Effects Of Physical Disturbance Research Articles

The effects of physical disturbance and sediment refuge on the growth of young native freshwater mussels Elliptio complanata (Eastern Elliptio)

Native freshwater mussels form a critical component of benthic foodwebs, but are endangered worldwide, making their study an important conservation issue. Many unionids live in shallow environments where they are potentially vulnerable to natural disturbances, but the impact of physical forces on their growth and the role of sediments as a refuge is poorly understood. Here, we validate the use of two types of shell internal lines (nacreous, prismatic) as indicators of physical disturbance and shell coloration as an indicator of sedimentary habitat. We use these indicators to test (1) whether the sediments provide an effective refuge for juvenile and young adult mussels from natural disturbances and (2) whether disturbance events affect their growth. Elliptio complanata (Eastern Elliptio) emerge from the sediments when they are 20–50 mm in size and 2.5–7 years old. Juvenile and young adults lay down more disturbance lines at more exposed nearshore sites, but also in small lake basins with dense mussel populations. Disturbance lines are produced during both endo- and epibenthic growth periods, but in contrast to adults, they are not associated with growth anomalies. Sediments accumulating in shallow nearshore areas of lakes provide an imperfect but effective refuge for native mussels that warrant protection.

Microbiome Aggregated Traits and Assembly Are More Sensitive to Soil Management than Diversity.

ABSTRACTHow soil is managed, particularly for agriculture, exerts stresses upon soil microbiomes, resulting in altered community structures and functional states. Understanding how soil microbiomes respond to combined stresses is important for predicting system performance under different land use scenarios, aids in identification of the most environmentally benign managements, and provides insight into how system function can be recovered in degraded soils. We use a long-established field experiment to study the effects of combined chronic (press) disturbance of the magnitude of organic carbon inputs with acute (pulse) effects of physical disturbance by tillage and chemical disturbance due to inorganic fertilization and pesticide application. We show that because of the variety of ways it can be assessed, biodiversity—here based on microbial small subunit rRNA gene phylotypes—does not provide a consistent view of community change. In contrast, aggregated traits associated with soil microbiomes indicate general loss of function, measured as a reduction of average genome lengths, associated with chronic reduction of organic inputs in arable or bare fallow soils and altered growth strategies associated with rRNA operon copy number in prokaryotes, as well as a switch to pathogenicity in fungal communities. In addition, pulse disturbance by soil tillage is associated with an increased influence of stochastic processes upon prokaryote community assembly, but fungicide used in arable soils results in niche assembly of fungal communities compared to untilled grassland. Overall, bacteria, archaea, and fungi do not share a common response to land management change, and estimates of biodiversity do not capture important facets of community adaptation to stresses adequately.IMPORTANCE Changes in soil microbiome diversity and function brought about by land management are predicted to influence a range of environmental services provided by soil, including provision of food and clean water. However, opportunities to compare the long-term effects of combinations of stresses imposed by different management approaches are limited. We exploit a globally unique 50-year field experiment, demonstrating that soil management practices alter microbiome diversity, community traits, and assembly. Grassland soil microbiomes are dominated by fewer—but phylogenetically more diverse—prokaryote phylotypes which sustain larger genomes than microbiomes in arable or bare fallow soil maintained free of plants. Dominant fungi in grassland soils are less phylogenetically diverse than those in arable or fallow soils. Soil tillage increases stochastic processes in microbiome assembly: this, combined with reduced plant biomass, presents opportunities for organisms with a capacity for pathogenesis to become established in stressed soils.

The First Cut Is the Deepest: Trawl Effects on a Deep-Sea Sponge Ground Are Pronounced Four Years on

Few studies have described the effects of physical disturbance and post-recovery of deep-sea benthic communities. Here, we explore the status of deep-sea sponge ground communities four years after being impacted by an experimental bottom trawl. The diversity and abundance of epibenthic megafauna of two distinct benthic communities in disturbed versus control areas were surveyed using a remotely operated vehicle on the Schulz Bank, Arctic Ocean. Four years after disturbance, megafaunal densities of the shallow (∼600 m depth) and deep (∼1,400 m depth) sites were significantly lower on the disturbed patches compared to the control areas. Multivariate analyses revealed a distinct separation between disturbed and control communities for both sites, with trawling causing 29–58% of the variation. Many epibenthic morphotypes were significantly impacted by the trawl, including ascidians, Geodia parva, Hexactinellida spp., Craniella infrequens, Lissodendoryx complicata, Haliclonia sp. Stylocordyla borealis, Gersemia rubiformis and Actiniaria sp. However, we found some smaller morphospecies to be equally abundant with control transects, including Polymastia thielei, Geodia hentscheli, and Stelletta rhaphidiophora, reflecting lower trawl impact for these morphotypes. Overall, our results suggest that these are fragile ecosystems that require much more time than four years to recover from physical disturbance typical of trawling activities.

Rapid recovery of native habitat-builders following physical disturbance on pier pilings offsets colonization of cryptogenic and non-indigenous species in a Chilean port

Examining the effects of disturbances within marine urban communities can shed light on their assembly rules and invasion processes. The effects of physical disturbance, through the removal of dominant native habitat-builders, were investigated in the recolonization of disturbed patches and colonization of plates on pier pilings, in a Chilean port. On pilings, disturbance substantially affected community structure after 3 months, although it slowly converged across treatments after 10 months. On plates, cryptogenic and non-indigenous species richness increased with removal severity, which was not observed in natives. Opportunistic taxa took advantage of colonizing at an early successional stage, illustrating a competition-colonization trade-off, although indirect effects might be at play (e.g. trophic competition or selective predation). Recovery of the habitat-builders then occurred at the expense of cryptogenic and non-indigenous taxa. Whether natives could continue winning against increasing propagule and colonization pressures in marine urban habitats deserves further attention. The interactions between disturbance and biological invasions herein experimentally shown in situ contribute to our understanding of multiple changes imposed by marine urbanization in a growing propagule transport network.

Oxygen Consumption and Sulfate Reduction in Vegetated Coastal Habitats: Effects of Physical Disturbance

Vegetated coastal habitats (VCHs), such as mangrove forests, salt marshes and seagrass meadows, have the ability to capture and store carbon in the sediment for millennia, and thus have high potential for mitigating global carbon emissions. Carbon sequestration and storage is inherently linked to the geochemical conditions created by a variety of microbial metabolisms, where physical disturbance of sediments may expose previously anoxic sediment layers to oxygen (O2), which could turn them into carbon sources instead of carbon sinks. Here, we used O2, hydrogen sulfide (H2S) and pH microsensors to determine how biogeochemical conditions, and thus aerobic and anaerobic metabolic pathways, vary across mangrove, salt marsh and seagrass sediments (case study from the Sydney area, Australia). We measured the biogeochemical conditions in the top 2.5 cm of surface (0–10 cm depth) and experimentally exposed deep sediments (>50 cm depth) to simulate undisturbed and physically exposed sediments, respectively, and how these conditions may affect carbon cycling processes. Mangrove surface sediment exhibited the highest rates of O2 consumption and sulfate (SO42-) reduction based on detailed microsensor measurements, with a diffusive O2 uptake rate of 102 mmol O2 m-2 d-1 and estimated sulfate reduction rate of 57 mmol Stot2- m-2 d-1. Surface sediments (0–10 cm) across all the VCHs generally had higher O2 consumption and estimated sulfate reduction rates than deeper layers (>50 cm depth). O2 penetration was <4 mm for most sediments and only down to ∼1 mm depth in mangrove surface sediments, which correlated with a significantly higher percent organic carbon content (%Corg) within sediments originating from mangrove forests as compared to those from seagrass and salt marsh ecosystems. Additionally, pH dropped from 8.2 at the sediment/water interface to <7–7.5 within the first 20 mm of sediment within all ecosystems. Prevailing anoxic conditions, especially in mangrove and seagrass sediments, as well as sediment acidification with depth, likely decreased microbial remineralisation rates of sedimentary carbon. However, physical disturbance of sediments and thereby exposure of deeper sediments to O2 seemed to stimulate aerobic metabolism in the exposed surface layers, likely reducing carbon stocks in VCHs.

The effects of the social environment and physical disturbance on personality traits

The environment can have a considerable impact on behaviour. The social environment is predicted to be a particularly important driver of behavioural variation and evolution through the indirect genetic effects that arise whenever individuals interact with conspecifics. We used male Australian field crickets, Teleogryllus oceanicus, to examine the effects of changes in the social environment (recorded acoustic sexual signals of other males) on the expression and consistency of boldness, activity and exploration, and their between-individual covariation. Switching from a silent environment to being exposed to male acoustic sexual signals resulted in crickets becoming less bold, active and explorative. Switching from an acoustic to a silent environment resulted in increased boldness and activity. We also looked at the effects of changes in the nonsocial environment via a physical disturbance that mimicked the presence of a potential predator (mechanical shaking). The effects of physical disturbance (and changes thereof) on behaviour were far less pronounced than the effects of changes in the social environment. Neither the repeatability of nor correlations between behaviours were affected by changes in physical disturbance. Only the average level of exploration was affected significantly when crickets were moved from an undisturbed to a disturbed environment, with crickets becoming less explorative. Although changes in the social and the nonsocial environment affected the repeatability of and correlations between some of the behaviours measured, changes in the social environment had the greater effect. We discuss the ecological and evolutionary implications of our findings and how they relate to our current understanding of social and nonsocial environmental effects on behaviour.

Growth, reproduction, and senescence of the epiphytic marine alga Phaeosaccion collinsii Farlow (Ochrophyta, Phaeothamniales) at its type locality in Nahant, Massachusetts, USA

Abstract The growth, reproduction, and senescence patterns of the epiphytic marine alga Phaeosaccion collinsii were tracked over two consecutive seasons at its type locality of Little Nahant, Nahant, MA (USA). We investigated the potential and/or combined effects of temperature and ambient nutrient supply (NO3 - and PO4 3-) on the phenology of this ephemeral species in its natural environment by collecting microscopic and macroscopic P. collinsii specimens from blades of eelgrass (Zostera marina) in a shallow coastal subtidal zone. Our results suggest that temperature is a strong driver of the alga’s in situ cycle and that the optimal temperature for P. collinsii growth and reproduction may be between 5 and 8°C, a narrower temperature threshold than previous laboratory studies on this subject have suggested. Several large winter storms also allowed us to observe the effect of physical disturbance on the integrity of the eelgrass beds and the population of microscopic and macroscopic P. collinsii. This study contributes the first in situ information on the abiotic conditions necessary for the successful growth and development of P. collinsii and a greater understanding of the life cycle of this unique golden brown alga.

Effects of physical disturbances on media and performance of household-scale slow sand (BioSand) filters

Point-of-use (POU) water treatment provides households in rural and remote communities with a means of obtaining greater control over their water quality and its effects on human health. One of the most prevalent POU interventions, the BioSand filter (BSF), is a household-scale, intermittently operated slow sand filter used by over 300,000 households. The sand and gravel media within BSFs can be housed in concrete (cBSF) or Hydraid plastic (pBSF) bodies, with the latter becoming increasingly popular due to their portability, durability, and anticipated scalability. This study evaluated whether pBSFs, which are lighter and thinner than their concrete counterparts, can maintain their integrity and performance after being subjected to disturbances that could occur in a typical household. Eight pBSFs and two cBSFs were run in parallel for 13 weeks, and three disturbances – one-time filter movement, one-time side impacts, and daily bucket impacts – were applied. Moving and side impacts affected pBSFs more dramatically than cBSFs, causing marked decreases in sand column height (6–29 mm decrease, p < 0.001) and decreases in maximum initial flow rate (18–84% decrease, p < 0.001). Brief spikes in pBSF effluent turbidity (0.98–15.2 NTU greater than mean effluent levels) also occurred immediately after disturbances.

Effects of physical disturbance on phosphorus uptake in temperate stream biofilms

Microbial biofilm nutrient uptake kinetics can provide insight into assimilative mechanisms that regulate stream primary productivity. While kinetic experiments are often performed, little work has estimated uptake in connection with disturbance (removal) or detailed the effects that common scour events may have on benthic microbes; therefore, the goal of this study was to evaluate physical disturbances on benthic stream biofilms to determine effects on phosphorus (P) uptake rate, physiological capability, and abiotic sorption. Artificial substrata were collected from 2 reaches along a temperate stream; resident biofilms were either removed via abrasion (disturbed) or left intact. A series of short-term radiotracer (H333PO4) experiments were then conducted to measure P uptake. In vivo autofluorescence was measured as a proxy of algal physiological condition. The experiments showed no difference in P-uptake rates (μgP/μgChl/d) between disturbed (x̄ = 0.77 ± 0.11 [SE] μgP/μgChl/d) and intact (x̄ = 0.91 ± 0.17 μgP/μgChl/d) biofilms (t = 0.69, p = 0.492, df = 33). Further, microbial physiology was not depressed by physical disturbance. While killed samples yielded significantly lower uptake compared to live biota (F = 17.51, p = 0.001), abiotic sorption still accounted for a moderate fraction (range = 0.021–0.038 μgP/μgChl/d) of total uptake and thus warrants estimation in metabolic studies. Overall, these findings lend credence to numerous experiments that investigate benthic microbial physiologic responses post-disturbance and highlight the importance of uptake following common physical disturbances that occur in turbulent environments. In addition, our work measured several kinetic constants across both disturbance and temporal gradients, and we discuss their significance within a physiological framework.

Physical and biological disturbances interact differently with productivity: effects on floral and faunal richness

Physical and biological disturbances are ecological processes affecting patterns in biodiversity at a range of scales in a variety of terrestrial and aquatic systems. Theoretical and empirical evidence suggest that effects of disturbance on diversity differ qualitatively and quantitatively, depending on levels of productivity (e.g., the dynamic equilibrium model). In this study we contrasted the interactive effects between physical disturbance and productivity to those between biological disturbance and productivity. Furthermore, to evaluate how these effects varied among different components of marine hard-substratum assemblages, analyses were done separately on algal and invertebrate richness, as well as richness of the whole assemblage. Physical disturbance (wave action) was simulated at five distinct frequencies, while biological disturbance (grazing periwinkles) was manipulated as present or absent, and productivity was manipulated as high or ambient. Uni- and multivariate analyses both showed significant effects of physical disturbance and interactive effects between biological disturbance and productivity on the composition of assemblages and total species richness. Algal richness was significantly affected by productivity and biological disturbance, whereas invertebrate richness was affected by physical disturbance only. Thus, we show, for the first time, that biological disturbance and physical disturbance interact differently with productivity, because these two types of disturbances affect different components of assemblages. These patterns might be explained by differences in the distribution (i.e., press vs. pulse) and degree of selectivity between disturbances. Because different types of disturbance can affect different components of assemblages, general ecological models will benefit from using natural diverse communities, and studies concerned with particular subsets of assemblages may be misleading. In conclusion, this study shows that the outcome of experiments on effects of disturbance and productivity on diversity is greatly influenced by the composition of the assemblage under study, as well as on the type of disturbance that is used as an experimental treatment.

Catastrophic regime shifts in coral communities exposed to physical disturbances: Simulation results from object-oriented 3-dimensional coral reef model

A 3-dimensional individual-based model, the ReefModel, was developed to simulate the dynamical structure of coral reef community using object-oriented techniques. Interactions among functional groups of reef organisms were simulated in the model. The behaviours of these organisms were described with simple mechanistic rules that were derived from their general behaviours (e.g. growing habits, competitive mechanisms, response to physical disturbance) observed in natural coral reef communities. The model was implemented to explore the effects of physical disturbance on the dynamical structure of a 3-coral community that was characterized with three functional coral groups: tabular coral, foliaceous coral and massive coral. Simulation results suggest that (i) the integration of physical disturbance and differential responses (disturbance sensitivity and growing habit) of corals plays an important role in structuring coral communities; (ii) diversity of coral communities can be maximal under intermediate level of acute physical disturbance; (iii) multimodality exists in the final states and dynamic regimes of individual coral group as well as coral community structure, which results from the influence of small random spatial events occurring during the interactions among the corals in the community, under acute and repeated physical disturbances. These results suggest that alternative stable states and catastrophic regime shifts may exist in a coral community under unstable physical environment.

Human trampling as short-term disturbance on intertidal mudflats: effects on macrofauna biodiversity and population dynamics of bivalves.

The effect of physical disturbance in the form of trampling on the benthic environment of an intertidal mudflat was investigated. Intense trampling was created as unintended side-effect by benthic ecologists during field experiments in spring and summer 2005, when a mid-shore area of 25 × 25 m was visited twice per month by on average five researchers for a period of 8 months. At the putatively-impacted location (I) (25 × 25 m) and two nearby control locations (Cs) (25 × 25 m each), three sites (4 × 4 m) were randomly selected and at each site, three plots (50 × 50 cm) were sampled after 18 and 40 days from the end of the disturbance. Multivariate and univariate asymmetrical analyses tested for changes in the macrofaunal assemblage, biomass of microphytobenthos and various sediment properties (grain-size, water content, NH4 and NO3 concentrations in the pore water) between the two control locations (Cs) and the putatively-impacted location (I). There were no detectable changes in the sediment properties and microphytobenthos biomass, but variability at small scale was observed. Microphytobenthos and NH4 were correlated at I to the number of footprints, as estimated by the percentage cover of physical depressions. This indicated that trampling could have an impact at small scales, but more investigation is needed. Trampling, instead, clearly modified the abundance and population dynamics of the clam Macoma balthica (L.) and the cockle Cerastoderma edule (L.). There was a negative impact on adults of both species, probably because footsteps directly killed or buried the animals, provoking asphyxia. Conversely, trampling indirectly enhanced recruitment rate of M. balthica, while small-sized C. edule did not react to the trampling. It was likely that small animals could recover more quickly because trampling occurred during the growing season and there was a continuous supply of larvae and juveniles. In addition, trampling might have weakened negative adult-juvenile interactions between adult cockles and juvenile M. balthica, thus facilitating the recruitment. Our findings indicated that human trampling is a relevant source of disturbance for the conservation and management of mudflats. During the growing season recovery can be fast, but in the long-term it might lead towards the dominance of M. balthica to the cost of C. edule, thereby affecting ecosystem functioning.

Effects of physical disturbance on the cold-water megafaunal communities of the Faroe–Shetland Channel

The response of a deep, cold-water benthic community to physical disturbance from offshore drilling was investigated using Remotely Operated Vehicle (ROV) video in the Faroe–Shetland Channel. This study presents a novel method for characterising changes in the ecosystem function of deep-water benthic megafaunal communities in response to physical disturbance. Ecosystem function is investigated using a range of community parameters, particularly abundance and diversity. High levels of physical disturbance, characterised by smothering of the seabed within a radius of 50 to 120 m around the drilling site, resulted in significant but variable reductions in megafaunal abundance (up to 92.3%). Reductions in diversity, particularly in species richness, were apparent between disturbed (Mao Tao estimated species richness of ES(500) = 12.9) and undisturbed areas (ES(500) = 20.6). The implications of selective removal of taxa on ecosystem function and recovery are discussed. Low level disturbance had comparatively little effect on the megafaunal communities; the effects varied in nature depending on motility and functional group (e.g. motile scavenger abundances were maximal at intermediate distances from disturbance). We conclude by comparing the functioning of the ecosystem under different regimes of disturbance and discuss implications for recovery of disturbed deep-sea habitats.

Effect of physical disturbance on reproductive performance in the spiny lobster, Jasus edwardsii

A study was conducted to examine the effects of physical disturbance on Jasus edwardsii spiny lobster broodstock during ovarian recrudescence through to larval hatch. Undisturbed brood‐stock were held in relative isolation and subjected to minimal human disturbance, which contrasted with weekly air exposure and handling of the disturbed animals. Broodstock behavioural response, phyllosoma number, competency, and survival in culture were examined. A higher proportion of broodstock from the undisturbed treatment were active during daylight hours compared with those in the disturbed treatment. All ovigerous females in the undisturbed group produced phyllosoma larvae compared with 72.7% of animals in the disturbed treatment. Individual undisturbed females produced greater numbers of phyllosoma larvae, which in turn performed better in larval competency tests, were larger at hatch, and survived better in culture than those from disturbed females. This study demonstrated that physical disturbance altered reproductive performance and larval competency of J. edwardsii. Protocols are suggested to minimise disturbance associated with handling in crustacean broodstock.

Effect of wave exposure on vegetation abundance, richness and depth distribution of shallow water plants in a New Zealand lake

SUMMARY 1. The effects of physical disturbance in terms of wave exposure, shore slope and substrate mobility on the presence, species richness, cover and depth limits of the low‐growing, shallow water macrophyte community (called the low mixed community) were examined at 41 shore sites in Lake Wanaka, South Island, New Zealand.2. Wave exposure at a site was described by the previous year's maximum depth of sediment motion and maximum vertical extension of waves, determined from a computer wave model. Shore slope at each site was recorded from −1 to 0 m depth, and sediment stability was assessed as the cover of small gravel.3. The low mixed community was only present on sites where the previous year's maximum depth of sediment motion was &lt;8 m, maximum wave run‐up was &lt;0.3 m, shore slope &lt;0.12 m m−1, and small gravel cover &lt;78%.4. Species richness, cover and depth limits of the low mixed community decreased with increasing disturbance on the sites. Sixty‐two percentage of the variation in species richness could be explained by physical disturbance variables when all sites were included (N=41). When only sites with a low mixed community were included (N=22), only 18% of the variation was explained. Species richness within sites supporting a low mixed community is thus poorly explained by physical disturbance, whereas presence or absence is better explained. There was no evidence to support the intermediate disturbance hypothesis in this study.5. Disturbance as a result of waves explained 86% of the variation in cover among sites with a low mixed community and accounted for 68 and 58% of the variation in upper and lower depth limit of the low mixed community, respectively.6. The models obtained in this study can be used as predictive models for the low mixed community in New Zealand lakes in relation to natural physical disturbance on the shore. By integrating these results with previous studies on the effect of water level fluctuation, we describe a generalised optimum physical habitat for the low mixed community in New Zealand lakes.

Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical distur bance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m-2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.

Effects of physical disturbance and granivory on establishment of native and alien riparian trees in Colorado, U.S.A.

Abstract.In western North America, the alien Elaeagnus angustifolia L. invades riparian habitats usually dominated by pioneer woody species such as Populus deltoides Marshall ssp. monilifera (Aiton) Eckenwalder. We conducted manipulative field experiments to compare the importance of physical disturbance and granivory for seedling establishment of these two species. We planted seeds of both species in disturbed and undisturbed study plots, and used exclosures, seed dish trials and live‐trapping to assess the role of granivory. Seedling establishment of both species was increased by physical disturbance and seeds of both species were subject to granivory. However, the relative importance of these two factors differed between species. For P. deltoides, lack of physical disturbance prevented seedling establishment in uncleared subplots, but granivory did not prevent seedling establishment outside of exclosures. For E. angustifolia, granivory prevented seedling establishment outside of exclosures, but lack of physical disturbance did not prevent establishment in uncleared subplots. The lesser dependence on disturbance may enable E. angustifolia to invade areas characterized by low levels of fluvial disturbance, such as floodplains along regulated rivers, where P. deltoides recruitment does not occur. Populations of granivorous rodents may affect the susceptibility of riparian ecosystems to invasion by E. angustifolia.

Effects of physical disturbance on an estuarine intertidal community: field and mesocosm results compared

The response of an estuarine macrobenthic community to different intensities of physical disturbance was examined using comparable field and mesocosm experiments. In the field study, areas of sediment (2500 cm2) were disturbed by surface-raking at a range of disturbance intensities: no disturbance, once every 2 mo, once every month, once every 2 wk, once or three times every week. These disturbance regimes were also imposed on buckets of sediment (855 cm2) collected from the field study site and maintained within a mesocosm system. In the univariate data analysis, neither field nor mesocosm communities displayed any trends in relation to the frequency of disturbance. Diversity indices were generally reduced in the mesocosm communities compared with the field communities. Multivariate analysis indicated significant differences between the field communities subjected to the most intense disturbance and those communities disturbed every ±2 wk or less. These differences were attributed to reductions in the five numerically dominant species in the high- disturbance treatments. More statistically significant differences were observed between the different disturbance intensities in the mesocosm experiment than in the field study. The implications of differences between field and mesocosm experiments results are discussed.

A Shell Gape Monitor to Study Effects of Physical Disturbance on Freshwater Mussels

ABSTRACT A bivalve shell gape monitor, previously used to detect toxic discharges in Europe, was used to measure response of the freshwater mussel Amblema plicata plicata (Say 1817) (Mollusca: Unionidae) to brief changes in water velocity. In the laboratory, overall behavior of eight A. p. plicata was virtually unaffected by exposure once every 2 hr to a 5-min pulse of high-velocity water (45 cm/s) created with a submersible pump. In the east channel of the upper Mississippi River near Prairie du Chen, WI, multiple passes of recreational craft reduced by only a few percent the total time that six A. p. plicata were opened during a 2-hr period. Only six of 42 events, caused by passage of a work boat or a 6.5-m-long skiff, caused a measurable response in mussel shell gape. It was concluded that A. p. plicata was not greatly affected by brief changes in ambient water velocity caused by vessel passage.

Effects of physical disturbance on nematode communities in sand and mud: a microcosm experiment

A microcosm experiment was carried out to evaluate the effects of continuous and spasmodic physical disturbance of differing frequency on the structure of nematode communities of intertidal sand and mud. There was a marked, characteristic change in abundance and diversity for both sediment types. In the sand microcosms, the majority of univariate measures of community structure, including species diversity, were lowest in the sediments subjected to a high frequency of disturbance. For the mud microcosms, most univariate measures reached their highest values in the treatments with an intermediate frequency of disturbance and were lower in treatments subjected to both higher and lower frequencies. Multivariate ordinations for both nematode assemblages showed a clear separation of undisturbed controls and disturbed treatments, but only for the muddy sediment was there a graded change in community composition with increasing frequency of disturbance. These results confirmed our a priori expectation that nematode assemblages from mobile sandy sediments would be more resilient to physical disturbance than those from sheltered muds, and these observations are considered in the context of Connell's intermediate disturbance hypothesis.