Benthic invertebrate recolonization of small-scale disturbances in the littoral zone of a subtropical Florida lake

Construction of spawning beds by the exotic cichlid fish, Sarotherodon aurea, produces numerous small-scale disturbances in shallow Florida lakes. Two experimental methods were used to simulate these disturbances (containers filled with defaunated sediment and disturbed natural substratum) and to determine benthic invertebrate recolonization patterns and rates.Recolonization of experimental containers was rapid and densities of macroinvertebrates reached equilibrium with the control habitat by Day 6 following disturbance. However, Chironomidae and miscellaneous taxa usually were more abundant (p < 0.05) in the experimental containers, and Oligochaeta always were more abundant (p < 0.01) in the control. Species of the planktonically dispersed insects either attained equilibrium rapidly or showed significantly higher densities in the experimental containers (i.e. opportunistic species).In the second experimental approach, disturbance of natural substratum removed 91% of the benthic invertebrates. The common species of chironomids reached equilibrium by Day 6 but the two predominant oligochaetes, Limnodrilus and Pristina, reached equilibrium at 9 and 15 days respectively. For the entire community, similarity on Day 6 was 87.1% and over the remainder of the sample period averaged 86.8%. Chironomid instar analyses showed significantly higher proportions of early instars in the disturbed patch indicating that colonization is principally by younger organisms. Thus, small-scale disturbance in freshwater benthic communities may be more important in ordering the biomass or age-group structure than the numerical abundance.     

Recolonization by benthic invertebrates after experimental disturbance in a Swiss prealpine river

1. Although the crucial point of disturbance experiments in streams is the extent to which they can simulate an actual spate, this aspect has been widely neglected in the design of such studies. Similarly, the influence of the specific hydrological disturbance regime of a stream on its benthic community has received much theoretical attention in recent years, but hypotheses have rarely been tested in the field. 2. Our field experiment compared the structure of the benthic invertebrate community in the prealpine River Necker in north-eastern Switzerland with predictions of the patch dynamics concept about the faunal composition of frequently disturbed streams. We also compared the resistance and resilience of the invertebrates between two sites in the River Necker. A similar substratum composition at both sites, but higher shear stress values both at baseflow and bankfull discharge at site 2, implied a higher disturbance frequency at the latter site. Five patches of stream bed of ≈ 9 m² were disturbed by kicking and raking at each site, while five similar areas served as controls. From each plot, six Surber samples were taken: the first immediately after the disturbance, and the following five 1, 3, 6, 10 and 30 days later. 3. Resilience of the total benthic invertebrate fauna was high. The total number of individuals recovered to undisturbed densities within 30 days at site 1 and 6 days at site 2. Taxon richness recovered within 3 days. In accordance with theory, taxa with high recolonization rates made up a major percentage of the total number of individuals, especially in disturbed plots. However, this percentage was lower at site 2 in spite of the higher disturbance frequency at this site. Rhithrogena spp., Leuctra spp. and the Simuliidae recovered faster to undisturbed densities at site 2. In contrast, absolute recolonization rates of these taxa were higher at site 1, where total invertebrate densities were more than twice as high as at site 2. 4. Our results suggest that the time since the last disturbance should be considered as an important factor in studies of benthic invertebrate communities in prealpine rivers, because disturbances can alter the community structure. In frequently disturbed streams, very short sampling intervals may be needed to detect differences in taxon-specific colonization rates. The specific hydrological disturbance regime of such streams is also important, because even within-stream differences in the resilience of the benthic invertebrate community are possible.     

Biotic disturbance and benthic community dynamics in salmon-bearing streams

1. Organisms can impact ecosystems via multiple pathways, often with positive and negative impacts on inhabitants. Understanding the context dependency of these types of impacts remains challenging. For example, organisms may perform different functions at different densities. 2. Anadromous salmon accumulate > 99% of their lifetime growth in marine ecosystems, and then return to spawn, often at high densities, in relatively confined freshwaters. While previous research has focused on how salmon nutrients can fertilize benthic communities, we examined how an ecosystem engineer, sockeye salmon Oncorhynchus nerka, influences seasonal dynamics of stream benthic communities through their nest-digging activities in south-western Alaska, USA. Benthic invertebrate and algal abundance were quantified every 7-14 days during the open water seasons of 10 streams in riffle and run habitats across multiple years, leading to 25 different stream-year combinations that spanned a large gradient of salmon density. 3. In streams with few or no salmon, benthic algal and insect biomass were fairly constant throughout the season. However, in streams with more than 0.1 salmon m(-2), algal and insect biomass decreased by an average of 75-85% during salmon spawning. Algal biomass recovered quickly following salmon disturbance, occasionally reaching pre-salmon biomass. In contrast, in streams with more than 0.1 salmon m(-2), aquatic insect populations did not recover to pre-salmon levels within the same season. We observed no positive impacts of salmon on algae or insects via fertilization from carcass nutrients. 4. Salmon, when their populations exceed thresholds in spawning density, are an important component of stream disturbance regimes and influence seasonal dynamics of benthic communities. Human activities that drive salmon densities below threshold densities, as has likely happened in many streams, will lead to altered seasonal dynamics of stream communities. Human activities that alter animal populations that are sources of biogenic disturbance can result in shifts in community dynamics.     

Influence of a simulated hurricane on aquatic insect recolonization in the phytotelma of Heliconia caribaea (Heliconiacaea)

Disturbances like hurricanes can affect diversity and community composition, which may in turn affect ecosystem function. We examined how a simulated hurricane disturbance affected insect communities inhabiting the phytotelma (plant-held waters) of Heliconia caribaea in the Luquillo Experimental Forest of eastern Puerto Rico, a tropical island that frequently experiences hurricanes. We hypothesized that disturbance would alter diversity and that larger Heliconia would attract more species following disturbance due to the area-diversity relationship described by the Theory of Island Biogeography. Individual flower parts (bracts) of Heliconia inflorescences (racemes) were artificially disturbed via removal of existing insect communities, then after refilling with water, cohorts of Heliconia were destructively sampled biweekly for 6 weeks to assess recolonization patterns of α (bract level), β, and γ (summed across bracts; raceme level) diversity over time and across raceme sizes. Although we found no support for our hypothesis about the effect of raceme size on recolonization, our hypothesis regarding recolonization patterns over time was supported; species richness, evenness, and abundance of bracts increased directly after the disturbance and then decreased below pre-disturbance levels, and community composition at the raceme level changed significantly over time during recolonization. β Diversity was also greater in smaller racemes compared to larger racemes, suggesting high heterogeneity across bracts of Heliconia racemes exacerbated by raceme size and age. Overall, our results highlight the importance of scale and appropriate measurements of diversity (particularly α) in experiments aiming to extrapolate conclusions about the ecological impacts of disturbances across different habitats and ecosystems.     

The contribution of epipelon to total sediment microalgae in a shallow temperate eutrophic loch (Loch Leven, Scotland)

Benthic microalgae are known to perform important ecosystem functions in shallow lakes. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. We tested the hypothesis that the positioning (across a depth gradient of 2–22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. all viable microalgae in the surface sediments) in a shallow eutrophic loch. Total sediment microalgal Chl a concentrations (TS-Chl; range: 5–874 μg Chl a g⁻¹ dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. Epipelic Chl a concentrations (Epi-Chl; range: <0.10–6.0 μg Chl a g⁻¹ dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5–5.5 m) in all seasons except autumn/winter. Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Epi-Chl was associated with intermediate light and habitat disturbance during spring/summer and summer/autumn and varied positively with habitat disturbance, only, in autumn/winter and winter/spring. The epipelon community structure also varied with depth; diatoms dominated shallow water sediments, cyanobacteria dominated deep water sediments, and sediments at sites of intermediate depth returned the highest biovolume estimates and the most diverse communities. This study has strengthened the hypothesis that the structure and biomass of benthic microalgal communities in lakes are regulated by habitat disturbance and water clarity, both of which are expected to respond to climate change and eutrophication. The degree to which these structural responses reflect functional performance requires clarification.     

Colonization of experimentally disturbed patches by stream macroinvertebrates in the Acheron River,Victoria

Twenty-five patches (1 m²) of natural stream substratum in the Acheron River, Victoria, were physically disturbed by kicking and raking during winter 1986 and summer 1987. The macroinvertebrate composition of these disturbed patches was examined at various times over the following 71 days, and compared with adjacent undisturbed control patches sampled concurrently. The disturbance did not alter the particle-size distribution (> 150 μm) of the disturbed patches. Organic material was reduced in the disturbed patches by about 70% in each season, but returned to control levels within 21 days in winter and 8 days in summer. The total number of species, and the density of species and individuals were all significantly reduced by the disturbance. Recovery of species density was complete after 21 days during winter and 8 days during summer, and the density of individuals recovered after 71 days during winter and 8 days during summer. The differences were due to the slower colonization rate of Chironomidae in winter, either because of a lower drift rate, or a slower recovery of detritus in winter. Individual species showed variations in colonization patterns, most increasing steadily at various rates, with some declining after an initial rapid increase (e.g. Baetis pp.). In the latter case, the density changes were mirrored in the control patches, emphasizing the need to take control samples concurrently with experimental samples. In each season, the species remaining immediately following the disturbance, and those subsequently colonizing the disturbed patches were in the same rank order (Spearman Rank correlation) as their occurrence in the control patches, suggesting that no taxa were differentially affected by the treatment. No evidence was found to allow the application of the Intermediate Disturbance Hypothesis to explain species diversity at the scale of this study. It appears that current hypotheses developed to explain the relationship between diversity and disturbance in sessile communities do not apply to highly mobile communities in streams.     

Influence of flood timing on the recovery of macrophytes in a former river channel

Over two vegetation cycles we compared the recovery of macrophytes from flood disturbances that occured at different seasons (July vs December) on patches of a former channel of the Rhône River, France. Some patches were disturbed twice; others were disturbed either in summer or in winter; others were never disturbed and were used as controls.The recovery rate of the vegetation was estimated from the duration of recolonization of the disturbed areas and of growth of the recolonizing species. The influence of the summer disturbance appeared to be strong because the disturbance occurred when the development of the vegetation was maximum. The influence of the winter disturbance was apparently much lower since most species had already declined at this time because of their phenology. The repetition of the two disturbances on the same patch had little influence on the vegetation community.In all cases, the recovery of the vegetation occurred rapidly, both for total vegetation cover and species richness. By the following spring, no significant differences appeared between disturbed and reference patches. The effect of the disturbances varied according to the phenology of the plants, and the macrophyte community studied was more sensitive in summer than in winter.     

THE EFFECTS OF PHYSICAL DISTURBANCE ON AFROPTILUM SUDAFRICANUM (EPHEMEROPTERA: BAETIDAE) POPULATION DENSITIES IN A KENYAN MOUNTAIN STREAM

Summary

The effects of physical disturbance on larval populations of the mayfly Afroptilum sudafricanum Lestage were examined in the Naro Morn River, Kenya. Disturbance was induced by stirring, shifting and relocating streambed substrates by hand. The method aimed to simulate natural physical disturbance. The effects of continuous disturbance were examined by sampling larval populations at minute intervals of up to 14 minutes. The effects of periodic disturbance were examined by sampling larval populations which had been disturbed at intervals of minutes (short-term), hours (medium-term) and days (long-term). About 85% of A. sudafricanum individuals was removed from the streambed within four minutes of continuous physical disturbance. There were higher densities at night than during the day. All densities on the control sites for the medium-term disturbance experiments were significantly higher than those on the disturbed sites. Most of the post-disturbance densities on the medium-term disturbance sites were significantly lower than the pre-disturbance densities (Tukey's honestly significance difference test, α=0.05). There was no distinct density pattern on the sites exposed to long-term disturbances. Sites with the same disturbance regime appeared to react differently, some having dramatic fluctuations in density. Disturbance effects on A. sudafricanum density were multi-faceted with different expressions in time and space. It is concluded that A. sudafricanum is highly mobile and is capable of recolonizing disturbed areas within six hours. Their high mobility enables them to crawl into the deeper sediment layers during floods. However, recovery time depended not only on species mobility but also on how densely the sources of recolonization were populated and the frequency of disturbance. Slight increases, strong declines and negligible disturbance effects on the density of A. sudafricanum were evident. Afroptilum sudafricanum is highly mobile and is capable of rapid recolonization. It is concluded that physical disturbance affects A. sudafricanum populations in the Naro Morn River.     

莱州湾小清河口近岸海域底栖生境健康评价

于2019年7月、8月和9月对莱州湾小清河口近岸海域的大型底栖动物进行调查,对该海域大型底栖动物群落的种类组成和群落结构等进行研究,利用丰度/生物量比较曲线法、AMBI指数法和M-AMBI指数法对研究海域大型底栖动物群落健康和底栖生境健康状况进行评价。研究结果表明,2019年7月、8月和9月共采集到大型底栖动物64种,其中多毛类24种,甲壳类19种,软体动物17种,其他类群4种。该海域的优势种为光滑河蓝蛤（Potamocorbula laevis）、彩虹明樱蛤（Moerella iridescens）、半褶织纹螺（Nassarius semiplicatus）、四角蛤蜊（Mactra veneriformis）,寡鳃齿吻沙蚕（Nephthys oligobranchia）、中蚓虫（Mediomastus californiensis）、东方长眼虾（Ogyrides orientalis）、尖齿拳蟹（Philyra acutidens）和短角双眼钩虾（Ampelisca brevicornis）,其中彩虹明樱蛤和寡鳃齿吻沙蚕在三次调查中均为优势物种,优势度明显。群落结构聚类分析和多维尺度分析表明,7月研究海域的大型底栖动物群落以10%的相似性可以分为3组,8月以18%相似性可以分为4组,9月以19%的相似性可以分为3组,三次调查的群落结构相似度均较低。丰度/生物量比较曲线法研究结果显示,底栖动物群落生物量优势显著,群落处于未干扰状态。AMBI指数及M-AMBI指数分析结果表明,莱州湾小清河口近岸海域底栖生境处于未干扰或者轻度干扰的状态,健康状况处于高等或者优良的状态。     

The effects of spate-induced disturbance, predation and environmental complexity on macroinvertebrates in a tropical stream

• 1 The combined effects of fish predation, substrate complexity and flow on benthic macroinvertebrates inhabiting riffles was investigated in a Hong Kong stream. Predation was manipulated using fish inclusion/exclusion cages containing complex (= many refuges) or simple (= few refuges) substrates. Experiments were undertaken during the winter dry season, when disturbance due to flow events was minimal, and repeated during the summer monsoon, when the stream experienced spates of varying intensity and duration.
• 2 Predation by the fish, Oreonectes platycephalus, significantly reduced the abundance of macroinvertebrates, especially chironomids and mayflies. Because chironomids (mostly Chironominae) were the dominant cage colonists, there was also a reduction in total macroinvertebrate density.
• 3 Predator impacts were significantly lessened during the wet season, when macroinvertebrate densities increased considerably, but significant reductions in the densities of vulnerable taxa and total macroinvertebrate abundance were nevertheless apparent.
• 4 Substrate complexity (the presence of prey refuges) had no significant effect on the ability of predators to reduce prey abundance.
• 5 Detritus accumulated in cages during the latter part of the study, and densities of most taxa were correlated with detrital standing stocks.
• 6 The results of this experiment indicate that biotic interactions such as predation may be suppressed during periods of spate-induced disturbance, although they can still influence benthic communities significantly. However, the effects of predation are highly taxon specific and may vary among streams in response to changes in predator and prey species composition, or the severity and duration of spates.

     

Resilience of Zostera marina habitats and response of the macroinvertebrate community to physical disturbance caused by clam harvesting

The consequences of physical disturbances to seagrasses depend on disturbance frequency relative to the capacity for recolonization and recovery following fragmentation. In a subtidal seagrass meadow of Zostera marina L., following a season of clam harvesting, we compared the temporal change of shoot density and biomass of this seagrass together with the community structure of the associated macroinvertebrates, at two sites differing in the intensity of the physical disturbance. The impacted site showed significantly lower shoot density and total biomass than the non-impacted site initially. The increase in above-ground biomass over four months (May to September) of this species was significantly higher (46%) at the impacted site than in the area not affected by the disturbance. Four months after cessation of the extraction activity, the biomass and density values of Z. marina reached similar values to those measured in the non-impacted site. The sexual reproductive effort of the seagrass population affected by the disturbance (4%) was significantly lower than at the non-impacted site (10%), which could influence genetic diversity and the seed bank. The community structure of molluscs showed 54% similarity between sites at the beginning of the study. Four months later, mollusc communities increased to a similarity of 74%. The current closure season (four months annually) established for the recovery of the exploited stocks of bivalves allowed the recovery of Z. marina density and biomass. Nevertheless, other population properties, such as those related to reproductive patterns, remained altered by the disturbance.     

Investigations into the soil mycoecology of Chambal ravines of India

Summary The present investigation deals with the seasonal succession of fungal communities in wheat fields. The studies were performed during the year 1975–76 and involved four study seasons,viz. winter (1975), summer (1975), rainy (1975) and winter (1976). The observations during two winter seasons were done to find out whether or not fungal communities exhibit definite cyclic pattern. Of the 69 isolated species, 25 showed wider ecological amplitude whereas some species were specific of a particular season. Each season was marked by different fungal mycoflora. The population ofAspergillus terreus, a dominant member of the community, along with many other autochthonous species, exhibited a definite periodicity during various seasons. Seasonal variations in the fungal communities, variations in the extent of similarity among the fungal communities during various seasons and the impact of climatic conditions on the components of fungal communities are discussed.     

Fauna of Natural Seagrass and Transplanted Halodule wrightii (Shoalgrass) Beds in Galveston Bay, Texas

Abstract We compared nekton and benthos densities and community compositions in a natural mixed seagrass bed dominated by Halodule wrightii (shoalgrass) with those found in three shoalgrass transplant sites and adjoining sand habitats in western Galveston Bay, Texas, U.S.A. Quantitative drop traps and cores were used to compare communities up to seven times over 36 months post‐transplant where transplant beds survived. Total densities of fishes, decapods, annelids, benthic crustaceans, and most dominant species were significantly higher in natural seagrass than in transplanted shoalgrass or sand habitats during most sampling periods. On occasion, fish and decapod densities were significantly higher in transplanted shoalgrass than in adjoining sand habitats. No consistent faunal differences were found among transplant sites before two of three sites failed. Taxonomic comparison of community compositions indicated that nekton and benthos communities in natural seagrass beds were usually distinct from those in transplanted beds or sand habitats, which were similar. We conclude that reestablishing a shoalgrass bed that resembles a natural seagrass bed and its faunal communities in the Galveston Bay system will take longer than 3 years, provided that transplants persist.     

Detectability of landscape effects on recolonization increases with regional population density

Variation in population size over time can influence our ability to identify landscape‐moderated differences in community assembly. To date, however, most studies at the landscape scale only cover snapshots in time, thereby overlooking the temporal dynamics of populations and communities. In this paper, we present data that illustrate how temporal variation in population density at a regional scale can influence landscape‐moderated variation in recolonization and population buildup in disturbed habitat patches. Four common insect species, two omnivores and two herbivores, were monitored over 8 years in 10 willow short‐rotation coppice bio‐energy stands with a four‐year disturbance regime (coppice cycle). The population densities in these regularly disturbed stands were compared to densities in 17 undisturbed natural Salix cinerea (grey willow) stands in the same region. A time series approach was used, utilizing the natural variation between years to statistically model recolonization as a function of landscape composition under two different levels of regional density. Landscape composition, i.e. relative amount of forest vs. open agricultural habitats, largely determined the density of re‐colonizing populations following willow coppicing in three of the four species. However, the impact of landscape composition was not detectable in years with low regional density. Our results illustrate that landscape‐moderated recolonization can change over time and that considering the temporal dynamics of populations may be crucial when designing and evaluating studies at landscape level.     

Community and population-level responses to disturbance in a sandflat community

The responses of soft sediment infauna were investigated in an intertidal sandflat to determine patterns of recolonization and succession at the community and population level. Experimental disturbance plots, 1 m², were initiated in August and sampled for 4.5 months along with ambient sediments. Sediment grain-size was used as a general indicator of the physical state of the disturbance patches, and grain-size distributions among disturbance and ambient patches became similar after ∼2.5 months. Recolonization varied among the dominant infaunal taxa. Densities of infauna that were most abundant in the habitat, primarily syllid polychaetes, did not recover to ambient levels until 3-4 months after disturbance, when ambient densities were falling to winter lows. Multivariate analysis indicated that community recovery occurred by the end of the study period after 4.5 months. Although community structure recovered by the end of the study, the population structure of the dominant species Parapionosyllis longicirrata remained significantly different among ambient and disturbed patches. On all sampling dates except one, disturbance patches had a higher number of larger individuals than ambient sediments. Previous studies have shown that bedload transport of juveniles and adults, and other processes, can cause recolonization to be relatively rapid on intertidal sandflats. However, our results indicate that recovery times may be on the order of months at large disturbance sizes. Therefore, rapid responses may occur primarily in the case of small-scale (<1 m²) disturbance patches. Secondly, recovery at the community level does not necessarily mean that population-level characteristics of species comprising the community have recovered. Population-level differences may be longer lasting than indicated by community level indicators of recovery.     

Responses of Benthic Infauna to Large-Scale Sediment Disturbance in Corpus Christi Bay, Texas

Responses of benthic infauna to large scale disturbance by dredged material placement were studied at eight paired (placement and reference) areas in Corpus Christi Bay, Texas in 1995 and 1996. Total infaunal abundance, taxa richness, and the biomass of annelids and molluscs in placement areas no longer differed from that of reference areas approximately one year after placement. Annual secondary production for annelids and molluscs did not differ between placement and reference areas, however mean (± standard error) total annual secondary production at reference areas (53.6 ± 5.8 g AFDW/m²/yr) was greater than that of placement areas (26.2 ± 2.5 g AFDW/m²/yr) due to higher brittle star densities in reference areas. Differences in community structure between placement and reference areas returned to pre-placement levels one year after disturbance. The magnitude of changes in sediment grain size related to disturbance was positively correlated with the degree of convergence of placement area communities with pre-placement and reference communities. Sediment compaction was consistently associated with infaunal taxonomic composition (BIOENV) and increased linearly with time after disturbance. The use of the term “recovery” with reference to recolonization of dredging-related disturbance implies a return to pre-placement ecological conditions that are frequently neither a reality nor a practical expectation for areas that are subject to repeated disturbances. Our characterization of biological responses to dredged material disturbance targeted benchmarks that were linked to both pre-disturbance conditions and differences between disturbed and neighboring undisturbed areas and indicate that impacts persisted less than one year.     

Effects of Macrobrachium hainanense predation on benthic community functioning in tropical Asian streams

1. Macrobrachium hainanense is a large predatory palaemonid shrimp, present at high densities in pools of low‐order forested streams in Hong Kong. The present study investigated the impacts of M. hainanense on benthic community structure and functions in pools of two streams: Tai Po Kau Forest Stream and Tai Shing Stream. 2. Repeated whole‐pool experiments involving shrimp density manipulations (removal, control and addition) were conducted in both streams between October 2000 and April 2002, and included a wet (May to September) and two dry (October to April) seasons. The three objectives of the study were to determine the effects of M. hainanense predation on benthic macroinvertebrate abundance and species richness, rates of leaf litter breakdown because of effects on detritivores, and periphyton standing stocks by way of an effect on herbivores. 3. Wet season results showed consistent reductions in benthos densities and species richness following heavy rainfall, irrespective of shrimp manipulation. These results suggested that spate‐induced disturbances might override biotic effects and play a dominant role in structuring benthic communities in stream pools in Hong Kong. 4. No significant, reproducible effects on any of the response variables measured in either stream were found during the dry season. Litter breakdown was reduced in the absence of shrimps during one experiment only, suggesting it might be a type I error. These results signified no effect of shrimp removal on benthic communities, or on the functional processes of litter breakdown, or on periphyton accumulation. The large scale of the experimental units (8–40 m²), refuge availability, and the presence of benthic predatory fishes that cropped excess prey made available by removal of M. hainanense, may have contributed to the lack of any effect, despite the abundance of the predatory shrimps.     

EFFECTS OF DISTURBANCE ON EPIPHYTIC COMMUNITY ARCHITECTURE1

Microdistributional patterns of attached algal communities on Cladophora and glass slides were compared under varying disturbance regimes in the Upper Mississippi River, using light and scanning electron microscopy. Localized physical disturbance, induced by boat traffic and wind-generated wave action in the main channel, inhibited development of complex attached algal communities and maintained an adnate flora with two-dimensional architecture. In contrast, communities protected from disturbance developed a more complex, three-dimensional architecture. A high degree of architectural similarity was observed between communities on Cladophora and glass substrates exposed to similar levels of disturbance. Communities exposed to severe disturbance resembled ones in early stages of colonization and development, whereas less disturbed communities were similar to ones in advanced stages of development. These results demonstrated that turbulence strongly influences the community structure of periphyton on both micro and macro scale levels.     

Response of benthic invertebrates to natural versus experimental disturbance in a Swiss prealpine river

1. The crucial point of disturbance experiments in streams is the extent to which they can simulate a natural spate. Ideally, disturbance experiments should proceed side by side with a phenomenological study to allow a direct comparison. In the present study conducted in a prealpine Swiss river, the River Necker, fortuitous events made such a comparison possible. 2. In summer 1994, we took Surber samples one day before and on several sampling dates after a major flood (recurrence interval ≈ 5 years), which was followed by a long period of uniform discharge in a river characterized by frequent spates. Beginning 19 days after this flood, patches of the stream bed (≈ 9 m²) were physically disturbed by kicking and raking. 3. The degrees of reduction in the total number of individuals and the dominant taxa were similar after both types of disturbance, as were the recolonization patterns of Rhithrogena spp., Leuctra spp. and Hydracarina. Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Corynoneura/Thienemanniella spp. showed a distinct lag phase after the flood before recolonization began, whereas there was no such lag phase after the experiment. Therefore, the time needed to recover to pre-flood densities was longer for these taxa. Nevertheless, recolonization rates and patterns after the lag phase were similar to those after the experimental disturbance. 4. Size-class measurements indicated that recruitment from egg hatching may have been more important after the flood than after the experimental disturbance for Rhithrogena spp., but not for Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Leuctra spp. Invertebrate drift was probably the most important pathway of recolonization after both types of disturbance. 5. Our experiment allowed a realistic simulation of several important effects of the large flood on the invertebrate community. Smaller spates that induce substratum movement at a spatial scale similar to our experimental plots are much more common than large floods in the River Necker. For these spates, our experiment should provide an even more realistic simulation of natural disturbance.     

Fish community responses to drying disturbances in an intermittent stream: a landscape perspective

1. Refugia are critical to the persistence of individuals, populations and communities in disturbed environments, yet few studies have considered how the position of refugia within the landscape interacts with the behavioural responses of component species to determine the influence of disturbance events on mobile animals. 2. An 18‐month quantitative electrofishing survey was undertaken on the Selwyn River, a stream that is intermittent in its middle reaches, to determine how the direction and distance to refugia affect the response of fish populations to drying, and how landscape context interacts with flow permanence to produce spatial patterns in communities. 3. Overall, the propensity of fish to take refuge in perennial reaches during drying episodes, and the rate and extent of recolonization from these refugia upon rewetting, depended upon the direction and distance to refugia and the behaviour of component species. 4. In the upper river, Canterbury galaxias (Galaxias vulgaris), upland bullies (Gobiomorphus breviceps) and brown trout (Salmo trutta) migrated upstream to permanent water as the stream dried from the bottom up, but frequent drying and slow recolonization by most species combined to produce a fish community in intermittent reaches that was quantitatively and qualitatively different to that in neighbouring perennial reaches. 5. In the lower river, fish did not appear to migrate downstream to permanent water as the stream dried from the top down, but a lower frequency of drying episodes and faster recolonization by upland bullies and eels (Anguilla spp.) from downstream refugia allowed the fish community in intermittent reaches to converge with that in neighbouring perennial reaches during prolonged wetted periods. 6. Longitudinal patterns of increasing fish density and species richness with flow permanence are interpreted as the product of species‐specific responses to drying events and the spatial position of refugia within the riverscape.