Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

Many coastal and estuarine environments are dominated by mixtures of non-cohesive sand and cohesive mud. The migration rate of bedforms, such as ripples and dunes, in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology. However, these models tend to ignore parameters describing the physical and biological cohesion (resulting from clay and extracellular polymeric substances, EPS) in natural mixed sediment, largely because of a scarcity of relevant laboratory and field data. To address this gap in knowledge, data were collected on intertidal flats over a spring-neap cycle to determine the bed material transport rates of bedforms in biologically-active mixed sand-mud. Bed cohesive composition changed from below 2 vol% up to 5.4 vol% cohesive clay, as the tide progressed from spring towards neap. The amount of EPS in the bed sediment was found to vary linearly with the clay content. Using multiple linear regression, the transport rate was found to depend on the Shields stress parameter and the bed cohesive clay content. The transport rates decreased with increasing cohesive clay and EPS content, when these contents were below 2.8 vol% and 0.05 wt%, respectively. Above these limits, bedform migration and bed material transport was not detectable by the instruments in the study area. These limits are consistent with recently conducted sand-clay and sand-EPS laboratory experiments on bedform development. This work has important implications for the circumstances under which existing sand-only bedform migration transport formulae may be applied in a mixed sand-clay environment, particularly as 2.8 vol% cohesive clay is well within the commonly adopted definition of “clean sand”

Influence of flow speed on the functional response of a passive suspension feeder, the spionid polychaete Polydora cornuta

Passive suspension feeders rely on surrounding flow to deliver food particles to them. Therefore, the classic conception of functional response (feeding rate vs. food concentration) may require modification to account for flow speed as a second independent variable. I compared the functional response of Polydora cornuta at different velocities and determined whether food capture was proportional to particle flux (concentration × velocity). To understand feeding responses at a mechanistic level, I measured the functional responses in terms of contact and capture rates and determined particle retention efficiency. Experiments were run separately with two sizes of food particles, and with juvenile or adult worms. For both worm sizes and both particle sizes, capture rate in weak flow was directly related to concentration, but in strong flow it was constant. Worms were therefore unable to benefit from abundant food when in strong flow. The critical velocity at which the capture rate became constant was lower for adult worms than for juvenile worms, and it was lower for small particles than for large particles. Retention efficiency was constant among all treatments, and the results for contact rate were essentially the same as for capture rate

Testing assumptions of the eukaryotic inhibitor method for investigating interactions between aquatic protozoa and bacteria, applied to marine sediment

Eukaryote-metabolic inhibitors have been applied in published incubation experiments to inhibit protozoans and infer their impacts on bacteria. This method has been used with water-column, sedimentary, biofilm, biofilter, soil, groundwater, wastewater, and sludge samples. The approach rests on assumptions that rarely have been tested: 1) the inhibitor is completely effective against all protozoans; 2) the inhibitor has no effects on other eukaryotes in the system (e.g., microalgae, metazoans) that could influence bacterial activity; and 3) the inhibitor has no effect on bacterial activity. We tested these assumptions for 10 inhibitors used in published studies (amphotericin B, anisomycin, colchicine, cycloheximide, cytochalasin B, fumagillin, griseofulvin, neutral red, nystatin, and thiram) and one combination of inhibitors (colchicine + cycloheximide) using marine sedimentary organisms. Only thiram was completely effective against the natural assemblages of ciliates and flagellates. With some inhibitors, partial inhibition of ciliates was followed by regrowth within 6-24 h or a bloom of a resistant species. Most inhibitors also killed infaunal invertebrates. Five inhibitors were further tested for direct effects on bacteria at concentrations that inhibited ciliates. Each inhibitor altered either esterase activity, denitrification, or both. All inhibitors thus violated one or more assumptions of the method. Conclusions in published work that did not demonstrate the validity of all assumptions underlying this method should not be accepted. Any future attempt to use a eukaryotic inhibitor against protozoans must confirm all assumptions of this method for the particular habitat, species, and processes to which the inhibitor will be applied

Impacts of the invasive grass Spartina anglica on benthic macrofaunal assemblages in a temperate Australian saltmarsh

Reported impacts of the invasive saltmarsh grass Spartina anglica on benthic macrofaunal assemblages around the world vary considerably, and there is little understanding of the reasons for this variation. We compared macrofaunal assemblages and sediment characteristics among patches of S. anglica and adjacent uninvaded habitats (bare mudflat and native saltmarsh) in southeastern Australia. Invaded patches showed reduced species richness (by 50%) and diversity compared to both uninvaded habitats. Macrofaunal abundance in S. anglica patches was also lower than in native marsh (by 60%), but not different from mudflat. There were no differences in biomass among habitats. Ordination clearly separated the species assemblage of invaded patches from uninvaded habitats, suggesting a unique community in the Spartina habitat. Molluscs and crustaceans were the most depleted in S. anglica patches, while the polychaete Nephtys australiensis was enhanced. Infauna and epifauna were both depleted in S. anglica, although the mechanisms for these impacts should differ. Burrowing by infauna in S. anglica patches was likely impeded by dense roots and rhizomes, because the below-ground plant biomass was 72% greater than in native saltmarsh. Epifauna were likely depleted in S. anglica patches due to shadinginduced inhibition of microphytobenthos growth, consistent with measured reductions of porewater salinity and increased mud content. Salinity and mud content were the sediment parameters that correlated most strongly with macrofaunal assemblage composition. These results, combined with a synthesis of published S. anglica impacts, suggest predictions of when S. anglica facilitates or inhibits macrofauna, considering infauna and epifauna separately

Community structure of marine sedimentary protists in relation to flow and grain size

Populations of unicellular, marine sedimentary protists are constrained by a variety of physical environmental factors, but influences of flow regime have rarely been studied. We compared community structure among 3 subtidal sites differing in flow strength and grain size in a coastal bay. We used denaturing gradient gel electrophoresis (DGGE) to assess eukaryotic diversity based on 18S rDNA, and quantitative Protargol staining (QPS) to examine ciliate communities by microscopy. Sedimentary 18S rDNA in mid-summer was dominated by diatoms. Analyses of gel bands by presence/absence among sites, dendogram, and multidimensional scaling showed that eukaryotic community structure was related to grain size more strongly than to flow regime. Among bands identified as diatoms by recovery and sequence analysis, 4 taxa (40%) differed among sites in relation to flow strength, and 2 taxa (29%) differed in relation to grain size. No bands had sequences matching ciliates, but QPS showed that 6 ciliate species (20 %) were distributed in relation to flow, and 10 species (33%), in relation to grain size. Ciliate species richness and community similarity were greatest for the 2 strong-flow sites, despite differences in mean grain size. The strong-flow, silty site had the high-est concentrations of chlorophyll a, total ciliates, karyorelictids, and scuticociliates, and the lowest ciliate species diversity. DGGE was run again for this site 1 mo later and revealed a shift in the rDNA pool to dominance by metazoans. Flow regime and grain size may be important factors structuring subtidal communities of sedimentary protists

Colonisation and succession of marine biofilm-dwelling ciliates in response to environmental variation

Protozoan assemblages and successional dynamics are important components of biofouling that require better understanding. We studied marine ciliates in temperate Australia as they colonised artificial substrates for 21 d during 2 different seasons, with 2 different aspects of orientation. Sessile and planktonic taxa established within 7 d, whereas vagile taxa colonised throughout the period. Abundances reached 366 ciliates cm-2. Colonies of the peritrichs Zoothamnium and Vorticella, and the hypotrichs Aspidisca and Euplotes were the most abundant. The north aspect received more light than the south aspect during summer, but assemblages did not differ significantly. Assemblage structure was different between seasons, and it developed more quickly and reached greater abundances during summer. A storm late in summer abruptly reduced abundances and affected functional groups differently, but diversity was largely unaffected. Thus, diversity of an established assemblage can be maintained through disturbances, despite abundances being subject to great fluctuation

Colonisation and succession of marine biofilm-dwelling ciliate assemblages on biocidal antifouling and fouling-release coatings in temperate Australia

Ciliate assemblages are often overlooked, but ubiquitous components of microbial biofilms which require a better understanding. Ciliate, diatom and bacterial colonisation were evaluated on two fouling-release (FR) coatings, viz. Intersleek 970 and Hempasil X3, and two biocidal antifouling (AF) coatings, viz. Intersmooth 360 and Interspeed 5640, in Port Phillip Bay, Australia. A total of 15 genera were identified during the 10 week deployment. Intersleek 970 displayed the most rapid fouling by ciliates, reaching 63.3(± 5.9) cells cm-2. After 10 weeks, all four coatings were extensively fouled. However, the toxicity of the AF coatings still significantly inhibited microbial fouling compared to the FR coatings. On all treatments, colonies of sessile peritrichs dominated the ciliate assemblage in the early stage of succession, but as the biofilm matured, vagile ciliates exerted more influence on the assemblage structure. The AF coatings showed selective toxic effects, causing significant differences in the ciliate species assemblages among the treatments

Resuspension of benthic protists at subtidal coastal sites with differing sediment composition

Mechanisms and consequences of sediment resuspension are well studied in coastal environments, but the associated resuspension of micro-organisms is poorly understood. Using an in situ flume, and by sampling the benthic boundary-layer during tidal cycles, we measured resuspension of protists and sediment at a subtidal, sandy site in Buzzards Bay, Massachusetts, USA, and compared it to previous studies at a silty site in the bay with similar current magnitude. At the sandy site, where the bottom shear stress typically is greater on flooding than on ebbing tides, resuspension occurred only with the flooding current, whereas at the silty site, where the bottom stress is relatively similar on flooding and ebbing tides, resuspension occurred during both flood and ebb. For sediment and most protistan taxa at the sandy site, erosion thresholds were higher and the total material resuspended in strong flow was lower than at the silty site. At both sites, organic-rich, flocculent surficial sediment resuspended in weak flow, but the composition of the protistan community associated with it differed. At the sandy site it included heterotrophic nanoflagellates (HNan), euglenoid flagellates, oligotrich ciliates, and scuticociliates, whereas at the silty site it included HNan, oligotrich ciliates, and the diatom Navicula distans. Differences in the resuspension thresholds of ciliate groups between sites were due largely to differences in taxonomic composition, whereas differences for diatoms were linked to sediment dynamics. At both sites, protistan taxa resuspended in sequence as flow accelerated, implying that the assemblage of species exchanged between benthic and planktonic communities depends on the maximal bottom stress. At the silty site, however, the composition of this group should be more sensitive to variations in tidal strength (e.g. during the spring-neap cycle) than at the sandy site. Resuspension of protists may have strong impacts on microbial food-web ecology in the water column and sediments by linking the communities and by creating strong fluctuations of cell concentrations (measured up to 24x in the boundary-layer in the case of oligotrichs). Tidal resuspension of protists is common in the subtidal of Buzzards Bay, but its dynamics depend on local sediment properties, hydrography, and taxonomic composition