Understanding factors structuring zooplankton and macroinvertebrate assemblages in ephemeral pans

Ephemeral aquatic ecosystems have a global distribution being most abundant in semi-arid and arid regions. Due to anthropogenic impacts threatening these environments, there is a need to understand various factors and processes structuring animal communities in these habitats. Macroinvertebrate and zooplankton assemblages were studied in different ephemeral (i.e. flood plain, large endorheic and small endorheic) pans in the south-eastern Lowveld of Zimbabwe in the wet season. Ten Cladoceran species, Calanoids and Cyclopoids taxa and thirty-three macroinvertebrate taxa were identified over the entire hydroperiod. Predator macroinvertebrates were the dominant taxa especially in endorheic pans. The pan categories differed significantly in both zooplankton and macroinvertebrates composition and richness, with zooplankton and macroinvertebrate taxa richness being high in flood plain pans. Conductivity, fish presence, hydroperiod, maximum depth, turbidity and vegetation cover played a major role in shaping both zooplankton and macroinvertebrate communities. The macroinvertebrate community assemblage reveals that small endorheic and flood plain pans represent extremes ends of the environmental gradient in the region while large endorheic pans represent an intermediate end.     

The role of an extreme flood disturbance on macrophyte assemblages in a Neotropical floodplain

Aquatic macrophytes produce large amounts of organic matter and have an essential structuring role in floodplains. This process highlights the importance of this community to aquatic biodiversity maintenance. We investigated the role of a flood disturbance on the response of macrophyte assemblages in regional and local structuring in the Upper Paraná River floodplain. Plant species were recorded before (November 2006) and after (March 2007) an uncommon increase in water level caused by the El Niño South Oscillation, which is considered a disturbance. Samples were taken in lakes and backwaters located in the floodplain and connected to three distinct rivers (that differentiate three sub-systems). Species richness and the assemblage structure of macrophyte patches underwent significant changes after the flood disturbance, depending on the specific sub-system (rivers) to which the lakes were connected. In addition, flood disturbance had a strong impact on community organization at the local scale. However, regionalization with respect to sub-systems remained significant after the flood disturbance. Our results emphasize the importance of connection to the river on macrophyte community composition and richness, and suggest that flood events in the Upper Paraná River floodplain disrupt community organization only at fine (local) scales.     

Spatial distribution and abundance of Unionidae mussels in a eutrophic floodplain lake

Although Unionidae mussels produce large biomass and reach high density in freshwater habitats, little is known about their ecology. We studied the distribution of 5 species of freshwater unionids in a eutrophic floodplain lake, on transects, along the lake shore and across the depth gradient. The clam distribution within the water body was not random: all species form a crowded zone along the lake shore, showing the highest density at ca. 0.5 m depth. The distribution of the most numerous species changed along the shore in Anodonta anatina and Unio pictorum but not in A. cygnea, whose numbers remained constant. The population numbers of the most numerous species showed a positive correlation with silt layer thickness. The generalized model of all the analyzed factors influencing the unionids’ distribution confirmed this relation and indicated a trade-off between water depth and distance from bank, which might be responsible for the occurrence of the zone at some optimum depth. Unionids have an important influence on freshwater ecosystem functions, thus their zonation implies that their functions are also spatially structured.     

Do hydraulic units define macroinvertebrate assemblages in mountain streams of central Argentina?

Methods to assess the physical habitat provide important tools for many aspects of river management. Hydraulic units (defined as a homogeneous patch of flow type and substrate) were described in mountain streams of Central Argentina and the distribution of macrozoobenthos in these habitat units was analyzed. Four streams from the upper Carcarañá River Basin (Córdoba, Argentina) were sampled in two hydrological periods. Hydraulic units (as substrate and flow type), current velocity, depth, macrophytes and macroalgae were assessed. Three benthic samples were taken in each hydraulic unit. A total of 12 hydraulic units were registered, which varied seasonally in their proportional abundance. The highest values of taxonomic richness, total abundance, diversity and evenness were found in the low-water period. The most heterogeneous hydraulic units (characterized by substrate of diverse grain size) presented the highest richness, diversity and evenness, whereas the highest total abundance was observed in hydraulic units with homogeneous substrate, such as bedrock or gravel sand. Canonical correspondence analysis grouped samples and taxa mainly in relation to the hydraulic units, and temporal variation in macroinvertebrate assemblages was observed. We found that the interaction between hydrological and geomorphological conditions affected benthic assemblages and that their organization is important at a mesoscale. Therefore, hydraulic units may be considered important tools in assessing stream integrity in lotic systems of central Argentina.     

Effects of nutrient enrichment derived from fish farming activities on macroinvertebrate assemblages in a subtropical region of Hong Kong

To study the correlation between nutrient enrichment derived from fish farming activities and changes in macrobenthic assemblages, a one-year field study was conducted in Kau Sai Bay marine fish culture zone of Hong Kong. Bimonthly sediment samples were collected at six stations: two at the fish cages, two near the boundary of the fish culture area, and two reference sites further away from the culture area. Sediment physico-chemical characteristics in terms of silt/clay fraction, moisture content, total organic carbon (TOC), total Kjeldahl nitrogen (TKN) and total phosphorus (TP) were analyzed. The macrobenthos (>0.5 mm) present in the sediment were sorted, identified and enumerated. On average, TOC, TKN and TP levels at the fish cage stations were 82.8%, 128.5% and 1315.7% higher than those at the reference stations, respectively. As a result, the N:P molar ratio was greatly reduced from 8.75 at the reference stations to 1.83 at the fish cage stations. Univariate and multivariate analyses revealed that diversity of macrofauna was significantly reduced and community structure differed at the fish cage stations relative to the reference sites. The intermediary stations near the fish culture area showed a transitional state of disturbance. Faunal diversity was negatively correlated with nutrient level, reflecting the adverse impacts of nutrient enrichment derived from fish farming activities on the benthic assemblages. Whilst in subtropical Asia-Pacific trash fish is the major feed for fish culture resulting in a higher nutrient loading and nutrient ratio accumulated in the sediment beneath the fish rafts, the effects of nutrient enrichment on macrobenthic assemblages are comparable to that in temperate waters owing to relatively high sediment metabolism rate and smaller fish culture scale in Hong Kong.     

Pressure-specific and multiple pressure response of fish assemblages in European running waters

We classified homogenous river types across Europe and searched for fish metrics qualified to show responses to specific pressures (hydromorphological pressures or water quality pressures) vs. multiple pressures in these river types. We analysed fish taxa lists from 3105 sites in 16 ecoregions and 14 countries. Sites were pre-classified for 15 selected pressures to separate unimpacted from impacted sites. Hierarchical cluster analysis was used to split unimpacted sites into four homogenous river types based on species composition and geographical location. Classification trees were employed to predict associated river types for impacted sites with four environmental variables. We defined a set of 129 candidate fish metrics to select the best reacting metrics for each river type. The candidate metrics represented tolerances/intolerances of species associated with six metric types: habitat, migration, water quality sensitivity, reproduction, trophic level and biodiversity. The results showed that 17 uncorrelated metrics reacted to pressures in the four river types. Metrics responded specifically to water quality pressures and hydromorphological pressures in three river types and to multiple pressures in all river types. Four metrics associated with water quality sensitivity showed a significant reaction in up to three river types, whereas 13 metrics were specific to individual river types. Our results contribute to the better understanding of fish assemblage response to human pressures at a pan-European scale. The results are especially important for European river management and restoration, as it is necessary to uncover underlying processes and effects of human pressures on aquatic communities.     

Long-term changes in cladoceran assemblages in the Danube floodplain area (Slovak–Hungarian stretch)

In this paper, the distribution of Cladocera species in the different sampling sites: the main channel/old river bed, parapotamal type side arms and plesiopotamal side arms, is described. The structure of cladoceran assemblages in the by-passed Danube section and in the adjacent floodplain water bodies has changed since the Gabčíkovo hydropower plant was put into operation. Great changes have been observed in the previous parapotamal side arm situated between river km 1840 and 1820, artificially fed with water from the head-race canal. The dominance of tychoplanktonic (benthic and phytophilous) species has increased, while the typical euplanktonic species have disappeared. Three characteristic groups of cladoceran assemblages were recorded when a different type of habitat was taken into consideration. Euplanktonic cladocerans prevailed on all sampling sites before damming. In periods after damming, littoral species, and later also a euplanktonic forms, dominated on the main channel sampling sites. In parapotamal and plesiopotamal side arms with rich littoral macrovegetation during periods after damming, phytophilous cladoceran species were the ones with the highest occurrence. The samplings from the first time period were rather homogenous. The samplings from the second and third period were more similar when considering the sample site than regarding the time period. In total, 64 cladoceran species were recorded in the course of 13 years (from 1991 to 2004). The increase in number of Cladocera species from 1991 to 2004 was significant. Chydorus sphaericus was found to be the most widely distributed species in the study area. The finding of Disparalona hamata is the first faunistic record from the central part of the Danubian watersheds.     

An index to assess hydromorphological quality of Estonian surface waters based on macroinvertebrate taxonomic composition

We developed an index (MESH – Macroinvertebrates in Estonia: Score of Hydromorphology) to assess hydromorphological quality of Estonian surface waters based on macroinvertebrate taxonomic composition. The MESH is an average score based on the affinities of selected indicator taxa to flow velocity and bottom type. As both parameters were highly correlated (r = 0.65) indicator response to both parameters were combined. The list of MESH indicators includes 394 freshwater macroinvertebrate taxa derived from 3282 samples collected from rivers and lakes during 1985–2009. The indicators were selected out of 690 taxa, by applying the information-theoretical Kullback–Leibler divergence. The individual scores of macroinvertebrates range from 0 to 3, the higher scores indicating faster flow and/or solid bottom substrate. For standing waters, flow velocity was always considered zero. Among the reference waterbodies, mean MESH was the highest for small streams followed by middle streams, large streams, and lakes. In lakes with medium water hardness (the prevailing type in Estonia), the MESH decreased gradually from stony to muddy bottom. The highest MESH values for standing waters were observed in the stony surf zone of very large lakes (area > 100 km²). The lowest values occurred for small lakes with exceptional hydrochemical characteristics (soft- and darkwater, and calcareous types). Similarly, MESH indicated stream degradation by damming. Mean MESH in reservoirs with a muddy bottom was significantly lower than in reservoirs with a hard bottom, or in unregulated stream sections.     

Larval fish assemblages in a tropical mangrove estuary and adjacent coastal waters: Offshore-inshore flux of marine and estuarine species

A total of 92,934 fish larvae representing 19 families were sampled monthly from the Sangga Kecil estuary (Matang Mangrove Forest Reserve) and adjacent coastal waters from May 2002 to October 2003. Larval fish assemblages were numerically dominated by Gobiidae (50.1%) and Engraulidae (38.4%). Canonical Correspondence Analysis (CCA) revealed that the larval fish assemblages, including their ontogenetic stages, differed between the mangrove estuary and adjacent offshore waters, and that salinity, turbidity and zooplankton food are the major environmental factors structuring the larval fish assemblages. Estuarine preflexion gobiid larvae were ubiquitous in the coastal and estuarine waters. Larval stages of euryhaline species that were spawned in offshore waters, such as Engraulidae and Clupeidae, were largely advected into mangrove areas at the postflexion stages. Larvae of other euryhaline fishes (Sciaenidae, Blenniidae and Cynoglossidae) that may have been spawned inside the estuary were, however, exported to offshore waters. Given that the collective number of juvenile and adult fish families in the Matang estuary was 53, while the number of larval families was only 17, the former is quite disconnected from the existing larval fish population in the estuary.     

Spatial homogenization of soil-surface pollen assemblages improves the reliability of pollen-climate calibration-set

Science China Earth Sciences - To make a reliable reconstruction of past climate from soil-surface modern pollen, it is necessary to reduce the sources of error. In this paper, pollen percentages...     

Dynamics of mesozooplankton assemblages in subtropical coastal waters of Hong Kong: A comparative study between a eutrophic estuarine and a mesotrophic coastal site

A monthly comparative study of mesozooplankton biomass and composition between a eutrophic Pearl River estuarine site (WE) and a mesotrophic coastal-oceanic site (EO) in Hong Kong waters was conducted to examine the response of mesozooplankton to nutrient-rich riverine discharge. Although the annual average mesozooplankton biomass was higher at WE than at EO, they were not statistically significant. Variations of mesozooplankton biomass at both stations followed similar trends of Chl a concentrations, with the exception of July at WE where mesozooplankton biomass was low but total Chl a was high. This mismatch may be due to the high flushing effect of the Pearl River discharge in summer and a time lag in mesozooplankton population growth. On the other hand, the composition of mesozooplankton was significantly modified by riverine discharge and eutrophication conditions. While small copepods dominated at both sites, the eutrophic estuarine water had a high abundance of barnacle and polychaete larvae, while cladocerans, bivalve larvae, gastropod larvae and chaetognaths mainly occurred at EO. Eutrophication increased the top-down role of copepods in the grazing community, revealed by an increase in the percentage of copepods in the total metazoan mesozooplankton, especially during the period of high river discharge. Moreover, mesozooplankton diversity at the two stations was similar, and they both showed relatively higher diversity during autumn and winter and lower diversity during summer, especially at WE. These results suggest that, despite high nutrient and Chl a concentrations in estuarine waters, mesozooplankton biomass were not enhanced compared to coastal waters with no river impact, possibly due to poor food quality and increased predation in the eutrophic estuarine waters.     

Spatial and temporal patterns of the infaunal community near a major ocean outfall in Southern California

Effects of waste water discharge from a major ocean outfall in Southern California to the benthic infaunal community were examined during a 5-year monitoring programme. Natural features, primarily water depth, accounted for 82% of the variability in the infaunal community, while discharge-related effects represented less than 8% of the variability. The area immediately adjacent to the diffuser had the strongest outfall effects. Infaunal abundance showed a pattern of enhancement centred at the outfall. Diversity was also high, indicating that the area was not characterized by a degraded community.     

Spatial variability of floodplain sedimentation at the event scale in the Rhine–Meuse delta,The Netherlands

This article addresses spatial variability of comtemporary floodplain sedimentation at the event scale. Measurements of overbank deposition were carried out using sediment traps on 11 floodplain sections along the rivers Waal and Meuse in The Netherlands during the high-magnitude flood of December 1993. During the flood, sand sheets were locally deposited behind a natural levee. At distances greater than 50 to 100 m from the river channel the deposits consisted mainly of silt- and clay-sized material. Observed patterns of deposition were related to floodplain topography and sediment transporting mechanisms. Though at several sites patterns were observed that suggest transport by turbulent diffusion, convection seems the dominant transporting mechanism, in particular in sections that are bordered by minor embankments. The average deposition of overbank fines ranged between 1·2 and 4·0 kg m⁻² along the river Waal, and between 1·0 and 2·0 kg m⁻² along the river Meuse. The estimated total accumulation of overbank fines (not including sand sheets) on the entire river Waal floodplain was 0·24 Mton, which is 19 per cent of the total suspended sediment load transported through the river Waal during the flood. © 1998 John Wiley & Sons, Ltd.     

Modelling floodplain flows using a two-dimensional finite element model

A prototype two-dimensional finite element flow model for depth-averaged free surface flows was developed for floodplain environments. Limited refinement of the model's physical representation was undertaken and the enhanced scheme applied to an 11 km river channel/floodplain reach in the U.K. Preliminary model results indicate that this modelling approach can be used to identify dynamic variations in the flow field parameters over length scales of the order of 10-100 m. Potentially, such data have the ability to permit detailed analysis of short-term floodplain sedimentary dynamics.     

Spatial patterns of terrestrial net ecosystem productivity in China during 1981–2000

As the third largest country in the world, China has highly variable environmental condition and ecological pattern in both space and time. Quantification of the spatial-temporal pattern and dynamic of terrestrial ecosystem carbon cycle in China is of great significance to regional and global carbon budget. In this study, we used a high-resolution climate database and an improved ecosystem process-based model to quantify spatio-temporal pattern and dynamic of net ecosystem productivity (NEP) in China and its responses to climate change during 1981 to 2000. The results showed that NEP increased from north to south and from northeast to southwest. Positive NEP (carbon sinks) occurred in the west of Southwest China, southeastern Tibet, Sanjiang Plain, Da Hinggan Mountains and the mid-west of North China. Negative NEP (carbon sources) were mainly found in Central China, the south of Southwest China, the north of Xinjiang, west and north of Inner Mongolia, and parts of North China. From the 1980s to 1990s, the increasing trend of NEP occurred in the middle of Northeast China Plain and the Loess Plateau and decreasing trends mainly occurred in a greater part of Central China. In the study period, natural forests had minimal carbon uptake, while grassland and shrublands accounted for nearly three fourths of the total carbon terrestrial uptakes in China during 1981–2000. Supported by the Ministry of Science and Technology of China (G2002CB412507), the Major Program of the National Natural Science Foundation of China (Grant No.30590384), the “Hundred Talent” Program of the Chinese Academy of Sciences, and K C WONE Education Foundation