The benthic communities of the saline lakes Abijata and Shala (Ethiopia)

Lake Abijata lies in a shallow depression (maximum depth 8–9.5 m); the water is green with phytoplankton and it supports large fish and bird communities. Lake Shala lies in a deep caldera (maximum depth reputedly 260 m); phytoplankton is sparse and fish and bird communities scanty.Lakes Abijata and Shala, sampled in January, 1985, had conductivities of 14 000 and 21 000 microSiemens cm^-1 at 25 °C respectively, mainly due to high sodium, carbonate and chloride ions. Calcium concentrations are very low.The benthic fauna was studied with an Ekman grab to a depth of 8.5 m in Abijata and 15.5 m in Shala and was found to be dense in both lakes but varying greatly in composition at different depths. In Abijata the benthos consisted mainly of Ostracoda and Chironomidae, and in Shala mainly of Tubificidae, Ostracoda and Chironomidae. There were very few Nematoda. No true halophilic species were found but the community consisted of euryhaline forms found also in non-saline waters. Predatory invertebrates were absent and many of the dominant species, notably of the Chironomidae, were different from those of non-saline lakes nearby.     

Evaluation of growth and biomass production of Arthrospira (Spirulina) fusiformis in laboratory cultures using waters from the Ethiopian soda lakes Chitu and Shala

The cost of nutrient media is the major challenge for biomass production of Spirulina. Although much effort has been made to use enriched seawater for the cultivation of this microalga, little attention has been given to the potential of water of soda lakes. In this study, growth (μ, day^?1) and biomass production (B) of Arthrospira fusiformis cultivated using waters of the soda lakes Chitu and Shala with or without supplementation were evaluated. Comparable μ and B values were achieved in both Lake Chitu water-based media (CBM) and Lake Shala water-based media (SBM), with slightly higher values in the latter. Both CBM and SBM supplemented with the standard Spirulina medium (SM) by 25 % and 50 % supported considerably higher μ and B. The pH and salinity of the cultures showed significant variations (P?μ and B. The observed higher μ and B were probably associated with the reduction in pH and salinity of the supplemented media due to addition of bicarbonate–carbonates and dilution, and provision of the limiting nutrient nitrogen. The higher μ and B in SBM may have resulted from some of their aggregate chemical parameters, which were closer to those in the SM, and abundant PO₄-P. This seems to suggest that Lake Shala water is more conducive to Arthrospira. We contend that 25 % and 50 % supplemented Lake Shala water can be preferably used to produce Arthrospira biomass, thereby reducing the cost of nutrients by 75 % and 50 %, respectively.     

First evidence of cryptic species diversity and significant population structure in a widespread freshwater nematode morphospecies (Tobrilus gracilis)

Free‐living nematodes are ubiquitous and highly abundant in terrestrial and aquatic environments, where they sustain ecosystem functioning by mineralization processes and nutrient cycling. Nevertheless, very little is known about their true diversity and intraspecific population structure. Recent molecular studies on marine nematodes indicated cryptic diversity and strong genetic differentiation of distinct populations, but for freshwater nematode species, analogous studies are lacking. Here, we present the first extensive molecular study exploring cryptic species diversity and genetic population structure of a widespread freshwater nematode morphospecies, Tobrilus gracilis, from nine postglacially formed European lakes. Taxonomic species status of individuals, analysed for fragments of the mitochondrial COI gene and for the large (LSU) and small (SSU) ribosomal subunits, were determined by morphological characteristics. Mitochondrial and nuclear markers strongly supported the existence of three distinct genetic lineages (Tg I–III) within Tobrilus gracilis, suggesting that this morphospecies indeed represents a complex of highly differentiated biological species. High genetic diversity was also observed at the population level. Across the nine lakes, 19 mitochondrial, and seven (LSU) and four (SSU) nuclear haplotypes were determined. A phylogeographical analysis revealed remarkable genetic differentiation even among neighbouring lake populations for one cryptic lineage. Priority and persistent founder effects are possible explanations for the observed population structure in the postglacially colonized lakes, but ask for future studies providing direct estimates of freshwater nematode dispersal rates. Our study suggests therefore that overall diversity of limnetic nematodes has been so far drastically underestimated and challenges the assumed ubiquitous distribution of other, single freshwater nematode morphospecies.     

Relation between nematode communities and trophic state in southern Swedish lakes

The aim of this study was to examine whether littoral nematode community patterns are shaped by lake trophic state. It was hypothesized that trophic level is associated negatively with the proportion of omnivores and positively with the percentages of bacterial feeders, but not at all with the diversity, abundance, and biomass of freshwater nematodes. Sediment samples were taken at littoral sites of eight southern Swedish lakes of different trophy in spring and autumn 2007. Trophic level was found to strongly influence species richness, as oligotrophic and mesotrophic lakes supported the greatest species numbers, whereas nematode abundance, biomass, and Shannon index were unaffected. Furthermore, our results indicated effects on the nematode community’s trophic structure, with a larger proportion of predatory nematodes in oligotrophic and mesotrophic lakes but no differences in the other feeding types (bacteria, algae and suction feeders, omnivorous species). Multivariate analysis indicated a shift in species compositions along the threshold from mesotrophic to eutrophic conditions, with the presence of Tobrilus gracilis, Monhystera paludicola, Brevitobrilus stefanskii, and Ethmolaimus pratensis related to the latter. Nematode communities in oligotrophic and mesotrophic lakes were characterized by a similar species composition, with pronounced occurrences of Eumonhystera longicaudatula, Semitobrilus cf. pellucidus, Prodesmodora circulata, and Rhabdolaimus terrestris. Overall, the results suggested that lake trophic state is a major factor structuring littoral nematode communities, although intra-lake variations might be of importance as well.     

The Ziway–Shala lake basin system, Main Ethiopian Rift: Influence of volcanism, tectonics, and climatic forcing on basin formation and sedimentation

In the central sector of the Main Ethiopian Rift, the Ziway–Shala lake basin system includes four present-day residual lakes, from north to south, lakes Ziway, Langano, Abijata, and Shala. This region of East Africa is under the influence of the Intertropical Convergence Zone seasonal migration. Thus it has been designated as a potential core site by the ERICA Project (‘Environmental Research for Intertropical Climate in Africa'). The four lakes have been subjected to strong changes in water level and water salinity at least during the Late Pleistocene. The purpose of this study is to produce a model of basin formation and sediment accumulation for this system of lakes, in order to separate the effects of climatic change from environmental variations induced by local or regional factors such as volcano-tectonic forcings. In addition to an exhaustive synthesis of available data, various investigations have been used to develop this model: 3D remote sensing, high-resolution seismics, coring, and structural, sedimentological, and hydrological field studies. New AMS radiocarbon dating helped to refine the pre-existing stratigraphic framework for this region, and basin age estimations were calculated using mean sediment accumulation rates. The history of the Ziway–Shala lake basin system has been reconstructed from the Late Pliocene–Early Pleistocene period (10⁶ yr), mainly characterized by catastrophic explosive volcanic eruptions. The early-middle Pleistocene–Late Pleistocene period (10⁴–10⁶ yr) was marked by a regional volcano-tectonic paroxysm, resulting in major changes in the morphology of the area, with the formation of the Abijata, Ziway and Shala lake basins. From 0.20 Ma, the Ziway–Shala basin history is marked by the eastward migration of volcano-tectonic activity, resulting in the development of the youngest basin of the Ziway–Shala system, the Langano Basin. The joint history of sedimentation in the Ziway, Langano, Abijata, and Shala lake basins started during the early-Late Pleistocene period (10¹–10⁴ yr) and is characterized from this period up to the present-day by a series of climatically controlled rises and falls of lake level.     

Effects of glaciers on nutrient concentrations and phytoplankton in lakes within the Northern Cascades Mountains (USA)

We compared nitrate concentrations, phytoplankton biomass, and phytoplankton community structure in lakes fed by glacier melt and snowmelt (GSF lakes) and by snowmelt only (SF lakes) within North Cascades National Park (NOCA) in Washington State, USA. In the U.S. Rocky Mountains, glacier melting has greatly increased nitrate concentrations in GSF lakes (52–236 µg NO₃–N L^?1) relative to SF lakes (1–14 µg NO₃–N L^?1) and thereby stimulated phytoplankton changes in GSF lakes. Considering NOCA contains approximately one-third of the glaciers in the continental U.S., and many mountain lakes that receive glacier meltwater inputs, we hypothesized that NOCA GSF lakes would have greater nitrate concentrations, greater phytoplankton biomass, and greater abundance of nitrogen-sensitive diatom species than NOCA SF lakes. However, at NOCA nitrate concentrations were much lower and differences between lake types were small compared to the Rockies. At NOCA, nitrate concentrations averaged 13 and 5 µg NO₃–N L^?1 in GSF and SF lakes, respectively, and a nitrate difference was not detectable in several individual years. There also was no difference in phytoplankton biomass or abundance of nitrogen-sensitive diatoms between lake types at NOCA. In contrast to the Rockies, there also was not a significant positive relationship between watershed percent glacier area and lake nitrate at NOCA. Results demonstrate that biogeochemical responses to global change in Western U.S. mountain lake watersheds may vary regionally. Regional differences may be affected by differing nitrogen deposition, climate, geology, or microbial processes within glacier environments, and merit further investigation.     

Chemical and algal relationships in a salinity series of Ethiopian inland waters

Most Ethiopian lakes are parts of closed drainage systems and collectively form an extensive salinity series, here treated comparatively for geographical, chemical and algal characteristics. Chemical data are presented for 28 lakes and numerous inflows, including original analyses for 15 lakes, in which total ionic concentration and electrical conductivity vary over 4 orders of magnitude. The principal determinant of a lake's position in the series is the open or closed nature of its individual drainage. At present there are three major closed systems (Awash R. — Afar drainage, northern rift lakes, southern rift lakes), numerous crater lakes with seepage -in and -out, and two cryptodepressions with marine inputs. Salinity is primarily determined by evaporative concentration, enhanced in lakes associated with past marine influence or recent volcanic activity by readily soluble materials in the catchment, and by some thermal-reflux pathways. However, anomalously dilute closed lakes exist, indicative of other processes of solute loss (e.g. past basin overflow, reverse weathering, seepage-out).There are strong positive correlations between increasing salinity and the concentrations of Na⁺, alkalinity and Cl^-. The last is used, in conjunction with other analyses of atmospheric precipitation, to estimate the marine and denudative contributions and the evaporative concentration factor, and to distinguish trends of ionic species during evaporative concentration. With several exceptions, affected by past penetration of sea water into the Danakil and L. Assal cryptodepressions, the most saline lakes are soda lakes with HCO₃ ^- + CO _inf3 ^sup2- and Na⁺ predominant and Ca²⁺ and Mg²⁺ largely eliminated. Soluble reactive silicate and phosphate tend to increase in concentration along the salinity series, but the unknown dynamics of algal growth are likely to introduce variance. Concentrations in some lakes are extremely high, e.g. > 40 mg SiO₂ l^-1 and > 1 mg PO₄-P l^-1.Phytoplankters recorded from individual lakes are tabulated and where available the community biomass concentration as chlorophyll a is given. Lakes of high salinity-alkalinity are typically very productive in terms of phytoplankton biomass and photosynthetic rates (exceptions: the very deep L. Shala and the very saline L. Abhe), supported in part by relatively high concentrations of phosphorus and inorganic carbon. Many species are of restricted salinity-alkalinity range, being characteristic of waters where levels are low (e.g. desmids, Melosira spp.), intermediate (e.g. Planctonema lauterborni), or high (e.g. Spirulina platensis). Phytoflagellates are most strongly represented in waters with higher concentrations of the bivalent cations Ca²⁺ and Mg²⁺. The common cyanophyte Microcystis aeruginosa can tolerate a wide salinity range, here as elsewhere.     

The Ethiopian Rift Valley lakes: chemical characteristics of a salinity-alkalinity series

The study on 10 lakes within the Ethiopian Rift Valley during March–May 1991 covered a range of conductivity (K₂₅) between 286 and 49100 µS cm^–1. HCO₃ ^– — CO _inf3 ^sup2– and Na⁺ were the dominant ions in all the lakes. Concentrations of K⁺, Cl^– and SO _inf4 ^sup2– increased with increasing salinity and alkalinity, whereas Ca²⁺ and Mg²⁺ decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12–222 mg SiO₂ 1^–1) and increased with increasing salinity, except for Lake Chamo which showed SiO₂ depletion (to < 1 mg SiO₂ 1 ^–1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a — total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.     

The meiofauna community of two small German streams as indicator of pollution

In order to validate the results of biomarkerstudies as tools for the assessment of smallstreams, the meiofauna community and inparticular the nematodes of two small Germanstreams (Baden-Württemberg) were analysedduring 1998. One site of Krähenbach (AB)and three sites of Körsch (KA, KD, KE) weresampled in March, May, July and November. Thelimnological analysis indicates that two sitesof Körsch (KD, KA: polluted sites) weredistinctly enriched by inorganic and organicsubstances compared with KE and AB (unpollutedsites). In the Krähenbach, nematodes,chironomids, oligochaetes and tardigradesdominated the meiofauna community (92%),whereas in the Körsch nematodes andoligochaetes were dominant (93%). The totalmeiofauna density showed seasonal patterns inboth streams. Nematodes were the most abundantgroup at the four investigated locations. Themean abundance of nematodes ranged from 28individuals per 10 cm² to 1205 individualsper 10 cm² and at least 103 species wereidentified, belonging to 25 families. Nosubstantial difference occurred in thedistribution of nematode feeding-types at thepolluted and unpolluted sites. All communitieswere clearly dominated by bacteria-feedingnematodes with percentages ranging from 70% to75%. In order to assess the sediment quality,the nematode community of each sampling sitewas interpreted using the ratio ofSecernentea/Adenophorea as well as the MaturityIndex (MI). Both the percentage of Adenophoreaand the MI decreased towards the pollutedsites, detecting a more disturbed nematodecommunity at the polluted than unpollutedsites. However, there are nematode species(e.g. Tobrilus diversipapillatus) whichdo not fit in this assessment concept. In thisstudy the disturbance of the nematode communitywas related to eutrophication, rather than tothe effect of contaminants. In summary, thisinvestigation confirmed that meiofaunaorganisms and in particular nematodes aresuitable for the assessment of the sedimentquality of running waters.     

The biogeochemistry and zoogeography of lakes and rivers in arctic Alaska

Water samples from 45 lakes and 8 rivers in arctic Alaska were analyzed for major anions, cations, nutrients, chlorophyll, zooplankton, and benthos. The waters were dilute (conductivities of 30 to 843 μS cm⁻¹), and their composition varied from Na-Ca-Cl waters near the Arctic Ocean to Ca-Mg-HCO₃ waters further inland. Sea salt input in precipitation was important in determining the chemistry of coastal lakes, partly because of low groundwater flow and less time for water to react with shallow unfrozen soils. Further inland, variations in water chemistry among sites were related mainly to differences in bedrock, the age of associated glacial drift, and the input of wind blown sediment. Variations in zooplankton species composition among the lakes were related more to latitude, lake morphometery, and biotic interactions than to water chemistry. The presence of fish as predators mostly determined the overall size structure of the zooplankton community. The chironomid taxa identified have been previously reported from the Neararctic, except for Corynocera oliveri which is a new record. The abundance of the widely distributed chironomid Procladius appears to be controlled by sculpin predation.     

A comparison of zooplankton communities in saline lakewater with variable anion composition

Although salinity and aquatic biodiversity are inversely related in lake water, the relationship between types of salts and zooplankton communities is poorly understood. In this study, zooplankton species were related to environmental variables from 12 lakes: three saline lakes with water where the dominant anions were SO₄ and CO₃, four saline lakes with Cl-dominated water, and five dilute, subsaline (0.5–3 gl^?1 total dissolved solids) lakes of variable anion composition. Although this study comprised only 12 lakes, distinct differences in zooplankton communities were observed among the two groups of chemically defined saline lakes. Canonical correspondence analysis identified total alkalinity, sulphate, chloride, calcium, sodium, potassium, and total phosphorus as all contributing to the first two ordination axes (λ₁ = 0.97 and λ₂ = 0.62, P<0.05). The rotifer Brachionus plicatilis and the harpactacoid copepod Cletocamptus sp. prevailed lakes with Cl-dominated water. In contrast, the calanoid copepods Leptodiaptomus sicilis and Diaptomus nevadensis were dominant in the SO₄/CO₃-dominated lake water with elevated potassium (79–128 mg l^?1) and total phosphorus concentrations (1322-2915 μg l^?1). The contrasting zooplankton species distribution among these two saline lake types is likely explained by variable selective pressure on zooplankton and their predators from differing physiological tolerances to salt stress and specific ions. While inland saline lakes with Cl as the dominant anion are relatively rare in Canada and SO₄/CO₃ are the common features, our study provided an opportunity to compare zooplankton communities across the two groups of lakes.     

Communities of anoxygenic phototrophic bacteria in the saline soda lakes of the Kulunda steppe (Altai Krai)

The saline soda lakes of the Kulunda steppe (Altai krai) are small and shallow; they are characterized by a wide range of salinity and alkalinity, as well as by the extreme instability of their water and chemical regimes. Accumulations of anoxygenic phototrophic bacteria (APB) visible to the unaided eye were noted only in several lakes with high rates of sulfate reduction in their bottom sediments. However, enumeration of APB cells by inoculation revealed their presence in all 17 lakes. APB cell numbers varied from 10³ to 10⁹ CFU cm^?3. In the APB communities of all lakes, purple sulfur bacteria of the family Ectothiorhodospi- raceae were predominant. In 14 out of the 17 lakes, purple nonsulfur bacteria of the family Rhodobacteraceae were also detected (10³–10⁷ CFU cm^?3). Purple sulfur bacteria of the family Chromatiaceae were less abundant: Halochromatium sp. (10⁴–10⁷ CFU cm^?3) were found in six lakes, while Thiocapsa sp. (10⁴ CFU cm³) were detected in one lake. On the whole, the APB communities of the soda lakes of the Kulunda steppe were characterized by the low diversity and evenness of their species compositions, as well as by the pronounced dominance of the members of the family Ectothiorhodospiraceae. There was no correlation between the structures of the APB communities and alkalinity. However, the dependence of the species composition of APB (mainly ectothiorhodospiras) on water mineralization was revealed. High mineralization (above 200 g l^?1) was a limiting factor that affected the APB communities on the whole, restricting the APB species diversity to extremely halophilic bacteria of the genus Halorhodospira.     

Long-term changes in chemical features of waters of seven Ethiopian rift-valley lakes

Chemical and chlorophyll a concentrations of seven Ethiopian rift-valley lakes were studied during 1990–2000. Results were compared with studies made between 1960 and 1990 in an attempt to detect long-term changes. Three different trends are apparent in the salinities (and the correlates conductivity, alkalinity, sodium concentration) of these lakes over the last 40 years: three lakes (lakes Zwai, Shalla and Abaya) have maintained their salinity levels from the 1960s, two lakes (lakes Langano and Awassa) have become more dilute, and the salinity levels of Lake Chamo and the soda lake Abijata have increased. Concentrations of silicate decreased in almost all the lakes whereas soluble reactive phosphorus (SRP) increased in most lakes. Chlorophyll a concentrations were higher in the recent samples from all lakes except two, which in conjunction with results from SRP and silicate analyses suggest eutrophication in four out of the seven lakes studied. The study relates salinization in lakes with closed drainage to increased human activities in their catchments, intensified by changes in climate during the last three decades in sub-Saharan Africa.     

Ecology of benthic and epiphytic nematodes in brackish waters

Abundances of benthic nematodes from shallow waters in Tvärminne, Finland and in Ringkøbing Fjord, Denmark averaged 1.2–1.5 10⁶ individuals m^?2. Less than 20 species were found. More than 80% of the individuals were made up by Anoplostoma viviparum, Chromadorita fennica, Axonolaimus spinosus, Daptonema trabeculosum and Sabatieria pulchra. S. pulchra is designated as an anoxybiotic species and is attracted to the root tips of young Potamogeton perfoliatus, whilst A. viviparum and C. fennica are oxybiotic species. Abundance of epiphytic nematodes extracted from submerged macrophytes in the Cladophora-Pilayella belt in the outer archipelago of the Finnish archipelago (July 1978) was estimated to reach 5 · 10⁶ individuals m^?2, equivalent to a biomass of 1.2 g org. C m^?2. Abundance and biomass are smaller in the Potamogeton meadows in the inner archipelago, in littoral algal vegetation of the sea zone, and on sublittoral vegetation. Epiphytic nematode species composition is influenced by salinity and food availability and not by physiognomic types or silt contents of the vegetation in the Baltic Sea, European estuaries and lakes. Dominant species are: Chromadorina bioculata, Punctodora ratzeburgensis, Chromadorita tenuis, Adoncholaimus thalassophygas, Theristus acer, Monhystera parva, M. aff. filiformis and Chromadora aff. nudicapitata. A theory is proposed to explain observed seasonal changes in the species composition of epiphytic and benthic nematodes. During winter months, the submerged vegetation is mostly destroyed, being found as detritus on the sea bottom. Sampling benthic nematodes during winter thus includes epiphytic species living in the detritus. In spring the epiphytic nematodes detect the newly growing vegetation, leave the sea bottom, colonize the submerged vegetation and spend their life there.     

Megalocypris ingens Delorme (Ostracoda) in Saskatchewan saline lakes: osmoregulation and abundance

Results of freezing point depression determinations show that the haemolymph of the ostracod, Megalocypris ingens Delorme, remains hyperosmotic to the medium as the concentration of the medium increases. Even though M. ingens is poikilosmotic, the median lethal concentration (LC50) for this species exceeds the LC50 for an insect species living in the same lakes that is capable of hyposmotic regulation. The LC50 for M. ingens occurred in sulphate-saline lake water with a specific conductance of 48 000 µS cm^-1 (at 25 °C). The absence of vegetation may be more important than salinity in limiting the distribution of M. ingens in Saskatchewan saline lakes.     

Life at the extreme: meiofauna from three unexplored lakes in the caldera of the Cerro Azul volcano,Galápagos Islands,Ecuador

On Isla Isabela, Galápagos Archipelago, three so far unexplored lakes were investigated in the caldera of Cerro Azul, one of the most active volcanoes in the world. The lakes face recurrent desiccation and eruption events and showed distinct differences in their water chemistry. Thirty cores from the upper 15 cm of sediment indicate distinct differences in the composition of meiobenthic communities between the lakes. In total, 27 different aquatic metazoan species could be distinguished. Numerically, rotifers dominated in two of the lakes, with mean densities up to 4.56 × 10⁶ individuals m⁻² while the third lake was dominated by a gastrotrich of the genus Chaetonotus (0.67 × 10⁶ individuals m⁻²). The largest lake harboured up to 14.4 × 10⁶ nematodes m⁻², which is the highest nematode density thus far reported for a freshwater habitat. The lakes yielded few nematode species (S = 7, N = 887) and calculation of the Shannon–Wiener index (H′) indicated an exceptionally low nematode diversity. The nematode community of one lake was clearly dominated by an undescribed suction-feeding Mesodorylaimus (59.6%), the community of the other lake by the epistrate feeder Achromadora pseudomicoletzkyi (89.3%), whereas the third lake surprisingly contained no nematodes. The benthic nematode biomasses for the two nematode-containing lakes differed by a factor 50. The food webs of the three lakes are presumed to have an exceptionable simply structure.

The nematode Neoaplectana carpocapsae and the beet armyworm Spodoptera exigua: Infectivity of prepupae and pupae in soil and of adults during emergence from soil

The nematode, Neoaplectana carpocapsae, infected >90% of the prepupae of Spodoptera exigua in soil even at concentrations as low as five nematodes/cm² of soil surface. Pupae were less susceptible to nematode infection in soil than prepupae, with mortality ranging from 10 to 24% and 10 to 83% for pupae exposed 3–5 days and 6–8 days to the nematode, respectively. Longer exposure (6–8 days) of the pupae to the nematode resulted in higher mortality with a positive relationship with increasing concentrations. Adults of S. exigua were susceptible to nematode infections as they emerged from the soil. The higher nematode concentrations (25 and 50 nematodes/cm²) resulted in higher adult mortality. The majority of nematode-induced mortality occurred within 24 hr after emergence. The susceptibility of emerging S. exigua adults to N. carpocapsae offers a new dimension for insect control.     

Limnology of naturally acidic,oligotrophic dune lakes in subtropical Australia,including chlorophyll — phosphorus relationships

This study provides the most comprehensive physico-chemical and phytoplankton data yet available for Australian dune lakes, which are among the world's most naturally acidic and oligotrophic freshwaters. Seasonal and spatial variations were examined in Blue Lagoon and Lake Freshwater, two ‘water-table window’ lakes in south-east Queensland. Like other dune lakes, they are acidic (minimum pH 4.20 and 4.55, respectively), polymictic water bodies with low concentrations of marine-derived major ions and almost undetectable levels of trace metals. While linmologically similar in winter, during spring-summer Lake Freshwater has significantly higher levels of chlorophyll-a, total phosphorus (TP) and turbidity than Blue Lagoon and other dune lakes, indicating seasonal mesotrophy. The key nutrient is TP, which has recently increased to a maximum of 17 μ· l⁻¹, due either to inputs from recreational sources, or to the death and decomposition of littoral vegetation resulting from falling water levels over the last decade. Inorganic nitrogen, though present only in small amounts, does not appear to limit the eutrophication process because of a shift in phytoplankton dominance from the usual desmids and dinoflagellates to N₂-fixing blue-green algae. A chlorophyll - TP linear regression derived for dune lakes indicates that at TP < 20 μg · l⁻¹ chlorophyll ‘yield’ is higher than in other lake types represented by regressions from the literature. This may be due to a more efficient utilization of the limited available phosphorus by dune lake algae which have adapted to the naturally oligotrophic environment. The implications of these findings for lake management are discussed.     

Pathogenicity of Two Species of Entomopathogenic Nematodes Against the Greenhouse Whitefly,Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), in Laboratory and Greenhouse Experiments

The greenhouse whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) is a polyphagous pest in greenhouse crops. The efficacy of two entomopathogenic nematodes (EPN), Steinernema feltiae and Heterorhabditis bacteriophora, as biological control agents against T. vaporariorum was evaluated using two model crops typical of vegetable greenhouse productions: cucumber and pepper. Laboratory tests evaluated adults and second nymphal instars for pest susceptibility to different EPN species at different concentrations of infective juveniles (IJ; 0, 25, 50, 100, 150, 200, and 250 IJ per cm²); subsequent greenhouse trials against second nymphal instars on cucumber and pepper plants evaluated more natural conditions. Concentrations were applied in combination with Triton X-100 (0.1% v/v), an adjuvant for increasing nematode activity. In laboratory studies, both life stages were susceptible to infection by the two nematode species, but S. feltiae recorded a lower LC₅₀ than H. bacteriophora for both insect stages. Similarly, in greenhouse experiments, S. feltiae required lower concentrations of IJ than H. bacteriophora to reach the same mortality in nymphs. In greenhouse trials, a significant difference was observed in the triple interaction among nematode species × concentration × plant. Furthermore, the highest mortality rate of the second nymphal instars of the T. vaporariorum was obtained from the application of S. feltiae concentrated to 250 IJ/cm² on cucumber (49 ± 1.23%). The general mortality caused by nematodes was significantly higher in cucumber than in pepper. These promising results support further investigation for the optimization of the best EPN species/concentration in combination with insecticides or adjuvants to reach a profitable control of this greenhouse pest.