Linking terrestrial and benthic estuarine ecosystems: organic matter sources supporting the high secondary production of a non-indigenous bivalve

The Asian clam, Corbicula fluminea, is among the most pervasive invasive species in freshwater ecosystems worldwide. Our objective was to study C. fluminea’s functional response in terms of feeding behavior and food selectivity, using the natural variation in organic matter (OM) sources that occur in estuarine environments. Using C and N stable isotopes, we identified and quantified the contribution of different OM sources supporting the production of C. fluminea along the salinity gradient occupied in the Minho River estuary (NW-Iberian Peninsula, Europe), where this species presently dominates the benthic macrofauna biomass. We observed a pronounced shift in the quality of OM available for C. fluminea along the estuarine mixing zone. Stable isotope analysis, POM C/N, and phytoplankton contribution estimates based on C:Chl a revealed that POM was largely comprised of terrestrial-derived OM in tidal freshwater stations (TFW) and was increasingly comprised of phytoplankton, a more palatable food source, towards the polyhaline estuary. A similar shift in the isotopic composition along the estuarine mixing zone was observed in C. fluminea, suggesting a shift in food resources. Accordingly, based on a Bayesian stable isotope mixing model, there was an upstream–downstream counter gradient in the contribution to C. fluminea biomass from terrestrial-derived OM (41–64 % in TFW) and phytoplankton (29–55 % in the brackish estuary). Although the majority of the food sources identified were filtered from the water column (70–80 %), reliance on sediment OM and microphytobenthos provided evidence for deposit feeding by C. fluminea. We conclude that C. fluminea has the ability to adapt to environments with low food quality because it can consume terrestrial-derived OM. This can be a competitive adaptation in systems with perennial low food quality such as the Minho River estuary. Moreover, its ability to couple benthic and pelagic environments and terrestrial ecosystems demonstrates a strong potential to alter food web flows in aquatic ecosystems.     

Distribution of picophytoplankton communities from brackish to hypersaline waters in a South Australian coastal lagoon

Background

Picophytoplankton (i.e. cyanobacteria and pico-eukaryotes) are abundant and ecologically critical components of the autotrophic communities in the pelagic realm. These micro-organisms colonized a variety of extreme environments including high salinity waters. However, the distribution of these organisms along strong salinity gradient has barely been investigated. The abundance and community structure of cyanobacteria and pico-eukaryotes were investigated along a natural continuous salinity gradient (1.8% to 15.5%) using flow cytometry.

Results

Highest picophytoplankton abundances were recorded under salinity conditions ranging between 8.0% and 11.0% (1.3 × 10⁶ to 1.4 × 10⁶ cells ml^-1). Two populations of picocyanobacteria (likely Synechococcus and Prochlorococcus) and 5 distinct populations of pico-eukaryotes were identified along the salinity gradient. The picophytoplankton cytometric-richness decreased with salinity and the most cytometrically diversified community (4 to 7 populations) was observed in the brackish-marine part of the lagoon (i.e. salinity below 3.5%). One population of pico-eukaryote dominated the community throughout the salinity gradient and was responsible for the bloom observed between 8.0% and 11.0%. Finally only this halotolerant population and Prochlorococcus-like picocyanobacteria were identified in hypersaline waters (i.e. above 14.0%). Salinity was identified as the main factor structuring the distribution of picophytoplankton along the lagoon. However, nutritive conditions, viral lysis and microzooplankton grazing are also suggested as potentially important players in controlling the abundance and diversity of picophytoplankton along the lagoon.

Conclusions

The complex patterns described here represent the first observation of picophytoplankton dynamics along a continuous gradient where salinity increases from 1.8% to 15.5%. This result provides new insight into the distribution of pico-autotrophic organisms along strong salinity gradients and allows for a better understanding of the overall pelagic functioning in saline systems which is critical for the management of these precious and climatically-stress ecosystems.     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Improvement of biomass and fatty acid productivity in ocean cultivation of <Emphasis Type="Italic">Tetraselmis</Emphasis> sp. using hypersaline medium

In this study, hypersaline media were used for ocean cultivation of the marine microalga Tetraselmis sp. KCTC12432BP for enhanced biomass and fatty acid (FA) productivity. Hypersaline media (55, 80, and 105 PSU) were prepared without sterilization by addition of NaCl to seawater obtained from Incheon, Korea. The highest biomass productivity was obtained at 55 PSU (0.16 g L^?1 day^?1) followed by 80 PSU (0.15 g L^?1 day^?1). Although the specific growth rate of Tetraselmis decreased at salinities higher than 55 PSU, prevention of contamination led to higher biomass productivity at 80 PSU than at 30 PSU (0.03 g L^?1 day^?1). FA content of algal biomass increased as salinity increased to 80 PSU, above which it declined, and FA productivity was highest at 80 PSU. Ocean cultivation of Tetraselmis was performed using 50-L tubular module photobioreactors and 2.5-kL square basic ponds, closed- and open-type ocean culture systems, respectively. Culturing microalgae in hypersaline medium (80 PSU) improved biomass productivities by 89 and 152% in closed and open cultures, respectively, compared with cultures with regular salinity. FA productivity was greatly improved by 369% in the closed cultures. The efficacy of salinity shift and N-deficiency to enhance FA productivity was also investigated. Lowering salinity to 30 PSU with N-starvation following cultivation at 80 PSU improved FA productivity by 19% in comparison with single-stage culture without N-deficiency at 30 PSU. The results show that salinity manipulation could be an effective strategy to improve biomass and FA productivity in ocean cultivation of Tetraselmis sp.     

Distribution and Diversity of Cyanobacteria and Eukaryotic Algae in Qinghai–Tibetan Lakes

Cyanobacteria and eukaryotic algae are important primary producers in a variety of environments, yet their distribution and response to environmental change in saline lakes are poorly understood. In this study, the community structure of cyanobacteria and eukaryotic algae in the water and surface sediments of six lakes and one river on the Qinghai–Tibetan Plateau were investigated with the 23S rRNA gene pyrosequencing approach. Our results showed that salinity was the major factor controlling the algal community composition in these aquatic water bodies and the community structures of water and surface sediment samples grouped according to salinity. In subsaline–mesosaline lakes (salinity: 0.5–50 g L^?1), Cyanobacteria (Cyanobium, Synechococcus) were highly abundant, while in hypersaline lakes (salinity: >50 g L^?1) eukaryotic algae including Chlorophyta (Chlorella, Dunaliella), Bacillariophyta (Fistulifera), Streptophyta (Chara), and Dinophyceae (Kryptoperidinium foliaceum) were the major members of the community. The relative abundance ratio of cyanobacteria to eukaryotic algae was significantly correlated with salinity. The algae detected in Qinghai–Tibetan lakes exhibited a broader salinity range than previously known, which may be a result of a gradual adaptation to the slow evolution of these lakes. In addition, the algal community structure was similar between water and surface sediment of the same lake, suggesting that sediment algal community was derived from water column.     

Taxonomy of halophilic Archaea: current status and future challenges

Several groups of Archaea, all Euryarchaeota, develop in hypersaline environments (from >10 % salt up to saturation). The cultured diversity of halophilic Archaea includes the family Halobacteriaceae of aerobic or facultative anaerobic, generally red-pigmented species (47 genera and 165 species as of February 2014) and seven representatives of four genera of methanogens, most of which obtain energy from methylated amines under anaerobic conditions. Metagenomic studies have identified an additional deep lineage of Archaea in salt lakes and ponds with brines approaching NaCl saturation. Genomic information is now available for representatives of these ‘Nanohaloarchaea’, but no members of this lineage have yet been cultured. Multilocus sequence analysis is becoming increasingly popular in taxonomic studies of the Halobacteriaceae, and such studies have demonstrated that recombination of genetic traits occurs at an extremely high frequency at least in some genera. Metagenomic studies in an Antarctic lake showed that large identical regions of up to 35 kb in length can be shared by members of different genera living together in the same environment. Such observations have important implications not only for the taxonomy of the Halobacteriaceae, but also for species concepts and questions on taxonomy and classification for prokaryotic microorganisms in general.     

Comparative limnology of Sambhar and Didwana lakes (Rajasthan,NW India)

Two alkaline saline inland lakes of Indian arid region were studied during 1984 and 1985, to assess functioning and interaction of various environmental and biological factors. Changes in physical and chemical variables, planktonic composition, chlorophyll content and phytoplankton primary productivity were examined.Salinity in both lakes fluctuated from almost fresh water (1.80), to hypersaline (300) and acted as the main controlling factor for almost all the biotic parameters. Maximum total alkalinities were 2162 mg l^–1 and 2090 mg l^–1, respectively in Sambhar and Didwana lakes. Dissolved oxygen ranged from completely anoxic conditions to maxima of 11.68 and 7.29 mg 1^–1, respectively in Sambhar and Didwana lakes. Nutrient enrichment in the lakes was low.The phytoplankton species composition of Sambhar lake was reduced from an earlier reported 20 genera to only 11 (Nostoc, Microcystis, Spirulina, Aphanocapsa, Oscillatoria, Merismopedia, Nitzschia, Navicula, Synedra, Cosmarium and Closterium). Phytoplankton of Didwana was composed of only 9 genera including Anabaena and Nodularia. Sambhar lake, which once contained Artemia, is now totally devoid of them. On the other hand, Artemia was the most dominant zooplankter in Didwana lake at a salinity range of 15–288. Other zooplankters such as Moina, Cyclops and Brachionus flourished at lower salinity levels in Didwana lake. The seasonal quantitative and qualitative phyto- and zooplankton changes in relation to salinity are documented.     

Performance of a new phytoplankton composition metric along a eutrophication gradient in Nordic lakes

A new phytoplankton metric is presented, which is developed from a large dataset of Norwegian lakes (>2,000 samples from >400 lakes). In contrast to previous metrics, this index is not built on selected ‘indicative’ taxa, but uses all available taxonomic information at genus and species level. Taxa optima with respect to lake trophic status (derived from total phosphorus concentrations) are used to calculate a phytoplankton trophic index (TI) for each sample. Analysis of the TI shows that phytoplankton communities exhibit highly non-linear responses to eutrophication in Norwegian lakes. Reference lakes are characterized by very similar TIs despite having considerable variation in total phosphorus and chlorophyll a concentrations. TI exhibits a non-linear distribution along the eutrophication gradient which separates unimpacted from impacted sites in the study area. We further show that TI exhibits smaller seasonal variations than chlorophyll a, making it a more reliable indicator for lake monitoring. Implications for its applicability within the WFD are discussed.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

Anammox bacterial populations in deep marine hypersaline gradient systems

To extend the knowledge of anaerobic ammonium oxidation (anammox) habitats, bacterial communities were examined in two hypersaline sulphidic basins in Eastern Mediterranean Sea. The 2 m thick seawater–brine haloclines of the deep anoxic hypersaline basins Bannock and L’Atalante were sampled in intervals of 10 cm with increasing salinity. ¹⁵N isotope pairing incubation experiments showed the production of ²⁹N₂ and ³⁰N₂ gases in the chemoclines, ranging from 6.0 to 9.2 % salinity of the L’Atalante basin. Potential anammox rates ranged from 2.52 to 49.65 nmol N₂ L^?1 day^?1 while denitrification was a major N₂ production pathway, accounting for more than 85.5 % of total N₂ production. Anammox-related 16S rRNA genes were detected along the L’Atalante and Bannock haloclines up to 24 % salinity, and the amplification of the hydrazine synthase genes (hzsA) further confirmed the presence of anammox bacteria in Bannock. Fluorescence in situ hybridisation and sequence analysis of 16S rRNA genes identified representatives of the marine anammox genus ‘Candidatus Scalindua’ and putatively new operational taxonomic units closely affiliated to sequences retrieved in marine environments that have documented anammox activity. ‘Scalindua brodae’ like sequences constituted up to 84.4 % of the sequences retrieved from Bannock. The anammox community in L’Atalante was different than in Bannock and was stratified according to salinity increase. This study putatively extends anammox bacterial habitats to extremely saline sulphidic ecosystems.     

Effects of salinity on strontium:calcium ratios in the otoliths of the West African black-chinned tilapia Sarotherodon melanotheron in a hypersaline estuary

The biology and the behaviour of fish populations in hypersaline environments are poorly known. The habitat occupation strategy of the tilapia Sarotherodon melanotheron has been studied along a salinity gradient in the Saloum hypersaline estuary in Senegal (salinity between 32 and 100). The individual migratory behaviour has been analysed from otolith strontium-to-calcium concentration ratios from fish sampled in five locations during the 2003 wet season and the 2004 dry season. In the upper part of the estuary (salinities > 50), the Sr:Ca ratio in the otolith showed high variations, from 2.51 to 33.30 × 10⁻³. These maximum observed values have never been reported in the literature. The individual mean of Sr:Ca ratios increased according to the salinity gradient in the estuary, with significantly higher values in the upper part (Sr:Ca mean = 16 × 10⁻³) than in the lower part (salinity < 50, Sr:Ca mean = 12 × 10⁻³). No significant difference in the Sr:Ca mean was observed between locations with comparable salinities. Sr concentration in the water at the different locations was positively correlated with ambient salinity. The mean of Sr:Ca in the otoliths was then in relationship with the level of Sr in the water. Thus, the Sr:Ca ratios in the otolith of S. melanotheron allowed to discriminate the populations of this species and to hypothesize that they did not undertake large scale movements within the estuary.     

The biology and evolution of Antarctic saline lakes in relation to salinity and trophy

. Five brackish to hypersaline lakes (Highway, Ace, Pendent, Williams and Rookery) in the Vestfold Hills, eastern Antarctica were investigated during the austral summer of 1999/2000. The aims were to characterise the functional dynamics of the plankton and gain an understanding of how the different environments in the lakes have led to the evolution of different communities. The plankton was dominated by microorganisms and differed across the salinity spectrum in relation to trophy, age and the presence of meromixis. However, some elements of the plankton were common to all of the lakes, e.g. the mixtrophic ciliate, Mesodinium rubrum, which reached abundances of 2.7᎒⁵ l^-1 and spanned a salinity gradient of 4-63‰. Marine dinoflagellate species also occurred in all of the lakes, often at high abundances in Highway Lake, Pendent Lake and Lake Williams. During December (mid-summer), primary production showed an increase along the salinity gradient from Highway Lake to Lake Williams; however, it was low in hyper-nutrified Rookery Lake because of the turbidity of the waters. Bacterial production followed the same trend and was extremely high in Rookery Lake (327 µg l^-1 h^-1 in January). The lakes possessed a marine microbial plankton that has become very simplified through time, and now contains a small number of highly successful species, which were pre-adapted to surviving in extreme Antarctic lakes.     

Morphological study of Cyclotella choctawhatcheeana Prasad (Stephanodiscaceae) from a saline Mexican lake

Background

Extracellular dissolved DNA has been demonstrated to be present in many terrestrial and aquatic environments, actively secreted, or released by decaying cells. Free DNA has the genetic potential to be acquired by living competent cells by horizontal gene transfer mediated by natural transformation. The aim of this work is to study the persistence of extracellular DNA and its biological transforming activity in extreme environments like the deep hypersaline anoxic lakes of the Mediterranean Sea. The brine lakes are separated from the upper seawater by a steep chemocline inhabited by stratified prokaryotic networks, where cells sinking through the depth profile encounter increasing salinity values and osmotic stress.

Results

Seven strains belonging to different taxonomic groups isolated from the seawater-brine interface of four hypersaline lakes were grown at medium salinity and then incubated in the brines. The osmotic stress induced the death of all the inoculated cells in variable time periods, between 2 hours and 144 days, depending on the type of brine rather than the taxonomic group of the strains, i.e. Bacillaceae or gamma-proteobacteria. The Discovery lake confirmed to be the most aggressive environment toward living cells. In all the brines and in deep seawater dissolved plasmid DNA was substantially preserved for a period of 32 days in axenic conditions. L'Atalante and Bannock brines induced a decrease of the supercoiled form up to 70 and 40% respectively; in the other brines only minor changes in plasmid conformation were observed. Plasmid DNA after incubation in the brines maintained the capacity to transform naturally competent cells of Acinetobacter baylii strain BD413.

Conclusion

Free dissolved DNA is likely to be released by the lysis of cells induced by osmotic stress in the deep hypersaline anoxic lakes. Naked DNA was demonstrated to be preserved and biologically active in these extreme environments, and hence could constitute a genetic reservoir of traits acquirable by horizontal gene transfer.     

Response of Aerobic Anoxygenic Phototrophic Bacterial Diversity to Environment Conditions in Saline Lakes and Daotang River on the Tibetan Plateau,NW China

Variations of the pufM gene [encoding the M subunit of the photosynthetic reaction center in aerobic anoxygenic phototrophic (AAnP) bacteria] diversity in response to environmental changes were investigated in waters of six aqueous regimes (including Daotang River and five saline/hypersaline lakes on the Tibetan Plateau) representing a full salinity gradient from freshwater to NaCl-saturation. AAnP bacterial community structures responded to salinity change: Gamma-like AAnP community was predominant in freshwater Daotang River (0.01% salinity). AAnP community structure shifted from Loktanella-like sequences of the Alphaproteobacteria in saline Qinghai Lake to Roseobacter-like sequences in hypersaline lakes (Gahai, Xiaochaidan and Charhan). In addition to salinity, other environmental variables (e.g. N and P availability, TOC and/or DOC, and HCO^? ₃/CO^2? ₃ ions) were also important in affecting the pufM gene diversity in hypersaline lakes. These data have important implications for our understanding of the response of AAnP bacterial community to environmental variables in high-altitude aquatic ecosystems.     

High salinity induced expression profiling of differentially expressed genes in shrimp (Penaeus monodon)

Four suppression subtractive hybridization (SSH) cDNA libraries were constructed to identify differentially expressed salinity stress responsive genes of black tiger shrimp, Penaeus monodon exposed to high (55 ppt) salinity conditions. One each of the forward and reverse SSH cDNA libraries were developed from the gill and gut tissues of shrimp and clones having inserts larger than 300 bp were unidirectionally sequenced. Based on the sequence homology search, the identified genes were categorized for their putative functions related to a wide range of biological roles, such as nucleic acid regulation and replication, immune response, energy and metabolism, signal transduction, cellular process, structural and membrane proteins, stress and osmoregulation. Gene expression levels in response to high salinity conditions at 2 weeks post salinity stress for some of the differentially expressed genes (Na⁺/K⁺-ATPase α-subunit, glutathione peroxidase, intracellular fatty acid binding protein, elongation factor 2, 14-3-3 like protein, penaeidin, translationally controlled tumor protein, transglutaminase and serine proteinase inhibitor B3) identified from SSH cDNA libraries were analysed by real-time RT-PCR. The highest gene expression levels was observed for Na⁺/K⁺-ATPase α-subunit in gill tissues (15.23-folds) and antennal glands (12.01-folds) and intracellular fatty acid binding protein in gut tissues (14.05-folds) respectively. The differential and significant levels of gene expression indicate the functional role of these genes in shrimp salinity stress adaptive mechanisms.     

Spatial distribution of diatom and cyanobacterial mats in the Dead Sea is determined by response to rapid salinity fluctuations

Cyanobacteria and diatom mats are ubiquitous in hypersaline environments but have never been observed in the Dead Sea, one of the most hypersaline lakes on Earth. Here we report the discovery of phototrophic microbial mats at underwater freshwater seeps in the Dead Sea. These mats are either dominated by diatoms or unicellular cyanobacteria and are spatially separated. Using in situ and ex situ O₂ microsensor measurements we show that these organisms are photosynthetically active in their natural habitat. The diatoms, which are phylogenetically associated to the Navicula genus, grew in culture at salinities up to 40 % Dead Sea water (DSW) (14 % total dissolved salts, TDS). The unicellular cyanobacteria belong to the extremely halotolerant Euhalothece genus and grew at salinities up to 70 % DSW (24.5 % TDS). As suggested by a variable O₂ penetration depth measured in situ, the organisms are exposed to drastic salinity fluctuations ranging from brackish to DSW salinity within minutes to hours. We could demonstrate that both phototrophs are able to withstand such extreme short-term fluctuations. Nevertheless, while the diatoms recover better from rapid fluctuations, the cyanobacteria cope better with long-term exposure to DSW. We conclude that the main reason for the development of these microbial mats is a local dilution of the hypersaline Dead Sea to levels allowing growth. Their spatial distribution in the seeping areas is a result of different recovery rates from short or long-term fluctuation in salinity.     

Experimental Detritus Manipulations Unite Surface and Cave Stream Ecosystems Along a Common Energy Gradient

Subsidies of detritus from donor habitats are important energy sources for many ecosystems, but understanding their role in structuring recipient food webs requires comparative experimental studies along the full spectrum of detrital fluxes. Here we report results from an experimental addition of maize (Zea mays L.) litter to a detritus-poor cave stream ecosystem, which we then compare with analogous, past experiments using detritus-rich surface stream ecosystems that similarly have detritus-based food webs and extremely low in situ primary production. Bulk-tissue and compound-specific stable isotope analyses showed that maize litter carbon (C) was rapidly assimilated by microbes and transferred via successive trophic levels to the top of the cave stream food web (omnivorous crayfishes and predatory salamanders). All trophic levels increased in abundance and biomass, but only facultative cave taxa, that is those also found in surface streams, contributed to this numerical response. The lack of response by obligate cave species presumably occurred because evolutionary trade-offs associated with adaptations to low-C environments constrained their population-level responses during the one-year period of the litter addition. Comparison of the responses of the cave community with the analogous litter manipulation experiments in surface streams showed strong convergence in the functional relationship between invertebrate and detritus biomass (R ² = 0.72, P < 0.0001). Our results suggest that these seemingly disparate stream food webs lie along a single, common gradient of detritus supply, occupied at its extreme minimum by communities of obligate cave taxa adapted to low-energy environments.     

Temporal variation of phytophilous Chironomidae over a 11-year period in a shallow Neotropical lake in southern Brazil

Small and shallow lakes make up most of the lake-covered area in the world, and immature Chironomidae generally constitute the dominant group among invertebrates in these systems, exhibiting a wide distribution and tolerance to various levels of water quality. The changes in trophic features associated with the dominance of different major primary producers (macrophyte and phytoplankton) are reflected in the functioning of these ecosystems and likely in their phytophilous Chironomidae species richness and abundance. These were thus studied in a Brazilian shallow lake over a 11-year period characterized by both clear and turbid water conditions in relation to water quality and dominant macrophytes. The results revealed differences in Chironomidae composition but not in richness associated with clear and turbid waters. The Goeldichironomus genera predominated during all years and for different macrophyte types (freefloating, floating leaf, and submerged). We attribute the variability of Chironomidae assemblages to changes in macrophyte dominance and water trophic status, such as the concentration of nutrients and chlorophyll-a, during the 11-year period. Submerged macrophytes were associated with clear waters and harbored the highest diversity, which emphasized the importance of these environmental conditions for preserving the biodiversity of Chironomidae in shallow lakes.     

Development of diatom-based salinity models for paleoclimatic research from lakes in British Columbia (Canada)

Diatoms were identified and enumerated from the surface sediments of 65 lakes located on the Cariboo and Chilcotin Plateaux (British Columbia, Canada). These lakes span a large gradient in lakewater ionic concentration (fresh through hypersaline) and composition, as well as other physical/chemical variables. Almost all of the study lakes had higher salinities in the late-summer than in the spring. The lakes with spring salinities >8 g l^–1 showed the largest seasonal increases in salinity. Ionic composition was similar in the spring and late-summer for most lakes. Both ionic concentration (i.e. salinity) and composition were important environmental variables that could account for the different diatom floras in the lakes. Diatom assemblages characteristic of carbonate-dominated and sulfate-dominated waters were identified. Other variables such as water depth and phosphorus concentration were also important.The majority (87%) of diatom taxa had estimated salinity optima < 3 g l^–1 Halophilic diatom taxa had broader tolerances to salinity when compared to the fresh water taxa, however taxa with narrow and broad tolerances could be identified across the salinity gradient. Species diversity was weakly but significantly correlated to lakewater salinity (r ² = 0.18 to 0.3, P < 0.05).Salinity inference models were developed based on the relationship between the diatom assemblages and the spring, late-summer and average salinity. The correlations between the measured and diatominferred salinity, based on the spring (r = 0.95), late-summer (r = 0.94) and average (r = 0.95) salinity data, are high because there was an extremely strong correlation (r = 0.98) between the log transformed spring and late-summer measured salinities. These salinity reconstruction models provide a tool that can be used to infer past climatic changes as part of paleolimnological studies from appropriate closed-basin lakes in British Columbia.