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

On predicting particle capture rates in aquatic ecosystems

Recent advances in understanding the capture of moving suspended particles in aquatic ecosystems have opened up new possibilities for predicting rates of suspension feeding, larval settlement, seagrass pollination and sediment removal. Drawing on results from both highly-resolved computational fluid dynamics (CFD) simulations and existing experimental data, we quantify the controlling influence of flow velocity, particle size and collector size on rates of contact between suspended particles and biological collectors over the parameter space characterising a diverse range of aquatic ecosystems. As distinct from assumptions in previous modeling studies, the functional relationships describing capture are highly variable. Contact rates can vary in opposing directions in response to changes in collector size, an organism's size, the size of particles being intercepted (related to diet in the case of suspension feeders), and the flow strength. Contact rates shift from decreasing to increasing with collector diameter when particles become relatively large and there is vortex shedding in the collector wake. And in some ranges of the ecologically relevant parameter space, contact rates do not increase strongly with velocity or particle size. The understanding of these complex dependencies allows us to reformulate some hypotheses of selection pressure on the physiology and ecology of aquatic organisms. We discuss the benefits and limitations of CFD tools in predicting rates of particle capture in aquatic ecosystems. Finally, across the complete parameter space relevant to real aquatic ecosystems, all quantitative estimates of particle capture from our model are provided here

Influences of nutritional state and temperature on suspension-feeding rates and mechanics in the spionid polychaete Polydora cornuta

Benthic suspension feeders can respond to variations in food resources by behavioral and physiological means, but little is known about their ability to adjust the mechanics of particle capture. We examined influences of nutritional state (i.e. growth rate as influenced by food level) and temperature on suspension-feeding mechanics of the spionid polychaete Polydora cornuta (previously P. ligni). Worms were conditioned for 5 to 10 d in 4 treatments including 2 levels of suspended algae (50 or 0.1 μg chl a l-1, representing bloom and non-bloom conditions, respectively) and 2 temperatures (15 or 5ºC , representing temperate spring/summer and winter, respectively). Both of these factors had significant, direct influences on growth rate. Worms of equal body size were then video taped while suspension feeding in a flume at their respective temperatures but with identical concentrations of food, including polystyrene beads for observing feeding mechanics. Worms with the lower growth rate captured beads at a rate 2.1x that of worms with the higher growth rate, and worms at 15ºC captured beads at a rate 1.9x that of worms at 5ºC, with no interactions between these factors. Particle contact rates did not differ among treatments; rather, the results were due solely to altered retention efficiencies (the proportion of contacted particles that were captured). Further experiments indicated that worms with the lower growth rate had enhanced adhesive strength of the mucous coating that retains particles on the palps and that low temperature inhibited the effectiveness of cilia in aiding retention. This is the first evidence that a benthic suspension feeder modifies the retention efficiency on its appendages, which it does in response to changes in its nutritional state. The enhancement of retention by mucus when under nutritional stress should act as a compensatory response at times or places of sparse food

Influences of biofilm-associated ciliates on the settlement of marine invertebrate larvae

Settlement of benthic marine invertebrate larvae often limits recruitment, influencing the structure and dynamics of natural populations as well as biofouling of marine infrastructure, ship hulls, and aquaculture operations

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