Muddy Sites Research Articles

In situ measurements of sediment sound speed in the frequency band of 2–10 kHz at target and reverberation experiment site

As part of the environmental measurements for TREX13 (Target and Reverberation EXperiment 2013), in situ measurements of surfacial sediment sound speed were carried out off Panama City, Florida, using a system called Sediment Acoustic-speed Measurement System (SAMS). SAMS consists of ten fixed sources positioned just above the seafloor, and one receiver which is driven into the seabed to a known depth. During TREX13, 10 deployments were made along the main reverberation track which is about 7.5 km in length. All measurements were made at a penetration depth of 3m between 2 to 50 kHz, focusing on 2–10 kHz. Preliminary sediment sound speed results show variation from low sound speeds (muddy sites) to high sound speeds (sandy sites). A 3–5% of dispersion was observed at coarse sandy sites between 2 and 10 kHz, whereas little dispersion was observed at muddy sites. [Work supported by ONR.]

Non-trophic Interactions Control Benthic Producers on Intertidal Flats

The importance of positive effects of ecosystem engineers on associated communities is predicted to increase with environmental stress. However, incorporating such non-trophic interactions into ecological theory is not trivial because facilitation of associated species is conditional on both the type of engineer and the type of abiotic stress. We tested the influence of two allogenic ecosystem engineers (lugworms, Arenicola marina L. and cockles, Cerastoderma edule L.) on the main primary producers (microphytobenthos) of the tidal flats, under different abiotic stresses controlled by reefs of blue mussels (Mytilus edulis L.). We added 25,000 cockles or 2,000 lugworms to 5 × 5 m plots, both in a muddy site with high sedimentation rates located coastward of a mussel bed, and in a sandy site without mussels and characterized by high hydrodynamic stress. After a year, cockles increased algal biomass in the sandy area, but not in the mussel bed site, where high values were measured in all plots. However, lugworms did not affect algal biomass in any of the sites. Field measurements suggest that cockles outweighed negative effects of water currents in the site without mussels by locally increasing sediment stability, whereas mussels overruled the effects of cockles in the wake of the reefs through hydrodynamic stress alleviation and/or biodeposition. Our results suggest that non-trophic interactions by ecosystem engineering bivalves control primary production of intertidal areas, and that the sediment-stabilizing effect of cockles plays a crucial role where the overruling effects of mussel beds are not present.

Twelve years of vegetation change in an artificial marsh after the transfer of plants and hydrological restoration

For 12 years starting from 1991, we performed vegetation surveys every 2–3 years at permanent plots located in an artificial marsh which was constructed in former rice paddies through sod transplantation from a natural marsh. Management of the artificial marsh was conducted to maintain the condition of the donor vegetation by removing unnecessary plants and ensuring a water supply of constant quality and quantity. However, the structure and floristic composition of the donor vegetation were destroyed during sod transplantation, and eutrophic water was supplied before the construction of a well in year 5. The transition of communities identified in the artificial marsh was monitored periodically at 34 fixed plots established three years after transplantation, with a further two plots added in year 7. Seasonal changes in surface water chemistry were also monitored. While the main communities of the artificial marsh resembled that of the donor marsh, two other communities were also identified at arid or muddy sites, where dominant plants had grown from seeds or propagules. After 12 years, we identified three communities (with one community containing two subunits), the dominance of which changed among the plots over the years. The communities developed along two main gradients, dry to wet and secondary succession. The first gradient was characterized by species groups favoring dry conditions, while the second gradient was characterized by species groups favoring disturbed conditions. The original composition recorded for donor marsh plants was not established by year 12 after transplantation to the artificial marsh.

In situ tensile fracture toughness of surficial cohesive marine sediments

This study reports the first in situ measurements of tensile fracture toughness, K IC, of soft, surficial, cohesive marine sediments. A newly developed probe continuously measures the stress required to cause tensile failure in sediments to depths of up to 1 m. Probe measurements are in agreement with standard laboratory methods of K IC measurements in both potter’s clay and natural sediments. The data comprise in situ depth profiles from three field sites in Nova Scotia, Canada. Measured K IC at two muddy sites (median grain size of 23–50 μm) range from near zero at the sediment surface to >1,800 Pa m1/2 at 0.2 m depth. These profiles also appear to identify the bioturbated/mixed depth. K IC for a sandy site (>90% sand) is an order of magnitude lower than for the muddy sediments, and reflects the lack of cohesion/adhesion. A comparison of K IC, median grain size, and porosity in muddy sediments indicates that consolidation increases fracture strength, whereas inclusion of sand causes weakening; thus, sand-bearing layers can be easily identified in K IC profiles. K IC and vane-measured shear strength correlate strongly, which suggests that the vane measurements should perhaps be interpreted as shear fracture toughness, rather than shear strength. Comparison of in situ probe-measured values with K IC of soils and gelatin shows that sediments have a K IC range intermediate between denser compacted soils and softer, elastic gelatin.

Effects of fish farming on the biological and geochemical properties of muddy and sandy sediments in the Mediterranean Sea

The aim of this paper was to test how benthic fauna and biogeochemical properties of sediment will vary in response to similar levels of organic enrichment (induced by fish farming) as a function of bottom-habitat type (i.e., mud versus seagrass/coarse sediments), distance from the enrichment source and depth. Our results showed that samples from silty sediments in the vicinity of fish farms have higher TOC and TON values, higher oxygen consumption, higher PO 4 release and lower benthic diversity. In this context muddy sites are more likely to be identified as impacted/critical, than coarse sediment ones.

Estimating areal production of intertidal microphytobenthos based on spatio-temporal community dynamics and laboratory measurements

In situ Microphytobenthic community dynamics were combined with laboratory measurement of predominant species by fluorescence methods to estimate the areal primary production. Field investigation of community dynamics of microphytobenthos (MPB) was conducted from August 2006 to August 2007 in intertidal flats of the Nakdong River estuary, Korea. MPB Biomass varied between 0.47 and 16.58 μg cm−3 in the surface 1 cm sediment, with two dominant diatom species, Amphora coffeaeformis and Navicula sp., occupying average 77.2 ± 14.9% of total number of MPB cells. The biomass was higher in the slightly muddy sand sites than that in the sand site, and showed different pattern of seasonal variation. The profile of vertical distribution of biomass was an exponential decrease trend with depth in sediments. The biomass proportions in the uppermost 3 mm were 57.6% and 37.8% with and without the presence of biofilm, respectively. The two dominant species were cultured in laboratory, and their photosynthetic parameters, rETRmax (relative maximum electron transport rate), α (light utilization coefficient) and Ek (light saturation parameter) were derived from rETR (relative ETR)-irradiance curves by Imaging- PAM (pulse amplitude modulated) fluorometry. The rETR-irradiance curves showed no significant difference of photosynthetic activities between the two species. The areal potential production ranged from 0.74 to 2.22 g C m−2 d−1.

Adaptations of microphytobenthos assemblages to sediment type and tidal position

Tagus intertidal microphytobenthos (MPB) assemblages were characterized over a wide range of sediment type and tidal height and the possible effects of these variables on MPB spatial distribution and photo-adaptation mechanisms were investigated. Two transects with six different sediment type and different tidal height sites were sampled once every two months from 2002 to 2004. Upper shore and sandy sites showed higher chlorophyll a (chl a) content, with sandy sediments showing a biomass peak in late winter–early spring, and muddy sites showing no obvious seasonal pattern. Stepwise multiple linear regressions showed that only SiO 2, tidal height and sediment particle size <63 μm were significant variables ( p<0.05), explaining 50% of MPB biomass spatial–temporal variability. However, when data were separated by transect, only tidal height remained significant at both transects. Sandier sediments exhibited higher zeaxanthin/chl a and lower fucoxanthin/chl a ratios characteristic of a mixed cyanobacteria/diatom assemblages, showing an alternate seasonality with cyanobacteria increasing in summer and diatoms dominating in spring. Diatom biofilms showed contrasting features depending on the sediment type. Epipsammic diatoms were small with an average length of around 10 μm, while epipelic diatoms showed a wider size range with size distribution peaks at 10–15 μm, 25–35 μm and >60 μm. Epipelic biofilms showed evidence of being low light-acclimated (high fucoxanthin/chl a) and of photo-regulating by vertical migration movements (presence of endogenous vertical rhythms and lower diatoxanthin/diadinoxanthin). Epipsammic biofilms showed higher diatoxanthin/diadinoxanthin ratios and no vertical migration rhythms. Thus, the two diatom biofilm types had distinct strategies to photo-regulate: epipelic diatoms using vertical migration to position themselves at the sediment depth of optimum light conditions, and epipsammic diatoms using the xanthophyll cycle to photo-regulate. Further studies comparing epipsammic and epipelic assemblages are necessary to better understand MPB photo-regulation mechanisms.

Analysis of Microbial Community in Tidal Flat for Investigation of Relationship between Microbiological Function and Physicochemical Characteristics of Sediment

Relationship between distribution of bacteria and physicochemical characteristics in tidal flat was studied. Sediment samples in Shirakawa river as sandy site and Midorikawa river as muddy site in Kumamoto Prefecture were investigated for the analysis of microbial community and the determination of sediment characteristics such as oxygen-redox potential, organic matter content, and so on. Midorikawa was seemed to be anaerobic and anaerobic bacteria were detected at that site. On the other hand, Shirakawa river was aerobic and aerobic bacteria were detected. Sulfur reducing bacteria, one of the anaerobic bacteria, might be the key group for understanding the relationship between microbial community and physicochemical characteristics in tidal flat.

Quantifying erosion rates and stability of bottom sediments at mussel aquaculture sites in Prince Edward Island, Canada

Downward fluxes of organic biodeposits under suspended mussel culture cause benthic impacts such as microbial mat production. Quantifying sediment erosion in these coastal ecosystems is important for understanding how fluxes of organic matter and particulates contribute to benthic–pelagic coupling. Critical shear velocity ( u⁎), erosion rates and particle size distributions of resuspended sediment were measured at two sites; an impacted muddy site with extensive mussel culture (site 1), and a coarser sandier site with less mussel influence (site 2), using a new method for assessing sediment erosion at Tracadie Bay, Prince Edward Island in August 2003. Shear forces were generated by vertically oscillating a perforated disc at controlled frequencies. These forces correspond to shear velocity, using a re-designed and calibrated Particle Erosion Simulator. Undisturbed sediment cores obtained by divers and grab (sub-cored using a Plexiglas™ cores) were exposed to shear stress to compare differences between collection methods. Microbial mats were present at site 1 which initially biostabilized sediment against erosion due to ‘armoring’ of the sediment, but onset of erosion was abrupt once these mats failed. Erosion sequences at site 2 (without mat cover) were smoother resulting in less material being eroded. Mean mass of material eroded was 47 and 23 g m − 2 min − 1 at sites 1 and 2 respectively. Mat area cover and shear velocity was strongly related. Critical shear velocities varied between 1.70 and 1.77 cm s − 1 , with no obvious differences between location or collection method, so sediments from these two contrasting sites had identical mean critical shear velocities. Significant differences existed in the concentrations of chlorophyll a, colloidal and bulk carbohydrates, between mats and bare sediment from site 1. Particle sizes measured by videography of resuspended sediment at different shear velocities ranged from 100 μm (the minimum diameter capable of being detected by the system), to large mat fragments of 1700 μm for both sites. These results provide evidence of the relevance of using a portable erosion device to indicate how sediment erodability is affected by mussel–microbial relationships.

Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension

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 358:125-136 (2008) - DOI: https://doi.org/10.3354/meps07338 Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension John Widdows1,*, Nick D. Pope1, Mary D. Brinsley1, Harald Asmus2, Ragnhild M. Asmus2 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, Germany *Email: j.widdows@pml.ac.uk ABSTRACT: The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ0). Despite this increase in τ0 there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τe] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z. marina having little effect on sediment erodability (bare: τe = 0.05 Pa; Z. marina: τe = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment. KEY WORDS: Seagrass beds · Zostera noltii · Zostera marina · Hydrodynamics · Sediment resuspension Full text in pdf format PreviousNextCite this article as: Widdows J, Pope ND, Brinsley MD, Asmus H, Asmus RM (2008) Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension. Mar Ecol Prog Ser 358:125-136. https://doi.org/10.3354/meps07338 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 358. Online publication date: April 21, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Nematode assemblages from submarine caves in Hong Kong

Nematode assemblages from sediments of two submarine caves in subtropical Hong Kong waters were investigated, and compared with those from other local sublittoral sediments. A total of 48 putative nematode species from 44 genera and 18 families were identified from the caves. Species richness of the cave nematode assemblages lay between those of adjacent sandy control sites and muddy control sites. Species diversity of the cave assemblages was much lower than those of the sandy control sites but was comparable with those of local muddy sediment. Nematodes at Conic Island Cave were dominated by non‐selective deposit feeders, while at Steep Island Cave most nematodes were omnivores/predators. This is in contrast with the more even composition of trophic guilds at the nearby non‐cave sites. Multivariate analyses revealed no clear demarcation of community structure among the cave, non‐cave and wreck nematode assemblages, indicating no nematode species association endemic to the cave sediment. Sediment characteristics and water depth are suggested as fundamental in determining nematode community structure and trophic composition in the area investigated. Submarine cave sediment provided a sheltered habitat with limited food resources favourable for non‐selective deposit feeders and omnivores/predators.

Growth variability and feeding of scallop Patinopecten yessoensis on different bottom sediments: Evidence from fatty acid analysis

Growth and feeding of the Japanese scallop, Patinopecten (= Mizuhopecten) yessoensis, from two populations inhabiting neighboring sites with similar water temperature, salinity, and hydrodynamic regimes but with different bottom sediment types were investigated. Fatty acid biomarkers were used to investigate the diets of scallops and to evaluate the food availability at the sites with different bottom sediments. Fatty acid composition of digestive gland and soft tissues of scallops from two populations were examined and compared to those of plankton, particular organic matter of near-bottom water and bottom sediments as its potential food sources. The high level of 20:5( n − 3) coupled with the high ratio of 16:1( n − 7) to 16:0 in the digestive gland of scallop indicated an important contribution of diatoms to scallop nutrition. The relatively high proportions of 18:2( n − 6), 20:4( n − 6), and 22:6( n − 3) suggested that heterotrophic flagellates, ciliates, and invertebrate larvae were also an important source of nutrition for the scallop. Trace amounts of branched and odd fatty acids were indicative of an insignificant contribution of detritus to scallop diet. Lipid and fatty acid compositions of scallops from two sites were characterized by pronounced differences pointing to the different food availability in near-bottom water at the muddy and sandy sites. Higher food availability at the muddy site resulted in an increase in the total lipid content in scallop's digestive gland due to increased proportion of triacylglycerols. Increased contents of diatom biomarkers indicated that scallops inhabiting muddy bottom were more reliant on diatom food sources. Scallops from the sandy site showed higher proportions of biomarkers derived from flagellates, ciliates, and invertebrate larvae, compared to scallops from the muddy site. This is consistent with the distribution of the fatty acids in near-bottom water POM at the study sites. In spite of the high microbial biomass in muddy bottom sediments and elevated proportion of bacterial fatty acids in near-bottom water POM, bacterial input in scallop diet increased slightly. Thus, the distribution of marker fatty acids in scallops collected at the two sites indicated spatial variability primarily due to the composition and abundance of near-bottom POM, suggesting that scallop feeding is dependent on food availability. Comparison of linear and weight parameters of scallops showed that, in spite of the best food potential at the muddy site, scallops grew significantly slower at the muddy site than at the sandy site. Thus, food potential was not the only factor regulating growth of scallops inhabiting different bottom sediments. It was found, that oxygen concentration in near-bottom water and bottom sediment resuspension depended on the grain-size composition of the sediments. Most probably, low oxygen concentration in the near-bottom water and high resuspension of inorganic fine-grained particles enriched with inedible dead organic matter are the main reasons of the reduction in growth of scallop inhabiting the muddy site.

Morphological Descriptions of New and Little‐Known Benthic Ciliates from Ganghwa Tidal Flat, Korea

Ciliates are highly diverse in the benthos where there are rare species to be unrevealed and described. By isolating species during successive and diversified cultivations, we discovered several new and interesting taxa from the top layer sediment of a muddy site in the Ganghwa tidal flat. These include three new species Spirodysteria ganghwaensis n. sp., Uronemella parafilificum n. sp., Zosterodasys minuta n. sp., and one poorly known form Loxophyllum chaetonotumBorror 1965. The morphology of live cells and infraciliature of these four species are described based on living observations, protargol impregnations, and morphometrics. Diagnoses and improved definitions are also provided. The newly established genus Spirodysteria n. g. differs from Dysteria mainly in its spirally twisted body shape. Spirodysteria kahli (Tucolesco 1962) n. comb. (formerly Dysteria kahliTucolesco 1962) has been included in this new genus.

Feeding and growth of Japanese scallop inhabiting different bottom sediment types

Two parts of the population of Japanese scallop Mizuhopecten yessoensis inhabiting sites with different bottom sediments in an open part of Peter the Great Bay of the Sea of Japan were compared. The scallops grew slower on muddy site compared to sandy site despite better food availability at muddy site. The food sources were determined using fatty acids as biomarkers. Analysis of the fatty acid composition of the scallops has identified diatom plankton, flagellates, and invertebrate larvae as the main scallop food sources. Benthic bacteria insignificantly contributed to the scallop diet. The food composition slightly varied in scallops on different sediment types. Most likely, low oxygen content in water, high resuspension of fine sediment particles rich in dead organic matter, and high content of contaminants accumulated in muddy sediments are the main factors of decelerated growth of scallops on muddy sites.

Morphological variation and habitat modification are strongly correlated for the autogenic ecosystem engineerSpartina anglica (common cordgrass)

We explored to what extent morphological variation and habitat modification are correlated for an autogenic ecosystem engineer, which is an organism that modifies its habitat via its own physical structures. The intertidal salt marsh species Spartina anglica is well known for its capacity to enhance sediment accretion within its canopy by reducing hydrodynamic energy. Sediment accretion is favorable to Spartina, as it reduces inundation stress, enhances soil drainage, and enhances nutrient availability. Shoot density and clonal architecture showed a large variation that was strongly correlated with the marsh elevation and sediment type. This correlation showed that at the lowest elevations at the muddy site, Spartina tussocks had the highest shoot density, which is known to be favorable for sediment accretion by reduction of hydrodynamic energy. There was also a strong positive correlation between the amount of sediment that accumulated within a tussock and gully formation around that Spartina tussock. The tussocks at the lowest elevations at the muddy site had the lowest lateral tussock growth. At the highest elevations at the sandy site, stem densities were lower and there was a relatively high rate of clonal expansion and marsh formation. At this location, we also observed tussock mortality due to erosion of several of these openly structured tussocks. Based on the observed correlations, we hypothesize that the morphology of Spartina represents trade-offs between the capacity to maximize habitat modification through sediment trapping by having a high shoot density versus the capacity to maximize clonal expansion by spreading shoots widely and the capacity of maximal clonal expansion by spreading shoots widely versus the risk of tussock mortality due to insufficient modification of the habitat that makes the tussock vulnerable to erosion. Our results indicated that morphological variation and habitat modification are strongly correlated for the autogenic ecosystem engineer S. anglica.

Microbial exopolymers in sediments of a second order stream: Njoro River Kenya

Determinations of concentrations of various carbohydrates associated with sedimentary biofilms were carried out in Njoro River between October 1998 and December 1999 in a muddy and sandy site. Significant temporal differences (P Journal of Tropical Microbiology and Biotechnology Vol. 2(1) 2006: 37−45

Seasonality and diversity patterns of microphytobenthos in a mesotrophic lake

Investigations on the littoral microphytobenthos populations of lakes are very rare. Based on new sampling techniques, allowing the undisturbed sampling of sublittoral sediments, we present information on limnic microphytobenthos populations of the Schohsee in Northern Germany (10° 26′ E, 54° 13′ N). Investigations on microphytobenthic communities at a sandy and a muddy site provided new insights into the community structure and seasonal variations of limnic benthic microalgae. The microphytobenthos was characterised by low chlorophyll-a contents (0.16 μg cm−2 to 0.74 μg cm−2) and low cell numbers (25 cells cm−2 to 266 cells cm−2). Both sites showed similar patterns in terms of algal biomass and seasonality; no site-specific characteristics were observed. The productivity of the sediment microflora in the Schohsee was low and most likely related to the mesotrophic lake character and to reduced light conditions in the near-shore sublittoral. The algal communities at both sites showed high diversity, distinct seasonality and succession patterns with shifts in community composition in spring, summer and autumn. The community was dominated by Fragilaria spp., Navicula spp., Nitzschia spp., and Stauroneis spp. in spring shifting to a Stauroneis spp.-dominated community in summer. In autumn the populations were dominated by Navicula spp., Nitzschia spp., and Stauroneis spp. Overall, the taxonomic composition showed a dominance of prostrate diatoms, whereas cyanobacteria, green algae and erect diatoms occurred rarely and with low abundance. Thus, the sediment microflora of the Schohsee can be characterized as a flat, two-dimensional community.

Biogeochemical implications for phosphorus cycling in sandy and muddy rhizosphere sediments of Zostera marina meadows (Denmark)

To date, few studies have investigated the implications of sediment biogeochemical con- ditions on differences in sediment phosphorus (P) pools and P availability in sandy (low-P) and muddy (high-P) siliclastic seagrass sediments. To determine the role of seagrass activity on seasonal changes in sedimentary P, we investigated the solid-phase P pools in seagrass Zostera marina sedi- ments in a Danish fjord. During the Z. marina growth season sediments were fractionated by sequen- tial extractions into 3 (and once into 5) chemically defined groups: loosely adsorbed inorganic P, inor- ganic P bound to oxidized metals (primarily Fe-bound), P adsorbed to clay minerals, Al and humic acids, Ca-bound P, and refractory organic P. Fe-bound P accounted for about 30% of total P in the muddy sediments, whereas this pool was small in the sandy sediment ( 80%). The Fe-bound P at the muddy site showed seasonal variations with lower pools during late summer, and it was always lower in the vegetated sediment, except dur- ing maximum biomass in July, where Fe-bound P was higher in the deep rhizosphere sediments. The seasonal variation was less at the sandy site, and there was little difference between unvegetated and vegetated sites, suggesting that the vegetation had a limited effect on the sedimentary P pools. How- ever, the presence of vegetation increased the availability of P in the water column at both types of site, as P was released across the sediment-water interface at the vegetated sites compared with an uptake during most of the sampling period at the unvegetated sites, although the fluxes were low and could only account for a minor fraction (<10%) of the seasonal changes in the sedimentary P pools. Estimates of P incorporation in seagrass biomass showed that the seasonal variation in sedimentary P pools were more than sufficient to support the P demand. We hypothesize that the higher P contents in seagrass tissues and the larger biomass at the muddy site are due to high P availability from the redox-sensitive Fe-bound and organic P pools at this site, whereas P may be a limiting factor for growth at the sandy site, where the mobility of P is lower due to binding in the Ca fraction.

Organic matter and benthic metabolism in Lake Illawarra, Australia

Carbon and nitrogen contents (total organic carbon and total nitrogen), chlorophyll-a concentrations in surface sediments and benthic sediment–water fluxes of oxygen and carbon dioxide were investigated at five stations in Lake Illawarra (Australia) to compare the sources/quality of sedimentary organic matter and the characteristics of diagenesis and benthic biogeochemical processes for different primary producers (e.g., seagrass, microphytobenthos and macroalgae) and/or sediment types (sand or mud). The unvegetated sediments showed lower C/N ratios (with the lowest value occurring in the deep organic-rich muddy site) than the seagrass ( Ruppia or Zostera) beds, which may be due to the contribution of microalgae (mainly diatoms) to the sedimentary organic matter pool. This was also supported by the detection of microalgal pigments in the bare sediments. On an annual basis, seagrass beds exhibited the highest gross primary productivity (O 2 or TCO 2 fluxes), while the lowest rates occurred in the deep central basin of the Lake. Seasonally, there was a general trend of highest production in spring or summer, and lowest production in winter or autumn. Organic carbon oxidation scenarios, evaluated by either calcium carbonate dissolution or sulfate reduction models, indicated that both models can explain organic matter mineralization. Trophic status was evaluated using different indices including benthic trophic state index, net O 2 fluxes and P/ R ratios for Lake Illawarra, which led to similar trophic classifications in general, and also the same trends in spatial and seasonal variations. Overall, these data indicated that the Lake was heterotrophic on an annual basis, as the total community carbon respiration exceeded production, and this supported an earlier LOICZ mass balance/stoichiometric modelling conclusion.

Variability of metal contents in the sea rush Juncus maritimus–estuarine sediment system through one year of plant’s life

Metal accumulation by Juncus maritimus and the effect this plant might have on the physico-chemical composition of sediments, from Douro river estuary (NW Portugal), were investigated through one year of plant's life. The contents of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were determined (by atomic absorption spectrophotometry) in sediments, rhizosediments (those among plant roots and rhizomes) and different tissues of J. maritimus. Two estuarine sites with different characteristics were studied, one a sandy sediment with relatively low organic matter content, the other a muddy site. The role of the plant seemed to be markedly dependent on the sediment composition. At the sandy site, significant variations throughout the year (a decrease from winter to summer and an increase in autumn) either in rhizosediment and sediment or in J. maritimus roots were only observed in Cd and Cu levels. These variations were the same in both compartments, probably caused by tidal water movements and the activity of the plant. At the muddy site, variations throughout the year were observed for all studied elements but Pb. Changes in the Cd and Zn contents of roots were opposite to those in the rhizosediment (uptake of Cd and Zn by J. maritimus roots in summer may have caused a decrease in rhizosediment levels). Fe and Mn showed similar patterns of variation in both roots and rhizosediment (contents significantly increasing in summer). Higher contents of Fe and Mn in roots in summer could be related to changes in mass, higher availability of the elements, or accumulation at the surface of the root in (hydr)oxides forms. For the remaining elements no significant correlations between rhizosediment and roots variations were observed, indicating that their uptake and release by roots were not based on single exchanges with the sediment. J. maritimus showed capacity to accumulate Cu, Zn and particularly Cd, thus appearing to be useful for the phytostabilization of these metals.