Tropical Freshwater Research Articles

Testing the prey naiveté hypothesis: Can native prey (Astyanax ruberrimus) recognize an introduced top predator, Cichla monoculus?

The prey naivete hypothesis (PNH) posits that prey will often fail to recognize and respond to introduced predators with whom they do not share a co-evolutionary history. We tested this hypothesis by examining anti-predator behaviour in the native characid fish Astyanax ruberrimus in response to its main native (Hoplias microlepis) and introduced (Cichla monoculus) fish predators in Panama. We observed the behaviour of wild-caught A. ruberrimus from an invaded and uninvaded site following exposure to chemical stimuli from: (1) injured conspecifics, (2) the native predator, and (3) the introduced predator. We found, first, that A. ruberrimus consistently responded to cues from injured conspecifics, suggesting that this species possesses an alarm signaling mechanism similar to that observed across Ostariophysan fishes. Second, A. ruberrimus responded to cues from their native predator, but only in one population, suggesting responses may be threat-sensitive. Third, A. ruberrimus lacking prior exposure to C. monoculus did not respond to cues from this predator, consistent with the PNH. In contrast, A. ruberrimus that have co-occurred with C. monoculus for several decades did respond to cues from this predator, suggesting that prior exposure to C. monoculus has led (either via local adaptation or learning) to acquired predator recognition. Overall, our findings are consistent with the PNH, although we cannot conclusively rule out alternate explanations for the observed differences between populations. Our work represents a first step towards understanding the role that behavioural naivete may have played in the initial stages of this important tropical freshwater introduction.

Copper Toxicity to Tropical Water Flea Daphnia carinata and Freshwater Micro-algae Scenedesmus

Sai Gon river is one most the most important river of South-eastern region of Vietnam by providing water resources and others vital natural services for millions of people in this area. In recent years, proportion with the sharply increasing of human activities in industry and agriculture, the river is continuously loaded with xenobiotics released by anthropogenic activities. Among pollutants, heavy metals are considered as the most toxic elements to aquatic living organisms and human health. The aim of this study is to assess the sensibility of freshwater microalgae Scenedesmus and water flea Daphnia carinata, two fresh water species from Vietnam to copper (Cu). After physical and chemical characterization, field water samples from upstream Sai Gon River was used as dilution water in toxicity tests. With water flea D. carinata, the EC50 value of 48h immobilization experiment was 1.90 µg/L. Growth inhibition of the Scenedesmus algae cells was determined following exposure for 72 h, and EC50 values was 78.2 µg/L. The results showed that Cu is highly toxic to both species, and water flea D. carinata was more sensitive than freshwater algae Scenedesmus. Based on the observed high sensitivity with Cu, both D. carinata and algae Scenedesmus are potential tools for the assessment of copper pollution in fresh water of Sai Gon river.

Soil organic carbon storage in a tropical freshwater wetland: the influence of vegetation type

The impact of different vegetation communities on soil organic carbon (SOC) in a tropical freshwater wetland in Uganda was investigated. SOC content, density and storage potential were determined under three different dominant vegetation communities: Cyperus papyrus L., Typha latifolia L. and Phragmites mauritianus Kunth. SOC content (123.7 ± 2.6 SE g C kg−1 dry soil) in C. papyrus was significantly higher (p < 0.05) than in both T. latifolia and P. mauritianus, whereas SOC density (kg C m−2) insignificantly varied (p > 0.05) among the three vegetation communities (C. papyrus = 7.2 ± 0.1, T. latifolia = 6.7 ± 0.1 and P. mauritianus = 6.2 ± 0.1). Similarly, for the entire sampled soil depth (0–50 cm), SOC storage potential was significantly higher (p < 0.05) in C. papyrus (36 118.08 ± 552.52 t C km−2), and was in the order of decreasing magnitude: C. papyrus > T. latifolia > P. mauritianus. Plant biomass density, and soil physico-chemical characteristics, bulk density, salinity, pH and temperature were significantly correlated (p < 0.05) with SOC. In conclusion, where climate change mitigation is considered as a wetland ecosystem service, restoration priorities for degraded/lost tropical freshwater wetlands need to consider C. papyrus plants. In addition, comparing SOC storage by ecosystems, Uganda’s wetlands contain three times more SOC than is contained in the country’s forests.

Salinization intensifies the effects of elevated temperatures on Channa striata, a common tropical freshwater aquaculture fish in the Mekong Delta, Vietnam

Increasing water temperatures and salinity intrusion due to climate change are serious challenges for freshwater aquaculture. In this study, we assessed the combined effects of salinization at 0, 6, and 9‰ and water temperatures of 28, 31, and 34 °C on the survival and growth rates, and feed utilization, of freshwater snakehead Channa striata. We also assessed feed digestibility in snakeheads to explore their physiological mechanisms. The lowest survival rate (66.7%) was found at 34 °C-9‰. The highest daily weight gain of 0.72 g/day was found at 34 °C-0‰, while the second highest (0.62 g/day) was at 31 °C-0‰. Chymotrypsin activity increased at higher temperatures; the activities of α-amylase and pepsin were lowest at 34 °C-9‰. The highest apparent digestibility coefficients (ADCs) for dry matter (80.3%), protein (95.3%), and lipid (97.9%) were observed at 31 °C-0‰ (p < 0.05). Elevated temperature (34 °C) combined with high salinity (9‰) was lethal for one-third of the fish. The lowest ADC and α-amylase activities led to the highest feed-conversion ratio. Snakehead growth rates were highest in the 31 °C-0‰ and 34 °C-0‰ treatments. The combined highest salinity and highest temperature treatment (34 °C-9‰) decreased snakehead growth, feed utilization efficiency, feed digestibility, and enzyme activities (except for chymotrypsin).

Community based actions save Yellow-spotted river turtle (Podocnemis unifilis) eggs and hatchlings flooded by rapid river level rises.

The conservation and recovery of increasingly threatened tropical freshwater turtle populations depends on effective management plans and actions. Here we show that community-based actions saved Yellow-spotted river turtle (Podocnemis unifilis) eggs submerged by unseasonal flooding and ensured the release of hatchlings. We recovered 926 eggs and 65 premature hatchlings from 74 submerged nests at 16 flooded nesting areas along 75 km of waterways. The rescued eggs were transferred to a rearing center and incubated. Hatchlings emerged from eggs that had remained underwater for up to two days. Hatchlings were maintained in 250–500 L nursery tanks until yolk sac scars had closed. Healthy hatchlings were then immediately released around the original nesting areas. We released 599 hatchlings (60.4%) from 991 submerged eggs and hatchlings. Egg survival (61.7% (571/926)) was substantially less than hatchling survival (94.2% (599/636)) but within the expected range of values reported for this species. These findings suggest that Yellow-spotted river turtle eggs and embryos are resistant to short-term submersion, which could help explain the widespread distribution of this species across highly seasonal Amazonian rivers. Management plans should take the possible survival of submerged eggs into consideration as part of species conservation and recovery actions.

Lacustrine responses to middle and late Holocene anthropogenic activities in the northern tropical Andes

A multi-proxy study on a sediment core from Pedro Palo Lake, a mid-altitude endorheic Andean Lake in northern South America, was carried out to understand the effects of human activities and climate variability on tropical mountain freshwater ecosystems. Results indicate that between ~ 7980 and 4110 cal year BP the lake maintained well-mixed, mesotrophic conditions, within which a diverse planktonic and benthic diatom community flourished. From 4110 to about 2050 cal year BP, benthic diatoms almost disappeared, and the planktonic community changed to include small Discostella spp. and large Fragilaria species, suggesting higher nutrient concentrations and possibly, more turbid waters. At this time, peaks in charcoal concentration and in the sedimentary C/N ratio imply that forest fires in the watershed intensified and that more organic matter, derived from terrestrial vegetation, accumulated in the lake. This period coincides with an increase in population, and in the social, technological and agricultural complexity of indigenous peoples. We propose that anthropogenic activities, burning and deforestation, increased the nutrient load to the lake causing the observed changes in the diatom communities. After 2050 cal year BP, periphytic diatoms returned, the intensity and frequency of fires diminished, and a secondary forest grew. We infer an associated decrease in nutrients, resulting from reforestation and/or lower water levels caused by drier climates. Estimations based on the age model of the core suggest that it took approximately 30 years for the lake to change from its most degraded state in the middle Holocene to its state in the late Holocene. However, the much-reduced diversity of the diatom community after ~ 2050 cal year BP indicates that the lake did not rebound to its previous middle Holocene conditions. The record from Pedro Palo is therefore a good example of the detrimental and long-lasting effects of anthropic deforestation.

Rhythmic activity of inshore migration of tropical freshwater glass eels of the genus Anguilla

Compared with temperate Anguilla, little is known about the early marine migration and recruitment mechanisms of these tropical eels. To understand the ecological and behavioural mechanisms of their inshore migration in tropical Anguilla, systematic research on their periodicity of inshore migration was conducted for 24 h over 9 to 13 consecutive days before and after the new moon dates four times in one year. Glass eel (juvenile) collections (151 to 4371 specimens during each survey period) in the Poigar River, north Sulawesi Island, Indonesia indicated a lunar periodicity, in which peak recruitment continued for 2 to 4 days during the new moon period; then, the recruitment sharply decreased before and after the new moon. Daily glass eel migration occurred in two peaks during the ebb (20:00) and flood (02:00) tides, while there was no significant different glass eel recruitment between the flood tide and ebb tides. Although the “selective tidal stream transport” that glass eels utilize during the flood tide for recruitment is widely accepted to be the main mechanism of inshore migration behaviour through estuaries to freshwater habitats for temperate glass eels, we did not find this mechanism during this study. These results suggest that the main recruitment mechanism for tropical glass eels may be associated with both endogenous rhythms (biological clock) and exogenous factors (tidal orientation) in the study site. The rhythmic activity of migrating tropical glass eels found in this study also contributes not only to understanding the biological and ecological implications but also provides clues for stock management and conservation of Anguilla, which have been critically declining in recruitment worldwide in recent decades.

Three-dimensional analysis of structural coloration by a slanted dielectric grating.

The stripe of the tropical freshwater fish "neon tetra" consists of many iridophores, in which tilted reflecting platelets are periodically arranged. The neon tetra has structural coloration and changes the color of a stripe in response to the surrounding conditions. The mechanism of the color change is thought to be controlling a slant angle of the platelets and changing the spacing between the platelets. This paper considers a slanted dielectric grating modeled on an iridophore of neon tetra, and formulates the matrix eigenvalues method as an analytical method for the three-dimensional scattering problem of a slanted grating having a grating vector. Calculating the reflection spectrum using the matrix eigenvalues method, the chromatic coordinates in standard red-green-blue color space, and the xy chromaticity coordinates for a slanted grating, it is shown that the color changes depending on the slant angles numerically.

Estimation of aboveground and belowground carbon stocks in urban freshwater wetlands of Sri Lanka

BackgroundThe occurrence of climate change at an unprecedented scale has resulted in alterations of ecosystems around the world. Numerous studies have reported on the potential to slow down climate change through the sequestration of carbon in soil and trees. Freshwater wetlands hold significant potential for climate change mitigation owing to their large capacity to sequester atmospheric carbon dioxide (CO2). Wetlands among all terrestrial ecosystems have the highest carbon density and are found to store up to three to five times more carbon than terrestrial forests. The current study was undertaken to quantify carbon stocks of two carbon pools: aboveground biomass (AGB) and belowground biomass (BGB). Chosen study sites; Kolonnawa wetland and Thalawathugoda wetland park are distributed within the Colombo wetland complex. Colombo was recognized as one of the 18 global Ramsar wetland cities in 2018. A combination of field measurements and allometric tree biomass regression models was used in the study. Stratification of the project area was performed using the normalized difference vegetation index (NDVI).ResultsThe AGB carbon stock, across strata, is estimated to be in the range of 13.79 ± 3.65–66.49 ± 6.70 tC/ha and 8.13 ± 2.42–52.63 ± 10.00 tC/ha at Kolonnawa wetland and Thalawathugoda wetland park, respectively. The BGB carbon stock is estimated to be in the range of 2.47 ± 0.61–10.12 ± 0.89 tC/ha and 1.56 ± 0.41–8.17 ± 1.39 tC/ha at Kolonnawa wetland and Thalawathugoda wetland park, respectively. The total AGB carbon stock of Kolonnawa wetland was estimated at 19,803 ± 1566 tCO2eq and that of Thalawathugoda wetland park was estimated at 4180 ± 729 tCO2eq.ConclusionsIn conclusion, the study reveals that tropical freshwater wetlands contain considerable potential as carbon reservoirs. The study suggests the use of tropical freshwater wetlands in carbon sequestration enhancement plans in the tropics. The study also shows that Annona glabra, an invasive alien species (IAS), has the potential to enhance the net sink of AGB carbon in these non-mangrove wetlands. However, further studies are essential to confirm if enhanced carbon sequestration by Annona glabra is among the unexplored and unreported benefits of the species.

Life-History Traits Response to Effects of Fish Predation (Kairomones), Fipronil and 2,4-D on Neotropical Cladoceran Ceriodaphnia silvestrii.

Aquatic environments are constantly exposed to a cocktail of contaminants mainly due to human activities. As polluted ecosystems may simultaneously present other multiple natural stressors, the objective of the present study was to evaluate joint effect of stressors (natural and anthropogenic) on life history traits of the Neotropical cladoceran,Ceriodaphnia silvestrii. For this purpose, the effects of water conditioned with predator kairomones (fish) and environmental concentrations (sublethal) of two pesticides widely used in sugarcane monoculture in Brazil, the insecticide Regent® 800 WG (active ingredient-a.i. fipronil) and the herbicide DMA® 806 BR (a.i. 2,4-D) were evaluated using chronic toxicity testing, isolated and in mixture, for this cladoceran species. The environmental risks of pesticides for tropical freshwater biota were also estimated from the risk quotient MEC/PNEC. Among the characteristics of the life history ofC. silvestriievaluated after 8days of exposure, compared with the mean value of control, the age of primiparous females was not affected by any evaluated treatment. However, species average survival decreased in the treatment of kairomones mixed with fipronil (FK) and in the treatment with a mixture of fipronil, 2,4-D, and kairomones (MFKD). The body length of maternal females was shorter than in the control after exposure in treatments with only kairomones (K) and FK. Fecundity of this cladoceran was reduced when exposed to FK and MFKD treatments, and the intrinsic rate of population increase significantly decreased for organisms exposed to treatment with fipronil (F) and to mixtures of fipronil and 2,4-D (MFD), MFDK, and FK. The results indicated that the combination of anthropogenic and natural stressors causes changes in C. silvestrii life history traits, which can contribute to the decline in populations, and our preliminary risk assessment results are a matter of concern regarding biota conservation.

Thermosensitive period for sex determination of the tropical freshwater turtle Malayemys macrocephala.

Many egg-laying reptiles possess temperature-dependent sex determination (TSD) in which outcome of gonadogenesis is determined by incubation temperature during a temperature-sensitive period of development. Prior studies on Malayemys macrocephala showed that incubation temperatures influence gonadal development and suggested that M. macrocephala exhibits TSD. However, information on the temperature-sensitivity period in this species was unknown until the current study. Turtle eggs were collected from rice fields in central Thailand from December 2016 to February 2017. In the laboratory, eggs were incubated at male-biased temperature (26°C) and shifted to female-biased temperature (32°C), or vice versa. Single shift experiments were performed systematically during embryonic stages 13-21. After hatching, sex of individual turtles was determined by histological analysis. We found that the sex determination of M. macrocephala is affected by temperature up to stage 16 of embryonic development.

Azolla pinnata: Potential Phytoremediation, Antimicrobial, and Antioxidant Applications

Azolla or the “green gold” is an aquatic nitrogen-fixing pteridophyte with a wide distribution in temperate and tropical freshwater ecosystems and paddy fields. Azolla is an ideal candidate for food, feed, and fodder applications. It can be utilized as a natural plant-based antimicrobial and also as a water purifier in a laboratory or industrial wastewater treatment. Its feasibility as a source for the development of health supplements was tested by analyzing the antioxidant and antimicrobial properties of the fern. The DPPH antioxidant activity of the various extracts shows the good presence of antioxidants. A fair antibacterial activity was shown against the disease, causing bacteria Staphylococcus sp. and Bacillus sp. Antioxidant and antimicrobial property of Azolla heightens the possibility of its use as food. The phytoremediation property of Azolla grown in a metal-containing sample was assessed using atomic absorption spectroscopy, and positive results indicated its prospective use in industrial or laboratory wastewater treatment. This can reduce the pollution of water bodies, like, rivers, where such water is discarded.

Characterization of tropical freshwater; River Ogun, Lagos Nigeria

Physico-chemical parameters and microbial contents of Ogun River at Magodo area, Lagos were investigated in relation to its safety for consumption. Water samples were weekly collected between August and December, 2016 at three equi-distance intervals for analysis on physico-chemical parameters, total bacterial count and biochemical tests following standard recommended procedures. Mean concentrations of the physico-chemical parameters were found in ranges for pH (6.68 ± 0.15 - 6.86 ± 0.21), temperature (26.78 ± 0.14 – 26.92 ± 0.31), electrical conductivity (355.40 ± 101.93 – 456.40 ± 115.77), total dissolved solids (166.80 ± 48.08 - 215.00 ± 54.29), dissolved oxygen (7.06 ± 0.01 - 7.08 ± 0.02), biochemical oxygen demand (25.59 ± 0.03 - 25.64 ± 0.01), chemical oxygen demand (98.75 ± 0.03 - 98.84 ± 0.02), total hardness (345.80 ± 1.28 - 349.40 ± 3.87), total alkalinity (9.28 ± 0.02 - 9.33 ± 0.02), nitrate (0.27 ± 0.00 - 0.28 ± 0.00), sulphate (0.55 ± 0.05 - 0.62 ± 0.00) and chloride (68.84 ± 0.77 - 74.97 ± 1.23). Total colony forming unit (Cfu) of bacteria in the river was found highest in descending order Centrimide Agar (CA) &gt; Mac-Conkey Agar (MCA) &gt; Plate Count Agar (PCA) &gt; Mannitol Salt Agar (MSA) at the first and second serial dilution respectively. Biochemical tests showed the presence of Staphylococcus species, Escherichia coli, Pseudomonas aeruginosa, Proteus sp., Salmonella sp. and Baccillus sp. in the river. Variations in the microbial load and physico-chemical parameters in Ogun River as a result of different human activities around the river was found within the standard limits which makes the River safe for human consumption.&#x0D; Key words: River Ogun, microbial load, bacterial count, biochemical tests Trop.

Urbanisation affects ecosystem functioning more than structure in tropical streams

Urbanisation poses a clear threat to tropical freshwater streams, yet fundamental knowledge gaps hinder our ability to effectively conserve stream biodiversity and preserve ecosystem functioning. Here, we studied the impact of urbanisation on structural and functional ecosystem responses in low-order streams in Singapore, a tropical city with a mosaic landscape of protected natural forests, managed buffer zones (between forest and open-country habitats), and built-up urban areas. We quantified an urbanisation gradient based on landscape, in-stream, and riparian conditions, and found an association between urbanisation and pollution-tolerant macroinvertebrates (e.g. freshwater snail and worm species) in litter bags. We also found greater macroinvertebrate abundance (mean individuals bag−1; forest: 30.3, buffer: 70.1, urban: 109.0) and richness (mean taxa bag−1; forest: 4.53, buffer: 4.75, urban: 7.50) in urban streams, but similar diversity across habitats. Higher levels of primary productivity (measured from algal accrual on ceramic tiles) and microbial decomposition (measured from litter-mass loss in mesh bags) at urban sites indicate rapid microbial activity at higher light, temperature, and nutrient levels. We found that urbanisation affected function 32% more than structure in the studied tropical streams, likely driven by greater algal growth in urban streams. These changes in ecological processes (i.e. ecosystem functioning) possibly lead to a loss of ecosystem services, which would negatively affect ecology, society, and economy. Our results point to possible management strategies (e.g. increasing vegetation density through buffer park creation) to reduce the impacts of urbanisation, restore vital ecosystem functions in tropical streams, and create habitat niches for native species.

Comparison of Sensitivity of Tropical Freshwater Microalgae to Environmentally Relevant Concentrations of Cadmium and Hexavalent Chromium in Three Types of Growth Media.

Sensitivity of tropical freshwater microalgae (Mesotaenium sp., Chlorococcum sp. and Scenedesmus sp.) to environmentally relevant concentrations of hexavalent chromium (Cr6+) and cadmium (Cd2+) was compared individually in three growth media viz. Bold's Basal Medium (BBM), Test Medium 1 (TM1) and Test Medium 2 (TM2) based on fluorescence reduction. Free metal content of growth media was determined by Visual MINTEQ (version 3.1). After 24h, relative fluorescence of microalgae in the three media decreased with increased metal concentration showing a concentration dependent graded toxicity response. All microalgae were more sensitive to the metals when grown in TM1, when compared, more sensitive to Cr6+ than Cd2+. Metal speciation indicated that TM1 and TM2 media have higher percentage of bioavailable Cd2+ than BBM, and chromium was present mainly as CrO42- and HCrO4-. The results suggest that the TM1 medium is more suitable under short term exposure of microalgae to metals in environmental monitoring.

Interactive persistent effects of past land‐cover and its trajectory on tropical freshwater biodiversity

Abstract Evidence indicating that ecological communities show delayed responses to environmental change has raised the need to better understand the effects of landscape history on biodiversity. We investigated how freshwater biodiversity is related to both recent and past land use change in tropical river catchments. More specifically, we analysed the relationship between biodiversity and change in forest cover that occurred across five decades, including landscape trajectories of forest gain and loss. The abundance of stream insects showed a more delayed response to landscape change than rarefied richness, whereas the Tsallis diversity index was not related to past forest cover. However, both alpha diversity and local abundance were related to mean forest cover across the five decades when conditioned by the trajectory of change. We found a negative relationship between the historical mean of forest cover and community descriptors in streams embedded in landscapes on a trajectory of forest loss, but a positive relationship in landscapes on a trajectory of forest gain. We provide the first evidence of delayed responses of tropical freshwater insect communities to landscape history and suggest that the magnitude of change in forest cover, mediated by its trajectory, is a major driver of delayed responses. We show that a trajectory of forest loss might result in both immediate loss of species and a set of species doomed for future extinction. Synthesis and applications. Delayed responses of multispecies communities to past landscape changes depend on their extent, frequency and intensity. Our findings indicate that, beyond the relative amount of forest cover within river catchments, the magnitude of change in forest cover mediated by its historical trajectory can be a major driver of delayed responses in stream communities. We suggest that, if biodiversity conservation is the main aim, stream restoration should be prioritized in catchments with higher forest cover, as even under a forest gain trajectory, stream communities continue responding to past landscape changes depending on how widespread and intensive changes were.

Climate change models predict decreases in the range of a microendemic freshwater fish in Honduras

Despite their incredible diversity, relatively little work has been done to assess impacts of climate change on tropical freshwater organisms. Chortiheros wesseli is a species of Neotropical cichlid (Cichlidae: Cichlinae) restricted to only a few river drainages in the Caribbean-slope of Honduras. Little is known about this species and few specimens had been collected until recently; however, our work with this species in the wild has led to a better understanding of its ecology and habitat preferences making it an excellent model for how freshwater fishes can be affected by climate change. This study assesses the distribution and habitats of Chortiheros wesseli using a combination of field data and species distribution modeling. Results indicate this species is largely limited to its current range, with no realistic suitable habitat nearby. Empirical habitat data show that this species is limited to narrow and shallow flowing waters with rapids and boulders. This habitat type is highly influenced by precipitation, which contributed the greatest influence on the models of present and future habitat suitability. Although several localities are within boundaries of national protected areas, species distribution models all predict a reduction in the range of this freshwater fish based on climate change scenarios. The likelihood of a reduced range for this species will be intensified by adverse changes to its preferred habitats.

Azolla pinnata: potential phytoremediation, antimicrobial and antioxidant applications

Azolla is an aquatic nitrogen-fixing pteridophyte with a wide distribution in temperate and tropical freshwater ecosystems and paddy fields. Its feasibility as a source for the development of health supplements was tested by analyzing the phytochemical, antimicrobial, and antioxidant properties of the fern. The plant was found to be rich in phenolic compounds like tannins, saponins, etc. which shows it will be of good medicinal value. The DPPH antioxidant activity of the various extracts shows the good presence of antioxidants, which shows its potential use as health supplements. A fair antibacterial activity was shown against the disease, causing bacteria Staphylococcus sp. and Bacillus sp. It can be concluded that the fern has good nutritional and antibacterial activity. The phytoremediation property of Azolla grown in a metal-containing sample was assessed using atomic absorption spectroscopy, and positive results indicated its prospective use in industrial or laboratory wastewater treatment. Thus Azolla is an ideal candidate for future research in food, feed, and fodder applications. It can be utilized as a natural plant-based antimicrobial and also as a water purifier.

Amelioration of copper toxicity to a tropical freshwater microalga: Effect of natural DOM source and season

Australian tropical freshwaters can experience extreme seasonal variability in rainfall and run off, particularly due to pulse events such as storms and cyclones. This study investigated how seasonal variability in dissolved organic matter (DOM) quality impacted the chronic toxicity of copper to a tropical green alga (Chlorella sp.) in the presence of two concentrations of DOM (low: ∼2 mg C/L; high: ∼10 mg C/L) collected from three tropical waters. Copper speciation and lability were explored using diffusive gradients in thin-films (DGT) and modelled maximum dynamic concentrations (cdynmax) using data derived from the Windermere Humic Aqueous Model (WHAM VII). Relationships between copper lability and copper toxicity were assessed as potential tools for predicting toxicity. Copper toxicity varied significantly with DOM concentration, source and season. Copper toxicity decreased with increasing concentrations of DOM, with 50% growth inhibition effect concentrations (EC50) increasing from 1.9 μg Cu/L in synthetic test waters with no added DOM (0.34 mg C/L) up to 63 μg Cu/L at DOM concentrations of 9.9 mg C/L. Copper toxicity varied by up to 2-fold between the three DOM sources and EC50 values were generally lower in the presence of wet season DOM compared to dry season DOM. Linear relationships between DGT-labile copper and dissolved copper were significantly different between DOM source, but not concentration or season. Modelled cdynmax consistently under-predicted labile copper in high DOM treatments compared to DGT measurements but performed better in low DOM treatments, indicating that this method is DOM-concentration dependent. Neither speciation method was a good surrogate for copper toxicity in the presence of different sources of natural DOM. Our findings show that DOM source and season, not just DOM concentration, affect copper toxicity to freshwater biota. Therefore, DOM quality should be considered as a toxicity-modifying factor for future derivation of bioavailability-based site-specific water quality guideline values.

Фактологическое обеспечение практик повышения эффективности выращивания тропических пресноводных видов

The article describes the possibility to grow tropical freshwater crayfish Cherax quadricarinatus (Von Martens, 1868) and shrimp Macrobrachium rosenbergii (De Man, 1879) &#x0D; in the climatic conditions of the Astrakhan region during 3-5 months, with further wintering in the ponds of fish farms. To achieve the maximum effect of the semi-intensive and intensive cultivation conditions it is necessary to apply additional feeding, observe the norms of seeding density and the diets of crustaceans. In two series of experimental works conducted in 2019-2020 on the base of Astrakhan State Technical University and a small innovative enterprise Modern Fishbreeding Complex Sharapovsky there was studied the method of increasing the biological and aquaculture potential of fast-growing tropical freshwater crayfish and shrimps by adjusting feed diets, chilled wet feed, and native formula feed based on feed components of local raw materials. Salicornia perennans Willd, a halophyte plant, was introduced into the formulation for the first time. As a result of experimental works the recommendations on semi-intensive and intensive cultivation of tropical freshwater species have been formulated, the possibility of increasing biological and aquaculture productivity of grown objects has been considered, which is recorded in the fish-biological standards and general management processes.