Tropical Crater Lake Research Articles

Assessment of branched glycerol monoalkyl glycerol tetraether (brGMGT)-based paleothermometry in the 250,000-year sediment record of Lake Chala, equatorial East Africa

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a relatively understudied group of bacterial membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), appear to be strongly influenced by temperature in terrestrial settings. In surficial bottom sediments of East African lakes, the abundance of brGMGTs relative to the sum of brGMGTs and brGDGTs (%brGMGT) and brGMGT distribution are strongly related to local mean annual air temperature (MAAT), stimulating development of new paleothermometers. However, applications of these methods to lake-sediment records are currently lacking. Here we investigate brGMGT concentrations and distributions in 916 samples throughout the 250,000-year (250-kyr) sediment sequence from Lake Chala, a presently fresh and permanently stratified (meromictic) tropical crater lake. All seven previously identified brGMGTs occur abundantly, reflected in a relatively high average %brGMGT of 19%. BrGMGTs and brGDGTs concentrations throughout the sequence are strongly correlated (R = 0.83, p < 0.001), suggesting that their producers and/or associated ecological niches substantially overlap. Clear distinction can be made between brGMGTs produced predominantly in the bottom sediments (H1034a and H1034c) versus the anoxic lower water column (H1020a-c and H1034b). Although a 17-month monitoring study of Lake Chala suggested brGMGTs are primarily produced in the sediments, down-core data assign greater importance to aquatic production than previously estimated. Instead of reflecting temperature, %brGMGT variations showed greatest similarity to GDGT proxies reflecting lake depth and/or mixing regime. BrGMGT-based temperature models produce ambiguous reconstructions, showing little similarity to known global temperature trends or the brGDGT-based mean summer temperature (MST) reconstruction from the same sediments.

High-throughput sequencing and marker pigment analysis of freshwater phytoplankton: A direct comparison with microscopic count data in the tropical crater lakes of Western Uganda

High-throughput DNA sequencing (HTS) and photosynthetic marker-pigment analysis can provide cost-and time-efficient alternatives for morphology-based microscopic identification and counting of lacustrine phytoplankton. However, application of particularly HTS in tropical lakes is still uncommon. We analyzed the community composition of cyanobacterial and eukaryotic phytoplankton in relation to trophic status and other environmental factors in 15 tropical crater lakes in western Uganda, using both high-performance liquid chromatography (HPLC) of photosynthetic marker pigments and HTS of 16 S and 18 S small subunit rRNA genes. We then compared the results with phytoplankton composition data based on microscopic identification and cell counting to evaluate whether the two former methods can be alternative or complementary approaches to analyze these lakes’ phytoplankton communities. HTS recorded 105 operational taxonomic units (OTUs) of cyanobacteria, and 197 OTUs of autotrophic eukaryotes mainly from dinoflagellates, cryptophytes and green algae. Ochrophyta (chrysophytes and diatoms) were present in low abundances only, and the few remaining sequences belonged to Streptophyta and Haptophyta. Marker pigment distribution among samples reflected the widespread and often dominant presence of cyanobacteria (primarily zeaxanthin, myxoxanthophyll and echinenone), followed by green algae (chlorophyll b, lutein, antheraxanthin, neoxanthin) and diatoms (fucoxanthin, diato-diadinoxanthin), and with cryptophyta (alloxanthin) important in some of the lakes. Direct and indirect ordinations of the OTUs and marker pigments revealed that, similar to patterns observed in cell counts, phytoplankton community composition differed between deep oligotrophic lakes and shallower more productive lakes, even though the distribution of cyanobacterial OTUs was more related to lake water conductivity and season than to trophic status. Procrustes analyses of the composition of green algae and Ochrophyta, the two dominant eukaryotic groups, based on HTS data showed patterns significantly similar to composition data based on cell counts, but no significant correlations were found between the complete eukaryotic or cyanobacterial HTS data and cell count data. While the latter discrepancies can be attributed to method-related biases in both HTS and microscopy, in contrast to the microscopic approach HTS failed to identify trophic indicator species and did not assign all lakes to the correct trophic category. Composition data based on marker pigments clustered the 15 lakes according to their trophic level only when pigment abundances were expressed as absolute concentrations, not when expressed as fractional abundances. We attribute this lower discriminating power partly to the wide distribution of cyanobacteria in these tropical crater lakes, besides the pigments’ low taxonomic resolution. Nevertheless, as the compositional patterns in relation to environmental variation observed in marker pigments and OTUs of the dominant eukaryotic phytoplankton groups are similar to those in cell count data, both methods hold potential for ecosystem-level monitoring of tropical crater lakes. However, the high-resolution HTS approach is handicapped by non-uniform amplification of gene sequences, while the low-resolution marker-pigment approach is handicapped by the widespread dominance of cyanobacteria in these systems irrespective of their trophic status.

Changing water quality and thermocline depth along an aquaculture gradient in six tropical crater lakes

Understanding how lakes respond to changes in nutrient loading along a productivity gradient can help identify key drivers of aquatic change, thereby allowing appropriate mitigation strategies to be developed. Physical, chemical and biological water column measurements combined with long-term water monitoring data for six closely located crater lakes, in Southeast Asia, were compared to assess the response of lakes along a productivity gradient equating to a transect of increasing aquaculture intensity. Increasing chlorophyll a (phytoplankton biomass) in the upper waters appeared to modify the thermocline depth and light availability causing a shift from a deep chlorophyll maximum at low aquaculture intensity to the emergence of algal dead zones lower in the water column with high aquaculture intensity. High phosphorus loading and light limitation from enhanced algal biomass, associated with high aquaculture intensity, exacerbated nitrogen drawdown, leading to the prevalence of potentially nitrogen-fixing cyanobacteria. Seasonal overturn during the cooler season resulted in low dissolved oxygen concentrations in the epilimnion, potential harmful algal blooms, a reduction in the habitable depth for fish and ultimately increased mortality amongst farmed fish.

Historical reconstruction of sediment accumulation rates as an indicator of global change impacts in a tropical crater lake

Historical reconstruction of sediment accumulation rates as an indicator of global change impacts in a tropical crater lake

Balancing ecosystem integrity and cultural values at sacred Lake Hora, Ethiopia: The need for conservation of wetland vegetation

AbstractAlthough macrophytes represent important habitats for the plankton communities of shallow lakes and ponds, the role of littoral macrophyte patches in deep crater lakes is not well described. The objective of the present study was to investigate the role of littoral macrophyte vegetation in structuring phytoplankton and zooplankton communities and, therefore, sustaining ecological integrity in a deep tropical crater lake (Hora Lake, Ethiopia). The lake has a surface area of 1.03 km2 and a mean depth of 16.5 m. The area surrounding the lake is a prime location for the celebration of the Oromo thanksgiving festival (Irreecha), which is inscribed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as an intangible cultural heritage, and also considered sacred by the Oromo people. Measurements of physical, chemical and biological parameters were conducted at fifteen sampling sites in both the littoral and central parts of the lake during the dry and wet seasons in 2017. A total of 28 phytoplankton and 19 zooplankton genera were identified. The phytoplankton and zooplankton communities were dominated by cyanobacteria and rotifers, respectively, at the open water sites. The wetland vegetation was found to be important in reducing potentially toxic and unsightly cyanobacteria. The submerged and emergent vegetation also served as refugia for zooplankton during the wet and dry seasons, respectively. Crustaceans accounted for about 30–45% of the total zooplankton abundance within the vegetation fringes. Their abundance decreased to 5% in the non‐vegetated areas, wherein the community was dominated by rotifers. As the population around the lake, particularly during the festival, may damage the aquatic vegetation, their access to the shoreline should be regulated for conservation and sanitary reasons (e.g. to reduce cyanobacteria growths).

Mixing regimes in the equatorial crater lakes of western Uganda

In order to understand the ecological functioning of lake ecosystems, little is more crucial than knowledge of their mixing regime. However in tropical regions, this information is scarce for all except the largest lakes, and often inferred from occasional observation of physical water-column properties. In this study, a total of 276 water-column profiles of temperature, conductivity and dissolved oxygen collected between 2000 and 2017 in 60 volcanic crater lakes in equatorial western Uganda are used to classify these lakes according to mixing regime. Most study lakes are small (on average 29 ha), but together they cover a spectacularly large gradient in maximum depth (Zmax) between 1.4 m and > 220 m, and consequently they display a wide range of mixing regimes. We identified four lakes as continuous warm polymictic, 12 as discontinuous warm polymictic, and 11 other lakes mix completely once or twice each year. We classify these 11 lakes as equatorial dimictic, because they experience deep mixing during both dry seasons separated by long periods of stable stratification, while the season of complete mixing may differ between years and among neighbouring lakes. The 33 remaining lakes do not usually mix completely and are thus meromictic. Relative depth (Zr) is the main factor controlling the mixing regime of these lakes. The presence of a shallow permanent chemocline reduces mixing depth, and the smallest meromictic lakes (< 3 ha) tend to occasionally mix deeper than predicted by Zr. Based on these variables, predictions can be made about the mixing dynamics of Ugandan crater lakes not covered by our dataset. Previous suggestions that the mixing depth of tropical crater lakes is strongly influenced by the wind shelter provided by a high crater rim or by water-column transparency could not be confirmed.

Anoxic in situ production of bacterial GMGTs in the water column and surficial bottom sediments of a meromictic tropical crater lake: Implications for lake paleothermometry

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are membrane-spanning lipids which were initially identified in marine settings and have more recently been found in peats and lake sediments. In the latter settings, their abundance relative to that of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) appears to increase under warmer climate conditions. Furthermore, their relative distribution in bottom sediments from an altitudinal transect of East African lakes has been linked to local mean annual air temperature (MAAT). To shed light on their sources and identify potential environmental factors driving their production in lakes, we investigated their occurrence and distribution in the water column, surficial lake-bottom sediments, and catchment soils of Lake Chala, a permanently stratified (meromictic) tropical crater lake in East Africa. Data from the water column comprise both suspended particulate matter (SPM) collected monthly at eight depth intervals between 0 and 80 m for 17 consecutive months, and settling particles collected monthly at a depth of 35 m over a period of 53 months. In catchment soils, brGMGTs are either relatively scarce or absent. In the water column, their occurrence is dominated by four of the seven known brGMGTs and mostly limited to the anoxic zone. Whereas the spatial distribution and concentration of brGMGTs in SPM were strongly influenced by the seasonal cycle of strong water-column stratification alternating with deep mixing, this trend is not consistent on the multi-year timescale of the settling-particles record. Moreover, their relative distribution in both SPM and settling particles did not respond to temperature variation during the studied interval. In contrast to the water column, surficial lake-bottom sediments contain the full suite of known brGMGTs in a distinct and spatially uniform fractional abundance, strongly suggesting that the majority of brGMGT production takes place there. BrGMGT-inferred MAAT for the Chala area reasonably estimates measured MAAT and conforms to the East African lake brGMGT calibration. Our results have important implications for the use of brGMGTs as proxy for MAAT, in that outside of the equatorial zone the presumably greater offset between bottom-water temperature and MAAT may create a bias towards cold-season air temperature. Regardless, the more singular source of brGMGTs in lake sediments is an advantage for paleotemperature reconstruction compared to brGDGTs, which several studies now indicate to have a mixed aquatic and terrestrial source of which the relative contribution cannot easily be disentangled or assumed to have been constant through time.

Limnological Characterization of Three Tropical Crater Lakes in the Archipelago of Samoa (Lanoto‘o, Olomaga, Mataulano)1

We present a first limnological study of three humic crater lakes in the archipelago of Samoa. The basins of Lakes Lanoto‘o (17.5 m deep) and Olomaga (12.2 m) on Upolu Island and of Lake Mataulano (5.6 m) on Savai‘i Island developed during consecutive periods of volcanic activity ranging from the middle Pleistocene to the late Holocene. Lake Olomaga may be a permanently stratified meromictic lake, while stratified lakes Lanoto‘o and Mataulano contained oxygen down to the bottom. Forty-seven phytoplankton and 8 zooplankton taxa were identified in the pelagic zones of the three lakes. The Samoan endemic Diaphanosoma samoaensis, the cyclopoid Mesocyclops roberti originally described from neighboring archipelagos, and a hitherto undescribed species of Microcyclops were recorded. Lake Lanoto‘o is the only lake with introduced fish: goldfish (Carassius auratus) and tilapias (Oreochromis niloticus) were stocked and it harbored more than four times more phytobiomass (8 mg L-1) than the smaller, unstocked lakes (1.2–1.6 mg L-1). Fish introduction poses a threat to unstocked lakes in Samoa. Measures should be taken to protect them from any alterations.

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Three new needle-shaped Fragilaria species from freshwater lake Apastepeque in El Salvador (Fragilaria salvadoriana sp. nov., F. maarensis sp. nov.) and subsaline lake Nam Co on the Tibetan Plateau (F. huebeneri sp. nov.) are described and compared based on light and scanning electron microscopy observations and morphometric analyses. Fragilaria salvadoriana sp. nov. is characterized by narrowly linear-lanceolate, sometimes centrally constricted valves, subcapitate to rarely capitate apices, and a distinct, dented appearing central area. Striae are composed of 2−5 occluded areolae. It can be differentiated from similar needle-shaped species by the valve outline, relatively low striae density, and shark fin-shaped spines. Characteristic of F. maarensis sp. nov. are a very narrowly lanceolate valve outline and subcapitate apices. The apical pore field is composed of 2–3 rows of poroids and acute, irregularly oriented spines are present at the junction between valve face and mantle. This taxon is clearly different from other Fragilaria species, displaying a high length-to-width ratio and a low number of areolae per stria. The Tibetan species, F. huebeneri sp. nov., forms long ribbon-like colonies linked together by spatula-shaped spines. Valves have subcapitate apices, a spindle- to needle-shaped outline and an indistinct central area. Striae are alternate and composed of 3–5 areolae per stria. Teratological forms of F. huebeneri sp. nov. were commonly observed in the sediment trap samples. Fragilaria salvadoriana sp. nov. and F. maarensis sp. nov. were found in a warm, tropical crater lake characterized by low conductivity and dissolved oxygen content, medium alkaline pH, and magnesium-calcium-bicarbonate-rich waters. Fragilaria huebeneri sp. nov. was frequent in a large, high elevation lake with increased specific conductivity, alkaline pH and sodium-bicarbonate-rich waters. The new species are compared to morphologically similar species from the genus Fragilaria Lyngbye and ecological preferences are discussed.

Rhythmic episodes of heating and cooling control thermal stratification of two tropical high mountain lakes

Continuous temperature monitoring for two adjacent tropical crater lakes in Mexico at 4200 m amsl shows that the lakes have rhythmic episodes of heating and cooling with a duration of ~ 30 days during the warmest months. The episodes were caused by rise and decline of solar irradiance reaching the lake surface. One lake, El Sol, showed over each heating and cooling episode a stable mixed layer (~ 20 days) and a deeper layer with a weak thermal gradient. Temperatures below the mixed layer warmed progressively by eddy diffusion after the mixed layer formed. Stratification was followed by full mixing of the water column. Within the same crater, an adjacent second lake, La Luna, showed the same cycles of heating and cooling; it stratified daily but not over multiple days. The difference between the lakes (discontinuous polymictic, continuous polymictic) is explained by the lower transparency of El Sol, which led to greater heat uptake near the surface than the more transparent La Luna. Lower transparency of El Sol was caused by modest anthropogenic effects on total suspended solids and nutrient loading, i.e., small deviations from the natural condition of El Sol caused it to differ qualitatively from La Luna. Events observed in these lakes would not have been evident from weekly temperature records.

Determining patterns of stratification and mixing in tropical crater lakes through intermittent water-column profiling: A case study in western Uganda

In tropical lakes, deep water-column mixing exerts great influence on nutrient dynamics and food-web structure, and on the probability for hazardous accumulation of bio- or geogenic gases in the hypolimnion. However, conventional criteria to infer a lake's mixing regime in temperate regions, such as the changing seasonal position of the thermocline or the depth to anoxia, often lead to faulty conclusions when applied to stratifying tropical lakes. The water column of these lakes tends to exhibit multiple thermoclines, and deep-water anoxia tends to spread quickly throughout the hypolimnion in all but the least productive lakes, complicating the interpretation of isolated temperature and dissolved-oxygen profiles. Here, multiple depth profiles of temperature, dissolved oxygen and conductivity from a diverse selection of 11 small tropical crater lakes in western Uganda are combined with time series of temperature-logger data to determine the occurrence and maximum depth of water-column mixing within a given time period. We propose three reliable indicators of past mixing: 1) presence of oxygen below the photic zone; 2) evidence of recent cooling at a given depth; and 3) disappearance of previously documented thermo- and chemoclines. The greatest depth of mixing can only be inferred with a fair degree of certainty from a chronosequence of at least two sets of profiles.

Long-Term Hydrologic Fluctuations and Dynamics of Primary Producers in a Tropical Crater Lake

Aquatic ecosystems in tropical regions remain understudied and their long-term dynamics poorly understood. In East Africa, a better understanding of how natural communities of primary producers in small freshwater ecosystems respond to climatic variability is needed to improve management and conservation of aquatic resources. This study explored the response of algae and bacteria communities to marked hydrological variation over the past 1500 years in a small western Ugandan crater lake, Lake Nkuruba. We analyzed sedimentary algal and bacterial pigments to evaluate the magnitude and direction of change in the autotrophic community in response to severe climatic perturbations in the region. The lithology of the Lake Nkuruba sediment core indicated that external forcing in the form of a major drought, associated with the Medieval Climate Anomaly, caused a heavy, short-lived detrital pulse to the basin that led to a brief but substantial disruption of the lake system in the second half of the 13th century. The system appears to have recovered rapidly, and then transitioned to a more productive state than the one preceding the drought. The considerable variation observed in the sedimentary pigment biomarkers is likely linked with climatically-induced changes in the water column structure of this small crater lake. Our results highlight the challenge of defining appropriate baselines or reference conditions in climatically-sensitive East African aquatic ecosystems and disentangling long-term anthropogenic impacts from the strong regional hydrological flux at the decadal to centennial scale.

Key Role of Alphaproteobacteria and Cyanobacteria in the Formation of Stromatolites of Lake Dziani Dzaha (Mayotte, Western Indian Ocean).

Lake Dziani Dzaha is a thalassohaline tropical crater lake located on the “Petite Terre” Island of Mayotte (Comoros archipelago, Western Indian Ocean). Stromatolites are actively growing in the shallow waters of the lake shores. These stromatolites are mainly composed of aragonite with lesser proportions of hydromagnesite, calcite, dolomite, and phyllosilicates. They are morphologically and texturally diverse ranging from tabular covered by a cauliflower-like crust to columnar ones with a smooth surface. High-throughput sequencing of bacterial and archaeal 16S rRNA genes combined with confocal laser scanning microscopy (CLSM) analysis revealed that the microbial composition of the mats associated with the stromatolites was clearly distinct from that of the Arthrospira-dominated lake water. Unicellular-colonial Cyanobacteria belonging to the Xenococcus genus of the Pleurocapsales order were detected in the cauliflower crust mats, whereas filamentous Cyanobacteria belonging to the Leptolyngbya genus were found in the smooth surface mats. Observations using CLSM, scanning electron microscopy (SEM) and Raman spectroscopy indicated that the cauliflower texture consists of laminations of aragonite, magnesium-silicate phase and hydromagnesite. The associated microbial mat, as confirmed by laser microdissection and whole-genome amplification (WGA), is composed of Pleurocapsales coated by abundant filamentous and coccoid Alphaproteobacteria. These phototrophic Alphaproteobacteria promote the precipitation of aragonite in which they become incrusted. In contrast, the Pleurocapsales are not calcifying but instead accumulate silicon and magnesium in their sheaths, which may be responsible for the formation of the Mg-silicate phase found in the cauliflower crust. We therefore propose that Pleurocapsales and Alphaproteobacteria are involved in the formation of two distinct mineral phases present in the cauliflower texture: Mg-silicate and aragonite, respectively. These results point out the role of phototrophic Alphaproteobacteria in the formation of stromatolites, which may open new perspective for the analysis of the fossil record.

Microbial Diversity and Cyanobacterial Production in Dziani Dzaha Crater Lake, a Unique Tropical Thalassohaline Environment.

This study describes, for the first time, the water chemistry and microbial diversity in Dziani Dzaha, a tropical crater lake located on Mayotte Island (Comoros archipelago, Western Indian Ocean). The lake water had a high level of dissolved matter and high alkalinity (10.6–14.5 g L-1 eq. CO32-, i.e. 160–220 mM compare to around 2–2.5 in seawater), with salinity up to 52 psu, 1.5 higher than seawater. Hierarchical clustering discriminated Dziani Dzaha water from other alkaline, saline lakes, highlighting its thalassohaline nature. The phytoplankton biomass was very high, with a total chlorophyll a concentration of 524 to 875 μg chl a L-1 depending on the survey, homogeneously distributed from surface to bottom (4 m). Throughout the whole water column the photosynthetic biomass was dominated (>97% of total biovolume) by the filamentous cyanobacteria Arthrospira sp. with a straight morphotype. In situ daily photosynthetic oxygen production ranged from 17.3 to 22.2 g O2 m-2 d-1, consistent with experimental production / irradiance measurements and modeling. Heterotrophic bacterioplankton was extremely abundant, with cell densities up to 1.5 108 cells mL-1 in the whole water column. Isolation and culture of 59 Eubacteria strains revealed the prevalence of alkaliphilic and halophilic organisms together with taxa unknown to date, based on 16S rRNA gene analysis. A single cloning-sequencing approach using archaeal 16S rDNA gene primers unveiled the presence of diverse extremophilic Euryarchaeota. The water chemistry of Dziani Dzaha Lake supports the hypothesis that it was derived from seawater and strongly modified by geological conditions and microbial activities that increased the alkalinity. Dziani Dzaha has a unique consortium of cyanobacteria, phytoplankton, heterotrophic Eubacteria and Archaea, with very few unicellular protozoa, that will deserve further deep analysis to unravel its uncommon diversity. A single taxon, belonging to the genus Arthrospira, was found responsible for almost all photosynthetic primary production.

Depth distribution of zooplankton in relation to limnological gradients under different stratification and interannual regimes in a deep, tropical crater lake

Zooplankton vertical distribution was studied in Lake Bosumtwi to understand how spatial and temporal variations in the water column affected diurnal and nocturnal patterns. Biweekly samples were collected for 24 consecutive months at the deepest point from 2005 to 2006. Water column properties shaping habitat suitability, namely temperature, light intensity, dissolved oxygen and phytoplankton biomass, varied markedly with depth but less with season. Most zooplankton stayed in the epilimnion continuously and their distribution was unaltered by lake stratification, mixing and restratification. Species dwelt permanently in the epilimnion without any necessity to migrate. Reduced water column variability and resultant weak selective pressures suppressed greater dispersion of species. Chaoborus ceratopogones (Theobald) was the main invertebrate predator and performed upward and downward movements on a diel cycle. Competing prey species overlapped extensively in spatial and temporal degree of habitat use. Chaoborus, however, segregated in deep water during the day, thereby minimizing prey vulnerability and promoting coexistence among predators and prey. During night-time residence, prey species may be exposed to increased predation risk from Chaoborus but gains in reproductive potential in warm, food-rich surface waters may offset losses to mortality. Our results suggest that high degree of habitat constancy and consequently weak selective pressures contributed to the lowered behavioural responses of zooplankton. The persistently skewed distribution of zooplankton is adaptive to predation and competition and promotes species coexistence. This behaviour will optimize growth and reproductive benefits over the cost of descent to suboptimal conditions in deep water.

Molecular diversity of plankton in a tropical crater lake switching from hyposaline to subsaline conditions: Lake Oloidien, Kenya

Salinity in the climate sensitive tropical endorheic crater lake Oloidien (Great African Rift Valley, Kenya) decreased from hyposaline to subsaline conditions during the period 2010–2015. The change in salinity was accompanied by a pronounced change in planktonic life forms—from blooms of the cyanobacterium Arthrospira supporting tens of thousands of Lesser Flamingos to highly diverse communities of cyanobacteria and algae which do not sustain the consumer birds. Besides the well-known macro- and microscopic lake life, a hidden diversity of microorganisms was detected using molecular methods. SSU rRNA gene clone libraries and data from Ilumina Miseq sequencing of samples collected at the two contrasting stages revealed distinct and highly diverse microbial communities. Different bacterial clades dominated the two samples. In 2011, Firmicutes (class Bacilli) whose origin was the fecal waste of birds were the dominant group. However, the Cyanobacteria and Chloroflexi were the most prevalent in 2015. From the microbial eukaryote samples obtained in 2011, rotifers and ciliates that feed on Arthrospira and rich bacterial food dominated the plankton, while the cryptophytes were the most prevalent in 2015. On the two occasions, a mixture of organisms previously not known to occur in saline or in freshwater habitats was found.

Des-A-lupane in an East African lake sedimentary record as a new proxy for the stable carbon isotopic composition of C3 plants

We studied the high-resolution and well-dated 25,000year sedimentary record of Lake Challa, a deep tropical crater lake in equatorial East Africa, to explore new proxies for paleoenvironmental and paleohydrological change. Sedimentary biomarker analysis revealed the presence of des-A-triterpenoids with oleanane, ursane and lupane carbon skeletons, microbial degradation products of angiosperm plant triterpenoids. Their increased influx from 16,000years ago corresponds with previously documented changes in the terrestrial vegetation of the Lake Challa basin during postglacial warming, in particular the relative increase in C3/C4 plant ratio inferred from the stable carbon isotopic signature (δ13C) of sedimentary n-alkanes derived from plant leaf waxes. In contrast to this n-alkane δ13C signature, the δ13C of des-A-lupane maintains a constant value of −27.4±1.1‰ across the glacial–interglacial transition. Since des-A-lupane is derived from C3 plants, its δ13C signature is here proposed to represent a novel and independent proxy for the time-variable carbon isotopic composition of local terrestrial C3 plants, which can improve estimates of the C3/C4 plant ratio based on two-end member mixing models of n-alkane δ13C values.

Dynamics of internal waves generated by mountain breeze in Alchichica Crater Lake, Mexico

Hydrophysical measurements carried out in the deep tropical crater lake Alchichica, Mexico, showed the presence of intense internal waves with a one-day period, generated by the regular influence of a mountain breeze. The diurnal waves effectively generate semidiurnal internal waves that are close to the resonance oscillations of the lake. The internal waves brake close to the shoreline and into the lake's bottom, which generates baroclinic circulation and water masses mixing. According to the data obtained with a ADCP, two diurnal and two semidiurnal inclined waves propagate in the lake towards each other in depth and can form fluctuations of a vertical mode, if the depth of the lake is a multiple of one-half the vertical wavelength. This multiplicity applies to both the diurnal and semidiurnal waves. The simultaneous existence of horizontal and vertical modes corresponds to a volumetric mode, or resonant lake oscillation. The waves collapse near the shore slopes and the lake's bottom, inducing baroclinic circulation and local mixing.

Small Changes in Climate Can Profoundly Alter the Dynamics and Ecosystem Services of Tropical Crater Lakes

African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R2adj = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions.

Phytoplankton community and physico-chemical seasonality and changes in Lake Hora-Kilole, Ethiopia, a tropical crater lake

Phytoplankton community structure and physico-chemical variables, sampled monthly at an offshore station in Lake Hora-Kilole from August 2007 to May 2008, were compared with previously reported data. In 1989 the Mojo River was temporarily diverted to flow into the lake. The lake subsequently changed markedly, first from being a small polymictic system to being a large monomictic system with a maximum depth of 29 m in 1990/1991, and then by 2007/2008 to again being a small continuously polymictic system less than 8 m deep. Secchi depth averaged 0.35 m and varied from 0.15 to 0.78 m, indicating a marked decline in water transparency from the levels recorded in 1990/1991. Nitrate levels, rarely detectable in the 1960s, subsequently became high, whereas phosphorus levels have declined since the 1960s. The phytoplankton community was dominated by cyanobacteria and dinoflagellates, in contrast to the dominance by cyanobacteria before 1989, and by dinoflagellates and chlorophytes shortly thereafter. Total phytoplankton Chl a averaged 83.5 mg m−3 and ranged from ∼36 to 148 mg m−3, peaking in October 2007 and February 2008. The impacts of river diversion and shoreline modification on the lake were reflected in changes in the physico-chemical conditions, including low light penetration and high nitrate, and changed phytoplankton community structure. The development of algal blooms in future is anticipated.