitat characteristics of reservoirs are expected to remain fairly unaffected. Chlorophylls and carotenoid pigments were extracted from biofilm samples and analysed using high performance liquid chromatography (HPLC). In the winter of 2003, during the period of average rainfall, biofilm biomass did not exceed 5 μg chlorophyll a cm−2 at any location. River biofilm biomass was roughly half of that measured in the reservoirs. In the winter of 2005 (drought), biofilm biomass increased by more than 5-fold in river locations and remained low or decreased in the reservoirs. Algal biofilms were either dominated by Bacillariophyceae or by Chlorophyceae regardless of the existence of drought. The relative contribution of Bacillariophyceae to total biofilm biomass was higher during the drought than under average hydrological conditions. The abundance of harpacticoids, cladocerans and ostracods was favoured by the drought only in the reservoirs where an increase in diatom proportion in biofilms was observed. The increase in the abundance of cyclopoid copepods, turbellarians, nematodes and chironomids in rivers during the drought could be explained by algal class composition and biomass of biofilms and environmental variables (organic matter sediment content, phosphorus availability content and sediment granulometry). The hydrological drought appears to regulate meiofauna abundance only in river locations, possibly through the promotion of the growth of biofilms and the availability of organic matter deposited in rivers during the drought.

Water quality and diversity of the Recent ostracod fauna in lowland springs from Lombardy (northern Italy)

The Po river plain (northern Italy) is delimited to two mountain ridges, the Alps and the Apennines. It hosts peculiar lowland man-modified springs, locally known as “fontanili”, which originate from natural resurgences occurring along the alluvial fans of the main watercourses, namely in the transition zone from the higher to lower plain which is characterized by changes in slope profile and sediment granulometry. These habitats usually show low variation in hydrological, hydrochemical and thermal conditions throughout the year. Twenty-eight springs, located in the provinces of Lodi and Cremona (Lombardy) in the alpine sub-catchment of the Po river were sampled in summer and autumn of 2004. Twenty-three of them were typical alluvial-fan springs, while the remaining five were terrace springs. The two groups of springs showed marked differences in their hydrochemical and hydrological characteristics. Sixteen ostracod species in three families (Candonidae, Ilyocyprididae, and Cyprididae) were identified. The most frequent species were Cypria ophtalmica (19 sites), Herpetocypris reptans (16), and Prionocypris zenkeri (13). Five species were found only once: Chlamydotheca incisa, Scottia pseudobrowniana, Pseudocandona compressa and Candona neglecta. Up to 6 taxa were recorded from a single site and the average number of taxa for each site was c. 3. The associations among ostracod taxa and their occurrence in relation to environmental factors were examined. Finally, the results of this survey were compared with a similar study previously conducted in 31 alluvial fan springs of the apennine sub-catchment of the Po river.

Physical, chemical and microbiological characterization of the intertidal sediments of Pereque Beach, Guarujá (SP), Brazil

Physical and chemical characteristics of intertidal sediments and their relationships with bacteria and cyanobacteria were analyzed at four stations at Pereque Beach. Granulometric analysis showed that Pereque beach has sediment that is classified as sand. The lowest value of the sediment C/ N rates (6.08), mainly due to a higher concentration of organic nitrogen, was found at the northern part of Pereque Beach, where organic matter of marine source was more prominent. In this area, density (9.6 × 10 6 cells cm −3), biomass (1992.04 ngC cm −3) and activity of bacteria were higher than at the southern end. In contrast, cyanobacteria density varied from 2.0 to 4.0 × 10 5 cells cm −3, with biomass and total chlorophyll a of the sediment being higher at the southern part, where there are water input from Pereque River and higher organic matter of continental origin. The variability in the microbial population is discussed in the light of the sediment granulometry, organic matter quality, fresh water inflow and pollution.

Impacts of boat-generated waves on macroinfauna: Towards a mechanistic understanding

Increasing evidence suggests that the waves produced by even low-wash vessels can have a sizeable impact on infaunal assemblages in otherwise sheltered estuaries. Although this impact is widely regarded to be a consequence of wash coarsening sediment grain-size, it may be due to a number of alternative mechanisms which include enhanced turbidity, decreased larval supply, changed resource availability and/or erosion of animals from the sediment. To increase understanding of impacts resulting from boat-wash, this study tested alternate mechanisms by which intertidal infauna along the Parramatta River, Sydney, Australia are impacted by wash from local ferries. Specifically, to discriminate between impacts of wash resulting from changes to sediment characteristics and impacts directly resulting from the altered hydrodynamic regime, cores of intertidal sediment were transplanted between a no-wash and a wash zone and within each of these zones. It was predicted that if wash indirectly determines the structure of assemblages by altering characteristics of the sediment: (1) sediment granulometry and/or mineralogy would differ between cores sourced from the wash zone and cores sourced from the no-wash zone; and (2) over a 6-week period during which sediment characteristics of transplanted cores do not appreciably change, assemblages transplanted across zones would remain more similar to assemblages at their origin than their destination. To the contrary, sediment grain-size did not differ among sites according to their location in wash or no-wash zones and the destination of sediment cores was more important than their origin in determining the structure of macroinvertebrate assemblages. Following transplant into the no-wash zone, the abundances of capitellids and nereids in cores of sediment from the wash zone rapidly increased in abundance. In cores of sediment transplanted from the no-wash zone to the wash zone, the abundances of these taxa decreased. The results of this study therefore indicate that impacts of boat-wash to infauna are not always a consequence of changes to sediment grain-size distribution. Thus, in order to successfully manage ecological impacts of boat-wash, other potential mechanisms such as erosion of sediments and changing resource availability also need to be considered.

Changes in the shelf macrobenthic community over large temporal and spatial scales in the Bohai Sea, China

Over the past 20 years, the Bohai Sea has been subjected to a considerable human impact through over-fishing and pollution. Together with the influence of the Yellow River cut-off, the ecosystem experienced a dramatic change. In order to integrate available information to detect any change in macrobenthic community structure and diversity over space and time, data collected during the 1980s and the 1990s from 3 regions of the Bohai Sea (Laizhou Bay, 16 stations, 37–38°N, 119–120.5°E; central Bohai Sea, 25 stations, 38–39°N, 119–121°E; eastern Bohai Bay, 12 stations, 38–39°N, 118.5–119°E) were reanalyzed in a comparative way by means of a variety of statistical techniques. A considerable change in community structure between the 1980s and the 1990s and over the geographical regions at both the species and family level were revealed. After 10 years, there was a considerable increase in abundance of small polychaetes, bivalves and crustaceans but decreased number of echinoderms. Once abundant in Laizhou Bay in the 1980s, a large echinoderm Echinocardium cordatum and a small mussel Musculista senhousia almost disappeared from the surveying area in the 1990s. Coupled with the increased abundance was the increased species richness in general whereas evenness was getting lower in central Bohai Sea and Bohai Bay but increased in Laizhou Bay. K-dominance plot showed the same trend as evenness J′. After 10 years, the macrobenthic diversity in the Bohai Sea as a whole was slightly reduced and a diversity ranking of central Bohai Sea > Laizhou Bay > eastern Bohai Bay over space was also suggested. Sediment granulometry and organic content were the two major agents behind the observed changes.

Numerical simulation of net longshore sediment transport and granulometry of surficial sediments along Chandeleur Island, Louisiana, USA

The formation and spatial evolution of Chandeleur Island, Louisiana, has been investigated extensively during the past several decades. No significant evaluation of the longshore sediment system, which is instrumental in the island's evolution and morphodynamic maintenance, has been completed. This paper provides the first quantitative description of the longshore transport system that operates along this transgressive, overwash-dominated barrier island system. The net longshore sediment transport system was investigated via the wave refraction model, WAVENRG, which provided estimates of the transport volumes and drift directions alongshore. Surficial samples were collected from the foredune, midtide and step environments in an effort to characterize the sediments along the island and determine if textural or compositional trends have developed in response to a predicted longshore sediment transport system. Data obtained during this research indicate that the longshore transport system along Chandeleur Island is characterized by a bi-directional drift system, with drift directed both north and south from a nodal point located in the south-central portion of the barrier island. Analysis of the predicted transport volumes indicates that the degree of wave refraction, and therefore the breaker angle, is more instrumental in controlling the alongshore volume rate of sediment transport than the breaker wave heights. Additionally, a larger magnitude of sediment transport is predicted in the southern portion of the barrier, which is in a greater state of deterioration than the north and central portion of the island. This apparent contradiction indicates that factors such as a variable subsidence rate along the island are contributing to the alongshore geomorphology. No significant textural or compositional trends were identified alongshore. This absence of granulometric trends is attributed to the lack of variability of the sediments that comprise the barrier and the frequency of overwash events which occur on this low-profile island.

Assessment of soft-bottom polychaete assemblages in the Gulf of Lions (NW Mediterranean) based on a mesoscale survey

Ninety-two stations (10, 20, 30, 40 and 50 m depth) located on 21 inshore–offshore transects between the French–Spanish border and the mouth of the Rhône River were sampled during the 1998 Fall. Their polychaete fauna was analyzed to: (1) describe the distribution pattern of polychaete assemblages at the scale of the whole Gulf of Lions, (2) identify the relationships between these assemblages and the main environmental parameters, (3) establish a link between these assemblages and the benthic communities already described for Northwestern Mediterranean, and (4) assess recent historical changes in trends in densities and α diversity between assemblages. Three polychaete assemblages were identified using cluster analysis and associated procedures. These assemblages were tightly associated with depth and sediment granulometry as indicated by the concordance between their spatial distributions and the sedimentary map of the Gulf of Lions. Assemblage I contained most of the 10 and 20 m deep stations and was associated with littoral fine sands. Assemblage I was found all over the Gulf of Lions and was characterized by high abundance and high biomass due to the presence of large numbers of the serpulid Ditrupa arietina. Assemblage II was mostly composed of 30 m deep stations and was associated with littoral sandy mud. Assemblage II was dominated by the lumbrinerid Lumbrineris latreilli. It was composed of two sub-assemblages (IIa and IIb) in relation with sediment granulometry. Sub-assemblage IIb was associated with finer sediment than sub-assemblage IIa. Sub-assemblage IIa was only found in the Southern part of the Gulf of Lions, whereas sub-assemblage IIb was mostly found in the Northern part of the Gulf of Lions. This pattern is probably indicative of the sedimentation of fine particles originating from the Rhône River at intermediate depth as suggested by: (1) the sedimentary map of the Gulf of Lions, and (2) the outputs of recent models of sediment transport in the Gulf of Lions. Assemblage III was composed of 40 and 50 m deep stations; it was associated with terrigeneous coastal mud and found all over Gulf of Lions. Assemblage III was dominated by the sternaspid Sternaspis scutata. These three assemblages were tightly related with the benthic communities already described independently by Picard [Picard, J.,1965. Recherches qualitatives sur les biocénoses marines des substrats meubles dragables de la région marseillaise. Recueil des Travaux de la Station Marine d'Endoume 52, 1–160] and Guille [Guille, A., 1971b. Bionomie benthique du plateau continental de la côte catalane française. II – Les communautés de la macrofaune. Vie et Milieu 21, 149–280] based on samples collected along the Provençal and the French Catalan rocky coasts, respectively. Unfortunately, no quantitative data were available to compare the α-diversity of the three newly defined assemblages with historical data collected by these two authors. We therefore compared our diversity measurements with the data collected by Desbruyères et al. [Desbruyères, D., Guille, A., Ramos, J.M., 1972/73. Bionomie du plateau continental de la côte catalane espagnole. Vie et Milieu 23, 335–363] in the Bay of Rosas (Spanish Catalan coast). Due to differences in the sampling effort between the two studies, this comparison was restricted to trends between assemblages. Our results show maximal densities and minimal α-diversity in the Littoral Fine Sand assemblage, which was not the case in Desbruyères et al. [Desbruyères, D., Guille, A., Ramos, J. M., 1972/73. Bionomie du plateau continental de la côte catalane espagnole. Vie et Milieu 23, 335–363]. Major temporal changes obviously reflect the increases of Ditrupa arietina (in the Littoral Fine Sands assemblage), and to a lesser extent Lumbrineris latreilli (in the Littoral Sandy Mud assemblage).

Environmental effects of dredging on sediment nutrients, carbon and granulometry in a tropical estuary

This monitoring study encompassed a period prior to dredging, during dredging and post dredging between July 1999 to June 2000 in Ponggol estuary located along the northeastern coast of Singapore. Mean concentrations of sediment nutrients in mg x Kg(-1) (+/- standard error of means) prior to dredging, during dredging and post dredging were 9.75 +/- 4.24, 8.18 +/- 4.29 and 11.46 +/- 4.74 for ammonium, 0.08 +/- 0.05, 0.06 +/- 0.02 and 0.09 +/- 0.01 for nitrite, 0.04 +/- 0.04, 0.11 +/- 0.17 and 0.25 +/- 0.30 for nitrate, 4.83 +/- 3.48, 0.77 +/- 0.48 and 8.33 +/- 9.73 for phosphate respectively. Pre dredge, dredge and post dredge levels of total carbon (TC) were 18.5 +/- 3.7, 20.2 +/- 3.5 and 34.6 +/- 12.0, of total organic carbon (TOC) were 10.5 +/- 2.9, 19.5 +/- 3.6 and 34.6 +/- 12.0 and of total inorganic carbon (TIC) were 7.9 +/- 1.0, 0.7 +/- 0.4 and non detectable in the sediments, respectively. Both, sediment nutrients and carbon registered lower concentrations with onset of dredging, with the exception of nitrate and TOC. A shift in sedimentary carbon from inorganic carbon to organic carbon was also observed with the onset of the dredging activities when the organically enriched historically contaminated layer was exposed. Sediment granulometry showed that the sediments in the estuary were predominantly silt and clay prior to dredging, which changed to sand with onset of dredging. Silt load in the sediments was highest post-dredge. Sediment nutrients and sediment organic carbon were observed to associate with the finer fractions (silt and clay) of sediments. Finer fractions of sediments get resuspended during a dredging event and are dispersed spatially as the result of tides and water movements. Prior to this study, the potential for nutrient release and sediment granulometry due to dredging have been suggested, but there have been few studies of it, especially in the tropics. The baseline information gathered from this study could be used to work out effective management strategies to protect similar tropical ecosystems elsewhere, should there be no other alternative to dredging.

Ecosystem Responses to Extreme Natural Events: Impacts of Three Sequential Hurricanes in Fall 1999 on Sediment Quality and Condition of Benthic Fauna in the Neuse River Estuary, North Carolina

A study was conducted in November 1999 to assess sediment quality and condition of benthic fauna in the Neuse River Estuary (NRE), North Carolina, USA, following the passage of three Atlantic hurricanes during the two months prior. Samples for analysis of macroinfauna (>0.5 mm sieve size), chemical contamination of sediments, and other abiotic environmental variables (salinity, dissolved oxygen, pH, depth, sediment granulometry) were collected at 20 sites from the mouth of the Neuse River at Pamlico Sound to approximately 90 km upstream. Results were compared to those obtained from the same area in July 1998 using similar protocols. Depressed salinity, caused by extreme rainfall and associated high freshwater flow, persisted throughout much of the estuary, which had experienced periods of water-column stratification and hypoxia of underlying waters. Fifteen of the 20 sites, representing 299 km2 (76% of the survey area), also showed signs of benthic stress based on a multi-metric benthic index of biotic integrity (B-IBI). Benthic impacts included reductions in the abundance, diversity, and numbers of species and shifts in taxonomic composition, with a notable increase in dominance of the opportunistic polychaete Mediomastus ambiseta as other former dominant species declined. There was no significant increase in the extent of chemical contamination compared to pre-hurricane conditions. Storm-related reductions in dissolved oxygen and salinity were the more likely causes of the observed benthic impacts, though it was not possible, based on these results, to separate storm effects from seasonal changes in the benthos and annual episodes of summer anoxia and hypoxia.

A multivariate analytical approach to granulometry of kaolinitic sediments from Botswana

Investigations on granulometric characteristics of kaolinitic sediments from Makoro and Kanye in Botswana were carried out in order to apply multivariate analyses in understanding grain size distribution trends. Kaolin samples from both study sites consisted mainly of the silt (2-49 µm) fraction: (2 µm) x =18.11%; (2-49 µm) x =79.55% and (50 µm) x = 2.35% for the Makoro samples, and 23.33%, 75.03% and 1.30% respectively for the Kanye samples Clay fractions ranged from 5.0% to 42.7% with a median of 20.35% among the 30 Makoro samples, and from 4.8% to 33.3% with a median of 24.85% among the 26 Kanye samples. Particle densities for samples from both Makoro and Kanye ranged from 2.64 to 2.66gcm -3 . Makoro samples contained significantly higher proportions of clay and sand, and lower percentages of silt than Kanye samples. The Kanye samples were quite distinguishable from the Makoro samples using modal size and surface area of particles. The results depicted values for particle density, mean, mode, and surface area of clayey grains to be consistent with those obtained from other kaolin deposits and occurrences from around the world. Keywords : granulometric characteristics, kaolinitic sediments, multivariate analyses, grain size distribution Botswana Journal of Technology Vol 14(1) 2005: 36-42

Environmental variables explaining structural and functional diversity of seagrass macrofauna in an archipelago landscape

Thirteen seagrass beds located over a 80-km range in the brackish waters of SW, Finland, northern Baltic Sea were investigated in order to determine the environmental variables important for univariate community measures and for number, composition and redundancy of functional groups of benthic macrofauna. For species assemblages, fetch and shore angle were the best explanatory variables, followed by sediment granulometry (fine gravel) and then sediment organics. Similarly, fetch, shore angle and Zostera marina shoot density were the best explanatory variables for functional group patterns. Small (< 50 m 2) inner-archipelago beds were functionally and structurally equal to the most extensive (500 to > 1000 m 2) seagrass beds in the study area. Community measures (density, number of species and diversity) and functional diversity (number of functional groups) equalled or exceeded levels previously recorded in deeper, non-vegetated communities in the northern Baltic Sea. In comparison with marine seagrass assemblages, the total number of species and number of species per function were low. However, species density and derived diversity measures (Shannon–Wieners index H′) equalled or exceeded those reported for other seagrass ecosystems. It is concluded that in terms of seagrass infauna, the Baltic Sea should not be regarded species poor, as is often generally stated, and that conservation initiatives and management strategies should consider both minor as well as more extensive occurrences of seagrasses in coastal waters.

Macrofaunal modification of porewater advection: role of species function, species interaction, and kinetics

Sedimentary habitats are complex associations of biotic, chemical, and physical pro- cesses comprising 'ecosystem function'. The relative importance of these processes to biogeochemi- cal cycling in highly reactive, permeable sediments remains poorly understood. We report results from several field experiments in a muddy-sand intertidal flat dominated by 2 functionally different types of bioturbating macrofauna in False Bay, Washington, USA: (1) the arenicolid polychaete Abarenicola pacifica and (2) 2 species of thalassinid shrimp (Upogebia pugettensis and Neotrypaea californiensis). Experimental plots composed primarily of one of the study taxa or mixed communities of both were evaluated for their effects on porewater advection, solute concentrations, and sediment characteristics. Fluorescein-impregnated acrylamide gels were used to infer rates of transport, and acrylamide gel peepers were used to record porewater concentrations of diagenetically important constituents among experimental plots. Laboratory studies evaluated rates of diffusive transport in non-bioturbated sediments for comparative analysis. We found that (1) functionally different macro- fauna affect rates of porewater advection in permeable sediments, (2) organism effects are not attrib- utable to changes in average measures of sediment granulometry, (3) species interactions may further complicate the advective environment and the resulting diagenetic processes, and (4) species effects vary according to reaction rate kinetics. We hypothesize that species-related effects on trans- port are due to inhibition of arenicolid feeding by thalassinid tubes that serve to block sediment fluidization and advective flow. Thus, specific behaviors and interactions among organisms appear to affect transport rates and sediment function in advectively permeable habitats. The results indicate the importance of integrating behavior, kinetics, and transport into future studies of sedimentary biodiversity and ecosystem function.