Muddy Sites Research Articles

Estimating the catching efficiency of a 2-m beam trawl for sampling epifauna by removal experiments

Abstract The catching efficiency of a standard 2-m beam trawl for sampling epifauna was estimated by removal experiments at two sites in the southern North Sea. In order to allow repeated sampling of the same area, a rig of three beam trawls one behind the other was constructed, the three being tied together by steel ropes. Catching efficiency was estimated on the basis of the sum of the catches of all three trawls relative to the numbers caught in the first trawl. The catching efficiency of the beam trawl for epifauna ranged from 36% to 44% of total abundance between sites. Efficiency was least for partly buried species (Liocarcinus holsatus, 9% and 18%; Buglossidum luteum, 27%; Arnoglossus laterna, 35%), and slightly better for species living on the surface of the sediment (Asterias rubens, 42% and 46%; Pomatoschistus minutus, 58% and 46%; Pagurus bernhardus, 51%). The abundance of epifauna will be underestimated by a factor of 1.4–11 relative to the abundance in the three trawls. Also, sediment type seems to influence the catching efficiency of the beam trawl, efficiency being less at the sandy study site than at the muddy site for most species.

Effect of sediment type on microphytobenthos vertical distribution: Modelling the productive biomass and improving ground truth measurements

Intertidal flats are frequently colonised by microphytobenthos (MPB) assemblages that form transient biofilms at the sediment surface which are responsible for large fractions of estuarine primary production. The large spatio-temporal variability in MPB biomass distribution in concert with the fact that tidal flats can cover many km 2 makes the use of remote sensing particularly useful in assessing MPB distribution. Water content, sediment type and MPB vertical migration are variables that affect the relationship between ground truth measurements and remote sensing of benthic chlorophyll. The effect of chlorophyll depth distribution (top 2 mm) on the relationship between benthic chlorophyll and several remote sensing indices (NDVI, PI, R562/R647, derivative indices and PAM fluorescence) was investigated over a 2 year sampling period at 6 sites (Tagus estuary, Portugal). Additionally, the effect of the dark adaptation time required to measure the minimum fluorescence parameter ( F 0) was also tested. Sediment type strongly affected MPB depth distribution with muddy sites showing a strong negative exponential decay in chlorophyll with distance from the surface while sandy sites had a homogenous distribution over the same scale (2 mm). Chlorophyll content (mass per unit mass, μg g − 1 ) in the top 2 mm was better correlated with remote sensing indices than concentration (mass per unit volume, mg m − 3 ), both for NDVI (0.72 vs. 0.45) and for PAM fluorescence (0.70 vs. 0.55). Separating the data by transect increased the correlation values in all situations. A fitted model of chlorophyll depth distribution showed that the effect of asymmetrical chlorophyll depth distribution was stronger on the correlations between chlorophyll concentration and NDVI than on chlorophyll content and NDVI (0.46–2 mm vs. 0.74–125 μm, muddy site) the same was valid for fluorescence (0.66–2 mm vs. 0.92–125 μm, muddy site). Dark adapting the samples for more than 5 min did not result in any significant difference in the relationship between F 0 and chlorophyll a. The residuals from the regression of chlorophyll content on NDVI were positively correlated (0.7) with the mass per unit of mass of sediment < 63 μm and negatively (− 0.6) with chlorophyll concentration, this indicates that if no correction is performed to account for chlorophyll depth distribution both units will be strongly affected by the mass of < 63 μm particles. The results demonstrate that although expressing chlorophyll a as concentration is generally a better option for ground truth measurements care should be taken to account for chlorophyll depth distribution since strong asymmetries within the sampling depth can introduce large errors.

Detecting Anthropogenic Stress in an Ecosystem: 3. Mesoscale Variability and Biotic Indices

Previous work in the Ria Formosa Lagoon has revealed that it is possible to use macrofauna community structure as an indicator of contamination effects. Communities at sites adjacent to sewage outfalls indicated contamination but no pollution gradients were observed. In this study, macrofauna within a sub-section of the earlier work were investigated to determine the extent of mesoscale (10–100 m) variability and new indices of biological community structure. The results were evaluated using univariate (Marine Biotic Index; BI) and multivariate statistical methods (PCA). The computer program Barrier (usually used with genetic data) was also applied to detect “barriers” between spatially referenced samples. PCA identified the most significant factor effecting the distribution of the organisms in the study area to be the mud content of the sediment (% < 63 μm). Communities at sandy sites consisted of suspension feeders and detrital browsers, whilst communities at muddy sites were composed mainly of deposit feeders. The second principal component produced a vector characterized by a contamination gradient. The effects of the sewage discharge appeared to be concentrated at the outfall as it had the highest TOC (10.4%) content and was the only site to be characterized by the BI as slightly polluted. With the use of the BI and Barrier two gradients were observed in the transition of sites from an unpolluted to a polluted state. These gradients appeared not to follow the flow of the channel as expected (high % TOC of sites along the channel), and instead extended west and south from the sewage source. In conclusion, weak gradients were observeable; the multiple approaches of BI, PCA and Barrier analyses in concert were able to confirm the sewage dispersion route through the area.

On-site measurements of hydrogen sulfide and sulfur dioxide emissions from tidal flat sediments of Ariake Sea, Japan

Emission variability of hydrogen sulfide and sulfur dioxide from tidal flat sediments to atmosphere in Ariake Sea, Japan was studied using a diffusion scrubber-based portable instrument. Ariake Sea is a typical closed sea consisting of a huge marsh area along the coast. Seasonal, spatial and diurnal variability in emission rates were examined. In addition, depth profiles of the gas emissions were examined with the profiles of anions, heavy metals, water and organic contents. Unexpectedly, SO 2 emission was much higher than H 2S in all measurements, while an opposite emission trend was observed in diurnal and spatial patterns of H 2S and SO 2 emissions. The mechanisms of these gas emissions are discussed. Total sulfur fluxes to the atmosphere as H 2S and SO 2 during the study averaged 7.1 μg S m −2 h −1 for muddy sites and 28 μg S m −2 h −1 for sandy sites. Sulfur fluxes from tidal flats were comparable to the artificial sulfur emission from the neighboring towns.

The Role of a Salt Marsh Plant on Trace Metal Bioavailability in Sediments. Estimation By Different Chemical Approaches * (7 pp)

The presence or absence of vegetation can condition sediment characteristics. The main aim of this work was to investigate the influence of the sea rush Juncus maritimus on metal (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) availability to organisms living on or in estuarine sediments, from Douro River (NW Portugal), by comparing the characteristics and chemical behaviour of rhizosediments (collected within the plant assemblage) and those of sediment (collected around the plant). In order to evaluate whether and how sediment characteristics condition the role of plants on metal availability, sandy and muddy sediments colonised by J. maritimus were studied in parallel. Metal availability was estimated by enzymatic digestion with pepsin (ED), which may provide an estimate of metal availability to organisms living at estuarine sediments. Nevertheless, since no consensus exists yet on the most suitable methodologies to estimate metal bioavailability in sediments, two more conventional approaches, BCR sequential extraction (SE) and AVS/ SEM model, were also used, in parallel, and the information these approaches provided was compared with that provided by ED. Total-recoverable metal contents were determined by atomic absorption spectrophotometry after sediment digestion using a high-pressure microwave system. Plants could concentrate metals around its roots and rhizomes. In addition, they were capable of oxidizing (release of oxygen by the roots) the anaerobic medium surrounding their roots in muddy sediment (reducing AVS). As sulphide oxidation renders metals (Cd, Cu, Ni, Pb and Zn) into more soluble forms, according to the AVS/SEM model, metals from muddy sites would be more available in rhizosediment than in sediment. The SE approach led to a similar conclusion. Nevertheless, the results provided by ED pointed at opposite conclusion, particularly for Cd and Zn, indicating less availability at rhizosediments than in the surrounding sediment. ED results were interpreted as a consequence of an enrichment of the rhizosediment in organic ligands exuded by the roots or liberated by dead plants. The effect of complexation of metals by organic compounds, which ED could not decompose/dissolve, seemed to overcome that caused by sediment oxidation. In general, a comparison of the informa tion about metal availability by ED, SE, AVS/SEM, showed that it did not always match and in few cases it was even contradictory. Therefore, a thorough evaluation of the metal availability in sediments requires a combination of different chemical approaches, so as to take into consideration differences in ways of organism exposure (interstitial water and/or ingestion of sediment particles).

Microphytic standing stocks at Kerguelen Islands (Subantarctic, Indian Ocean), annual variations in relation to environmental factors: II, intertidal and subtidal microphytobenthos

The microphytobenthic standing crop in marine coastal sediments from Kerguelen main island (from surface to 8 cm deep) varied in relation to the tidal position, grain size, and shelter of sediments. A time-series analysis was performed at three distinct sites: a sheltered fjord (PRAY), a moderately exposed embayment (PAF) and a deep fjord with higher open-ocean influence (PN). Temporal variations on all studied parameters were observed on intertidal, as well as on subtidal sediments, but with a great range in variations and pattern. Compared to phytoplankton blooms at the same location and stations, the microphytobenthos productive periods were not so marked, especially on intertidal sediments. Nevertheless, high productivity periods were in most cases linked to austral spring and summer (up to 30–50 μg Chl a g−1 dw, in intertidal sheltered sands; >170 μg Chl a g−1 dw, in sheltered subtidal muds). On subtidal muddy sites, the high phaeopigments concentrations (up to 195 μg Phaeo a g−1 dw at PRAY site) were attributed to kelp and epiphyte degradation and sedimentation, and also to macrofauna trophic influence, whereas at Portes-Noires fjord (PN) a Phaeophytine a late spring increase in 1991 was suspected to be of plankton origin. The PN site exhibited a lower productivity than at the other two sites, regardless of the tidal position of the sediment, due to a delayed annual cycle in surficial temperature and standing crop. Compared to the phytoplankton blooms, the microphytobenthos productivity at Kerguelen main island was less restricted in time and may occur throughout the year, not just to the benefit of the benthos food-chain, but also to the pelagic one via resuspension, as well as to filter feeders (i.e. mussels).

The use of HPLC pigment analysis to study microphytobenthos communities

The use of HPLC pigment analysis has become a widespread tool for investigating the taxonomic composition of phytoplankton. However, for micropytobenthos, due to the complexity of these communities and the accumulation of vegetal senescent material, the application of this technique presents further problems. Samples of unialgal cultures from representative taxa of microphytobenthos and samples of surface sediment from intertidal mudflats were collected, extracted in methanol and analyzed for photosynthetic pigments in a HPLC system, with a fluorimeter and a photodiode array detector. A total of 11 chlorophylline pigments and 19 carotenoids were identified. Absorbance chromatograms from cultures and natural populations are displayed. Pigment ratios from five species belonging to different taxonomic classes are compared. Pigment composition and diversity of intertidal muddy sites are discussed; the increase of degraded compounds of chlorophyll a and carotenoids along a vertical profile of 5 cm is shown on different sediment types.

Fish assemblages in the Hauraki Gulf, New Zealand

Species assemblages in the Hauraki Gulf, New Zealand, and the factors affecting them, were determined from abundance data collected from 1381 trawl tows by two research vessels during 1964–97. Correspondence Analysis and Ward's Cluster Analysis were used to identify assemblages, and Canonical Correspondence Analysis (CCA) to explore the relationships between the assemblages and depth, sediment type, latitude, longitude, and year. Twenty‐six species that occurred in more than 5% of the stations were assigned to four assemblages, and a further four species showed no affinities with other species. For four commercial species, there was no evidence of ontogenetic variation in assemblage membership. The CCA models explained only 11–12% of the variance in the data. Year was the most important explanatory variable, suggesting that between‐year variation in species abundance affected assemblage composition. Sediment type was also important for many species. Assemblage A species were most abundant at shallow (<50 m) mud sites, Assemblage B species at sand and deep (>50 m) mud sites, and Assemblage D species at deep mud sites. Assemblage C species had varied habitat preferences. The commercially important Pagrus auratus and Nemadactylus macropterus preferred mud, and Chelidonichthys kumu preferred sand. Zeus faber was most abundant at muddy sites, but also occurred (sometimes abundantly) at sandy sites.

Radionuclide distribution in tidal flat sediments of the Westerschelde estuary

Depth profiles of the particle-reactive radionuclides 210Pb, 137Cs and 7Be were established in sediments of a tidal flat in the Westerschelde. Cores were collected at different seasons at selected sites with contrasting characteristics. Accumulation rates estimated from 137Cs profiles are consistent between the five sites with values comprised between 1.1 at the marginal sites and 2.5 cm y-1 for the central part. Although the very low level of excess 210Pb limits the precision of this method, profiles of 210Pbxs with depth allow calculating a range of sedimentation rates equivalent to those derived from 137Cs data. At the muddy site, the vertical distribution of radionuclides revealed the existence of an important accumulation layer of fine particles at a depth of around 10 cm at the muddy site, may be in relation with biological and physical processes. The vertical distribution of 7Be in the top 6 cm exhibits wide variability on seasonal scale, with the highest inventories and bioturbation rates observed in summer. This trend is clearly related to the seasonal variation of silt content of surface sediments.

Sediment transport on the Palos Verdes shelf over seasonal to decadal time scales

We combine direct observations, longer-term wave data, and model calculations to characterize resuspension and transport of fine-grained, effluent-affected sediment on the Palos Verdes shelf. Near-bed waves, currents, and suspended sediment concentrations were monitored during the winter of 1992–93 with a bottom tripod and current-meter mooring at a 63-m-deep site. Wave conditions that winter were moderate (∼2 year recurrence interval), and mean current was alongshelf to the northwest; currents were not significantly correlated with wave conditions. Seven wave events during the winter (December–March) produced near-bed wave orbital velocities at the study site in excess of 14 cm s −1, the observed threshold for significant resuspension. Three of these events occurred during the bottom tripod deployment and are characterized by the highest persistent suspended sediment concentrations in the tripod record. Suspended sediment flux was alongshelf to the northwest for 5 of the 6 wave events for which current data were available; one event occurred during low southeast currents. Measured suspended sediment concentration and grain size generally agree with values that were calculated using a shelf sediment transport model with no adjustment of parameters from values determined for two muddy sites on the northern California shelf. We extend our seasonal observations to a period of almost 2 decades by applying the observed thresholds for wave-driven resuspension to near-bed wave conditions calculated from NDBC Buoy 46025 surface wave data. An average of 10 resuspension events per year, with an average duration of 1.6 days, were identified at a water depth of 60 m; the number of events dropped to 3 per year at 90 m, beyond the shelf break. For the majority of these events, calculated net suspended sediment flux is toward the northwest (alongshelf) at an average rate of 140 kg m −1 h −1; about a third of the events have net southeastward flux at an average rate of 30 kg m −1 h −1. The calculated thickness of the resuspended surface layer of the bed was less than 1 cm for all events at 60 m.

Physico-biogeochemical controls on benthic-pelagic coupling of nutrient fluxes and recycling in a coastal upwelling system

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 235:15-28 (2002) - doi:10.3354/meps235015 Physico-biogeochemical controls on benthic pelagic coupling of nutrient fluxes and recycling in a coastal upwelling system Andrew W. Dale1,2,*, Ricardo Prego1 1Marine Biogeochemistry Research Group, Institute of Marine Research (CSIC), 6 Eduardo Cabello, 36208 Vigo, Spain 2Department of Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, United Kingdom *Present address: Dept of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, United Kingdom. E-mail: a.w.dale@plymouth.ac.uk ABSTRACT: Sediment incubation microcosms, multitrap apparatus and water column variables have been employed to describe the dynamic changes in benthic-pelagic coupling between nutrient pools in the Pontevedra ria, NW Spain, during spring and summer 1998. A comparison of the chemical characteristics of suspended and bed sediment together with sediment nutrient effluxes revealed that particulate organic nitrogen and carbon were progressively depleted upon transit through the nutrient pools. The main fate of particulate organic nitrogen reaching the bed sediment is denitrification, although resuspension is also important. An estimate of net denitrification (Dnet) was made at 2 muddy sites in the ria with a mass balance at the benthic boundary layer. First-order approximations calculate Dnet to be 178 and 182 µmolN m-2 h-1 at the 2 stations, and agree well with previous modelling estimates. Denitrification is highest when upwelling relaxes and the flux of organic matter to the sediment increases. Regular inputs of offshore water ensure water renewal and re-oxygenation of bottom waters, thus preventing anoxia, particularly in the summer. With upwelling, large quantities of ammonium are effluxed to the water column (250 µmolNH4+ m-2 h-1), probably as a result of the bed sediment resuspension engendered by upwelling and stirring of phytodetrital fluff held in suspension as neutrally buoyant material above the sediment surface. We hypothesise that hydrodynamical processes play an important role in determining the quantity of nutrients remineralised in the Pontevedra ria and, in the case of nitrogen, the rate of denitrification at the benthic boundary layer. KEY WORDS: Upwelling · Denitrification · Nitrogen · Sediment · Hydrodynamics · Benthic-pelagic coupling · Pontevedra ria · Galicia Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 235. Online publication date: June 19, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Temporal and spatial patterns of intertidal sediment-water nutrient and oxygen fluxes in the Douro River estuary, Portugal

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 233:55-71 (2002) - doi:10.3354/meps233055 Temporal and spatial patterns of intertidal sediment-water nutrient and oxygen fluxes in the Douro River estuary, Portugal Catarina M. Magalhães1,*, Adriano A. Bordalo1, William J. Wiebe2 1Laboratory of Hydrobiology, Institute of Biomedical Sciences, University of Porto, 4099-003 Porto, Portugal 2Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA *Present address: Largo Abel Salazar 2, 4099-003 Porto, Portugal. E-mail: c_magalhaes@yahoo.com ABSTRACT: The Douro River watershed drains 17% of the Iberian Peninsula; the present average freshwater discharge into the estuary is 488 m3 s-1. The Douro River estuary receives largely untreated sewage from about 1 million inhabitants of Greater Porto. A major portion of the intertidal sediments is currently scheduled for removal by dredging. In order to assess their role in nutrient dynamics and productivity, sediment-water fluxes of ammonium, inorganic phosphorus, nitrate, nitrite, silicate and oxygen were determined quarterly in muddy and sandy intertidal sediments of the lower estuary. Fluxes were estimated with both light and dark incubations. The range of net nutrient uptake rates from all sites was as follows: ammonium 28 to 978 µmol m-2 h-1; nitrate 40 to 810 µmol m-2 h-1; nitrite 1 to 128 µmol m-2 h-1; dissolved orthophosphate 1 to 152 µmol m-2 h-1 and silicate 69 to 369 µmol m-2 h-1. Stepwise multiple regression analysis revealed strong coupling between the concentrations of most nutrients in the water column and the corresponding flux into the sediment. Mean net nutrient effluxes to the overlying water were observed in only 6 cases out of a total of 120, and only at the muddy sediment site. Net oxygen production (light incubation) ranged from 58 to 618 mgO2 m-2 h-1, while oxygen consumption (dark incubation) ranged from 11 to 64 mgO2 m-2 h-1. Primary production correlated significantly with chlorophyll a concentrations in the top (0.5 cm) sediment layer at the sandy sites, but showed no correlation at the muddy site. All sampling sites were net daily sources of oxygen to the water. The net productivity of the intertidal flats and their capacity for water-column inorganic nutrient removal assumes particular importance in the Douro River estuary, a eutrophic system with a permanently heterotrophic water column. KEY WORDS: Douro River estuary · Intertidal flat · Sediment-water interface · Nutrient uptake · Oxygen fluxes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 233. Online publication date: May 21, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

The Effect of Muddy Bottom Sediment on the Abundance and Life Span of the Barnacle, Hesperibalanus hesperius, Epizoic on Scallop Shells

Barnacles are an important fouling component of molluscs of commercial value, and they may adversely affect their hosts. The abundance of living and dead individuals, survival of newly-settled young and the maximum life span of the barnacle, Hesperibalanus hesperius, encrusting the shells of the scallop, Patinopecten (Mizuhopecten) yessoensis, were compared for two sites differing in bottom sediment composition. It was found that muddy sediment (mud content > 25.5%) had an adverse effect on the survival of this barnacle. The valves of scallops collected from a sandy substratum had significantly more barnacles than scallops sampled from muddy sand. At the sandy site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was 378.4 ± 9.6 individuals per valve, and on the lower valves, 92.3 ± 1.8 individuals per valve; of these, 38.5-48.3% were alive. At the muddy sand site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was only 154.5 ± 4.5 individuals per valve, and on the lower valves, 31.7 ± 1.4 individuals per valve. However, living barnacles made up only 20.1-22.6% of the total number counted. On muddy sand, the number of barnacles per cm2 of scallop valve increased with scallop age up to 3-5 years, after which it declined. However, on sand, the number of barnacles per cm2 of scallop valve did not change with increase in host age from 5 to 8 years. The young barnacles settled in significantly greater numbers on the shells of scallops living on sand than on muddy sand. The mortality of barnacles during the first month after settlement on the upper scallop valves was about 45 and 80% on sand and muddy sand respectively. Old, large barnacles (with a shell base >10 mm), both living and dead, occurred more often on scallops living on the sandy substratum. The largest barnacles had a base length of 15-20 and 13-15 mm, at the sandy and muddy sand sites respectively. The possible reasons for these differences are discussed.

The role of colonization in establishing patterns of community composition and diversity in shallow-water sedimentary communities

To determine whether pattern and diversity in benthic sedimentary communities are set primarily at colonization or by post-settlement biological interactions, we collected faunal cores and conducted reciprocal sediment transplant experiments at a sandy and a muddy site at 12 m depth, ∼3 km apart off New Jersey. Multivariate analyses of cores collected at these sites in September 1994 indicated differences in the taxa determining local pattern, with the bivalve Spisula solidissima and the polychaete Polygordius sp. being dominant at the sandy site, and oligochaetes, several polychaete species, and the bivalve Nucula annulata dominant at the muddy site. Individual cores from the sandy site were significantly less diverse than those at the muddy site. Short-term experiments (3-5 d) were deployed by divers at three different times (August-September, 1994). Replicate trays (100 cm 2 ) filled with azoic sand or mud were placed flush with the ambient seafloor at both sites. Multivariate comparisons indicated that sediment treatment in trays played a greater role in determining colonization patterns in the first experiment, site played a greater role in the second, and both variables contributed in the third. This pattern suggests that larval settlement and habitat choice played an important role in the first and third experiments, and that local transport of recently settled juveniles from the surrounding sediments was important in the second and third experiments. Sandy-site trays had significantly lower diversity than muddy-site trays, but there was no effect of sediment type in trays on diversity of colonizers. These experiments focused on small spatial scales and three short time periods, but they demonstrate that species patterns in some environments may be set by habitat selection by larvae and by juvenile colonization from the surrounding community. Post-colonization processes such as predation and competition likely play a major role for some species, but patterns of initial colonization corresponded well with those in the local community.

Variable effect of a large suspension-feeding bivalve on infauna: experimenting in a complex system

In soft-sediment habitats there are many examples of species that modify their habitat and thus can be expected to have an important influence on macrobenthic community structure. The large, suspension-feeding pinnid bivalve Atrina zelandica adds complexity to soft-sediment habitats by pro- truding into the water column and altering boundary-flow conditions and by providing predation refuges and substrates for epifaunal settlement. To investigate effects of A. zelandica density on macrobenthic community composition, we conducted a density manipulation experiment in 4 different habitat types in and around Mahurangi Estuary, New Zealand. Our experiment incorporated a comparatively large spatial and temporal scale: each habitat was separated by at least 1.75 km, and was sampled 3 times over 16 mo. Based on previous work, we predicted that macrofaunal community responses would differ be- tween sites and would be stronger at sandy sites than muddy sites, and that variability in site hydro- dynamic and sediment characteristics would help explain differences in benthic community responses to the density manipulation. While these predictions were supported, there was considerable temporal variation in the response. We also made predictions of the response of different aggregate macrofaunal groups to the A. zelandica manipulation (i.e. total numbers of individuals and taxa, suspension feeders, deposit feeders, top 2 cm dwellers, and mobile, short and long-lived species). Whether these predictions were supported varied spatially as well as temporally. To be able to generalise results, larger scale experiments, conducted at more than 1 site and at more than 1 time, are generally considered preferable. Although our A. zelandica manipulation experiment has these attributes, the results have demonstrated that the influence of this large suspension feeder on the associated macrofaunal community is not sim- ple (except perhaps in sandy, relatively non-tidal environments), and illustrates our limited success in 'reducing' the complexity of this system using a field experiment. However, we were able to demonstrate that interactions between A. zelandica, site hydrodynamic conditions and sediment characteristics were all important in influencing macrofauna, rather than there being a simple A. zelandica density-macro- fauna relationship. Thus, where multi-species interactions, indirect effects, non-linear biotic/abiotic interactions and threshold effects play an important role, specific experiments may not always lead to generalisable results, simply because the system is too complex.

Fine-scale acoustic tomographic imaging of shallow water sediments

In acoustic tomographic system capable of performing in situ two-dimensional (2D) acoustic imaging of shallow water sediments is described. This system is capable of resolving inhomogeneities greater than 10 cm and differentiating sound-speed variations greater than 2%, A tomographic inversion is performed in a 2D vertical slice of about 1 m/sup 2/ (1 m/spl times/1 m) using three identical probes, with each consisting of 70 evenly distributed transducers. In normal deployments, two of the probes are oriented vertically and are separated by about 1 meter, and the third is positioned horizontally right above the two vertical probes. The additional horizontal probe greatly improves the horizontal resolution of the system compared to conventional crosshole tomographic setups. Numerical simulations are performed to evaluate the influences of arrival time detection error and transducer position error on the performance of the tomography system. For an arrival time of 500 ns (standard deviation) and a position error of 4 mm (standard deviation), sound-speed anomalies of greater than 0.8% can be correctly predicted near the upper portion (close to the horizontal probe) and are resolvable near the lower portion. A controlled laboratory experiment was conducted to evaluate the performance of the system. The location of a polyurethane block (Conap EN22) used as a known target is correctly predicted while the inverted sound speed is about 9% lower than that from its actual value. Field data taken from a saturated muddy site are presented and analyzed. The inverted mean sound speed and attenuation are about 1480 ms/sup -1/ and 20 dBm/sup -1/, respectively.

Effects of habitat complexity and relative larval supply on the establishment of early benthic phase red king crab (Paralithodes camtschaticus Tilesius, 1815) populations in Auke Bay, Alaska

Between September 1996 and May 1998, the influence of habitat complexity and larval supply on the establishment of early post-settlement populations of red king crab (Paralithodes camtschaticus) was studied in situ in Auke Bay, southeast Alaska. Dive transects and suction dredge surveys conducted during fall 1996 and spring 1998 indicated that late age 0 to 1+ red king crabs were located only in the most complex habitat. This pattern was similar to patterns observed for early age 0 crabs, using settlement pails, during the summer of 1997. Early instars recruited into settlement pails containing ambient sediment at both the rocky cobble and shell-hash sites, but no settlement could be detected in muddy habitat. Population density of benthic age 0+ crab peaked in mid-July, then dropped throughout the summer, and greater densities were always observed in rocky cobble than in shell-hash. Simultaneous use of passive larval collectors ruled out the possibility that these patterns were simply a reflection of larval supply. Rather, the highest levels of larval supply were associated with the muddy site at which no settlement could be detected. The availability of complex habitat, defined simply as substrate rich in available crevice space that is scaled to the body size of the crab instars, appeared to be the primary determinant of the value of nursery habitat, and it is likely to be the critical factor determining early post-settlement survivorship within the population. Such considerations are vital to management of red king crab fisheries where complex nursery habitat is likely to be relatively rare and where conflicts with trawl fisheries and other anthropogenic disturbances to bottom habitat are a potential concern.

Seasonal benthic organic matter mineralisation measured by oxygen uptake and denitrification along a transect of the inner and outer River Thames estuary, UK

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 197:103-119 (2000) - doi:10.3354/meps197103 Seasonal benthic organic matter mineralisation measured by oxygen uptake and denitrification along a transect of the inner and outer River Thames estuary, UK M. Trimmer1, D. B. Nedwell1,*, D. B. Sivyer2, S. J. Malcolm2 1Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom 2Centre for Environment Fisheries and Aquaculture Sciences, Lowestoft NR33 OHT, United Kingdom *Corresponding author. E-mail: nedwd@essex.ac.uk ABSTRACT: Seasonal measurements of organic matter mineralisation by oxygen uptake and denitrification were carried out from July 1996 to March 1998 along a ~200 km transect of the River Thames estuary, UK. There was a distinct gradient of decreasing rates of organic matter mineralisation seaward, which was related to the concentration of suspended solids and sedimentary organic carbon (C) at each site. There was clear seasonality and highest rates of oxygen uptake (10056 µmol O2 m-2 h-1) at the muddy sites, but lower rates and non-temperature-dependent oxygen uptake at the sandier sites. Denitrification, both that driven by nitrate from the overlying water (Dw) and that coupled to nitrification in the sediment (Dn), followed a similar trend to oxygen uptake, from negligible rates of approximately 1 µmol N m-2 h-1 for both Dw and Dn at the furthest offshore site, Site 12, to 11407 and 8209 µmol N m-2 h-1, respectively, at the inner muddy Site 1. The Thames estuary is heterotrophic and a very efficient organic C filter, trapping and remineralising 77% of its organic C input. Attenuation of fluvial nitrate loads was regulated by freshwater flow. Minimal attenuation (3%) occurred during peak flows (i.e. during periods of shortest freshwater flushing time) and >100% attenuation during periods of lowest freshwater flow (longest flushing times). Including the sewage treatment works (STWs) nitrate load in this calculation reduced the degree of attenuation of the nitrate load to, on average, 11%. Annual rates of Dw and Dn for an inner area of 125 km2 were 112 and 85 Mmol N yr-1, respectively, with a total rate of 196 Mmol N yr-1 (2744 t), which was equivalent to 9% of the total dissolved inorganic nitrogen (DIN) load for 1995-96. A mean denitrification rate (Dw) of 0.64 mol N m-2 yr-1, based on measurements in 4 east coast estuaries, was used to estimate a total rate of denitrification for the entire area of UK east coast estuaries. The total rate of 0.81 Gmol N yr-1 represented 16% attenuation of the total fluvial discharge of nitrate (~6 Gmol N yr-1) to the UK¹s east coast estuaries (1995-96) and hence a 16% reduction in the UK nitrate load to the North Sea. KEY WORDS: Organic mineralisation · Oxygen uptake · Denitrification · Nutrient attenuation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 197. Publication date: May 12, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Physical determinants of the distribution and abundance of the burrowing ocypodid crab Paratylodiplax blephariskios Stebbing in the St. Lucia and Mhlathuze estuaries, KwaZulu-Natal, South Africa

The burrowing ocypodid crab, Paratylodiplax blephariskios, is endemic to the southeast coast of southern Africa where it forms an important component of the estuarine benthic fauna in muddy substrata. Two estuaries in KwaZulu-Natal, namely St. Lucia and the Mhlathuze, were chosen to investigate the influence of temperature, salinity, distance from the mouth and nature of the substratum (median phi value, sorting coefficient and organic content) on the distribution and abundance of the crab. Multiple and stepwise regression showed that particle size, sediment sorting coefficient and distance from the mouth were significantly related to crab abundance in both estuaries, while both temperature, salinity and the organic content of the substratum had little effect. At St. Lucia, distance from the mouth, rather than the nature of the substratum appeared to be the main determinant influencing crab distribution and abundance. However, at the Mhlathuze P. blephariskios was only recorded from muddy sites located further from the mouth. It was concluded that a combination of median particle size and sorting coefficient best explained crab distributions within the two estuaries. The absence of this crab species from muddy substrata in the lake compartments of the St. Lucia system suggests that its dispersion is tidal here, which leads to higher densities in the lower Narrows region near the mouth.

Temporal and spatial variation of sulfur-gas-transfer between coastal marine sediments and the atmosphere

The spatial and temporal variability of sulfur gas fluxes (H 2S, COS, CH 3SH, DMS, and CS 2) at the sediment–air interface were studied in the intertidal Wadden Sea area of Sylt-Rømø (Germany/Denmark) during eight measuring campaigns between June 1991 and September 1994. Measurements were performed mainly at four sites in a sheltered intertidal bay of approximately 6 km 2 (Königshafen) and discontinuously in a wider range of the 400 km 2 Sylt-Rømø tidal flat area. In situ fluxes of the S-gases were determined by a dynamic chamber technique focusing on dry sediment periods. Additional experiments were conducted in order to determine changes in S-gas concentrations in the sediment between the surface and 70 cm depth. In most cases H 2S was the dominant S-gas emitted from the sediment to the atmosphere, contributing up to 70% of the total S-emission at this interface. Mean H 2S emission rates ranged between 0.07 and 9.95 μg S m -2 h -1. Both emission rates and relative contribution of H 2S were lowest from fine sand and highest from muddy sites. Diurnal variation of H 2S emission was evident in summer and fall with up to 10-fold higher rates during night than during the day. Distinct seasonal variation of H 2S-transfer between the sediment and the atmosphere was observed with higher emission rates in the summer than in spring or fall. The emission of H 2S to the atmosphere was smaller by a factor of 1600–26 000 than the H 2S produced from sulfate reduction. Apparently, the efficiency by which H 2S produced in the sediment is retained and reoxidized by biogeochemical sediment processes is extremely high. Carbonyl sulfide (COS) was emitted with relatively constant rates in space and time with mean flux rates ranging between 0.24 and 2.0 μg S m -2 h -1. Carbon disulfide emission rates were comparable to those of COS and varied between 0.3 and 2.23 μg S m -2 h -1. DMS played a minor role in the S-gas transfer from uncovered sediment areas contributing between 3.1 and 23% to total S-emission from the sediment to the atmosphere.