Seasonal patterns of growth and nutrient status of the macroalga Adamsiella chauvinii (Rhodophyta) in soft sediment environments

The seasonal growth and nutrient status of the temperate subtidal macroalga Adamsiella chauvinii was determined at three soft sediment sites with different ranges of tidally-driven current speeds. A. chauvinii thalli exhibited maximum growth rates during the late summer (February) and no evidence of nitrogen limited growth in summer as often exhibited by macroalgae growing on hard substrata in temperate environments. We suggest that growth was maintained during the summer period, when seawater nitrogen concentrations were low, by localized sources of ammonium probably produced by sediments and fauna within the canopy. Growth rates were up to 50% lower at the site with the slowest water velocity throughout most of the experimental period. However a greater tissue nitrogen content of A. chauvinii thalli at the slow flow site did not support evidence for mass-transfer limitation. It is suggested that as a consequence of slow flow, sedimentation on blade surfaces reduced the amount of light available for photosynthesis and growth. Lower growth rates at the slow flow site reduced metabolic demand for nutrients resulting in accumulation of tissue nitrogen and phosphate.     

Growth Responses of Tropical Cladocerans to Seston from Lake Monte Alegre (Brazil) Supplemented with Phosphorus, Fatty Acids, a Green Algae and a Cyanobacterium

The cladocerans Ceriodaphnia richardi, Daphnia ambigua, D. gessneri and Moina micrura were used to access food quality of Lake Monte Alegre’s seston. Experiments were carried out in summer and autumn as growth assays with lake seston only (control) and seston supplemented with phosphate, fatty acids or Synechococcus, and Scenedesmus. In summer, high C:P ratios in seston suggested strong phosphorus limitation, however, contrary to the expectations of stoichiometric theory, the addition of phosphate to seston did not improve cladoceran growth. Addition of PUFA increased growth rates and clutch size of D. gessneri, suggesting a possible deficiency in essential fatty acids in summer. Addition of Scenedesmus increased significantly growth rates of the cladocerans D. gessneri and C. cornuta, suggesting energy limitation in summer. In autumn, C:P ratios were lower than in summer, but still above the threshold ratio for Daphnia. At this time, addition of phosphate increased significantly growth rates of Daphnia suggesting strong P limitation, especially in D. gessneri. However, energy limitation was still important in autumn, as suggested by a further increase in growth rates in +Syn and +Sce treatments. Energy limitation was especially strong for Moina micrura, which is a fast-growing species, with high P content. Algal digestion resistance is a plausible hypothesis for energy limitation, since carbon concentrations in both seasons were above incipient limiting levels. These results show that the seston C:P ratio was not a consistent predictor of cladoceran P limitation and that factors other than P and energy limitation seem to be also important, such as PUFA or other biochemical factors. An erratum to this article is available at .     

Seasonal abundance,breeding and sex-structure of populations of Tilapia nilotica (L.) and Labeo niloticus (F.) in the Jebel Aulia Dam area of the White Nile

Seasonal abundance, maturation and sex-structure were studied in populations of T. nilotica and L. niloticus in the Jebel Aulia reservoir and in the White Nile below the dam. L. niloticus showed a single annual breeding cycle from mid-July to December and was most abundant in October in the lake, and in early November in the river. T. nilotica showed two distinct annual seasons of abundance: summer (April to June) and autumn (November to October). Spawning activity was highest in summer (April to June) and autumn (late July to September). The sex-ratio varied inversely with age (size); males were predominant in populations of older fish, perhaps because of their greater endurance to adverse conditions as evidenced by their better survival in hypoxic water.     

紫茎泽兰入侵前后抗虫物质含量差异及其对泽兰实蝇寄生的响应

【目的】紫茎泽兰是我国重要的入侵杂草,对我国生态环境和农、林、牧业造成重大危害。本研究通过比较紫茎泽兰原产地与入侵地种群地上部抗虫物质含量的差异及其对泽兰实蝇寄生的响应,为探明紫茎泽兰入侵种群持续扩张的化学生态机制提供依据。【方法】选取5个紫茎泽兰种群(3个中国云南种群:C1,C2,C3;2个墨西哥种群:M1,M2),比较原产地和入侵地种群地上部分碳氮比、单宁类(单宁酸、儿茶素、鞣花酸)和类黄酮物质(槲皮素、异槲皮素、山奈酚)含量差异,以及泽兰实蝇寄生前后这些抗虫物质的含量变化。【结果】入侵地种群地上部分碳氮比不同程度地低于原产地种群,茎秆部位差异最明显。紫茎泽兰入侵地种群的单宁类和类黄酮物质含量亦低于原产地种群,其中,芽尖部位单宁酸和儿茶素含量差异尤为明显,分别比原产地种群低26.4%和32.3%。泽兰实蝇寄生后,原产地和入侵地种群单宁类和类黄酮物质含量基本呈上升趋势,其中,M1种群寄生植株在虫瘿破膜前,儿茶素含量比未寄生植株升高了163.2%。【结论】紫茎泽兰入侵种群在抗虫特性方面产生了资源再分配的适应性变化,有利于其在新生境天敌缺乏条件下的持续扩张。虽然泽兰实蝇寄生会诱导紫茎泽兰原产地种群和入侵地种群抗虫物质的增加,但原产地种群增加的幅度更大,表明紫茎泽兰入侵后对专食性天敌胁迫的适应性明显下降。     

In vivo nuclear magnetic resonance studies on the lugworm Arenicola marina. II Seasonal changes of metabolism

In vivo ³¹P- and ¹³C-NMR spectra of the lugworm Arenicola marina (Polychaeta, Annelida) gathered between 1988 and 1994 at different times of the year were evaluated for seasonally dependent metabolic changes. Beside the typical ³¹P-NMR signals of ATP and (phospho)taurocyamine, other seasonally dependent signals were observed: from January to March an intensive signal at 1.4–1.8 ppm was identified as inorganic phosphate compartmented in an acidic intestinal lumen. Between April and September signals at 1.2–1.4 ppm were assigned to phosphodiester. Starting in July males showed a second phosphagen signal [(phospho)creatine of spermatozoa, cf. Kamp and Juretschke (1989a)] whose intensity increased until spawning in September. The (phospho)taurocyamine/ATP ratio was also dependent on the season. In January or February the ratio reached 11, while in summer and autumn the ratio was between 4 and 5. As verified by biochemical assays this effect was caused by a significant decrease of ATP in the lugworm body wall during winter (December–February). The phosphagen (phospho)taurocyamine and the respective unphosphorylated guanidine taurocyamine remained constant throughout the year. Levels of free inorganic phosphate incurred similar changes to ATP. ¹³C-NMR spectra of lugworms showed a dramatic change in lipid stores. They were below the detection limit between January and March but developed into the most intensive signals during summer. The most abundant amino acids, glycine and alanine, were observed throughout the year while glycogen could not be detected in the ¹³C-NMR spectra. After treating tissue extracts with amyloglucosidase, the signals of the hydrolytic product glucose were recorded indicative of NMR-invisible glycogen stores.Abbreviations AN adenine nucleotides - NMR nuclear magnetic resonance - P _i inorganic phosphate - (P)Cr (phospho)creatine - PDE phosphodiester - PME phosphomonoester - PPA phenylphosphonic acid - (P)Tc (phospho)taurocyamine     

Restoration of Botshol (The Netherlands) by reduction of external nutrient load: recovery of a characean community,dominated by Chara connivens

Till about 1965 a rich characean community occurred in the shallow peat lake Botshol with six species of which the rare Nitellopsis obtusa and Chara major dominated at many sites. In the period 1980–1988 characean biomass strongly decreased and only two species, Chara globularis and C. connivens, remained in small populations at a few localities. Of the macrophyte Najas marina also some small populations remained, while the aquatic moss Fontinalis antipyretica and the filamentous alga Vaucheria dichotoma predominated at many sites. These phenomena may have been due to eutrophication by the input of polluted water. This process of impoverishment was stopped by restoration measures in 1989, resulting in a lower phosphorus concentration (ca 25 µg l^-1) and a higher water transparency. Immediately after these measures the Characeae community increased in abundance and number of species. During the summer of 1990, and especially 1991, a spectacular growth occurred of Chara connivens. C. connivens was often accompanied by C. major. Other species with scattered occurrence were C. aculeolata, C. aspera, C. contraria and C. globularis. The reasons for the shift in dominance from Nitellopsis obtusa to Chara connivens are discussed. From growth experiments evidence was obtained that neither the recent higher chloride level, nor the lowered phosphate concentration were the main factors for the domination of Chara connivens.     

Seasonal changes in agar characteristics of two populations ofPterocladia capillacea in Gran Canaria,Spain

Agar characteristics ofPterocladia capillacea were examined seasonally at two intertidal populations exposed to different wave energy on the northern rocky shore of Gran Canaria Island. Plants were collected monthly from August 1991 to July 1992. Agar yield, gel strength, melting and gelling temperature and chemical properties such as sulphate and pyruvate content were measured. Percent epiphytism was determined on both populations, together with the changes in biomass as dry weight. Specimens in the sheltered habitat were larger and more epiphytized than ones in the exposed area. There was a clear seasonal change in agar characteristics in both populations. Agar yields decreased in late spring and early summer, although in the sheltered habitat fluctuations were more erratic. Gel strength increased in winter, reaching a maximum in December–February. No significant differences were found in agar yield, gel strength or melting and gelling temperatures, but there was a difference between fresh to dry weight ratio. The role of the exposure degree as a possible environmental factor responsible for this behavior is discussed. Agars ofPterocladia capillacea from Canary Islands show characteristics for industrial use.Author for correspondence     

Floral biology and reproductive ecology ofClusia nemorosa (Clusiaceae) in northeastern Brazil

Between March 1993 and May 1995 the floral biology and the phenology ofClusia nemorosa G. Mey. (Clusiaceae) were studied in two dioecious populations in Pernambuco State, and some observations were made in gynodioecious populations, that were encountered only in Bahia State. The species flowers from June to January/February. The flowers are bowl shaped, and last only one day. The floral reward is resin which is produced by staminodia in all types of flowers. The sex ratio was 1:1 for the studied populations. The pollen viability was approximately 98%. Apomixis was not observed. The rate of fruit set, in hermaphrodite individuals, under natural conditions and the sex ratio of the gynodioecious populations studied suggest that the hermaphrodite individuals may behave as males.Clusia nemorosa is melittophilous and was visited by 16 species of bees. The main pollinators wereEuplusia mussitans, E. iopyhrra, Eulaema cingulata, E. nigrita, Euglossa cordata andEuglossa sp.     

Life-history variation in Crepis tectorum (Asteraceae)

Summary Populations of the monocarpic plant Crepis tectorum were grown in a series of uniform environments to test the hypothesis that weedy populations are more r-selected than populations from a more natural habitat. Weedy populations exhibited a combination of r- and K-selected traits. The relatively rapid growth, the potential for a summer annual habit, and the relatively high fecundity that characterized at least one of the two weed populations studied were considered as r-selected traits favored in habitats of unpredictable duration. However, high levels of competition from other weedy species or from the crop in arable fields may explain at least some presumably K-selected traits observed in the weedy populations, e.g. relatively large seeds and late flowering in the summer. Results indicated that stress due to abiotic factors (strong winds, desiccation and nutrient deficiency) has been a more important selective factor than r- or K-selection, in non-weedy populations from calcareous grasslands (alvars) on the Baltic islands.     

Prevalences of fungal pathogens and othernatural enemies of cereal aphids (Homoptera:Aphididae) in wheat under dryland and irrigatedconditions in South Africa

Surveys were conducted in the summer andwinter rainfall wheat producing regions of SouthAfrica in a first attempt to investigate theidentity and impact of entomopathogenic fungi withinthe cereal aphid complex. Wheat produced underdryland and irrigated conditions was surveyed duringthe 1996 and 1997 seasons. Six cereal aphid specieswere recorded of which the Russian wheat aphid, Diuraphis noxia, was the most abundantunder dryland conditions in the summer rainfallregion as opposed to the oat aphid, Rhopalosiphum padi, in the winter rainfallregion. Rose grain aphid, Metopolophiumdirhodum, was most prevalent underirrigated conditions in the summer rainfall region.Five species of entomopathogenic fungi were recordedincluding four entomophthorales and the hyphomycete,Beauveria bassiana. TheEntomophthorales included Pandora neoaphidis, Conidiobolus obscurus, C.thromboides, and Entomophthoraplanchoniana. Pandora neoaphidis wasthe most important etiological agent recorded fromD. noxia, with up to 50% mycosis recordedunder dryland conditions in the Bethlehem summerrainfall region. Similarly, P. neoaphidis wasthe most prevalent species within populations ofM. dirhodum. under irrigated conditions in theBergville/Winterton summer rainfall region (up to77% mycosis). However, mycoses of R. padi didnot exceed 1.7% in samples from these areas,suggesting that R. padi may be lesssusceptible to P. neoaphidis than M.dirhodum. Epizootics in populations of D.noxia under dryland conditions in both the winterand summer rainfall regions indicated a high levelof susceptibility to P. neoaphidis.Occurrences of hymenopterous parasitoids andpredators in populations of D. noxia were low,although a parasitism level of 25% was recorded inone small sample of R. padi collected from anirrigated field in the summer rainfall region.     

Food limitation and seasonal population declines of rotifers

Summary Keratella cochlearis exhibited consistent seasonal abundance patterns during a four-year study in Little Rock Lake, Wisconsin, U.S.A. In each year, spring population peaks were followed by strong summer reductions. Concomitant with population declines, there were reductions in rotifer egg ratios. Keratella taurocephala abundance patterns were similar to those of K. cochlearis during 1984 and 1985, but not in 1986 and 1987, when spring peaks and summer declines were not apparent. However, summer declines in the egg ratio of K. taurocephala were observed during each year. The reduction in rotifer populations simultaneously with decreased egg ratios suggested that population declines were caused by food limitation. Food-addition experiments conducted in situ in small enclosures indicated that food was limiting for K. cochlearis when its populations were declining, but not during other periods of the year. Keratella taurocephala did not show a consistent response to food addition.     

Genetic variation and population structure in Korean populations of sand dune speciesSalsola komarovi (Chenopodiaceae)

Salsola komarovi lljin is a herbaceous annual native to the sand dunes and beaches of Japan, northern China, Sakhalln and Korea. Starch-gel electrophoresis was conducted on leaves and stems collected from 300 plants in eight Korean populations. The mean number of alleles per locus (A _p=1.51), mean expected heterozygosity (He _p=0.116), and total genetic diversity (H _T=0.279) were comparable with those for species with similar life history and ecological traits. A general conformance of genotype frequencies to Hardy-Weinberg expectations (meanF _IS=−0.030) indicates thatS. komarovi is an outcrossing species. Slightly more than 20% of the genetic variation was found among populations (F _ST=0.204). In addition, significant differences in allele frequency were detected between populations at all 11 polymorphic loci (P<0.001). Nei's genetic identities range from 0.885 to 0.985 with a mean of 0.942. However, indirect estimates of the number of migrant per generation (0.97, calculated fromF _ST and 0.31, calculated from seven private alleles) indicate that the levels of gene flow is low among Korean populations. Although the species maintains a moderate level of genetic variation within populations, the small, isolated natural populations of the species have been severely destructed by human activities, particularly in summer season. If this is true, conservation efforts should be focused on those populations that currently maintain the most genetic diversity (e.g., populations of Cheju Island and coast of the southwestern Korean Peninsula).     

A distinct temporal separation of maximum bacterial numbers and phytoplankton biomass in the open-water zone of Lake Maarsseveen I: Results and literature review

Summary Starting in March there was an increase in bacterial number until a maximum was reached at the end of April; from then there was a decrease in number until the end of summer. The biomass of the phytoplankton followed an opposite curve,viz. a maximum in February, a minimum at the end of April and several pulses in summer. Two hypothetical models are described to explain this phenomenon. (a) Competition for phosphate between phytoplankton and bacteria (with subsequent grazing by herbivorous zooplankton on bacteria), and (b) excretion by bacteria of compounds that inhibit phytoplankton growth.     

Recolonization and restoration of upper intertidal Fucus gardneri (Fucales,Phaeophyta) following the Exxon Valdez oil spill

The Exxon Valdez oil spill in March 1989 and subsequent cleanup caused injury to intertidal Fucus gardneri populations especially in the upper intertidal. A survey in 1994 in Prince William Sound, Alaska showed that the upper boundary of Fucus populations at oiled sites was still an average of 0.4 m lower than the upper boundary at unoiled sites.Restoration of severely damaged Fucus populations was started on a small-scale at a heavily oiled rocky site in Herring Bay, Prince William Sound. Experiments employed mats of biodegradable erosion control fabric to act as a substratum for Fucus germlings and to protect germlings from heat and desiccation stress. A series of plots was covered with mats made from a resilient coconut-fiber fabric in June 1993. Half of the mats were inoculated with Fucus zygotes. A series of uncovered control plots was also monitored. There was no enhancement of Fucus recruitment on the rock surfaces under the mats. Dense populations of Fucus developed on the surface of all of the mats by the summer of 1994. The natural rock surfaces in the control plots, both inoculated and not, were barren of macroscopic algal cover. By September 1994, the juvenile thalli on the mats were approximately 2 cm in length. Inoculating the mats had an effect only in the upper region of the intertidal. It is expected that the thalli will become fertile during the 1995 season. These thalli may serve as a source of embryos to enhance the recovery of new Fucus populations in this high intertidal area.     

Rotifer vertical distributions in a meromictic basin of Lake Banyoles (Spain)

The vertical distribution of rotifer populations has been analyzed in a meromictic basin (III) of the karstic Lake Banyoles. This basin had, at the time of the study, an anaerobic monimolimnion from around 17 m in summer or 21 in winter to 25 m, its maximum depth. However, below this layer a warmer microaerophilic zone with suspended marls is found in a chimney to a depth of 130 m.The rotifer distributions of several years are compared. The presence of large populations of rotifers at the oxic-anoxic interface is constant through the years, but highly variable regarding the total abundance and relative proportions of the species, depending on the importance of the vertical mixing in winter and the posterior segregation of water layers as a function of production and decomposition processes. Filinia hofmanni, Filinia longiseta longiseta are species with a restricted occurrence at the oxycline and are not present in the other non meromictic basins of the lake. Filinia terminalis, more broadly distributed, also has dense populations in summer at the oxycline in basin III, but disappears from the plankton during this season in the other basins. Other species, such as Polyarthra dolichoptera, Anuraeopsis fissa, Keratella cochlearis and Kellicottia longispina, also showed maxima at the oxycline in summer, as well as other maxima in the metalimnion. Populations at the oxycline are probably different ecotypes adapted to low oxygen conditions, but having the advantage of high food availability.     

Seasonal changes in plasma membrane H<Superscript>+</Superscript>-ATPase activity of <Emphasis Type="Italic">Vaccinium myrtillus</Emphasis> (L.) and <Emphasis Type="Italic">Pinus sylvestris</Emphasis> (L.)

Seasonal changes in vanadate sensitive plasma membrane H⁺-ATPase activity of bilberry (Vaccinium myrtillus L.) and Scots pine (Pinus sylvestris L.) were studied in a period from February to August in northern Finland. The plasma membrane isolation was performed by sucrose gradient centrifugation, and the H⁺-ATPase activity was assayed by spectrophotometrical determination of released inorganic phosphate. The studied species showed seasonal changes from high winter to low spring activity, indicating probable physiological changes between hardened and dehardened tissue. ATPase activity of bilberry peaked up at the beginning of the growth period, obviously due to active phloem loading of photosynthates.     

Distribution and population dynamics of Porphyra (Bangiales, Rhodophyta) in the southern Western Cape, South Africa

Although Porphyra is commercially farmed in many countries, in South Africa only small harvests of wild populations for sale as nori have been carried out. The discovery that Porphyra improves growth of South African abalone (Haliotis midae) farmed inland-based tanks has led to increased pressure to harvest wild populations. This paper reports on a survey of the distribution and seasonality of Porphyra in the southern Western Cape. Porphyrawas present at all sites surveyed, and showed considerable temporal variation. A significant amount of the Porphyra present is in reserves and therefore protected from harvesting. Close rexamination of one site revealed seasonal populations of Porphyra that occupied different niches dependent on season. Recruitment peaked in spring and autumn, leading to dense summer and winter populations. Summer populations generally grew lower in the eulittoral than winter populations. No pattern in the mortality of larger thalli wasde tected, though sporeling mortality was high following recruitment peaks. Although it seems that most sites in the southern Western Cape are suitable for harvesting, the taxonomy of the genus in the region urgently needs revision if populations are to be appropriately managed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synergistic effects of an extreme weather event and habitat fragmentation on a specialised insect herbivore

Habitat fragmentation is considered to be one of the main causes of population decline and species extinction worldwide. Furthermore, habitat fragmentation can decrease the ability of populations to resist and to recover from environmental disturbances such as extreme weather events, which are expected to occur at an increasing rate as a result of climate change. In this study, we investigated how calcareous grassland fragmentation affected the impact of the climatically extreme summer of 2003 on egg deposition rates, population size variation and survival of the blue butterfly Cupido minimus, a specialist herbivore of Anthyllis vulneraria. Immediately after the 2003 summer heat wave, populations of the host plant declined in size; this was paralleled with decreases in population size of the herbivore and altered egg deposition rates. In 2006 at the end of the monitoring period, however, most A. vulneraria populations had recovered and only one population went extinct. In contrast, several butterfly populations had gone extinct between 2003 and 2006. Extinction probability was significantly related to initial population size, with small populations having a higher risk of extinction than large populations. These results support the prediction that species of higher trophic levels are more susceptible to extinction due to habitat fragmentation and severe disturbances.     

Seasonality of VAM infection in three populations of Agrostis capillaris (Gramineae) on soil with or without heavy metal enrichment

Three populations of the perennial grass Agrostis capillaris, growing on limestone derived clay with and without natural enrichment of the heavy metals cadmium, lead, and zinc, and on a sandy soil polluted by a metal smelter have been investigated with regard to the percentage and seasonality of infection with vesicular-arbuscular mycorrhizal (VAM) fungi and its impact on mineral nutrition.In all populations VAM infection was lowest during winter, and highest during late summer and autumn. The population at the metal smelter site was less infected by VAM fungi than both other populations. The concentration of mineral nutrients for the three populations was clearly related to the soil concentration, but hardly modified by the degree of VAM infection.     

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m^–2 d^–1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.