Patterning of impoundment impact on chironomid assemblages and their environment with use of the self-organizing map (SOM)

The paper assesses the impact of the Jeziorsko dam reservoir on chironomid assemblages and selected environmental factors in the Warta River, Poland, by means of patterns recognized with the self-organizing map (SOM, Kohonen unsupervised artificial neural network). Over 1988–1996, in four annual cycles, a total of 233 monthly samples were collected in a seven order section of the river at two sites: WAA (backwater) located about 2 km upstream from the Jeziorsko Reservoir, and WAB (tailwater) located about 1.5 km downstream from the reservoir's dam. At each site three habitats were selected: H₁, H₂ and H₃ at WAA, and H₁₁, H₁₂ and H₁₃ at WAB. H₁ and H₁₁ were located in the depositional area close to the banks, H₂ and H₁₂ about 6–7 m towards the mid-river and H₃ and H₁₃ in the mid-river. SOM effectively vertically separated H₁ and H₁₁ (bank habitats) from H₃ and H₁₃ (the mid-river zone of both sites) and H₂ (the transition zone of the upstream site). The H₁₂ samples were scattered all over SOM but still exhibited a slight temporal gradient. At the end of the study the water discharge, especially in summers, stabilized at WAB at a level lower than natural and as a result submerged macrophytes appeared at H₁₂ making the abundance of macroinvertebrates increase very quickly. Moreover, a weaker horizontal grouping of samples by season and by site of collection (upstream or downstream from the reservoir) was observed over SOM: 1) bank upstream habitat H₁, with hydrological regime resembling natural, was separated from the downstream H₁₁, which enlarged and contracted in response to dam operation, 2) deeper habitats were less dependent on water level and this is why they underwent seasonal fluctuations. To sum up, the deepest habitats were most resistant to water level fluctuations, while the formerly most productive habitat at the tailwater WAB site, H₁₁, became the most negatively impacted. Nevertheless, the reservoir has not negatively influenced chironomid density, because the latter increased closer the mid-river, at H₁₂, where large patches of macrophytes developed.     

Large dam reservoirs are probably long-period oscillators of fish diversity

Long‐term fish population time series obtained annually in a backwater (upstream) and a tailwater (downstream) site of the large lowland Jeziorsko Reservoir, Poland, were analysed and compared to detect impoundment influence on the fluctuation of fish population density and on the fluctuation of a fish diversity measure. The partial‐rate correlation function (PRCF) of all constantly occurring fish species displayed negative first‐order feedback of past density on present density, both upstream and downstream. Weak and irregular cyclic patterns were observed in the autocorrelation function (ACF) of populations of several species upstream and of two species downstream. Most fish species exhibited no significant concordant variations between the two sites, but there were no data supporting different regulation. Probably, different exogenous (environmental) factors control fishes at the two sites. The ACFs of the time series of the exponential of Shannon entropy (N₁ diversity measure) obtained at the Jeziorsko tailwater site, and in the main body of the smaller ?ímov Reservoir, Czech Republic, revealed strong fluctuation patterns. The fluctuation was well fitted by a sine‐wave model of a 9 year period at the ?ímov, although it co‐occurred with a positive linear trend. A sine‐wave model also fitted well with the trend at the Jeziorsko tailwater site, where, however, only one 16·5 year period occurred; hence, it is indicative of cyclicity. The results indicate that dam reservoirs may be oscillators of fish diversity and that part of the literature controversy over the effect of a reservoir on fish populations may be due to a too short sampling period: it is shown that both increase and decrease in diversity may be observed on the basis of samples selected from one diversity time series obtained at the tailwater.     

Patternizing of impoundment impact (1985–2002) on fish assemblages in a lowland river using the Kohonen algorithm

Impoundment impact on fish assemblage structure was investigated in the dammed middle course of the Warta River. A backwater site (AB) was located 2 km upstream of the Jeziorsko Reservoir, and a tailwater site (CD) 1.5 km downstream of the dam. Both sites were studied for 3 years in the pre‐impoundment period (1985–1987) and 15 years after damming (1988–2002). Quantitative electrofishing in spring and autumn assured obtaining yearly average biomass for each population. Most of the data analysis aimed to assess the dam impact on the fish assemblage structure but other accompanying impacts such as discharge manipulations, revetment, different forms of engineering, and water quality improvement in the tailwater and backwater reaches were also discussed. The Kohonen algorithm (self‐organizing map, SOM) was used for the analysis, and perfectly separated AB and CD samples into two clusters. Samples from the backwater (AB) proved that this reach of the Warta River had maintained its almost natural character and that fish assemblages had changed moderately, now occupying only five neighbouring hexagons out of a total of 16. In the tailwater (CD), however, because of considerable fluctuations in fish assemblages the SOM produced three subclusters, which engaged nine hexagons: (i) the pre‐impoundment period (1985–1987, two hexagons); (ii) 7 years after the definite closure of dam sluices (1988–1994, five hexagons); and (iii) the past 8 years of sampling (1995–2002, two hexagons), when stabilization in the assemblage was observed. The SOM also definitely proved profound changes in fish assemblage composition: most lithophilous species declined and many phytolithophilous and phytophilous species became dominants, particularly in the tailwater site where downstream migration of 0+ of successfully spawned species from the reservoir took place.     

Influence of dam removal on trichopteran assemblages in the lowland Drzewiczka River,Poland

The aim of this study is to assess the response of Trichoptera in a patchy tailwater stretch of the Drzewiczka River (Poland) to dam removal as compared to the previous pulse discharge disturbance. The study was carried out in the river at the end of a whitewater slalom canoeing track located downstream of the dam reservoir called Drzewieckie Lake (area 0.84 km²). Between February and April 2002, the dam reservoir was gradually emptied before its dredging, and the Drzewiczka River recovered its natural discharge (1.7–4.5 m³ s⁻¹) for several years (temporary renaturisation). Altogether, 120 monthly samples of macroinvertebrates and their environments were collected in two sampling cycles, S1 in 2000–2001 during high discharge fluctuations (2.1–12.0 m³ s⁻¹ daily) and S2 in 2002–2003 during renaturisation after emptying the reservoir, from the following five habitats: H_P—pool habitat, H_S—stagnant habitat, H_M—macrophyte habitat, H_B—bank habitat and H_R—riffle habitat. On the basis of trichopteran abundance, the patterns in their assemblages were recognised with use of a self-organising map that was a Kohonen artificial neural network. The obtained classification of trichopteran samples was found to be based in general on the spatial criterion, i.e. dependent on habitats, irrespective of which sampling cycle the samples originated from, which clearly showed that the habitat mosaic of the river bed was observed both in S1 and S2. In addition, a very important function was noted for riparian and land plants, which developed intensively at the bottom of the Drzewieckie Reservoir immediately after it was emptied. They restricted extensive transport of reservoir sediments to the downstream river reach, thus reducing differences between S1 and S2 in the tailwater. It is also worth noting that in S1, in comparison to S2, Cyrnus trimaculatus, Mystacides azurea and Lype reducta flowing downstream from the reservoir were observed more frequently in the river, which confirmed that impoundments can be conducive to the presence of certain species downstream of dams. Summing up, although artificial short-term flow fluctuations usually diminish the quality and quantity of benthos, the article presents a case in which they were small and short enough to allow the formation of a mosaic of bed patches and positively affected certain parameters of macrobenthic communities according to the intermediate disturbance hypothesis. Handling editor: David Dudgeon     

Response of Unionid Mussels to Dam Removal in Koshkonong Creek, Wisconsin (USA)

Dam removal is a potentially powerful tool for restoring riverine habitats and communities. However, the effectiveness of this tool is unknown because published data on the effects of dam removal on in-stream biota are lacking. We investigated the effects of a small dam removal on unionid mussels in Koshkonong Creek, Wisconsin (USA). Removal of the dam led to mortality both within the former impoundment and in downstream reaches. Within the former reservoir, mortality rates were extremely high (95&p e r c n t;) due to desiccation and exposure. Mussel densities in a bed 0.5km downstream from the dam declined from 3.80±0.56 musselsm^?2 in fall 2000 immediately after dam removal to 2.60±0.48 musselsm^?2 by summer 2003. One rare species, Quadrula pustulosa, was lost from community. Mortality of mussels buried in deposited silt was also observed at a site 1.7km below the dam. Silt and sand increased from 16.8 and 1.1% of total area sampled in fall 2000 to 30.4 and 15.9%, respectively, in summer 2003. Total suspended sediment concentrations in the water column were always higher downstream from the reservoir than upstream, suggesting that transport and deposition of reservoir sediments likely contributed to downstream mussel mortality. Thus, while benefits of the dam removal included fish passage and restoration of lotic habitats in the former millpond, these changes were brought about at some cost to the local mussel community. Pre-removal assessments of potential ecological impacts of dam removal and appropriate mitigation efforts should be included in the dam removal process to reduce short-term negative ecological effects of this restoration action.     

Summer co‐existence of small‐sized cyprinid and percid individuals in natural and impounded stretches of a lowland river: food niche partitioning among fishes

Due to changes of discharge regime downstream of a dam reservoir, an alluvial natural stretch of the Warta River changed to a macrophyte‐dominated ecosystem. Large patches of submersed, aquatic macrophytes appeared in summer and their effect is analysed in this study. These patches contained enriched macroinvertebrate assemblages (epiphyton and benthos) and they were refuge for both zooplankton and young fishes released from the reservoir. Despite these altered conditions in this stretch, roach Rutilus rutilus, perch Perca fluviatilis and ruffe Gymnocephalus cernua dominated, as they did in the natural backwater. Fishes were sampled every 2 weeks from June to August, together with their food resources to assess the partitioning of the diet among small individuals of the three species in both stretches (the natural and affected ones). The aim of the analysis was to answer how animal food associated with water plants was partitioned between the species. In both stretches, G. cernua were primarily benthivorous, but epiphytic fauna, zooplankton and large‐sized benthic chironomid larvae replaced lack of many large, benthic insects in the tailwater. Levins' food breath index decreased from 0·36 in the backwater to 0·29 in the tailwater. An opposite trend was observed for P. fluviatilis occurring among macrophytes. Perca fluviatilis were competitors of R. rutilus and took food not only in or on the river bed, but also in the water column. They ate zooplankton and epiphytic fauna and Levins' index increased from 0·32 to 0·44 in the tailwater. Rutilus rutilus fed on adult insects, algae and plant fragments in the natural stretch. In the tailwater, these food types were chiefly complemented by zooplankton. Despite this, the niche breadth of R. rutilus was similar at the two sites. Abundance of food associated with the macrophytes appeared to facilitate cohabitation in the abundant fish populations, but P. fluviatilis appeared to benefit the most in the altered river stretch.     

Upstream and downstream responses of fish assemblages to an eastern Amazonian hydroelectric dam

相似文献   

A long-term study (1985–1995) of fish populations in the impounded Warta River,Poland

Three sites, two in the tailwater and one upstream of the backwater of a large man-made reservoir in the middle course of the 808 km long lowland Warta River (Oder catchment), were studied. The pre-impoundment study lasted one (1985), the post-impoundment one 10 yr (1986–1995). The dam contributed to the extinction of the anadromous Vimba vimba, and of the rheophilous Chondrostoma nasus, both cyprinids. A number of directly or indirectly negative changes related to the construction of the reservoir were recorded in the tailwater. These changes affected the species number, spawning groups, diversity, density, standing crop, mean body weight, ABC index, and proportional stock density – PSD. Numerous impacts related to engineering, water chemistry and climate were more negative for fish at the tailwater than at the backwater sites. The hydrology of the site upstream of the backwater continued to be dependent on natural factors. Fish populations there suffered only from accidental impacts of the construction of the dam, such as bank revetment or the clearance of trees and shrubs. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of two differently sized dams on mussel assemblages and growth

Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures.     

The effect of cascaded huge dams on the downstream movement of <Emphasis Type="Italic">Coreius guichenoti</Emphasis> (Sauvage &amp; Dabry de Thiersant, 1874) in the upper Yangtze River

Huge dams (installed capacity > 100 MKW) are generally built on large rivers that display high biodiversity and include major migration routes for many diadromous and potamodromous fish. As a result, these dams lead to severe ecological impacts and receive more attention than smaller dams. Most previous work on the impact of huge dams on fish downstream movement has focused on a single dam and reservoir, so little is known about the effect of cascaded huge dams and reservoirs on downstream movement. During the period 2012–2014, two huge dams (the Xiangjiaba and the Xiluodu) were constructed on the upper Yangtze River and the reservoir impoundments began, respectively in October 2012 and May 2013. These cascaded hydroelectric projects could have a large adverse effect on the downstream movement of an important potamodromous fish species, Coreius guichenoti. To study the effect of cascaded impact of sequential huge dams and reservoirs on passive and active downstream movement of C. guichenoti, eggs and larvae were collected in the Yibin section during 2012–2014 and fish were collected monthly at a site in the Hejiang section from June 2012 to July 2014. Our results showed that, compared to one huge reservoir and dam, cascaded dams exert a more serious effect and obstruct downstream movement of eggs, larvae and young fish (particularly the yearlings and two-year-olds) of C. guichenoti. Individual C. guichenoti were able to pass with relative ease through one reservoir and dam, but passing through both reservoirs and dams was very difficult. To allow access to the spawning grounds upstream, a fish passage should be built on the Xiluodu dam. However, due to the hydropower development in the whole upper basin, captive breeding and maintaining at least 60 km of riverine habitat upstream of a spawning ground could be a more cost-effective approach to maintaining C. guichenoti populations in the Upper Yangtze River.     

Movement of walleye in an impounded reach of the Au Sable River,Michigan, USA

Synopsis We estimated long-range spawning and foraging movements of walleye and observed their use of river and reservoir habitats between two large hydroelectric dams on the Au Sable River, Michigan. We used radiotelemetry to monitor seasonal and daily movements of 11 large walleye. Walleye ranged throughout the entire reach between the two dams. Eight of the 11 fish used both river and reservoir locations. Walleye migrated upriver in April or May and presumably spawned near the dam tailwaters. After spawning, walleye remained in the river for up to 6months, usually establishing local ranges. During this time, they occupied low-velocity refuges within the first 25 km of the upstream dam. They seldom occupied the downstream area, which has higher variation around the mean temperature. Food availability and water temperature may have affected the length of time that walleye remained in the river after spawning. All walleye overwintered in the reservoir. We designed this study to evaluate if walleye have a potential negative impact on brown trout, Salmo trutta. In summer, walleye were often present near sites where fingerling trout were stocked. We found the highest potential for interaction between the two species occurs within the first 25km of the upstream dam during summer. The movement patterns of and habitats used by large walleye validate concerns that walleye could compete with adult brown trout for food and resting sites and prey on juvenile brown trout.     

The influence of Dworshak Dam on epilithic community metabolism in the Clearwater River, U.S.A.

Epilithic community metabolism was determined on a seasonal basis over two years in nonregulated and regulated reaches of the Clearwater River in northern Idaho, U.S.A. Metabolism was estimated using three, 12-liter recirculating chambers and the dissolved oxygen method, with parameters expressed as g O₂ m^?2 d^?1. In the nonregulated reach above the reservoir, gross community productivity (GCP) ranged from 0.8 to 3.2, community respiration (CR₂₄) from 0.3 to 1.2, and production/respiration (P/R) ratios from 1.2 to 3.3. Epilithic metabolism in the regulated reach immediately below the dam increased sharply; GCP ranged from 4.2 to 25.5, CR₂₄ from 1.9 to 9.7, and P/R ratios from 1.4 to 5.7. Increased primary production and respiration in the regulated reach was a result of extensive growth of an aquatic moss (Fontanalis neo-mexicanus). The influence of the dam on epilithic community metabolism was mitigated 2.5 km downstream of the dam due to the regulated North Fork of the Clearwater River (NFCR) merging with the larger, nonregulated Clearwater River. While the regulated Clearwater River below the confluence was somewhat affected by the regulated NFCR flows upstream, metabolism was similar to that found above the reservoir (GCP = 1.2 – 2.6, CR₂₄ = 0.6 – 1.3, and P/R = 1.4 – 2.2). This study demonstrates that while Dworshak Dam has altered both primary production and respiration directly below the dam, the placement of the dam only 2.5 km upstream from a nonregulated reach greatly mitigates its effects on stream metabolism downstream. %     

Community structure of larval Chironomidae (Diptera) in a backwater area of the River Danube

1. The larval chironomid community was studied quantitatively in three cross-sections of an impounded area of the River Danube between July 1985 and July 1986. 2. Sixty-six species were identified, most of which belonged to the subfamily Orthocladiinae, but species of the tribe Chironomini were numerically dominant. 3. The sections differed distinctly in terms of species number and diversity; a tendency towards increasing species richness and diversity was observed in the upstream sections of the impoundment. Marked fluctuations in these two parameters characterized the area close to the dam; the temporal oscillations in diversity values differed significantly between the sections (ANOVA: P<0.05). 4. Clear differences were found between sampling sites in terms of community resilience, the minimum eigenvalue of the community matrix defined as the rate of return of a disturbed system. Longer return times after ‘disturbances’ could be expected at sites further upstream from the dam. 5. Temporal persistence of species composition was estimated for each section with Kendal's correlation coefficient from 1985 to 1986. Sites further upstream of the dam illustrated a significantly greater temporal species persistence over time and a smaller degree of fluctuation, compared to the dam area (ANOVA: P<0.05). A significant negative relationship was found between overall persistence and cumulative monthly water discharge, at the dam site (P<0.01). 6. Classification of the larval chironomid community gave evidence of generally low spatio-temporal overlap between species in the impoundment area.     

Estimation of transport distance of fine particulate organic matter in relation to channel morphology in tailwaters of the Lake Biwa and reservoir dams

Knowledge on transport and deposition of fine particulate organic matter (FPOM) from reservoir dams is increasingly required for habitat management and restoration of dam tailwater ecosystems. Variations in the transport distance of FPOM, however, have never been studied well, particularly in relation to channel morphology, due to channel size restrictions of artificial tracers such as corn pollen when applied to larger river channels. This study aims to show the relations between FPOM retention efficiency and channel morphology in dam tailwaters using lentic plankters as tracers. We estimated the mean transport distance, S _p, by calculating downstream reduction ratios of lentic tracer plankters and calculated the deposition velocity, v _dep. Suspended FPOM samples were collected in tailwaters of two river channels below reservoir dams and two artificial canals below Lake Biwa in the Yodo River system. The longest S _p (19.2 km) and the shortest one (2.2 km) were recorded in the deep canal and shallow canal, respectively, showing a positive correlation with channel hydraulic radius. The values of v _dep were 4.7–6.4 times higher in river channels than in artificial canals. These results indicate that increasing complexity of bed morphology can minimize S _p, whereas bed degradation and armored bed materials may lead to increased S _p. Advantages of lentic plankters as tracers for estimating distance ranges of reservoir dam impact on river ecosystems are also discussed.     

Senescent Reservoirs and Ecological Restoration: An Overdue Reality Check

The recent publication of Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy has generated much scientific, public, and political discussion. Although the book emphasizes the restoration of entire aquatic ecosystems, discussion of senescent dams and human-made reservoirs is absent. The important societal and ecological roles of reservoirs warrant a closer examination of the potential ecological restoration of aging reservoirs. Problems with long-term reservoir management include lack of long-term management strategies, sedimentation, hazardous waste accumulation, impacts of recreational use, and the creation of new aquatic and riparian habitats. Policy conflicts may arise when habitats created in the reservoir are destroyed to restore the downstream habitats or when created habitats upstream undergo successional changes that impact the commercial or recreational value of the reservoir. Rare or endangered species may also create similar conflicts. The establishment of an ecological restoration bonding program that includes environmental education and conservation prior to new dam construction may aid in resolving potential conflicts in the future.     

Variation in the summer diet of age-0 brown trout in a regulated mountain stream

We describe the age-0 brown trout (Salmo trutta) collected from a mountain stream downstream from a reservoir in June, July, and August 1990. Sample sites were 1, 12, and 20 km downstream from the reservoir at elevations of 2840, 2600, and 2230 m above mean sea level. The mean length of fish in the samples increased with downstream progression and from June through August. Zooplankton predominated the diet of fish collected in June from the most upstream sampling site, while Diptera larvae predominated the diet of fish from more downstream sampling sites in June. In July and August, the diet of fish from the upstream site was predominated by Diptera larvae, while the diet of fish from the other two sites was predominated by Ephemeroptera nymphs. Variation in diet among sampling sites and sampling months was related not only to increasing fish sizes over the summer but also to the availability of zooplankton flushed from the reservoir.     

Microbial colonization of seston and free bacteria in an impounded river

A headwater river-reservoir system was studied from March–November, 1982 to determine effects of impoundment on seston quality. We used epifluorescent microscopy to compate and contrast microbial colonization of seston and abundance of free microbes. There was a significant relationship between colonization density and particle size at all sites. Smaller particles were colonized more densely by bacteria than larger particles. Total counts of bacteria (free plus attached) did not differ significantly beween sites. However there were significantly more free bacteria (# ml^–1) in the reservoir and 5 km below the dam than upstream of the impoundment. Density of attached bacteria (# µm^–2) was similar upstream and downstream of the reservoir but slightly higher at lentic sites. Proportionally, attached bacteria were more important upstream (mean 42–45% of total counts) than in the reservoir and downstream (19–31%). We concluded that reservoir seston was of higher quality than riverine seston and major downstream effects included decreases in seston concentration (due to sedimentation) and shifts in proportional abundance of free and attached bacteria. Seston consumers capable of using ultrafine particles (<25 µm), including free bacteria, had higher quality food resources below the dam than in upstream areas.     

Food consumption by fish populations in the Warta River,Poland, before and after impoundment

The impact of impoundment on the energy budget and gross ecological efficiency (K ₁) of fish populations in sites upstream from the backwater (AB) and downstream of the dam (CD) of a manmade reservoir (17.6 km²) located in the middle reaches of the lowland Warta River (808 km long), was investigated.Consumption (MJ ha^–1 yr^-1) in four years following the reservoir's impoundment was 34–74% lower in site AB, and 43–75% in site CD than before the impoundment. K ₁ of investigated species was higher before the impoundment and only pike exhibited an inverse efficiency trend. Significant differences were also observed in the diet of certain dominant fish species, which resulted from a change in the food supply as well as a reduction in their life span after the river was dammed.     

Benthic invertebrate assemblage change following dam removal in a Wisconsin stream

Small dams (height <10 m) have transformed stream networks across the United States. Shopiere Dam was removed from Turtle Creek, a fourth order stream in Southeastern Wisconsin in the fall of 1999. We sampled three sites (upstream of the impoundment, immediately below the dam, and farther downstream) before and after dam removal to identify changes in the invertebrate assemblage following removal. Prior to removal, upstream and downstream sites had similar taxonomic composition. In contrast the dam site had more taxonomic variation. The upstream, dam and downstream sites responded differently to dam removal in analyses of diversity, functional feeding groups, and invertebrate composition. Upstream at the reference site, changes in functional feeding group composition appeared to be associated with a decrease in silt coverage. At the dam site, taxonomic composition changed following dam removal, however diversity and functional feeding groups remained similar. At the downstream site, the invertebrate assemblage remained similar in all analyses. Our observations indicate that the effects of dam removal were not uniform through the stream, rather each site responded in a different way.     

Culturable and VBNC <Emphasis Type="Italic">Vibrio cholerae</Emphasis>: Interactions with Chironomid Egg Masses and Their Bacterial Population

Vibrio cholerae, the etiologic agent of cholera, is autochthonous to various aquatic environments. Recently, it was found that chironomid (nonbiting midges) egg masses serve as a reservoir for the cholera bacterium and that flying chironomid adults are possible windborne carriers of V. cholerae non-O1 non-O139. Chironomids are the most widely distributed insect in freshwater. Females deposit egg masses at the water's edge, and each egg mass contains eggs embedded in a gelatinous matrix. Hemagglutinin/protease, an extracellular enzyme of V. cholerae, was found to degrade chironomid egg masses and to prevent them from hatching. In a yearly survey, chironomid populations and the V. cholerae in their egg masses followed phenological succession and interaction of host–pathogen population dynamics. In this report, it is shown via FISH technique that most of the V. cholerae inhabiting the egg mass are in the viable but nonculturable (VBNC) state. The diversity of culturable bacteria from chironomid egg masses collected from two freshwater habitats was determined. In addition to V. cholerae, representatives of the following genera were isolated: Acinetobacter, Aeromonas, Klebsiella, Shewanella, Pseudomonas, Paracoccus, Exiguobacterium, and unidentified bacteria. Three important human pathogens, Aeromonas veronii, A. caviae, and A. hydrophila, were isolated from chironomid egg masses, indicating that chironomid egg masses may be a natural reservoir for pathogenic Aeromonas species in addition to V. cholerae. All isolates of V. cholerae were capable of degrading chironomid egg masses. This may help explain their host–pathogen relationship with chironomids. In contrast, almost none of the other bacteria that were isolated from the egg masses possessed this ability. Studying the interaction between chironomid egg masses, the bacteria inhabiting them, and V. cholerae could contribute to our understanding of the nature of the V. cholerae–egg mass interactions.