Inductive reasoning and forecasting of population dynamics of Cylindrospermopsis raciborskii in three sub-tropical reservoirs by evolutionary computation

Seven-day-ahead forecasting models of Cylindrospermopsis raciborskii in three warm-monomictic and mesotrophic reservoirs in south-east Queensland have been developed by means of water quality data from 1999 to 2010 and the hybrid evolutionary algorithm HEA. Resulting models using all measured variables as inputs as well as models using electronically measurable variables only as inputs forecasted accurately timing of overgrowth of C. raciborskii and matched well high and low magnitudes of observed bloom events with 0.45 ≤ r² > 0.61 and 0.4 ≤ r² > 0.57, respectively. The models also revealed relationships and thresholds triggering bloom events that provide valuable information on synergism between water quality conditions and population dynamics of C. raciborskii. Best performing models based on using all measured variables as inputs indicated electrical conductivity (EC) within the range of 206–280 mS m⁻¹ as threshold above which fast growth and high abundances of C. raciborskii have been observed for the three lakes. Best models based on electronically measurable variables for the Lakes Wivenhoe and Somerset indicated a water temperature (WT) range of 25.5–32.7 °C within which fast growth and high abundances of C. raciborskii can be expected. By contrast the model for Lake Samsonvale highlighted a turbidity (TURB) level of 4.8 NTU as indicator for mass developments of C. raciborskii.Experiments with online measured water quality data of the Lake Wivenhoe from 2007 to 2010 resulted in predictive models with 0.61 ≤ r² > 0.65 whereby again similar levels of EC and WT have been discovered as thresholds for outgrowth of C. raciborskii. The highest validity of r² = 0.75 for an in situ data-based model has been achieved after considering time lags for EC by 7 days and dissolved oxygen by 1 day. These time lags have been discovered by a systematic screening of all possible combinations of time lags between 0 and 10 days for all electronically measurable variables. The so-developed model performs seven-day-ahead forecasts and is currently implemented and tested for early warning of C. raciborskii blooms in the Wivenhoe reservoir.     

The effects of phosphorus and temperature on the competitive success of an invasive cyanobacterium

Rising lake temperatures and changing nutrient inputs are believed to favour the spread of a toxic invasive cyanobacterium, Cylindrospermopsis raciborskii (Woloszynska) Seenayya and Subba Raju, in temperate lakes. However, most evidence for these hypotheses is observational or based on physiological measurements in monocultures. We lack clear experimental evidence relating temperature and nutrients to the competitive success of C. raciborskii. To address this, we performed a 2 × 2 factorial laboratory experiment to study the dynamics of mixed phytoplankton communities subjected to different levels of temperature and phosphorus over 51 days. We allowed C. raciborskii to compete with ten different species from major taxonomic groups (diatoms, green algae, cryptophytes, and cyanobacteria) typical of temperate lakes, under low and high summer temperatures (25 and 30 °C) at two levels of phosphorus supply (1 and 25 µmol L^?1). Cylindrospermopsis raciborskii dominated the communities and strongly decreased diversity under low-phosphorus conditions, consistent with the hypothesis that it is a good phosphorus competitor. In contrast, it remained extremely rare in high-phosphorus conditions, where fast-growing green algae dominated. Surprisingly, temperature played a negligible role in influencing community composition, suggesting that changes in summer temperature may not be important in determining C. raciborskii’s spread.     

Recent invader or indicator of environmental change? A phylogenetic and ecological study of Cylindrospermopsis raciborskii in New Zealand

Cylindrospermopsis raciborskii is a diazotrophic and potentially toxic cyanobacterium that was initially thought to be confined to tropical freshwaters. Recently it appears to have expanded its range to more temperate regions of the globe. There are contrasting hypotheses to explain this spread including; dispersal of highly adapted strains or localised spread from warm refuges as climatic or environmental conditions change. C. raciborskii was first detected in the isolated island nation of New Zealand in 2003, providing a unique opportunity to explore whether this recent identification is due to a new incursion or resultant from climatic or environmental change. Phylogenetic analysis (nifH, ITS1-L, ITS1-S, and rpoC1) of six strains isolated from two New Zealand lakes showed they were most closely related to those from South America, and suggest that the recent detection of this species was not due to a new incursion. Ten years of environmental data from three lakes (Waaki, Waikare and Whangape) experiencing blooms were analysed to identify potential reasons for recent C. raciborskii blooms. This analysis showed that the relatively recent (within the last 20–30 years) collapses of extensive macrophyte stands in lakes Waaki, Waikare and Whangape have resulted in increased turbidity’, low water column dissolved reactive phosphorus and seasonal shifts in the dissolved inorganic nitrogen availability, all conditions known to facilitate C. raciborskii dominance. Collectively these data indicate that C. raciborskii has always been present in New Zealand, and that recent changes in environmental conditions in these lakes are now facilitating bloom events.     

Can overwintering versus diapausing strategy in <Emphasis Type="Italic">Daphnia</Emphasis> determine match–mismatch events in zooplankton–algae interactions?

Mismatches between predator and prey due to climate change have now been documented for a number of systems. Ultimately, a mismatch may have far-reaching consequences for ecosystem functioning as decoupling of trophic relationships results in trophic cascades. Here, we examine the potential for climate change induced mismatches between zooplankton and algae during spring succession, with a focus on Daphnia and its algal food. Whereas the development of an overwintering population of daphnids may parallel shifts in phytoplankton phenology due to climate warming, changes in the photoperiod–temperature interaction may cause the emerging population of daphnids to hatch too late and mismatch their phytoplankton prey. A decoupling of the trophic relationship between the keystone herbivore Daphnia and its algal prey can result in the absence of a spring clear water phase. We extended an existing minimal model of seasonal dynamics of Daphnia and algae and varied the way the Daphnia population is started in spring, i.e., from free swimming individuals or from hatching resting eggs. Our model results show that temperature affects the timing of peak abundance in Daphnia and algae, and subsequently the timing of the clear water phase. When a population is started from a small inoculum of hatching resting eggs, extreme climate warming (+6°C) results in a decoupling of trophic relationships and the clear water phase fails to occur. In the other scenarios, the trophic relationships between Daphnia and its algal food source remain intact. Analysis of 36 temperate lakes showed that shallow lakes have a higher potential for climate induced match–mismatches, as the probability of active overwintering daphnids decreases with lake depth. Future research should point out whether lake depth is a direct causal factor in determining the presence of active overwintering daphnids or merely indicative for underlying causal factors such as fish predation and macrophyte cover. Priority program of the German Research Foundation—contribution 5.     

Cold tolerance of the maize orange leafhopper,Cicadulina bipunctata

Cicadulina bipunctata was originally distributed in tropical and subtropical regions of the Old World. This leafhopper recently expanded its distribution area to southern parts of temperate Japan. In this study, factors affecting the overwintering ability of C. bipunctata were examined. A series of laboratory experiments revealed that cold acclimation at 15 °C for 7 days enhanced the cold tolerance of C. bipunctata to the same level as an overwintering population, adult females were more tolerant of cold temperature than adult males, and survival of acclimated adult females was highly dependent on temperature from −5 to 5 °C and exposure duration to the temperature. The temperature of crystallization of adult females was approximately −19 °C but temperatures in southern temperate Japan rarely dropped below −10 °C in the winter, indicating that overwintering C. bipunctata adults in temperate Japan are not killed by freezing injury but by indirect chilling injury caused by long-term exposure to moderately low temperatures. An overwintering generation of C. bipunctata had extremely low overwinter survival (<1%) in temperate Japan; however, based on winter temperature ranges, there are additional areas amenable to expansion of C. bipunctata in temperate Japan.     

The cyanobacterium Cylindrospermopsis raciborskii is facilitated by copepod selective grazing

Blooms of the toxin-producing cyanobacterium Cylindrospermopsis raciborskii occur in tropical and subtropical lakes during spring-summer but the mechanisms behind bloom formation are unclear. This study tests the hypothesis that C. raciborskii accumulations in freshwater systems are facilitated by selective copepod grazing. Prey selection was examined in a series of experiments with C. raciborskii and the green alga, Chlamydomonas reinhardtii, as well as within natural phytoplankton assemblages. Clearance rates of the copepod Boeckella sp. on a C. raciborskii diet were 2–4 times lower than that of a common cladoceran Ceriodaphnia sp. when both grazers had prey choice. More C. raciborskii was cleared by Boeckella sp. when in mixed natural phytoplankton assemblages, but the clearance rate declined when nutrient replete C. reinhardtii was added, demonstrating that when alternate “high quality” algae were present, so did C. raciborskii consumption. The clearance rates of Boeckella sp. on two toxic C. raciborskii strains were significantly lower than on a non-toxic strain, and on C. raciborskii with low cellular P content. When we tested the grazing preference of a copepod dominated mixed zooplankton community on C. raciborskii during the early bloom period, clearance rates were relatively low (0.05–0.20 ml individual⁻¹ h⁻¹), and decreased significantly as the proportion of C. raciborskii increased above 5%. These results suggest that C. raciborskii persistence could be promoted by copepods preferentially grazing on other algae, with significant loss of top-down control as C. raciborskii abundance increases.     

Effects of thermal acclimation and photoacclimation on lipophilic pigments in an invasive and a native cyanobacterium of temperate regions

The freshwater cyanobacterium Cylindrospermopsis raciborskii spreads from tropical to temperate regions worldwide. This entails acclimation to varied light and temperature conditions. We studied the thermal and light acclimation of the photosynthetic machinery of C. raciborskii by monitoring alteration of the chlorophyll a and carotenoid content in German strains of C. raciborskii, in African and Australian strains of C. raciborskii, and in German strains of Aphanizomenon gracile, a native cyanobacterium belonging to the same order (Nostocales). Our results showed that temperate and tropical C. raciborskii strains did not differ in pigment acclimation to light and temperature. In contrast, the ratio of photoprotective carotenoids (namely the carotenoid glycoside 4-hydroxymyxol glycoside [aphanizophyll]) to chlorophyll a increased significantly more in C. raciborskii in comparison with A. gracile (1) with decreasing temperatures from 20 to 10°C and a moderate light intensity of 80?µmol photons m^?2?s^?1 and (2) with increasing light intensities at a suboptimal temperature of 15°C, compared to 20°C. We conclude that below 20°C photoinhibition is avoided by greater photoprotection in the invasive species C. raciborskii compared to the native species A. gracile.     

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

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CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) INVASION AT MID‐LATITUDES: SELECTION,WIDE PHYSIOLOGICAL TOLERANCE,ORGLOBALWARMING?1

The tropical bloom‐forming cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) Seenaya et Subba Raju is causing increasing concern because of its potential toxicity and invasive behavior at mid‐latitudes. This species has recently been identified in several temperate areas and was first reported in France in 1994, but the mechanisms underlying this acclimation process remain to be elucidated. We performed a range of laboratory experiments in an attempt to identify the physiological characteristics that could account for this behavior. We investigated the three following hypotheses: 1) clones of C. raciborskii adapted to temperate climate have been selected as it advanced north, 2) C. raciborskii has high physiological tolerance that allows it to prosper in a wide range of conditions, and 3) changes inclimate (global warming) have favored the development of C. raciborskii in mid‐latitudes. Ten strains of C. raciborskii from Australia n=1), Brazil (2), France (2), Germany (1), Hungary (1), Mexico (1), and Senegal (2) were cultured at different temperatures and light intensities. The in vitro growth parameters (μ and T_opt) were the same for the tropical and temperate strains. All 10 strains displayed positive net growth in a wide range of temperatures (from 20 to 35°C) and light intensities (from 30 to 400 μmol photons·m^?2·s^?1), with maximum growth rates at around 30°C and 80 μmol photons·m^?2·s^?1. This suggests that the colonization of mid‐latitudes by C. raciborskii may result from a combination of its ability to tolerate a rather wide range of climatic conditions and the global warming phenomenon, which provides this species with better environmental conditions for its growth.     

Climate change affects timing and size of populations of an invasive cyanobacterium in temperate regions

Cylindrospermopsis raciborskii, an invasive freshwater cyanobacterium, originated from the tropics but has spread to temperate zones over the last few decades. Its northernmost populations in Europe occur in North German lakes. How such dramatic changes in its biogeography are possible and how its population dynamics in the newly invaded habitats are regulated are still unexplained. We therefore conducted a long-term (1993–2005) study of two German lakes to elucidate the mechanisms behind C. raciborskii population dynamics and to identify the abiotic constraints on its development. Our data revealed that pelagic populations of C. raciborskii thrived for three months during the summer, contributing up to 23% of the total cyanobacteria biovolume. Population sizes varied greatly between years without exhibiting any distinct long-term trends. In the annual lifecycle, C. raciborskii filaments emerged in the pelagic habitat when the temperature rose above 15–17 °C. At that time, mean photosynthetically active radiation in the mixed water column (I _mix) overstepped its maximum. Rates of population net increase were highest at the beginning of the season (0.15–0.28 day⁻¹), declined continuously over time, and were significantly positively correlated with I _mix. This indicates that the onset of the pelagic population is temperature-mediated and that I _mix controls its growth. Since I _mix peaks before the population onset, the time of germination is of crucial importance for successful development. To test this hypothesis, we designed a model to simulate pelagic population size, starting at different dates in the annual cycle. Moving the population onset forward by 30 days resulted in a doubling of the population size. We therefore conclude that an earlier rise in water temperature associated with climate change has promoted the spread of C. raciborskii to the temperate zone. Earlier warming permits earlier germination, thereby shifting the pelagic populations to a phase with higher I _mix, which advances growth and the population establishment.     

Substantial differences in littoral fish community structure and dynamics in subtropical and temperate shallow lakes

1. Fish play a key role in the functioning of temperate shallow lakes by affecting nutrient exchange among habitats as well as lake trophic structure and dynamics. These processes are, in turn, strongly influenced by the abundance of submerged macrophytes, because piscivorous fish are often abundant at high macrophyte density. Whether this applies to warmer climates as well is virtually unknown. 2. To compare fish community structure and dynamics in plant beds between subtropical and temperate shallow lakes we conducted experiments with artificial submerged and free‐floating plant beds in a set of 10 shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N), paired along a gradient of limnological characteristics. 3. The differences between regions were more pronounced than differences attributable to trophic state. The subtropical littoral fish communities were characterised by higher species richness, higher densities, higher biomass, higher trophic diversity (with predominance of omnivores and lack of true piscivores) and smaller body size than in the comparable temperate lakes. On average, fish densities were 93 ind. m⁻² (±10 SE) in the subtropical and 10 ind. m⁻² (±2 SE) in the temperate lakes. We found a twofold higher total fish biomass per unit of total phosphorus in the subtropical than in the temperate lakes, and as fish size is smaller in the former, the implication is that more energy reaches the littoral zone fish community of the warmer lakes. 4. Plant architecture affected the spatial distribution of fish within each climate zone. Thus, in the temperate zone fish exhibited higher densities among the artificial free‐floating plants while subtropical fish were denser in the artificial submerged plant beds. These patterns appeared in most lakes, regardless of water turbidity or trophic state. 5. The subtropical littoral fish communities resembled the fish communities typically occurring in temperate eutrophic and hypertrophic lakes. Our results add to the growing evidence that climate warming may lead to more complex and omnivory‐dominated food webs and higher density and dominance of smaller‐sized fish. This type of community structure may lead to a weakening of the trophic cascading effects commonly observed in temperate shallow lakes and a higher risk of eutrophication.     

The relationship between temperature,competition and the potential for colonization of a subtropical pond by Daphnia magna

The role that temperature plays in excluding large daphnid species from subtropical and tropical ponds, and competitive relationships between Daphnia laevis and Daphnia magna, are explored. D. magna, a large temperate species is probably not excluded from subtropical or tropical systems by elevated temperatures. However, D. magna was a poorer competitor, under a restricted set of laboratory conditions, than D. laevis, the only pond dwelling daphnid in subtropical Florida. Competition is proposed as one mechanism that may limit the number of daphnid species in subtropical and tropical ponds and lakes. Reduced environmental fluctuations in subtropical and tropical systems (compared with temperate systems) may allow zooplankton populations to reach an environmental carrying capacity where competition limits the number of similar, coexisting species in a habitat.     

Inter-annual variation in macroinvertebrate communities in a shallow forest lake in eastern Finland during 1990–2001

Cylindrospermopsis raciborskii undergoes characteristic morphological changes during its annual population development. Primary filaments (those that derive directly from the akinetes) possess the following morphological features: the trichomes have one or two acuminate ends; they are narrower (2.4–2.6 μm) tfhan older filaments, their lengths vary in a wide range (from 40 to 300 μm); some of them are straight but others are slightly coiled even in populations that later consist exclusively of straight filaments; cell walls between the cells are not or only hardly visible; the filaments have a fine granulation but no gas-vacuoles, polyphosphate- or any other contrasting bodies are seen. The secondary filaments deriving from the first division of primary filaments are similar but with only one acuminate end. This way, the number of germinating akinetes can be estimated as the number of primary filaments + 1/2 number of secondary filaments. The described morphological method to estimate inoculum size of C. raciborskii has limitations. (1) The number of akinetes that we get in this way is only a minimum number since germination is not synchronous under field conditions and probably not each akinete germinates that is present in the sediments. (2) The method is applicable only if germination occurs suddenly (relatively synchronously). This is often the case in a temperate region where germination is triggered by temperature. (3) The method cannot be used in tropical or subtropical lakes where water temperature is permanently rather high and akinetes do not develop or develop only very rarely. (4) Being an a posteriori method it cannot be used as predictive tool. Estimations for Lake Balaton are in a good agreement with results of studies with other methods (germination in laboratory cultures and direct counting) and have the advantage that they do not need time-consuming experimenting or difficult direct countings.     

Distribution patterns of pelagic euphausiids in the East China Sea

Distribution patterns and abundance of the euphausiids were examined in the East China Sea (23°30′ –33°00′N, 118°30′ –128°00′ E) in relation to temperature and salinity. The data were collected in 4 surveys from 1997 to 2000. The density or yield density model was used to predict optimum temperature and salinity of water for euphausiid distribution, and thereafter distribution patterns of euphausiids were determined based on the predicted parameters. Of 23 species, Euphausia pacifica, E. nana, Pseudeuphausia sinica and P. latifrons were numerically dominant. The analyses indicate that Euphausia pacifica is an offshore temperate water species, E. nana is an offshore temperate warm water species, P. sinica is a coastal subtropical water species and P. latifrons is an oceanic tropical water species. The 4 species occupied 4 different water masses, respectively, namely, cold water mass, cold and warm water mixed masses in winter and spring, cold and warm water mixed masses in summer and autumn, and warm water mass, which could be the good designators of individual water masses, respectively. The predicated optimal temperatures for E. tenera, S. carinatum, E. diomedeae, Stylocheiron affine, Nematoscelis sp., N. gracilis, N. atlantica, Stylocheiron sp. and S. suhmii are all > 25°. These species are mainly distributed in southern Kuroshio in winter and spring, Kuroshio, the Taiwan Warm Current and Tsushima Current in summer and autumn, the equatorial waters of Pacific Ocean and the eastern waters of the Taiwan Strait. They are called as oceanic tropical water species. Nematoscelis tenella and T. tricuspidata are referred to as offshore subtropical water species according to their geographic distributions even if they are halobionts. Euphausia sanzoi is considered as a typical offshore subtropical water species, which inhabited waters below 25°. Stylocheiron microphthalma, occupying warm current waters where temperature and salinity are nearly 25° and 34 in summer and autumn, belongs to oceanic tropical water species. In the same way, E. similes, E. mutica, Euphausia sp., E. brevis and E. recurva are classified into offshore subtropical water species in accordance with the optimum temperature and salinity of waters as well as locations and seasons of their occurrence. Optimum temperature, rather than salinity, is a better parameter in determining the distribution patterns of euphausiids.     

Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

Molecular Characterization of the Toxic Cyanobacterium Cylindrospermopsis raciborskii and Design of a Species-Specific PCR

Cylindrospermopsis raciborskii is a toxic-bloom-forming cyanobacterium that is commonly found in tropical to subtropical climatic regions worldwide, but it is also recognized as a common component of cyanobacterial communities in temperate climates. Genetic profiles of C. raciborskii were examined in 19 cultured isolates originating from geographically diverse regions of Australia and represented by two distinct morphotypes. A 609-bp region of rpoC1, a DNA-dependent RNA polymerase gene, was amplified by PCR from these isolates with cyanobacterium-specific primers. Sequence analysis revealed that all isolates belonged to the same species, including morphotypes with straight or coiled trichomes. Additional rpoC1 gene sequences obtained for a range of cyanobacteria highlighted clustering of C. raciborskii with other heterocyst-producing cyanobacteria (orders Nostocales and Stigonematales). In contrast, randomly amplified polymorphic DNA and short tandemly repeated repetitive sequence profiles revealed a greater level of genetic heterogeneity among C. raciborskii isolates than did rpoC1 gene analysis, and unique band profiles were also found among each of the cyanobacterial genera examined. A PCR test targeting a region of the rpoC1 gene unique to C. raciborskii was developed for the specific identification of C. raciborskii from both purified genomic DNA and environmental samples. The PCR was evaluated with a number of cyanobacterial isolates, but a PCR-positive result was only achieved with C. raciborskii. This method provides an accurate alternative to traditional morphological identification of C. raciborskii.     

Nutrient–chlorophyll relationships in tropical–subtropical lakes: do temperate models fit?

In tropical lakes relatively little is known about the general relationship between nutrient concentration and phytoplankton biomass. Using data from 192 lakes from tropical and subtropical regions we examine the relationship between total P (TP) and chlorophyll (Chl). The lakes are all located between 30° S to 31° N include systems in Asia, Africa, and North and South America but are dominated by Brazilian (n=79) and subtropical N. American (n=67) systems. The systems vary in morphometry (mean depth and lake area), trophic state as well total N (TN) to␣total P (TP) ratios and light extinction. Despite a nearly 500-fold range in TP concentrations (2–970 μg P l⁻¹), there was a poorer relationship between log TP and log Chl (r ²=0.42) than is generally observed for temperate systems from either narrow or broad geographic regions. N limitation is not a likely explanation for the relatively weak TP–Chl relationship in the tropical–subtropical systems. Systems had high average TN:TP ratios and neither a multiple regression with log TP and log TN nor separating systems with high TN:TP (>17 by weight) improved the predictive power of the log TP–log Chl relationship.     

Sediments,not plants,offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large‐bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes. 2. We performed ‘habitat choice’ experiments to test the response of Daphnia magna to predation cues at both the horizontal and vertical level by mimicking a ‘shallow littoral’ zone with plants and a ‘deeper pelagic’ zone with sediments. 3. Initial separate response experiments showed that natural plants, artificial plants and predation cues all repelled D. magna in the absence of other stimuli, while sediments alone did not trigger any significant response by D. magna. 4. The habitat choice experiments showed that, in the presence of predation cues and absence of plants, Daphnia moved towards areas with sediment. In the presence of both plants and sediments, Daphnia moved away from the plants towards the sediments under both shallow and deep water treatment conditions. 5. Based on these results, we suggest that Daphnia in Mediterranean shallow lakes avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, as also observed in subtropical shallow lakes. This pattern is not likely to change with water level alterations, a common feature of lakes in the region, even if the effectiveness of the refuge may be reduced.     

Methane emissions from Mexican freshwater bodies: correlations with water pollution

The literature concerning methane (CH₄) emissions from temperate and boreal lakes is extensive, but emissions from tropical and subtropical lakes have been less documented. In particular, methane emissions from Mexican lakes, which are often polluted by anthropogenic carbon and nutrient inputs, have not been reported previously. In this work, methane emissions from six Mexican lakes were measured, covering a broad range of organic inputs, trophic states, and climatic conditions. Methane emissions ranged from 5 to 5,000 mg CH₄ m^?2 day^?1. Water samples from several depths in each lake were analyzed for correlation between water quality indicators and methane emissions. Trophic state and water quality indexes were most strongly correlated with methane fluxes. The global methane flux from Mexican freshwater lakes was estimated to be approximately 1.3 Tg CH₄ year^?1, which is about 20% of methane and 4.4% of total national greenhouse gas emissions. Data for untreated wastewater releases to the environment gave an emission factor of 0.19 kg CH₄ kg^?1 of Biochemical Oxygen Demand, which is superior to that previously estimated by the IPCC for lake discharges. Thus, the large volume of untreated wastewater in Mexico implies higher methane emission than previously estimated.     

Genetic and eco-physiological differences of South American Cylindrospermopsis raciborskii isolates support the hypothesis of multiple ecotypes

The global distribution of the toxic cyanobacterium Cylindrospermopsis raciborskii has recently increased, and it has now been identified in tropical, subtropical and temperate freshwater bodies. The mechanisms underlying its success and expansion are still unknown. Several hypotheses have been proposed, including climate change, natural selection and physiological tolerance to different environmental conditions. In this study, we determined the phenotypic and genotypic characteristics of two recently isolated South American strains of C. raciborskii obtained from Uruguay. We analyzed the morphology, growth preferences, tolerance to low temperature (14 °C) and toxin production of the strains and performed phylogenetic analyses based on the ITS and nifH gene sequences. Both isolates showed significantly different morphology and growth behavior under different light intensities and phosphate supply. When genetic differences were assessed by BOX PCR, cluster analyses revealed that they could also be distinguished genotypically and were clearly distinct from C. raciborskii isolated from other continents. Phylogenetic analysis showed that the Uruguayan strains were closely affiliated to other C. raciborskii isolated from the Americas, especially to those from Brazil. Similar to previous studies, we found three solid clusters (Africa-Australia, Europe and America) according to the geographical origin of the isolates. Interestingly, based on nifH sequences, subclusters were identified in American populations indicating an early spread of the species within the continent. We propose that phenotypic and genetic variability of C. raciborskii populations is linked to the existence of different ecotypes whose success is subject to the local environmental conditions.