Vegetation changes in a shallow African Lake: Response of the vegetation to a recent dry period

This paper shows how the bed of Lake Chilwa in Malawi was colonised by plants during a recent dry period. Aeschynomene pfundii and Diplachne fusca were the principal colonising species on the alkaline mud flats. The subsequent changes in the vegetation after the lake refilled are described. Aeschynomene pfundii survived the flooded conditions for four years and eventually died out as seed germination could not occur underwater. The salinity of the lake during the low water and drying period prevented the lake bed from being extensively colonised by Typha domingensis.     

THERMAL SENSITIVITIES OF MALATE DEHYDROGENASE ISOZYMES IN TYPHA

Two species of cattail. Typha latifolia L. and T. domingensis Pers., occur naturally along the shoreline of Par Pond, an 1,128 ha impoundment subjected to intermittent thermal stress at the Savannah River Plant in South Carolina. T. domingensis is not tolerant of the stresses imposed at the hottest section of this lake. Typha latifolia is established throughout the lake but at the hottest section displays an altered morphology compared with stands from ambient areas. Both T. latifolia and T. domingensis are electrophoretically monomorphic, and no variation in isozyme expression in ten enzyme systems was recorded in populations from Par Pond. The malate dehydrogenase (MDH) isozyme patterns for T. latifolia and T. domingensis are identical, but their thermal sensitivities differ sharply. All six major T. domingensis MDH isozymes are denatured by heating at 50 C; whereas, T. latifolia has three isozymes which are stable at 50 C and three isozymes which are denatured at this temperature. Tests of thermal stability are more useful than measurements of electrophoretic mobility in detecting enzyme differences and a possible biochemical basis for differences in thermal tolerance in these natural populations.     

Nutrient and plant dynamics in Lake Agmon Wetlands (Hula Valley,Israel): a review with emphasis on Typha domingensis (1994–1999)

Shallow lake Agmon is a newly created subtropical wetland in north-eastern Israel. The lake is part of the Hula Project aimed at slowing down deterioration processes of the peat soils, to establish infrastructure for ecotourism as an income for the land owners, and nutrient removal from Lake Kinneret inputs. An onset of benthic filamentous macro-green algae during late winter–spring season, followed by submerged macrophytes vegetation during spring–summer was documented. The phosphorus summer loads are mostly plant–mediated internal fluxes and nitrogen intensively removed from lake waters by sedimentation and denitrification. The summer phytoplankton, mostly colonial cyanobacteria, are P limited. During 1995 and early 1996, dense Typha domingensisstands were developed in the southern half of the Lake (chalk-marl bottom sediments). The P-limited Typhavegetation collapsed within less than half a year and reappeared in the south-eastern part of the lake where sediments were exposed and oxidized. It is hypothesized that phosphorus cycle is a strong dependant of macrophyte mediation, and P deficiency in the sediments predominantly affected Typhadecline and an increase of P availability later enabled the reappearance of the Typhastands.     

Abundance and distribution of fish in the lower Mnembo River, Malawi-Mozambique

Endorheic Lake Chilwa is one of the most productive lakes for fisheries in Africa. High human population densities and agricultural practices in the Chilwa catchment have been linked to observed declines of the annual fish catch in the lake. The Mnembo River is a major inflow into Lake Chilwa which has received little scientific study to date. In 2003/2004, fish diversity, abundance and distribution were monitored monthly at three sites in the Mnembo River. Twelve fish species were collected, a subset of the fourteen fish species present in Lake Chilwa. Although Barbus spp. were the most abundant species sampled in the Mnembo, total biomass was higher for Labeo cylindricus (6709 and 10,434 g, respectively). Total biomass of Pareutropius longifilis (1741 g) and Brycinus imberi (1174 g) were also high in the river. Catches of Barbus and Labeo were highly correlated (r = 0.763). Compared with other inflows into Lake Chilwa (Likangala and Domasi), the Mnembo River appears to be less adversely influenced by human populations, with high abundance of Barbus and strong presence of the riverine species Labeo, Pareutropius and Brycinus.     

Development of polymorphic microsatellite DNA loci for characterizing Oreochromis shiranus subspecies in Malawi

Six sets of microsatellite primers were developed from Oreochromis shiranus nuclear DNA. These cross‐primed in species of the genera Sarotherodon and Tilapia, and were polymorphic in most of the species. The total number of alleles ranged from 18 to 30 per locus; the mean heterozygosity per population ranged from 0.51 ± 0.12 to 0.82 ± 0.03, which is higher than reported in allozyme studies. Five sets of primers were used to characterize five putative populations of O. shiranus, an indigenous mouth brooding tilapia which has been widely distributed in fish farms and reservoirs in Malawi. Two postulated subspecies of O. shiranus (subspecies shiranus and subspecies chilwae) have been difficult to distinguish morphologically. Lake Chilwa, Lake Chiuta and Bunda Reservoir populations cluster together as O. sh. chilwae, while the Lake Malombe and Bishop Reservoir populations form a second cluster as O. sh. shiranus. The assignment of the Chiuta and Chilwa populations of O. sh. chilwae to a single subspecies is consistent with the fact that the two lakes were a single open lake until about 8000–9000 _bp. There is no connection between them and the Lake Malawi‐Malombe drainage system where the O. sh. shiranus subspecies is found.     

Seasonal allocation of carbohydrates between above‐ and below‐ground organs of Typha domingensis

Seasonal carbohydrates allocation by Typha domingensis was evaluated to identify the potential physiological weaknesses in the growth cycle of this plant in Lake Burullus, Egypt. Monthly plant samples (February–October 2014) were separated into shoots, roots and rhizomes to evaluate the seasonal changes in water‐soluble carbohydrates (WSC), starch and total non‐structural carbohydrates (TNC) for each plant organ. The present study indicated that rhizomes are strong carbohydrates sink during the life cycle of T. domingensis. Starch represented the greatest part of the TNC pool, surpassing the concentration of WSC 1.8–4.3 times. The WSC, starch and TNC concentrations of T. domingensis below‐ground organs (rhizomes and roots) were high at the beginning of the vegetative period (February); they reached their minima in March to support the shoots growth, then were followed by a gradual increase due to the translocation from shoots. The time when T. domingensis is expected to be most susceptible to a management technique is at the point in the seasonal cycle when the stored carbohydrates are at the lowest (in March).     

Littoral habitat loss caused by multiyear drought and the response of an endemic fish species in a deep desert lake

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Community structure of rotifers in two environments of the upper River Paraná floodplain (MS) - Brazil

We evaluate the influence of abiotic and biotic factors on the community structure of rotifers across a regional hydrological cycle in lotic and lentic environments of the upper River Paraná. Depth, transparency, temperature, pH, electrical conductivity, dissolved oxygen, chlorophyll a and densities of rotifers were measured at two stations in Lake Guaraná (littoral and open water regions) and at one station in the River Baía (open water region). Highest densities of rotifers were found at the lake littoral. Canonical correlation analysis related environmental variables with the densities of the most abundant rotifers. The strongest relationship was with chlorophyll a, dissolved oxygen, hydrological level and water temperature. Diversity of rotifers at each station was mainly explained by fluctuations in hydrological level. Results of grouping analysis suggested the formation of groups according to phases of the hydrological cycle.     

Swamps,springs and diatoms: wetlands of the semi-arid Bogoria-Baringo Rift,Kenya

Lakes Bogoria and Baringo lie in a semi-arid part of the Kenya Rift Valley between 0° 15′–0° 30'N and 36° 02′–36° 05′E. Nevertheless, the area around these lakes contains numerous wetland systems that have been formed: along lake shorelines; along faults where hot, warm and cold springs have developed; and along river systems that cross the rift floor. Six major types of wetland are recognized: Proximal Hot Springs; Hot Spring Marshes; Blister Wetlands; Typha and Cyperus papyrus Swamps; Floodplain Marshes; Hypersaline Lake Littoral Wetlands; and Freshwater Lake Littoral Wetlands. These show significant variability in terms of geomorphic setting, water chemistry, temperature, plant communities and diatom floras. They are variously dominated by macrophytes, such as Cyperus laevigatus, Typha domingensis and Cyperus papyrus. In some cases macrophytes are absent. In hot spring settings and in hypersaline lake littoral zones bacterial mats are common. Although absent in some samples, diatoms occur in at least parts of all of the wetlands, varying in diversity, abundance and species composition. Canonical correspondence analysis indicates that diatom floras show a close relationship with pH, temperature, and specific conductivity, with other environmental variables such as Si and nitrate being of secondary importance. Common diatoms include: Anomoeoneis sphaerophora var. guntheri, Navicula tenella, N. cuspidata, and Nitzschia invisitata in hot springs, where diversity is generally low and abundance is variable. Other wetland types contain distinctive diatom floras that variously include: Fragilaria brevistriata, Gomphonema parvulum, Navicula tenelloides, Nitzschia communis, N. latens, N. sigma, Rhopalodia gibberula, and Stauroneis anceps.     

Establishment of the Baikalian endemic amphipod Gmelinoides fasciatus Stebb. in Lake Ladoga

Gmelinoides fasciatus Stebb., a small amphipod from the Lake Baikal basin, was discovered in July 1988 in Lake Ladoga, the largest European lake. G. fasciatus likely invaded Lake Ladoga as a consequence of its intentional introduction, aimed at enhancing fish production, in some Karelian Isthmus lakes close to Lake Ladoga's western shore in the early 1970's. Benthos studies conducted in 1989 and 1990 revealed that G. fasciatus was well established in littoral communities along the western and northern shores of Lake Ladoga. G. fasciatus was the dominant species in these littoral communities and contributed over 70% of the macroinvertebrate biomass. The species was abundant in different macrophyte beds and stony littoral areas, both in heavily polluted and undisturbed sites. The maximum abundance and biomass of G. fasciatus was about 54,000 ind m^–2 and 160 g (wet wt.) m^–2. Negative impacts of G. fasciatus on native species have been observed. Studies are needed to identify the effects of G. fasciatus on the functioning of littoral communities and to predict and control the spread of this amphipod.     

Taxonomy of the genus Megacyclops (Crustacea,Copepoda): morphometry and the use of enzyme electrophoresis

Morphometric investigations on the populations of Megacyclops in Lake Constance showed, that in a sublittoral region from about 20 to 50 m depth an intermediate type between the two species M. viridis (littoral) and M. gigas (benthal) could be distinguished. Using enzyme electrophoresis it was evident, that these specimens belonged to M. viridis.The annual range of water temperatures in the sub-littoral region corresponds to that found in the shallow areas of the lake or in ponds during winter and early spring, when the adults of M. viridis are markedly larger than in summer and fall (seasonal variation). Nevertheless the sublittoral population represents a different local type with special characteristics in morphometric details.     

A CONCEPTUAL MODEL OF A SILTATION SYSTEM IN SHALLOW LAKES WITH LITTORAL VEGETATION

SUMMARY

In shallow lakes a siltation system develops when wind driven water turbulence erodes the lake floor, silt is resuspended and transported by means of waterdrift into the littoral vegetation. Here, due to the resistance of the vegetation, the turbulence is reduced and the silt deposited. Through this mechanism, the prevailing winds have a major rôle in the erosion and siltation pattern, lake morphology, vegetation pattern and in the mixing of water and nutrients from the open water and the reedswamp and vice versa. The model explains the uneven distribution of vegetation at the lakes Neusiedlersee (Austria) and Chilwa (Malawi) satisfactorily, if one assumes a positive correlation between siltation and reedswamp growth and spread.     

Structure and dynamics of zooplankton communities in the littoral zone of some North Carolina lakes

Species composition and seasonal, dynamics of zooplankton in the littoral zone were studied in three piedmont North Carolina lakes for one year. Nygaard's compound index indicated oligotrophic conditions in Belews Lake and eutrophic conditions in Salem Lake and Lake 150. A total of 32 species of zooplankton were found in the samples. Significantly fewer species of truely littoral zooplankton were present in the oligotrophic lake regardless of season (P < 0.05). Eutrophication appears to favor increased diversity in zooplankton of the littoral zone. Habitat heterogeneity, provided by a well developed littoral zone containing aquatic macrophytes, may be the major factor contributing to the maintenance of more taxa of zooplankton in eutrophic systems.Distinct taxa of Cladocera and Copepoda are present in the littoral zone, exhibiting seasonal dynamics that are largely different from the limnetic fauna. Such dynamics may result from predation by characteristically limnetic species of cyclopoid copepods which seek prey in the littoral zone. The numerical and seasonal abundance of predators and prey are inversely related and the true littoral taxa, represented in the present study by the genera Alona, Chydorus, Pleuroxus, Sida, Simocephalus, and Eucyclops, contained no predaceous species. The seasonal dynamics of these genera are very similar even in widely separated geographical regions, indicating that the factor(s) responsible operate independently of climatic and chemical variables. Intrazooplankton predation appears to be a more plausable explanation than segregation along depth which, although consistent with data from studies of limnetic waters, cannot be related to populations of zooplankton in shallow littoral areas.     

Hydrologic and anthropogenic influences on aquatic macrophyte development in a large,shallow lake in China

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The ciliated protozoa of the monomictic Lake Kinneret (Israel): species composition and distribution during stratification

The ciliate taxa from epilimnion, hypolimnion, and littoral of Lake Kinneret were studied from November to December 1987 during stratification. Two collections of planktonic and benthic ciliates were taken at each of six sampling stations. Benthic ciliates from the littoral zone were collected at seven stations along the lake shore. Densities were determined for each ciliate species and for total protozoans at each station and sampling date. Thirty-six species of ciliates, representing 31 Genera, 18 Orders and 7 Classes, were identified. Planktonic protozoans characteristic of the epilimnion included Coleps hirtus and Vorticella mayeri. The more abundant taxa of benthic ciliates in the profundal of the lake included Saprodinium dentatum, Plagiopyla nasuta and Dexiotricha plagia. The predominant ciliate in the littoral was Pleuronema coronatum. Principal component analysis, performed on the correlation matrix of both sampling stations and species, revealed that epilimnion, hypolimnion, and littoral belt were colonized by different ciliate communities.     

FACTORS REGULATING THE SPATIAL DISTRIBUTION OF THE FILAMENTOUS ALGA PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) IN AN INDIANA LAKE1

Pithophora oedogonia (Mont.) Wittr. biomass in Surrey Lake, Indiana was greater in the littoral than in the pelagial region. Although mean soluble reactive phosphorus concentrations did not differ between the two areas, nitrate concentrations were almost six times higher in the cove than in the open water. Using laboratory cultures of Pithophora, the half saturation constant (K_s at 20° C relating filament growth to external concentrations of nitrate-nitrogen was determined to be 1.23 mg L^?1 (=88 μM)and for phosphate-phosphorus, 0.1 mg L^?1 (=3.22 μM). These values were used to calculate a NO₃-N/PO₄-P atomic ratio of 27.6. Comparison of this value with NO₃-N/PO₄-P ratios in Surrey Lake showed that nitrogen limiting conditions were prevalent in the open water section of the lake. Alkaline phosphatase and dark ammonia uptake analyses on field collected filaments from the shallow and deep water sections confirmed the hypothesis that nitrate is the major factor limiting growth of Pithophora in Surrey Lake.     

Sedimentation of photosynthetic pigments during the bloom of the green sulfur bacterium <Emphasis Type="Italic">Chlorobium phaeobacteroides</Emphasis> in Lake Kinneret: spatial patterns

Sediment fluxes were studied in the subtropical Lake Kinneret (Israel) in 2006, 2007, and 2008 from mid-June to October, when lake was chemically stratified and the green sulfur bacterium Chlorobium phaeobacteroides formed a dense population in the anoxic metalimnion. The rate of seston accumulation in traps was measured with sedimentation traps positioned along an offshore transect connecting the littoral zone and lake center. The sediment fluxes increased from the lake center towards the littoral, while the percentage of organic material (OM) decreased correspondingly. High fluxes of bacteriochlorophyll e (BChl e—a signature pigment of Chl. phaeobacteroides) and chlorophyll a (Chl a—a marker for eukaryotic algae and cyanobacteria) were detected in all locations. The relative contributions of Chl a and BChl e to the bulk of the accumulated OM were higher in traps positioned below the thermocline in the pelagic zone than in traps located near the shore line. The presence of BChl e in traps exposed to oxic conditions in the littoral, where Chl. phaeobacteroides does not develop, implies horizontal translocation of cells from the lake center towards its periphery. We assume that seiche-mediated movement of particles embedded in the metalimnetic waters is the most probable explanation for the existence of Chl. phaeobacteroides tracer in an oxic environment, but do not exclude the possibility of resuspension of settled particles as source for BChl e in littoral traps. The green sulfur bacteria are potentially important component of the sediment flux of photosynthesizing organisms in a thermally stratified lake and should be taken into account when carbon budget are constructed.     

Investigation of distribution,species composition,and degree of resistance to antibiotics of the bacteria of the <Emphasis Type="Italic">Enterococcus</Emphasis> genus in Lake Baikal

The paper presents the result of investigation of species composition, distribution and degree of resistance to antibiotics of the Enterococcus bacteria in Lake Baikal. It is shown that they live in littoral zones of the lake. In the deep-water part of the lake the studied microorganisms are not found. We isolated 120 strains identified as E. faecium, E. avium, E. faecalis, E. mundii, E. hirae, E. durans, E. gallinarum. On the whole, the strains of enterococcus isolated from Lake Baikal are characterized as antibiotic-sensitive. Nevertheless, the antibiotic-resistant strains of enterococci are found in the studied collection, including those with typical intermediate level of resistance to vancomycin.     

Macrophyte species composition,distribution and diversity in relation to some physicochemical factors in the littoral zone of Lake Ziway,Ethiopia

The species composition, distribution and diversity of macrophytes in Lake Ziway were determined at the end of dry and wet seasons along nine littoral sites during 2010 using a belt transect method. Some physicochemical parameters were also measured. Fourteen macrophyte species were identified with low species diversity for the lake (H' value of 1.805). The macrophyte species composition of the lake has undergone many changes during the last few decades. Arundo donax, which was never reported earlier, attained the highest relative density (30.7%) and frequency (30.5%) in this study, followed by Echinochloa colona, Potamogeton schweinfurthii, Cyperus articulatus, Typha latifolia and Cyperus papyrus. The average densities of A. donax, C. articulatus, E. colona and Pistia stratiotes were positively correlated with nutrients, whereas the density of the other species had negative correlation with physicochemical variables (CCA). Generally, the present macrophyte composition and abundance indicates healthy ecological condition of the lake. But if nutrient levels continue to increase and water levels continue to decline, we expect further changes in macrophyte composition and especially a shift towards invasive floating species.     

Lake Superior: an invasion coldspot?

Lake Superior receives a disproportionate number of ballast water discharges from transoceanic ships operating on the Laurentian Great Lakes. Although this provides dispersal opportunities for nonindigenous species (NIS), relatively few NIS were initially discovered in this lake prior to being recorded elsewhere in the basin. A lack of NIS records from this lake may be an artefact of sampling bias. We tested this hypothesis by sampling benthos and plankton from littoral and deepwater habitats around the perimeter of Lake Superior during June and August 2001. Using morphological analysis techniques, we identified a total of 230 invertebrate taxa representing planktonic, benthic and nektonic lifestyles. Five species with invasion histories in the lower Great Lakes, the bivalves Sphaerium corneum, Pisidium amnicum and P. moitessierianum, gastropod Potamopyrgus antipodarum and amphipod Echinogammarus ischnus, were identified for the first time in Lake Superior. In addition, records of expanded distributions within this lake are presented for the amphipod Gammarus fasciatus and oligochaetes Ripistes parasita and Vejdovskyella intermedia. Recently introduced NIS in Lake Superior were found near international ports, implicating shipping as the vector of their introduction. Intrinsic physical-chemical aspects of Lake Superior may account for the scarcity of NIS in this lake as compared to the lower Great Lakes.