Nutrient enrichment experiments in three central Florida lakes of different trophic states

Seasonal nutrient enrichment experiments (short-term bioassays) were conducted in three Florida lakes of different trophic states to determine the effects of addition of various nutrient combinations upon chlorophyll a and phytoplankton standing crops. Nutrient enriched surface water samples with crustacean zooplankton removed were incubated in situ in clear polyethylene bags for 3 to 6 days. The 2⁵ factorial design employed two levels (ambient and enriched) of each of five nutrients [NH₄ ⁺, PO _inf4 ^sup3− , Fe⁻ -EDTA, SiO _inf3 ^sup2− and a cation (Ca²⁺ or K⁺) or trace elements]. Ammonium produced significant increases in chlorophyll a and phytoplankton standing crops in all experiments. Phosphate produced similar results in the mesotrophic lake, but the eutrophic lakes had both positive and nonsignificant responses which varied seasonally between lakes. Iron increased chlorophyll a in most experiments but affected total phytoplankton standing crop only during the summer and fall. Silicon had negative effects in some experiments. Cations and trace elements produced marked differences between lakes for chlorophyll a, but total phytoplankton standing crop showed few significant responses. Synergistic responses to two- and three-factor interactions were observed in all lakes. Differences in the responses of phytoplankton taxonomic divisions to enrichment may be responsible for much of the between lake variation in chlorophyll a and total phytoplankton volume responses. Nutrient limitations in these lakes are discussed and related to limnological factors and predictive models.     

Changes in Vertical Distribution of Pratylenchus scribneri under Potato and Corn

The vertical distribution of Pratylenchus scribneri populations was monitored under irrigated corn and potato grown in loamy sand soil. population estimates were based on the number of nematodes recovered from 100-cm³ soil samples and the roots contained therein. Reproduction was assessed by counting the number of second-stage juveniles. An index of population maturity was computed to evaluate the age structure of populations. At no time were nematodes distributed uniformly among five soil depths from 0 to 37.5 cm deep. During the summer (June-September), changes in the total number of P. scribneri and the number of second-stage juveniles recovered were not consistent among the depths sampled. Early (April-June) and late (September-November) in the season, changes in the abundance, reproduction, and maturity of populations were similar among depths. The timing and pattern of increases in numbers of nematodes suggests that variation in the abundance of P. scribneri in the soil profile beneath potato and corn was caused primarily by reproduction rather than the movement of nematodes.     

辽宁省丹东地区吸血蠓类(双翅目:蠓科)生态调查

吸血蠓类对人畜危害较大,不仅是多种疾病的传播媒介,而且刺叮吸血骚扰性很大。1988年我们在辽宁省丹东地区东沟和风城二县郊区进行了采集和生态习性的调查,现将结果报道如下。材料和方法器材由军事医学科学院微生物流行病研究所提供的诱蠓帐、捕虫网和吸虫器。调查方法     

Downhill movement of litter and its implication for ecological studies in three types of forest in Japan

The amount of litter moving down the slope was measured in three types of forest, together with an examination of rain as a factor in bringing this about. The three forest types were a natural mixed stand ofPinus densiflora and hardwood trees (plot A), aCryptomeria japonica plantation (plot S) and aChamaecyparis obtusa plantation (plot H). The amount of moved litter was quite large in plots A and H, but relatively small in plot S. The rain factor had little influence on litter movement in plot A, but was the main cause of movement in plot S and (especially) plot H. It is suggested that measurement of litter input and output not only by traps above ground level, but also by ones on the ground is essential for determining the cycling of elements inC. obtusa forests. It is also suggested that the decomposition of leaf litter should be studied both on the soil surface and in the soil inC. obtusa forests.     

Seasonal variation in copper and zinc concentrations in three talitrid amphipods (Crustacea)

Cu and Zn concentrations were determined for three talitrid amphipods, Orchestia gammarellus (Pallas), O. mediterranea Costa and Talitrus saltator (Montagu), collected at two-monthly intervals in 1987 from sites on Great Cumbrae Island, Firth of Clyde, Scotland. To account for size effects, log transformed Cu and Zn concentrations were regressed against log dry weight for each bimonthly sample of each species, and compared by analysis of covariance. Copper concentrations in O. gammarellus were significantly raised in March 1987, and lowered in November 1987. Copper concentrations in O. mediterranea and T. saltator were significantly lowered in November 1987. Cu concentrations differed interspecifically in the order O. gammarellus > O. mediterranea > T. saltator. There was no significant intraspecific seasonal variation in zinc concentration in any of the three species. Zn concentrations differed interspecifically in the order T. saltator > O. gammarellus > O. mediterranea.     

Stir-up effect of wind on a more-or-less stratified shallow lake phytoplankton community,Lake Balaton,Hungary

Microstratification of phytoplankton in the large shallow Lake Balaton (Hungary) was studied during a 24 h period. Dissolved O₂ showed biological stratification; flagellates exhibited a definite circadian rhythm. In the middle of the investigation a heavy storm broke out which was followed by the disappearance of differences between different layers of water. Storm-induced destratification is described by cluster-analysis. Abundances of dominant species changed differently in connection with the storm. Numbers of Nitzschia sp. increased due to stirring up from the sediment surface. Numbers of single-celled or colony-forming species (Cyclotella comta, Crucigenia quadrata, Coelosphaerium kuetzingianum) practically did not change. Numbers of all the three dominant filamentous species (Aphanizomenon fos-aquae f. klebahnii, Lyngbya limnetica, Planctonema lauterbornii) significantly decreased, which might be attributed to an unknown loss process and was followed by a competitive displacement by algae of small cell size.     

Effects of eel (Anguilla anguilla L.) removals from selected sites of a stream on its subsequent densities

We assessed the effect of eel (Anguilla anguilla) removal from three sites of a Cantabrian stream upon its subsequent densities. In the first sample (Sept. 1986) numbers and densities were estimated as 43, 45 and 84 ind and 3490, 3030 and 3750 ind ha ⁻¹. Removal of these eels reduced the subsequent numbers and densities which, except on two occasions, were never reached again during the two years (eleven estimates) of study. Highest densities were recorded in the uppermost site in May and July, 1987, coincident with a strong drought and the lowest densities occurred in 1988 during a normal wet year. We hypothesize first that, because of a selective underground homing behaviour of eels, electro-fishing is inefficient and results in underestimates of the population. Second, seasonal variations of water discharge and droughts may not influence the homing behaviour of'eels until a threshold of dryness is reached. If this occurs, eels abandon their refuges and move towards the stream bottom. It seems that in Arroyo Chabatchos this threshold was exceeded in the summer of 1987 when the highest densities were estimated. The re-colonization of these sites experimentally depleted of eels, is a slow procces that lasts for, at least, two years.     

Seasonal coupling between phyto- and bacterioplankton in a sand pit lake (Créteil lake,France)

Phyto- and bacterioplankton biomass and activity were simultaneously measured during the course of one year in the shallow Créteil Lake (France).Phytoplankton was dominated, during the whole year, by small-sized organisms (10 to 25 µm). Bacteria were in a majority small coccoids (<0.3 µm). Phyto -and bacterioplankton abundances averaged respectively 3.3 × 10⁶ cells l^–1 and 6 × 10⁹ cells l^–1.The phasing of the activity and biomass periods suggest a close coupling between phyto- and bacterioplankton. There were two distinct periods of high activity and biomass. Maximal values were observed in summer but an early increase occurred also in winter. Low or undetectable phytoplankton excretion rates, when heterotrophic activity was maximum, indicated a bacterial uptake of up to 100% of the released algal products during the incubation period. Heterotrophic uptake measurements with both glucose and amino acids revealed a seasonal change of the substrates in the lake, glucose uptake being associated more with the maximum activity of the algae, while the amino acids uptake was relatively higher during their decline.The maximal photosynthetic rate averaged 21.5 mgC m^–3 h^–1 and mean Vmax values were 0.056 and 0.050 mgC m^–3 h^–1 respectively for glucose and amino acids uptake.     

The role of leaf surface wetness in larval behaviour of the sorghum shoot fly, Atherigona soccata

The susceptibility of sorghum to the shoot fly Atherigona soccata Rondani, (Diptera: Muscidae) is affected by seedling age and is highest when seedlings are 8–12 days old. This corresponds with high moisture accumulation on the central leaf which is the path of newly hatched larva as it moves downwards from the oviposition site, towards the growing apex. Studies showed that leaf surface wetness (LSW) of the central shoot leaf was higher in 10-day old seedlings than in seedlings of other ages. Similarly, LSW was much higher in the susceptible sorghum genotype CSH 1 than in the resistant genotype IS 2146. Larvae moved faster towards the growing point and produced deadhearts much earlier in CSH 1 than in IS 2146. They also moved faster in 10-day old seedlings than in seedlings of other ages. It was also shown that the leaf surface wetness of the central shoot leaf is a more reliable parameter of resistance than the glossy leaf trait or trichome density.

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L'influence de la humidité de la surface foliaire sur le comportement de la mouche des pousses du sorgho

Résumé La sensibilité du sorgho à la mouche des pousses du sorgho, Atherigona soccata Rondani, est liée à l'âge de la plantule. Elle est plus forte lorsque la plantule est âgée de 8 à 12 jours et la sensibilité est maximale à 10 jours. A ce stade de croissance on observe une forte accumulation d'humidité sur la feuille centrale de la tige. Les jeunes larves traversent cette zone humide lorsqu'elles descendent vers la zone de croissance à partir des pontes déposées sur la face ventrale des feuilles déroulées.Des études ont été menées à l'ICRISAT (Inde) sur la relation entre l'humidité de la feuille centrale de la tige des plantules du sorgho et les dégâts provoqués par la mouche des pousses. L'humidité de la surface des feuilles (HSF) a été estimée grâce à une échelle visuelle graduée 1 à 5 où, 1 = pas d'humidité apparente et 5 = surface de la feuille recouverte de gouttes d'eau. La HSF est plus élevée sur des pousses de sorgho âgées de 10 j que sur les pousses appartenant à d'autres classes d'âge. Les valeurs observées sont également plus fortes pour les variétés non résistantes à ce ravageur (CSH 1,4.8) que pour les variétés résistantes (IS 2146, (2)). La vitesse du déplacement larvaire entre le cornet et la zone de la croissance varie en fonction de l'âge de la plante et des cultivars. Les larves migrent plus rapidement vers la zone de croissance et provoquent la mort du coeur du sorgho plus tôt dans la variété CSH 1 que dans IS 2146. Les larves se déplacent plus rapidement dans les pousses âgées de 10 j que dans les pousses appartenant à d'autres classes d'âge.Des études ont également démontré que la HSF n'est pas directement liée au caractère feuille lisse où à la densité des trichomes. La HSF est faible pour les génotypes résistants présentent où non le caractère feuille lisse. Par contre la HSF est élevée pour les génotypes non résistants présentant le caractère feuille lisse ou non. Aucune relation directe entre la densité des trichomes et les dégâts provoqués par la mouche des pousses n'a pu être mise en évidence. L'analyse des correlations établie pour les caractères de surface des feuilles avec la mort du cur des sorghos indique que les correlations sont faibles et non-significatives pour le caractère feuille lisse (0.49) et la densité des trichomes (0.39 et 0.2). Par contre les correlations sont fortes et significatives pour la HSF (0.82).On conclue que la HSF de la feuille centrale de la tige est un facteur important dans le déterminisme de la résistance du sorgho vis à vis de la mouche des pousses. Les relations entre les processus physiologiques de la plante et les facteurs impliquées dans l'accumulation d'eau sur la surface des feuilles font actuellement l'objet d'études détaillées.

     

Dissolved and suspended mercury. species in the Wabigoon River (Ontario,Canada): seasonal and regional variations

Seasonal and regional variations in the speciation, sediment-water partitioning, and dynamics of mercury (Hg) were studied at selected sites along the Hg-polluted Wabigoon River, and at unpolluted headwater and tributary sites, during April–September, 1979. ‘Dissolved’ and ‘particulate’ forms of Hg in the water were separated by continuous-flow centrifugation in the field. The Hg and other pollutants such as wood chips and salt had been discharged from a chlor-alkali plant and paper mill at Dryden, Ontario. Concentrations and loadings of particulate methyl mercury (CH₃Hg⁺) and total particulate Hg (and loadings of total ‘dissolved’ Hg) were greatest during the spring flood (April-May) owing to accelerated resuspension and transport of sediments. Concentrations of ‘dissolved’ CH₃Hg⁺, however, were highest in the summer (July–September), probably reflecting stimulation of microbial methylating activity by elevated temperatures, together with factors such as reduced levels of metal-scavenging particulates and minimal dilution by runoff. Total dissolved Hg concentrations were relatively high in September at polluted sites only, possibly because of desorption from sediments due to elevated concentrations of Cl⁻ ions. Loadings of dissolved CH₃Hg⁺ tended to be high in the summer but were generally depressed (suggesting sorption by suspended particles) during the major spring-flood episode in May. During July–August dissolved CH₃Hg⁺ was a function of total dissolved Hg, suggesting rapid biomethylation of desorbed inorganic Hg; but in general dissolved and suspended CH₃Hg⁺ levels depended on environmental variables and were unrelated to total Hg concentrations. In the summer only, total dissolved Hg was a function of dissolved Cl⁻. Hg species in particulates were associated with sulfides, hydrated Fe and Mn oxides, organic matter (notably high molecular weight humic and humic-Fe components), and selenium (Se); but CH₃Hg⁺ and total Hg differed in their specific preferences for binding agents, implying that binding sites discriminate between CH₃Hg⁺ and Hg²⁺ ions. CH₃Hg⁺ was associated with sulfide and (in the spring only) with Fe oxides, whereas total Hg was associated with organic matter and Se and with DTPA- and NaOH-extractable Fe in the spring but with Mn oxide and NaOH-extractable organics in the summer. Sulfides were most abundant in May, indicating that they were eroded from bottom sediments, but Fe and Mn oxides were most abundant in the summer, probably owing to activities of filamentous iron bacteria and other micro-organisms. Particulate Hg was 98–100% nonextractable by mild solvents such as Ca acetate, CaCl₂, dilute acetic acid, and (at polluted sites only) DTPA solutions, suggesting that the particulate Hg mobilized in the spring may not be readily available to organisms; association with Se and high molecular weight humic matter also supports this hypothesis. Hg probably becomes more bio-available in the summer, as suggested by the upsurge in dissolved CH₃Hg⁺ and total dissolved Hg levels, and by increases in the solubility of particulate Hg in acetic acid, DTPA, H₂O₂, and NaOH solutions, as well as an increase in the relative importance of lower molecular weight fractions of NaOH-extractable Hg (in September). Regional variations in Hg speciation and partitioning reflected a gradient in sediment composition from wood chips near Dryden to silt-clay mud further downstream. Hg in silt-clay mud relatively far (> 35 km) downstream from the source of pollution or in unpolluted areas appeared to be more readily solubilized by Cl⁻ ions or chelators such as DTPA, more readily methylated (as indicated by downstream increases in dissolved CH₃Hg⁺ levels and CH₃Hg⁺/total Hg ratios), and was to a greater degree organically bound (H₂O₂-extractable), and thus was probably more bio-available, than Hg in wood-chip deposits. Possible explanations include weaker binding of Hg by the mud, the more finely divided state of the mud, and improved microbial growth at lower concentrations of toxic pollutants. Owing to enrichment in sulfides and Fe oxides, resuspended wood-chip sediments were especially efficient scavengers of CH₃Hg⁺. The results indicate that in any pollution abatement plan aimed at lowering the Hg levels in the biota of lakes fed by the Wabigoon River, immobilization, removal, or detoxification of dissolved as well as particulate forms of Hg in the river would probably have to be considered. Possibly, Hg species could be ‘scrubbed’ from the river water by increasing the suspended load and by sedimentation and treatment with Hg-binding agents in special receiving basins.