Macrophyte Patches Research Articles

Inter-annual and habitat variation of the mayfly assemblage in grassland and pine afforested headwater streams (Córdoba, Central Argentina)

Mayflies are widely used as indicators of stream conditions since their presence and abundance are closely associated with changes in environmental characteristics. We aim to analyze the richness and abundance patterns of mayflies in headwater streams of the Ctalamochita river basin in central Argentina, where some sub-basins have been fully afforested with exotic pines. We sampled 3 grassland streams and 3 pine afforested streams in 2008, 2012, 2014, and 2015. We took Surber samples in riffles and in 2012 we also sampled 2 additional habitats: pools and macrophyte patches. We found ten mayfly genera. Americabaetis and Leptohyphes were the most frequent and abundant and Cloeodes and Tricorythodes were rare, with a relative frequency < 3%. Richness and abundance were higher in grassland streams (ANOVA, p < 0.05), but the abundance pattern of different mayfly genera varied. In 2012 mayfly abundance was higher in pools and macrophytes in the afforested streams (ANOVA, p < 0.05), but it was similar among habitats in grassland streams. Different species dominated different habitats and stream types, and some shared the same habitat. In riffles and pools, the dominant genera was similar for afforested and grassland streams, while in macrophyte the dominance varied according to the riparian vegetation. Distribution patterns are influenced by microhabitat characteristics, which may be conditioned by changes in riparian land uses.

Interactions between Aquatic Plants and Cyanobacterial Blooms in Freshwater Reservoir Ecosystems

Climate change and nutrient pollution are echoed by worldwide increasing trends in the frequency, duration, and toxicity of cyanobacterial (blue-green algal) blooms. Therefore, searching for the best options to mitigate blooms is relevant and timely. Aquatic vascular plants offer a promising solution through biological control. In this study, we use reservoirs regularly affected by intensive blooms (the Kyiv and Kaniv Reservoirs of the Dnipro River, Ukraine) to investigate whether macrophytes may inhibit or reduce the massive development of cyanobacteria. Special attention was paid to plants with floating leaves and free-floating plants since data on their effects on cyanobacteria are controversial. On the basis of field and satellite observations, the spatial distribution of cyanobacterial blooms and aquatic macrophyte patches was assessed. Multispectral images captured by satellites Sentinel-2a (S2A) and Sentinel-2b (S2B) were used. In addition, based on data from field observations, a comparative analysis of phytoplankton and physical and chemical parameters between areas of the reservoirs overgrown and not overgrown by macrophytes was carried out. The obtained results indicate that in macrophyte patches phytoplankton structure differed from that observed in open waters. However, in areas of reservoirs dominated by floating-leaf plants or free-floating plants, a significant decrease in phytoplanktic or cyanobacterial biomass was not observed. This is most likely due to the fact that these macrophytes did not reduce the concentration of biogenic substances to a level that would limit cyanobacterial growth. On the contrary, intensive overgrowth of floating-leaf plants (in particular, Trapa natans) along the river sections of the reservoirs, as well as other factors, contributed to nitrogen and phosphorus enrichment. Therefore, in the face of relevant nutrient supply, these ecological groups of macrophytes (floating-leaf plants and free-floating plants) have not shown statistically significant effectiveness in controlling the process of cyanobacterial blooms in reservoir ecosystems.

Balancing ecosystem integrity and cultural values at sacred Lake Hora, Ethiopia: The need for conservation of wetland vegetation

AbstractAlthough macrophytes represent important habitats for the plankton communities of shallow lakes and ponds, the role of littoral macrophyte patches in deep crater lakes is not well described. The objective of the present study was to investigate the role of littoral macrophyte vegetation in structuring phytoplankton and zooplankton communities and, therefore, sustaining ecological integrity in a deep tropical crater lake (Hora Lake, Ethiopia). The lake has a surface area of 1.03 km2 and a mean depth of 16.5 m. The area surrounding the lake is a prime location for the celebration of the Oromo thanksgiving festival (Irreecha), which is inscribed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as an intangible cultural heritage, and also considered sacred by the Oromo people. Measurements of physical, chemical and biological parameters were conducted at fifteen sampling sites in both the littoral and central parts of the lake during the dry and wet seasons in 2017. A total of 28 phytoplankton and 19 zooplankton genera were identified. The phytoplankton and zooplankton communities were dominated by cyanobacteria and rotifers, respectively, at the open water sites. The wetland vegetation was found to be important in reducing potentially toxic and unsightly cyanobacteria. The submerged and emergent vegetation also served as refugia for zooplankton during the wet and dry seasons, respectively. Crustaceans accounted for about 30–45% of the total zooplankton abundance within the vegetation fringes. Their abundance decreased to 5% in the non‐vegetated areas, wherein the community was dominated by rotifers. As the population around the lake, particularly during the festival, may damage the aquatic vegetation, their access to the shoreline should be regulated for conservation and sanitary reasons (e.g. to reduce cyanobacteria growths).

Environmental filtering and low trait redundancy characterise fish assemblages in Lake Mamo, Orinoco River floodplain, Venezuela

The study of assembly rules of fish communities in Neotropical floodplain lakes represents a major interest in community ecology due to their high species diversity and environmental variation, both spatial and temporal. In this study, assembly rules of freshwater fishes in Lake Mamo were explored. The main goals of this research were to analyse the seasonal variations in taxonomic and functional diversities of fishes in sandbanks and patches of aquatic macrophytes, in order to identify whether the fish assemblages are organised according to environmental filtering or limiting similarity, and to explore the relationships between taxonomic and functional diversity to determine if fish assemblages are either functionally redundant or complementary. In both mesohabitats, fishes were collected using seine nets during four hydrological seasons corresponding to low waters, rising waters, high waters and falling waters. Taxonomic and functional structure of fish assemblages in each habitat and across hydrological seasons was examined through multidimensional scaling (MDS), PERMANOVAs and principal co-ordinate analyses. Seasonal variations in taxonomic diversity were determined through Fligner tests of Shannon diversity indices, and for functional diversity, of functional richness (FRich), functional dispersion (FDisp), functional evenness (FEve) and functional divergence (FDiv). Null model analyses were used to determine if the functional indices diverged from randomness across hydrological seasons. Finally, relationships between functional indices and Shannon diversity were explored using linear, power, asymptotic and logistic models. There were significant differences in taxonomic and functional composition between mesohabitats and more markedly across hydrological seasons. All fish assemblages were significantly underdispersed at all instances, while functional evenness was higher than expected in both mesohabitats during low waters, signaling both a strong environmental filtering and niche packing. There were significant linear associations between the Shannon diversity and functional dispersion and between the Shannon diversity and functional evenness, suggesting that functional redundancy in these assemblages is low. In Lake Mamo, fish assemblages are filtered first by seasonality and then by habitat type. As more species are added to the assemblages, niches are more packed in the functional space, but the number of traits increases at a constant rate. This means that if a lake becomes impaired as a consequence of potential oil spills, the loss of fish species could represent also the loss of traits and functions in the ecosystem.

Changes in ecosystem functions generated by fish populations after the introduction of a non-native predator (Cichla kelberi) (Perciformes: Cichlidae)

Abstract The introduction of non-native predators is a matter of great concern, but their impacts on ecosystem functions remain poorly understood. We investigated how changes in fish diversity following the invasion of Cichla kelberi affected ecosystem functions generated by fish populations. Fish assemblages were sampled in macrophyte patches in a Neotropical impoundment over a 5-year period, before and after the introduction of the predator. We assigned seven ecosystem functions (26 trait-states) to each fish species, and examined how these functions behaved after the invasion. We collected 577 fish belonging to 25 species. Species richness, fish biomass and main species declined significantly over periods. The biomass of ecosystem functions changed significantly over time, and most trait-states declined. Few trait-states were lost, but all functions had at least one trait-state reduced by more than 85%. A null model analysis showed that changes in functions were not driven by species identities, while species richness correlated positively with total biomass and with most functions, suggesting that the loss of taxa and biomass drove observed changes in ecosystem functions. Our study provided evidence that community disassembly associated with the invasion of C. kelberi translated to the decline of several ecosystem functions, affecting energy mobilization and transference.

Spatial and seasonal variation in ecosystem metabolism are associated with aquatic macrophyte traits, shading, and water temperature in a shallow riparian pond

How much do traits of aquatic macrophytes affect rates of ecosystem metabolism in a shallow riparian pond? The factors that drive patterns of primary production and ecosystem respiration (ER) in wetlands remain poorly understood even though metabolism is one of the few ways that we can measure the response of an entire ecosystem to human intervention. We measured the metabolic rates of macrophyte patches in the spring, summer, and autumn by placing oxygen sondes in clear mesocosms embedded in the mud of a small, shallow pond (0.3–0.6 m deep) in central Utah, USA. We hypothesized that all patches (Chara, metaphyton, and Potamogeton foliosus) would be autotrophic (gross primary production [GPP] would exceed ER) in spring, whereas all patches (Chara, metaphyton, and Lemna) would be heterotrophic (ER > GPP) in summer. We also hypothesized that shading by the floating macrophyte, Lemna, would reduce the GPP of all submersed macrophytes, thereby producing heterotrophic conditions throughout the pond in autumn. As predicted, all macrophyte patches were autotrophic in spring, but the GPP of Chara and metaphyton was 1.5 and 1.4× greater than the GPP of P. foliosus, respectively. In summer, Chara and metaphyton maintained high rates of GPP, but net aquatic production (NAP = GPP – ER) was heterotrophic in all patches (Chara, metaphyton, and Lemna), and all patches produced long periods of nighttime hypoxia (between 9.5–15 h). Hypoxia in summer was likely caused by warm temperatures often exceeding 35°C. In autumn under a mat of Lemna, GPP and ER were small, and hypoxia declined as temperatures declined. ER in this pond was driven by macrophyte respiration and the decomposition of macrophytes, not by the decomposition of allochthonous carbon as in most freshwater ecosystems. Light saturation of photosynthesis was common in summer when Chara produced the highest rates of GPP. Overall, rates of GPP were closely balanced by rates of ER such that NAP fluctuated between slight heterotrophy and slight autotrophy in all macrophyte patches in spring and summer before the pond was covered with Lemna. Traits of aquatic macrophytes (e.g., presence of vascular tissue, growth form) produced patchiness in rates of metabolism within a season, whereas hydrology and interactions among macrophytes (shading and allelopathic compounds) drove seasonal changes in metabolism.

Interactions Between Physical Template and Self-organization Shape Plant Dynamics in a Stream Ecosystem

Both internal feedbacks and preexisting heterogeneity of a physical template can produce biological patchiness in ecosystems. The relative importance of the two drivers might change over time, in response to changes in the external environment. This is especially relevant for ecosystems experiencing high environmental variability. We investigated the interaction between these two drivers—internal feedbacks and template heterogeneity (represented by variation in water permanence in this study)—using 9-year data on macrophyte (wetland) patch distribution collected from Sycamore Creek, a desert stream in Arizona, USA. Sycamore Creek is highly variable both in space and in time due to its flashy hydrograph. We used a lattice simulation model that considered both the spatial heterogeneity of the geomorphic template and the influence of local positive feedbacks on plant growth. We showed that the relative strengths of local feedbacks and the template effect varied with the hydrological condition in a given year. Overall, the effect of template heterogeneity increased in wetter and more frequently flooded years and the effect of internal positive feedback decreased. Averaging over the 9-year study period, the effect of local feedbacks on wetland distribution and abundance in Sycamore Creek was about twice that of template heterogeneity. In the driest years, it reached up to ~ 12 times. Our study suggested that compared to catastrophic floods, severe droughts—the type of hydrological change projected to become more frequent for the region under a changing climate—have a stronger legacy effect on the biotic community in subsequent years.

The amphibian macrophyte Polygonum punctatum as a temporary habitat and feeding ground for fish

Plants that colonize wet habitats may become temporarily submerged forming a new habitat for fish in the flood period. In this study, the amphibian macrophyte Polygonum punctatum was evaluated as a temporary habitat and feeding ground for fish (≤ 108 mm) during the flood period in a river–floodplain ecosystem. The fish were sampled in January and February of 2015 in a lake of the Upper Parana River floodplain, inside patches dominated by P. punctatum (flood-formed, temporary habitat) and Eichhornia azurea (permanent aquatic habitat, used as control). The abiotic data did not differ between the two plants, unlike their biomass, which was higher for E. azurea. Fish abundance did not differ between macrophytes but the composition and richness did, with more species registered in E. azurea. The foraging efficiency of fish was significantly lower in individuals using the amphibian macrophyte for most species. Differences in the diet composition were verified for half of the species, but the main items consumed were the same in both macrophytes, changing only the proportion. The niche breadth was similar in the patches of both macrophytes. The results suggest that during flood periods, the new habitat provided by the macrophyte P. punctatum was colonized by small fish that probably used this new habitat as a refuge and feeding ground. However, the lower foraging efficiency in this macrophyte suggests that it does not provide an amount of food resources comparable to the permanent habitat represented by E. azurea.

Morphological traits of submerged macrophytes reveal specific positive feedbacks to water clarity in freshwater ecosystems

Positive feedbacks are generally related to the interactions between biotic processes and abiotic drivers and may lead to the emergence of alternative stable states in ecosystems. Understanding the mechanisms of self-reinforcing feedbacks in a macrophyte-dominant clear state is critical for lake management. Based on a survey of 35 lakes in the Yangtze River floodplain and canonical correspondence analysis (CCA) with forward selection, the results showed that water clarity is the most limiting factor that influences the community structure and biomass of submerged macrophytes. The canopy length of tall macrophytes (i.e., Myriophyllum spicatum L. and Potamogeton malaianus Miq.) showed positive allometry with plant height, while the canopy length of small macrophytes (e.g., Potamogeton maackianus A. Benn.) showed isometry. Our results indicated the existence of positive feedbacks between macrophyte vegetation and water clarity in a “more vegetation, higher water clarity” pattern. We found that the relationships between monospecific community biomass and water clarity differed among community types, indicating that the strength of the positive feedback was interspecific. Furthermore, we found significant differences in the Secchi depth (SD), chlorophyll a (Chl a), light attenuation coefficient (K) and dissolved oxygen (DO) associated with monospecific macrophyte patches. Plant height had significant relationships with the mean values of SD, Chl a, total phosphorus (TP) and K, suggesting that plant height was one of the mechanisms underlying the positive feedbacks. In management practices, efforts to build and maintain the resilience of an ecosystem should be trait-based rather than merely focusing on vegetation abundance.

Association Between the Success of an Invasive Macrophyte, Environmental Variables and Abundance of a Competing Native Macrophyte.

The success of invasive species depends on the overcoming of abiotic and biotic filters. Abiotic variables likely have greater relative importance over invasion at broad spatial scales, while biotic interactions are more important at fine spatial scales. In this study, we tested the hypotheses that (i) the abundance of the invasive Hydrilla verticillata is more correlated with abiotic factors than with competing native species at broad spatial grain; and that (ii) H. verticillata abundance is more correlated with competing native species than with abiotic factors at fine spatial grain. Here, we considered spatial scale as the grain size (i.e., the extent of sampling unit) assuming broad spatial scales as a large area encompassing the entire patches of macrophytes, and fine spatial scales as a small area inside one macrophyte patch. We collected the abundance of hydrilla and the competing native species along with environmental variables in a large subtropical reservoir. To evaluate how the relative importance of the abiotic factors and the competing native species vary between spatial grains we used Bayesian Generalized Linear Models. At broad grain, the abundance of the competing native species, maximum fetch (positive correlation), turbidity and conductivity (negative correlation) were the most important factors to explain the hydrilla abundance. At fine grain, alkalinity, total organic matter of the sediment and the abundance of a competitive native species (all negative correlations) were the most important variables. Our results indicate a greater importance of abiotic factors at broader grains while competitive interactions seem to be important only in the finer spatial grains. Environmental heterogeneity may explain the positive correlation between native and invasive abundances at broad grain, while the negative correlation at fine grain suggests the effect of competition. In synthesis, we show that the abiotic factors that explain the invasion success of a submerged invasive macrophyte are the same in two spatial grains, but the importance of biotic interactions changed with grain. Thus, our data suggest that models that attempt to explain the success of invasive plants, should consider spatial scales.

Sentinel-2 for mapping the spatio-temporal development of submerged aquatic vegetation at Lake Starnberg (Germany)

Submerged aquatic vegetation (SAV) plays an important role in freshwater lake ecosystems. Due to its sensitivity to environmental changes, several SAV species serve as bioindicators for the trophic state of freshwater lakes. Variations in water temperature, light availability and nutrient concentration affect SAV growth and species composition. To monitor the trophic state as required by the European Water Framework Directive (WFD), SAV needs to be monitored regularly. This study analyses the development of macrophyte patches at Lake Starnberg, Germany, by exploring four Sentinel-2A acquired within the main growing season in August and September 2015. Two different methods of littoral bottom coverage assessment are compared, i.e. a semi-empirical method using depth-invariant indices and a physically based, bio-optical method using WASI-2D (Water Colour Simulator). For a precise Sentinel-2 imaging by date and hour, satellite measurements were supported by lake bottom spectra delivered by in situ data based reflectance models. Both methods identified vegetated and non-vegetated patches in shallow water areas. Furthermore, tall- and meadow-growing SAV growth classes could be differentiated. Both methods revealed similar results when focusing on the identification of sediment and SAV patches (R² from 0.56 to 0.81), but not for a differentiation on SAV class growth level (R² <0.42).

Flow-divergence feedbacks control propagule retention by in-stream vegetation: the importance of spatial patterns for facilitation

Facilitation (enhancement of propagule retention in this case) is increasingly recognized as an important driver of biodiversity, but it is still unknown if facilitation during dispersal and colonization is affected by self-organized spatial pattern formation. We investigated the ability of in-stream submerged macrophyte patches to trap the vegetative propagules of three species (Berula erecta, Groenlandia densa, Elodea nuttallii in two size classes: 13–22 and 40–48 cm long), and to potentially benefit the colonization of these three species. We tested the effects of propagule traits, hydrodynamic forcing, and spatial patch configuration on propagule trapping. Propagule buoyancy was negatively correlated with trapping chance, while propagule size did not influence trapping. Species-specific differences in buoyancy were maintained for weeks after fragmentation. Propagule retention was interactive and conditional upon the interplay between incoming flow velocities and vegetation spatial patterning. In the flume experiment at low flows, a patchy configuration (one patch filling 66% of the flume width) retained more surface-drifting propagules (B. erecta, G. densa), than near-homogeneous cover (two patches close together, filling the entire flume width). In contrast, retention of sinking E. nuttallii propagules increased in the two-patch configurations. In flume and field releases where patches did not completely fill the channel width, water flowed around the patches rather than over or through them. This resulted in low-flow velocity areas within patches where canopies were upright and propagules were retained, and higher velocity flows around patches. In contrast, when vegetation filled the channel width, water could not be diverted laterally around the patches and preferentially flowed over them, causing the canopies to bend and reduce their trapping capacity. In flume experiments at high flows, retention of all species decreased, regardless of vegetation configuration, as propagules passed over the reconfigured vegetation canopies. These findings on the interplay of water movement and patch reconfiguration suggest that environmental heterogeneity generated by the self-organizing behavior of aquatic plants might enhance colonization of sessile organisms, calling for landscape-scale processes like dispersal to be better investigated.

Plants face the flow in V formation: A study of plant patch alignment in streams

AbstractInteractions between biological and physical processes, so‐called bio‐physical feedbacks, are important for landscape evolution. While these feedbacks have been quantified for isolated patches of vegetation in aquatic ecosystems, we still lack knowledge of how the location of one patch affects the occurrence of others. To test for patterns in the spatial distribution of vegetation patches in streams, we first measured the distance between Callitriche platycarpa patches using aerial images. Then, we measured the effects of varying patch separation distance on flow velocity, turbulence, and drag on plants in a field manipulation experiment. Lastly, we investigated whether these patterns of patch alignment developed over time following locations of reduced hydrodynamic forces, using 2‐yr field observations of the temporal patch dynamics of Ranunculus penicillatus in a lowland chalk stream. Our results suggest that vegetation patches in streams organize themselves in V‐like shapes to reduce drag forces, creating an optimal configuration that decreases hydrodynamic forces and may therefore encourage patch growth. Downstream patches are more frequently found at the rear and slightly overlapping the upstream patch, in locations that are partially sheltered by the established upstream vegetation while ensuring exposure to incoming flow (important for nutrient availability). Observations of macrophyte patch dynamics over time indicated that neighboring patches tend to grow in a slightly angled line, producing a spatial pattern resembling the V‐formation in migratory birds. These findings point to the general role of bio‐physical interactions in shaping how organisms align themselves spatially to aerodynamic and hydrodynamic flows at a range of scales.

Diversidad de macroinvertebrados fitófilos en arroyos de cabecera en pastizales de altura en Córdoba, Argentina

Macrophytes play an important role in lotic systems increasing habitat heterogeneity and providing food and shelter for wildlife. The aim of the study was to analyze the structure and composition of the invertebrate community associated with macrophytes in headwater streams in mountain grasslands (Ctalamochita sub-basin, Córdoba, Argentina) in two hydrological periods. The structure of the invertebrate assemblages in different macrophyte species (Hydrocotyle spp., Rorippa nasturtium-aquaticum and Ranunculus flagelliformis) was also evaluated. Three streams were sampled during high water (March 2012) and low water (August 2012) period. In each period, samples of invertebrates associated with the dominant macrophyte in each stream were collected. Seventy-two taxa of phytophilous macroinvertebrates were identified. The most frequent and abundant taxa were Naidinae, Hyalella curvispina, Americabaetis spp., Corynoneura spp. and Rheotanytarsus spp. Assemblages composition differed between hydrological periods and greater abundance of invertebrates was detected in R. nasturtium-aquaticum. The results showed that macrophyte patches in grassland headwater streams harbor an abundant and diverse invertebrate fauna and provide important base information for biomonitoring.

Emergent Macrophytes Support Zooplankton in a Shallow Tropical Lake: A Basis for Wetland Conservation.

Understanding the biodiversity value of littoral zones of lakes is a priority for aquatic biodiversity conservation. However, less emphasis has been given to the littoral part of tropical African lakes, with many of the previous researches focusing only on the open water side. The aim of the present study was, therefore, to investigate the impact of the littoral zone of a shallow freshwater tropical lake (Ziway, Ethiopia), dominated by two emergent macrophytes, on zooplankton community structure. We hypothesized that the wetland vegetation serves as a preferred microhabitat for zooplankton communities. A lake with substantial coverage of emergent macrophytes was monitored monthly from January to August, 2016. The monitoring included the measurements of physical, chemical, and biological parameters. Sampling sites were selected to represent areas of the macrophyte vegetation (Typha latifolia and Phragmites australis) and the open water part of the lake. Sites with macrophyte vegetation were found to be the home of more dense and diverse zooplankton community. However, during the period of high vegetation loss, the density of crustacean zooplankton showed significant reduction within the patches of macrophytes. From biodiversity conservation perspective, it was concluded that the preservation of such small areas of macrophytes covering the littoral zone of lakes could be as important as protecting the whole lake. However, the rapid degradation of wetland vegetation by human activities is a real threat to the lake ecosystem. In the not-too-far future, it could displace and evict riparian vegetation and the biota it supports.

Seasonal aquatic macrophytes reduce water temperatures via a riverine canopy in a spring-fed stream

AbstractMaximum water temperatures in streams throughout the western USA typically occur in late summer and early autumn, coinciding with low stream flow. However, in the spring-fed Big Springs Creek in northern California, where constant-temperature groundwater springs provide relatively stable stream flow throughout the year, peak water temperatures and maximum diurnal variability occur in spring. We attribute this anomaly to the riverine canopy provided by emergent aquatic macrophytes (e.g., Polygonum hydropiperoides and Nasturtium officinale), which mimics the shade function of a riparian canopy. Macrophyte biomass increased 264% between January and August 2011. This increase coincided with a 111% reduction in flow velocity and a 53% increase in stream depth. Solar radiation was reduced by an average of 88% in patches of macrophytes that covered ~50% of the water surface during the summer. Decreased solar radiation reduced rates of stream heating, maximum temperatures, and temperature variability. We ...

The role of macrophyte structural complexity and water flow velocity in determining the epiphytic macroinvertebrate community composition in a lowland stream

Habitat structural complexity provided by aquatic macrophytes in lowland streams affects the associated epiphytic macroinvertebrate assemblages in both direct (increased microhabitat diversity, refuge against predation) and indirect ways (e.g. current attenuation by physical structures). In a correlative field study carried out in two different years in a Belgian stream, we investigated the effects of the factors macrophyte identity, macrophyte complexity (represented as fractal complexity) and current velocity on the composition of the macroinvertebrate community associated with monospecific macrophyte patches, consisting of plants with differing structural complexity; Sparganium emersum Rehmann (least complex), Potamogeton natans L. (intermediate) and Callitriche obtusangula Le Gall (most complex). In addition to significantly lower within-patch current velocity being observed, vegetation stands consisting of complex macrophytes also harboured significantly richer macroinvertebrate communities than stands of simpler macrophytes. A significant part of the variation in the macroinvertebrate community composition could be explained by plant identity, macrophyte complexity and current velocity. However, it was not possible to determine the relative importance of these three factors, because of their high degree of intercorrelation. Additionally, the explanatory power of these factors was higher under conditions of high current velocity, suggesting a role of macrophyte patches as instream flow refugia for macroinvertebrates.

The trapping of organic matter within plant patches in the channels of the Okavango Delta: a matter of quality

The role of in-stream aquatic vegetation as ecosystem engineers in the distribution of organic matter was investigated in the Okavango Delta, one of the world’s largest oligotrophic wetlands. The Okavango channel beds are covered up to 50% with submerged macrophyte patches. By accumulating and concentrating organic matter in the sediments below the patches, macrophytes are likely able to locally forestall a deficiency of nutrients. Up to 21 times more N, 18 times more C, 13 times more P and 6 times more Si can be found in vegetated sediments compared to non-vegetated sediments. Nutrient specific accumulation relates to its relative scarcity in the overlaying water. There is a depletion of dissolved N relative to P, whereas Si is relatively abundant. The Okavango Delta water can generally be characterised as oligotrophic based on plant species composition (e.g. presence of carnivorous plants and absence of floating plants), low plant N:P ratios, and low nutrient- and element-concentrations. Local mineralization and intensified nutrient cycling in the sediments is hypothesized to be crucial for the macrophytes’ survival because it provides a key source of the essential nutrients which plants otherwise cannot obtain in sufficient quantities from the nutrient poor water. By engineering the ecosystem as such, channel vegetation also retards the loss of elements and nutrients to island groundwater flow, contributing to one of the key processes driving the high productivity of the Okavango Delta, making it unique among its kind.

Benefits of adjacent habitat patches to the distribution of a crayfish population in a hydro-dynamic wetland landscape

Aquatic macrophyte patches are natural features of wetland ecosystems that serve as habitat for aquatic animals. Previous studies suggest animal densities in Everglades, USA, wetlands are generally less numerous in sawgrass ridges than in deeper lily sloughs. We studied the density distribution of a population of Procambarus fallax in ridge and slough habitat types over a 2-year period, spanning two wet–dry cycles and estimated growth and survival rates under flooded conditions to understand comparative value of each to the crayfish population. Procambarus fallax individuals inhabited and recruited in both marsh habitats. During periods of high water, crayfish densities were similar in both habitats; however, densities in both habitats varied seasonally, leading us to postulate some degree of population redistribution in response to fluctuating water depths. Analysis of size distributions over time revealed juveniles in both habitats and two major recruitment periods each year; distinct juvenile cohorts were present in early winter (Nov–Dec) and mid-summer (July–Aug). An in situ experiment of juvenile growth demonstrated that slough habitat type supported faster growth over ridge habitat. To understand habitat-specific mortality risk, a tethering study during flooded conditions indicated that relative predation risk by aquatic predators was greater in sloughs for all sizes and higher for smaller individuals in both habitats. The comparative importance of ridge and slough balances growth potential and survival probability during flooded conditions. This is the first study through time and across both habitat types analyzing the distribution and size structure of P. fallax population in the Everglades.

Spatial and temporal variation of fish assemblage associated with aquatic macrophyte patches in the littoral zone of the Ayapel Swamp Complex, Colombia

Abstract: Aim The purpose of the present study was to examine spatial and temporal variation in fish assemblage structure associated with aquatic macrophytes in the littoral zone of the ASC. Methods Specimens were caught between January 2008 and February 2009, over four limnimetric moments, using both cast net and seine net. Data on the temperature, electrical conductivity, pH and dissolved oxygen was recorded for the characterization of the water mass in the sites. Results A total of 34,151 specimens from 44 species were collected. The most abundant species were Eigenmannia virescens, Astyanax caucanus, Astyanax fasciatus, Roeboides dayi and Cyphocharax magdalenae, which together accounted for more than 75% of the sample. Temporal and spatial comparisons showed variation in the environmental conditions and highlighted the existence of heterogeneous abiotic conditions (p<0.05). However, the multiple comparisons test showed the existence of homogeneous spatial conditions (p>0.05) regarding the fish assemblage structure. The multivariate analysis showed no significant relationship between existing environmental conditions and the fish assemblage (p=0.04). The analysis also showed the absence of a relationship between the fish assemblage and environmental variables with respect to the flood pulse and sampling sites (p>0.05). Conclusion The uniformity of the fish communities that inhabit aquatic macrophyte patches in the littoral region of the ASC may be related to the availability of suitable habitat in structural terms, that probably supports a more abundant and varied wildlife.