Benthic macroinvertebrates associated with four species of macrophytes

Benthic macroinvertebrates associated with four species of macrophytes (Nymphoides peltata, Ceratophyllum demersum, Polygonum amphibium and Carex sp.) were investigated during two growing seasons (2001 and 2002) in the slow-flowing Čonakut Channel in the Kopački rit Nature Park in Croatia. A total of 31 macroinvertebrate taxa were found. C. demersum, a submerged plant with dissected leaves, supported the highest macroinvertebrate abundance, almost seven times more than N. peltata, a floating plant with undissected leaves, which harboured the lowest abundance during the research period. Chironomidae larvae (50–83%) and Oligochaeta (14–46%) were the most abundant groups recorded on all macrophyte species. Water-level fluctuation, because of its influence on the appearance and growth of aquatic vegetation, and the trophic state of water within the macrophyte stands seemed to be the main factors which affected the taxonomic composition and abundance of macroinvertebrates.     

Distribution and abundance of macroinvertebrates within two temporary ponds

We investigated the distributions of macroinvertebrates within two temporary ponds (Spring Peeper Pond and Taylor-Ochs Pond) in central Ohio and examined what environmental factors may be driving those distributions. We sampled macroinvertebrates in Spring Peeper Pond three times from May to July 2001, and Taylor-Ochs Pond two times from May to June 2001. Macroinvertebrate distributions were significantly aggregated on all sampling dates in both ponds. Bivalve abundance in Spring Peeper Pond was higher in shallower water. The distribution of bivalves in Taylor-Ochs Pond was not correlated with any variable we measured. Dragonfly nymph abundance in Taylor-Ochs Pond decreased between the first and second sampling dates, whereas in Spring Peeper Pond no factor examined was correlated with dragonfly nymph density. Snail densities in Spring Peeper Pond were negatively related to dissolved oxygen and depth. In Taylor-Ochs Pond, snail abundance was positively related to temperature. The densities of damselfly nymphs in Spring Peeper Pond were positively related to dissolved oxygen and depth and declined across the study. In Spring Peeper Pond, hemipteran densities were negatively related to depth and increased across the study. Damselfly nymphs and hemipterans were not common enough in Taylor-Ochs to analyze. In general, the abiotic and biotic factors we examined explained relatively little (<37% in all cases) of the within pond distribution of the macroinvertebrates in our two study ponds.     

Predation and drift of lotic macroinvertebrates during colonization

Summary A field experiment was carried out to determine the effect of an invertebrate predator on the colonization and drift of benthic macroinvertebrates in experimental stream channels. Lotic invertebrates colonized four replicate channels: two controls with no predators, and two channels with low densities (2.8 m^–2) of predatory stonefly nymphs, Doroneuria baumanni (Perlidae). Immigration rates were measured at the inflow of two other channels. Drift rates of invertebrates immigrating to and emigrating from channels were measured daily, and benthic samples were collected every five days. Over a 25-day colonization period, benthic densities of Baetis nymphs and larval Chironomidae were reduced by D. baumanni. Colonization curves were fit with a power function and significantly different colonization rates were indicated for both Baetis and chironomids in predation and control channels. A predator-induced drift response was exhibited by Baetis only and this response was size-dependent. In the presence of D. baumanni, large Baetis drifted more frequently than small nymphs and, correspondingly, small nymphs were more frequent in the benthos. Net predator impacts on invertebrate densities in channel substrates were partitioned into predator-induced drift and prey consumption. These estimates suggest that predator avoidance by Baetis is a prominent mechanism causing density reductions in the presence of predators. Reductions in the density of Chironomidae, however, were attributed to prey consumption only. A rainstorm during the experiment demonstrated that stream flow disruptions can override the influence of predators on benthic invertebrates, at least temporarily, and re-set benthic densities.     

The effect of river restoration on fish,macroinvertebrates and aquatic macrophytes: A meta-analysis

An increasing number of rivers have been restored over the past decades and several studies investigated the effect on biota. The published monitoring results have already been summarized in narrative reviews but there are few quantitative reviews and a comprehensive meta-analysis on different organism groups and factors influencing restoration effect is missing. We compiled monitoring results and information on catchment, river and project characteristics from peer-reviewed literature and unpublished databases to (i) quantify the effect of restoration measures on fish, macroinvertebrates and macrophytes, and (ii) identify predictors which influence restoration effect. Results indicated significant effects of restoration on all three organism groups, especially of widening projects on macrophyte richness/diversity, instream measures on fish and macroinvertebrates, and higher effects on abundance/biomass compared to richness/diversity. Restoration effect was most strongly affected by agricultural land use, river width and project age. Effects were smaller but restoration generally still increased richness/diversity and abundance/biomass in agricultural catchments. Since land use is a proxy for different pressures, the underlying causal relationships have to be investigated in more detail. Project age was the most important factor but had non-linear and even negative effects on restoration outcome, indicating that restoration effects may vanish over time. The meta-analysis indicated that river managers in general can expect an increase of richness/diversity and abundance/biomass of all three organism groups investigated, especially of macrophytes in widening projects and of fish and macroinvertebrates if instream measures are applied. However, variability was high, stressing the need for adaptive management approaches. Furthermore, the large but non-linear and different (even negative) effects of project age stressed the need for long-time monitoring to better understand the trajectories of change caused by restoration measures and to identify sustainable measures. The meta-analysis was restricted to metrics commonly reported in literature and future studies would greatly benefit from authorities and scientists reporting original monitoring data, which would allow to use functional metrics to investigate the effect of restoration measures and to infer causal relationships.     

Comparisons among colonization of artificial substratum types and natural substratum by benthic macroinvertebrates

A field study was designed to concurrently evaluate differences in colonization by benthic macroinvertebrates on a range of artificial substratum types (single particles of natural rock or clay brick and baskets of natural substratum) after three colonization periods (1, 8 and 29 days). Fauna on the artificial substrata were compared to natural substratum and the effect of natural epilithic cover on colonization by zoobenthos was determined. Densities of total number of organisms and the seven most abundant taxa, total number of taxa and quantity of organic material were greater on the natural substratum than on the artificial substratum types. Relative abundances of taxa on pairs of the artificial substratum types, unlike pairs of each artificial substratum type and the natural substratum, were statistically correlated. Among the artificial substratum types densities of total number of organisms and about one-half of the most abundant taxa, total number of taxa and quantity of organic material were greatest in the substratum baskets. Natural epilithic cover on the single rock particles and substratum baskets affected the densities of total number of organisms and two of the seven most abundant taxa. These taxonomic groups were at approximately two to six-fold greater densities on the substrata with fine sediment. consistent patterns in densities of the zoobenthos on the substrata were found after each colonization period. In our study all measures of the macroinvertebrate assemblages (densities of each taxon, total number of organisms, total number of taxa and relative abundances of taxa), with few exceptions, were different between each artificial substratum type and natural substratum. This result showed the abundance and composition of the macroinvertebrate fauna on artificial substratum types were different from the natural substratum. Therefore, the choice of using artificial substrata instead of direct sampling of the natural substratum should be carefully made. Among the artificial substratum types relative abundances of taxa were similar on the single substratum particles and substratum baskets indicating single particles instead of baskets might be used to sample the zoobenthos. Investigators should consider the potential effect of the natural epilithic cover of substratum particles on colonization by zoobenthos when choosing the type of artificial substratum.     

Epiphytic algae and macroinvertebrates on submerged and floating-leaved macrophytes in an Italian lake

1. The presence of contiguous beds of submerged (Myriophyllum spicatum, Ceratophyllum demersum and Najas marina) and floating-leaved (Trapa natans) vegetation in a north Italian lake allowed us to test the effect of the different host architecture on epiphytic algae and invertebrates and to predict the consequences for the lake of changes in the predominant vegetation. 2. Epiphyton development, measured as carbon, nitrogen, phosphorus, chlorophyll a (Chl a), phaeophytin and as algal and macroinvertebrate density, was significantly higher on submerged plants than on T. natans. The C : Chl a ratio, a proxy of the ratio of heterotrophs to autotrophs, was higher on the floating-leaved plants. The elemental (C : N : P) and pigment (Chl a : phaeophytin) ratios were not significantly different between the two vegetation types. 3. The taxonomic composition of epiphytic algae and invertebrates was similar on the different plants. The more varied morphology of the floating-leaved T. natans resulted in a higher diversity of epiphytic algae, however, but not of macroinvertebrates. 4. There was a significant inverse relationship between epiphyton biomass and the standing crop of the host plant, suggesting a key role for light and water exchange in epiphyton development. 5. Replacement of floating-leaved by submerged plants would increase the total biomass of epiphytic algae and invertebrates.     

The effects of shoot age on colonization of an emergent macrophyte (Typha latifolia) by macroinvertebrates

1. The colonization and dynamics of epiphytic aquatic macroinvertebrates are described on first-year and second-year shoots of an emergent macrophyte (Typha latifolia) in a Swiss pond. Effects of shoot senescence on composition, richness, density and biomass of the macroinvertebrates are quantified. 2. There were two phases of colonization: a short-term colonization process which corresponded with that usually observed on inert substrates and a longer term colonization process related to the attractiveness of the shoots for the colonizers. In this second process, the older shoots showed a higher attractiveness for most invertebrate taxa. 3. Taxa colonized the shoots at different rates. Rapid colonizers included the limpet Ferrissia wautieri and the mayfly Cloeon dipterum. Conversely, Oligochaeta, Chironomidae and Ceratopogonidae were particularly slow colonizers. 4. The older shoots supported a higher mean annual richness, abundance and biomass of invertebrates. Compared with younger shoots, the older shoots demonstrated a higher carrying capacity for most invertebrate taxa, as is the case for many other freshwater macrophytes.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.     

Dry weight loss and colonization of plant litter by macroinvertebrates: plant species and lake types compared

Rates of dry weight loss and of colonization of litter samples of different plants by macro-invertebrates were estimated using litter bags submerged in different types of lake. Samples of Carex rostrata litter were submerged for 3 months, in summer, in two lakes, one with an acidic and nutrient-poor water and one with near-neutral and nutrient-rich water. More plant litter remained (75%) in the former lake than in the latter (51–62%, varying with the mesh size of the litter bags used). The biomass of invertebrates in the litter samples from the acid lake was about 10 times less than in those from the near-neutral. Oligochaeta and Chironomidae were the dominant taxa found in the litter bags from the acidic lake, whereas Hirudinea and Isopoda predominated in those from the near-neutral lake.Litter samples from four different plants (Carex rostrata, Typha latifolia, Triticum sp. and Phleum pratense) were immersed in litter bags in another near-neutral lake further south for 12 months. When recovered, about 45% of the Triticum litter remained compared with only 7% of the Phleum. The values for Carex and Typha fell between these two extremes. When expressed as per g dry weight of residual plant litter, the greatest biomass of invertebrates was recorded in the Phleum litter samples and the least in the Typha samples. The faunal composition of the invertebrates in the different types of litter was approximately the same, with Isopoda and Hirudinea predominating.     

Distribution of selected macroinvertebrates in a mosaic of temporary and permanent freshwater ponds as explained by autologistic models

We investigated the aquatic macroinvertebrate fauna of 76 ponds and small pools in an urban fringe landscape, and related the presence of ten species to measures of water permanence, pond area and environmental conditions using logistic models. The incidence of all the species was strongly associated with variation in hydroperiod, but patterns were more variable with the other explanatory variables. To determine whether the presence of a species at neighbouring ponds increased its probability of occurance at a pond we constructed a series of autologistic models, that differed from the aspatial logistic model in that they included a spatial autocovariate in the predictor terms. The improvement of model fit on inclusion of this autocovariate, measured as the decline in deviance compared to the aspatial models, was determined across a range of lag distances. In seven of the ten species, the autologistic models explained the incidence of the species amongst the ponds better than the aspatial models. Spatial effects were typically over short distances (<200 m) before declining, but in two species appeared to reach an asymptote, and we propose that variation in dispersal ability is the most likely factor producing these spatial effects. We conclude that it is essential that some measure of spatial autocorrelation is considered when evaluating the distribution of aquatic macroinvertebrates at small or medium scales.     

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.     

Experimental colonization of sand, gravel and stones by macroinvertebrates in the Acheron River, southeastern Australia

SUMMARY. 1. The contribution of drift to macroinvertebrate colonization was studied over 10 days using pairs of substratum-filled trays, one suspended above the streambed (and thus able to be colonized by drift only) and one buried flush with the streambed (and thus open to colonization from all directions). Trays were filled with sand, gravel or stones.
2. Colonization of sand-filled trays was rapid, being completed within 24h (i.e. no subsequent change in numbers of individuals per tray); colonization of gravel or stone-filled trays was not completed by the end of the experiment.
3. Drift contributed an average of 86% of the colonizing fauna of the sand-filled trays, 36% of the fauna of the gravel-filled trays and 25% of the fauna of the stone-filled trays. The low contribution of the drift to the gravel and stones is at odds with some studies which, using similar techniques, suggest that drift is the primary source of colonizing individuals.
4. Most individual species showed low contributions by drift to colonization, with the majority having contributions under 66%.
5. Distinct differences were noted between the fauna colonizing the sand-filled trays and those colonizing the gravel or stone-filled trays (which were similar).
6. Several species common in the drift were rare in the colonizing fauna and several species which were common colonizers were absent or rare in the drift.     

Assessing trout farm pollution by biological metrics and indices based on aquatic macrophytes and benthic macroinvertebrates: A case study

In this study we examine the ecological responses of aquatic macrophytes and benthic macroinvertebrates to a trout farm effluent located in the upper Tajuña River (Guadalajara province, Central Spain), comparing the suitability of biological metrics and indices based on these two communities to assess trout farm pollution in the upper reaches of fluvial ecosystems. Sampling surveys were carried out in winter and summer 2006, and in spring 2007. Four sampling sites were selected along the study reach: S-1, placed upstream from the trout farm, was used as a reference station; S-2, S-3 and S-4 were placed about 10, 100 and 1000 m downstream from the trout farm outlet, respectively. The stream bottom was mainly stony with cobbles and pebbles at all sampling sites, except at S-2 where it was covered by a thick layer of organic sediment. Aquatic macrophytes and benthic macroinvertebrates significantly increased in abundance downstream from the trout farm effluent, particularly at S-3 and S-4. However, while the diversity of aquatic macrophytes increased downstream (at S-3 and S-4, but not at S-2), the diversity of benthic macroinvertebrates significantly decreased, particularly at S-2. Regarding aquatic macrophytes, moss coverage values, as well as values of the Index of Macrophytes (IM) and the Macroscopic Aquatic Vegetation Index (MAVI), were higher at S-1 than at S-2. In contrast, values of moss coverage and IM were significantly lower at S-1 than at S-3 and S-4. MAVI values were similar at S-1, S-3 and S-4. Regarding benthic macroinvertebrates, the abundance percentage of oligochaetes and chironomids (expressed as OC%) was significantly higher at downstream polluted sites (particularly at S-2) than at S-1. Conversely, the abundance percentage of ephemeropterans, plecopterans and trichopterans (expressed as EPT%), and scores of the total Biological Monitoring Water Quality (t-BMWQ) index and the average Biological Monitoring Water Quality (a-BMWQ) index were much lower at S-2 than at S-1, with a clear tendency to increase with increasing downstream distance from the trout farm. Correlation analyses showed that changes in physicochemical parameters (inorganic nutrients, dissolved oxygen and turbidity) along the study reach were better correlated with changes in metrics and indices based on benthic macroinvertebrates. Overall it is concluded that metrics and indices based on benthic macroinvertebrates (such as OC% and EPT% metrics and t-BMWQ and a-BMWQ indices) can be more suitable than those based on aquatic macrophytes for the biological monitoring of trout farm pollution in the upper reaches of fluvial ecosystems, since the higher diversity of benthic macroinvertebrates inhabiting the river bottom would permit this community to exhibit a better indicator performance regarding the environmental stress caused by trout farm effluents. However, further studies are needed in order to generalize this conclusion.     

Strong top-down effects of omnivorous fish and macroinvertebrates on periphytic algae and macrophytes in a tropical reservoir

Aquatic Ecology - Top-chain predators play a fundamental role in the functioning of shallow reservoirs. Studies to elucidate the cascade effects produced by fish on macroinvertebrates and...     

How consumption and fragmentation of macrophytes by the invasive crayfish Procambarus clarkii shape the macrophyte communities of temporary ponds

Crayfish are major herbivores in freshwaters, and their activity may change invaded freshwater ecosystems. Macrophyte removal by crayfish results from consumption and fragmentation of plant material and may depend upon the palatability of each species. We studied the impact of Procambarus clarkii on the macrophyte community of Mediterranean temporary ponds, testing in laboratory if consumption and fragmentation of five macrophyte species were correlated in palatability tests and in a preference test. We performed an experiment in a natural pond, where we introduced crayfish and estimated its effects on macrophyte biomass, number of rooted stalks and number of floating cut stalks. P. clarkii consumed preferably Juncus heterophyllus in both tests and avoided Carex divisa and Ranunculus peltatus in the preference test. In the presence of preferred species, consumption and fragmentation of the non-preferred species were heavily reduced. In the field trial, the “Crayfish” compartment had 31% less macrophyte biomass, 41% less rooted stalks and 92% more floating cut stalks. Crayfish may remove macrophyte species from the community sequentially, from the most to the least preferred species. Impacts of crayfish in temporary ponds may depend on time of invasion and on the composition of the macrophyte community.     

Comparative monitoring of diatoms, macroinvertebrates and macrophytes in the Woluwe River (Brussels, Belgium)

The river Woluwe in Brussels and Flanders (Belgium) is a small tributary of 15 km length that drains an area of 9400 ha in the Schelde river basin. The headwaters of the Woluwe are highly fragmented by diverse pond systems and are vaulted in the Brussels agglomeration. Hyporheic zones locally influence the water quality. The downstream stretch of the river receives sewage waters from households and industry. As the river Woluwe within a short distance represents a typical gradient from groundwater-fed sources in the forest towards severely polluted water, a comparative monitoring using diatoms, macroinvertebrates and macrophytes was done. The saprobic index based on diatoms, the Belgian Biotic Index (BBI) for macroinvertebrates and a macrophyte index based on the N-values of Ellenberg were used in this comparison and for estimating the correlation with the bimonthly measured chemical variables in 16 sampling stations. The diatom saprobic index and the macrophyte index were strongly correlated. Both groups showed strong correlations with phosphate, ammonium and chemical oxygen demand. The Belgian Biotic Index showed lower correlations with the nutrient variables, but was slightly better correlated to chemical oxygen demand, chloride and dissolved oxygen. None of the indices showed a correlation with nitrate. Local substrate or light conditions could interfere with the indicator system, especially for the macrophytes and occasionally for the macroinvertebrates. It was concluded that at least in this particular river system, the indices based on the primary producers were more indicative for the trophic status, whereas the BBI showed a broader relationship to the general degree of pollution. Therefore, these three indices are considered as complementary for monitoring the biological quality and the ecological status of a river system.     

Surveying biodiversity in protected and managed areas: Algae, macrophytes and macroinvertebrates in boreal forest streams

Extensive surveys of biodiversity in protected and managed areas have not been conducted for a majority of taxonomic groups and ecosystem types, which makes it difficult to assess how large a portion of biodiversity is at least potentially under protection. The situation is the same in boreal regions, and only preliminary analyses of the biodiversity patterns of less well-known organism groups, including many freshwater taxa, within the protected area network have been conducted. We studied patterns of species richness and community composition of algae, macrophytes (bryophytes and vascular plants), and macroinvertebrates of headwater streams draining protected areas and managed forests in a boreal drainage basin in Finland. We found no significant differences in the species richness and community composition of these organism groups between the protected and managed streams. Gamma- and beta-diversity varied strongly among the protected and managed stream groups, yet this variation was contingent on the organism group and the beta-diversity measure used. In general, there was much species turnover within both protected and managed stream groups, masking any between-group differences. However, we found a number of redlisted and rare species in our surveys. Of these species, several macrophyte species occurred more frequently in the protected streams. By contrast, rare species of algae and macrophytes did not generally show such inclinations to the protected streams. We found no strong congruence in species richness or community dissimilarity between algae, macrophytes, and macroinvertebrates, suggesting that the main anthropogenic gradient in terms of forestry is not strong enough to modify stream environmental conditions and thereby shape biodiversity in the focal drainage basin. This finding also suggests that surveys of aquatic biodiversity across protected and managed landscapes should not rely too heavily on the surrogate taxon approach, but instead should consider patterns shown by multiple taxonomic groups that represent biologically and ecologically disparate organisms. Our results indeed suggest that the levels of alpha-, beta-, and gamma-diversity show differing among-taxon responses to forest management and naturalness of headwater streams.     

Influence of macroinvertebrates and macrophytes on waterfowl utilization of wetlands in the Prairie Pothole Region of northwestern Wisconsin

Waterfowl and limnological data were monitored on Waterfowl Production Area (WPA) wetlands in northwestern Wisconsin over a 6-yr period (1983–88) to determine the impact of macroinvertebrates and macrophytes on waterfowl utilization. Interrelationships between limnological conditions and Waterfowl Breeding Pair Densities (BPDs reported as pairs/ha water surface) were analyzed using correlation and general linear model analysis techniques.Annual changes in waterfowl BPDs differed between wetlands according to differences in the structure of macrophyte communities and basin morphometry. The strength of associations differed between the two dominant waterfowl species. In a wetland dominated by dense stands of submersed vegetation, annual fluctuations in blue-winged teal (Anas discors) BPDs corresponded directly with changes in macrophyte biomass, but not with changes in macroinvertebrate density. In a nearby less densely vegetated wetland of similar water chemistry and trophic status, fluctuations in teal BPDs corresponded directly with changes in macroinvertebrate density, but not with changes in macrophyte biomass. These associations occurred despite a significant positive correlation between macroinvertebrates and macrophyte biomass in the latter habitat. Annual fluctuations in mallard (Anas platyrhynchos) BPDs were not correlated significantly with either macrophyte biomass or macroinvertebrate density in either wetland.     

An experimental test of the effects of food resources and hydraulic refuge on patch colonization by stream macroinvertebrates during spates

1. The passive or active movement of organisms between habitat patches plays important roles in achieving ecosystem resilience to disturbance and long-term control of population levels. However, causal mechanisms of disturbance-induced movements of mobile biota across heterogeneous habitat patches at a relatively short time-scale are little understood. 2. We experimentally tested the effects of food resource values on macroinvertebrate colonization of hydraulic refugia from spates in a second-order creek. Experimental cages were colonized by macroinvertebrates with combinations of resource types (natural or polyester leaves), and extent of exposure to stream flow (exposed to or sheltered from current); one half of each set was collected before and after a spate. This experiment was repeated over three spates of varying magnitude and seasonal contexts. 3. Pre-spate colonization was consistently greater for the cages with natural leaves relative to artificial leaves regardless of the extent of flow exposure. Two autumn spates with relatively low and stable antecedent flow conditions caused large movements of organic matter and macroinvertebrates across the stream, showing community-level accumulations into hydraulically sheltered patches independent of food treatment. The smallest spate with high and variable antecedent flows during winter resulted in negligible responses, which we interpret to be a result of depletion of easily transportable organic matter and organisms. 4. Two detritivorous taxa, the mayfly Paraleptophlebia spp. and stonefly Despaxia augusta (Banks) were the most responsive to autumn spates, and had disproportionately higher colonization rates of cages when provided with natural leaves during the largest autumn spate. Preferential settlement in food-enriched hydraulic refugia was attributable to taxon-specific mobility related to efficient acquisition of detritus resource, whose availability varies spatially and temporally. 5. Our findings suggest (1) detritivorous macroinvertebrate colonization of hydraulic refugia can be influenced by hydraulic controls as well as food resource value, and (2) pre-spate environmental conditions in terms of resource distribution and availability may pre-condition organisms' susceptibility to spates and also affect refugium usage, at least in food-limited, detritus-based stream systems.