Density fluctuations in populations (1982–1986) and biological observations ofPotamopyrgus Jenkinsi in two trophically differing lakes

The hydrobiid snailPotamopyrgus jenkinsi (E.A. Smith), characterized by parthenogenesis and ovovivipary, was quantitatively sampled monthly between June, 1982, and December, 1986, on sandy bottoms in the shallow zones of the meso-oligotrophic Lake Maarsseveen I and the eutrophic Lake Maarsseveen II. The snail demonstrated a very clumped distribution in both lakes. The mean numbers of juveniles and adults taken together fluctuated strongly. Organisms in Lake I showed relatively high densities (up to 25,000 per m²) in 1982, followed by a sudden drop to values approaching zero in December, 1982, with a subsequent rapid increase in densities, fluctuating between 2,000 and 200 per m². In Lake II, densities of snails fluctuated between 13,000 and 300 per m² with decreases in the spring of 1985 and 1986. The various types of decreases in the lakes are extensively discussed, but no explanation is presently available. The reduction in Lake I was of catastrophic proportions, but the speed of recovery of the population was remarkable.Floating was observed only in Lake I, and only during the occurrence of the highest densities on the sediment. Burrowing behaviour was very common, but strongly suppressed under an uninterrupted dark regime. A shift of temperature from 15 to 22°C had the same effect. A number of submerged macrophyte species from Lake I proved to attractP. jenkinsi in the absence of sandy substrate, though these plants were only covered by the snail during the period of the highest densities in 1982. Temperatures of 20°C or lower were well tolerated, unlike temperatures of 25 and 30°C. Growth was distinct at 10, 15 and 20°C. Keeled individuals were encountered in much higher numbers in Lake I than in Lake II.     

窦性节律下家兔心率与在位心室肌纤维有效不应期的关系

在自然呼吸和窦性节律下,用浮置式玻璃微电极引导在体单个左心室肌纤维动作电位,作为兴奋的指标,以其 0相触发产生期前的试验刺激,测定有效不应期(ERP)。38只家兔的测定结果表明,在R-R间期为205—330ms的范围内,随着心率加快(R-R间期缩短),ERP减小,而 ERP/RR 间期增大,说明 ERP 与心率直接有关。并且,在较快的心率时,ERP 相对延长。通过相关与回归分析,制出了能够删除心率影响的校正公式。静脉注射酒石酸锑钾(50mg/kg)发现,在窦性和起搏节律下,酒石酸锑钾均能轻度延长 ERP(P<0.001)。窦性节律下的校正后值与起搏节律的测定结果完全一致。证明校正后值能够用来比较处理前后不同心率条件下,各种药物、离子及其它因素对ERP 的影响。 本文的校正公式虽然只适用于同种动物和方法,但此校正公式的制作原理也可以广泛应用到其它多种动物。     

Noxious floating weeds of Malaysia

After ten years of field surveys on various water bodies ranging from stagnant water ponds, pools and man-made lakes to flowing waters such as rivers, streams and canals, there is a clear evidence of four problematic weeds in Malaysia. These species are Eichhornia crassipes, Salvinia molesta, Lemna perpusilla, and Pistia stratiotes. Among these weeds, E. crassipes and S. molesta are widely distributed througout Malaysia. E. crassipes generally dominates canals and rivers although, recently, this species has spread to man-made lakes. The favourable tropical climate and conducive environmental factors help to trigger the massive growth of these weeds. The high nutrient concentrations, notably phosphate which has a soluble reactive concentration greater than 0.1 mg l^–1, initiate a high productivity. Manual control methods are generally used and several herbicides including 2,4-D and glyphosate are frequently employed to eradicate these weeds.     

Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado

Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with¹⁵N-N₂. The average ratio (± SD) of moles C₂H₄ reduced per mole N₂-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N₂-N·m^–2·d^–1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N0₃ ^– concentrations as low as 0.5 M, and was below detection limits at concentrations of 4 M, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N₂-N·m^–2·d^–1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO₃ ^–), but rates were undetectable (< 0.05 mmol N₂-N·m^–2·d^–1) when there was river flooding (high NO₃ ^–). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.     

Hydrologic,vegetation, and substrate characteristics of floating marshes in sediment-rich wetlands of the Mississippi river delta plain,Louisiana, USA

Floating marshes occur over 70% of the western Terrebonne Basin, Louisiana, USA, freshwater coastal wetlands. They are of several types: A free-floating thick-mat (45–60 cm) marsh dominated by Panicum hemitomon and Sagittaria lancifolia; a thick mat marsh dominated by Panicum hemitomon and Sagittaria lancifolia that floats part of the year, but whose vertical floating range is damped compared to adjacent water; and an irregularly-floating thin mat (< 30 cm) dominated by Eleocharis spp. in the spring and Ludwigia leptocarpa and Bidens laevis in the summer and fall. Floating mats must be almost entirely organic in order to be buoyant enough to float. The western Terrebonne wetlands receive large winter/spring supplies of suspended sediments from the Atchafalaya River. Even though sediment concentrations in the adjacent bayou are as high as 100 mg l^–1, the Panicum hemitomon/Sagittaria lancifolia free-floating marsh probably receives no over-surface sediments since it floats continuously. The bulk density data of the damped-floating marsh, however, suggest some mineral sediment input, probably during winter when this marsh is submerged. These two types of floating marsh could not have developed in the present sediment regime of the Atchafalaya River, but as long as they remain floating can continue to exist. Thin floating mats are found in areas receiving the least sediment (<20 mg 1^–1 suspended sediment concentration in adjacent bayous). This low sediment environment probably made possible their formation within the past 20 years. They may represent a transitional stage in mat succession from (1) existing thick-mat floating marsh to a degrading floating marsh, or (2) a floating marsh developing in shallow open water.Corresponding editor: D. Whigham     

Connections Between Benthic Populations and Local Strandings of the Southern Bull Kelp Durvillaea Antarctica Along the Continental Coast of Chile1

Floating seaweeds are important dispersal vectors in marine ecosystems. However, the relationship between benthic populations and stranded seaweeds has received little attention. After detachment, a fraction of floating specimens returns to the shore, resulting in strandings that fluctuate in space and time. It has been hypothesized that the availability of stranded seaweeds is related to their benthic abundance on adjacent coasts. Using the large fucoid Durvillaea antarctica, we tested whether stranded biomasses are higher at sites with dense adjacent benthic populations. Benthic abundance of D. antarctica along the continental coast of Chile was estimated using three approximations: (i) availability of potentially suitable habitat (PSH), (ii) categorical visual abundance estimates in the field, and (iii) abundance measurements in the intertidal zone. Higher PSH for D. antarctica was observed between 31° S–32° S and 40° S–42° S than between 33° S and 39° S. Lowest benthic biomasses were estimated for the northern latitudes (31° S–32° S). Regression models showed that the association between stranded biomass and PSH was highest when only the extent of rocky shore 10 km to the south of each beach was included, suggesting relatively short-distance dispersal and asymmetrical transport of floating kelps, which is further supported by low proportions of rafts with Lepas spp. (indicator of rafting). The results indicate that stranded biomasses are mostly subsidized by nearby benthic populations, which can partly explain the low genetic connectivity among populations in the study region. Future studies should also incorporate other local factors (e.g., winds, currents, wave-exposure) that influence stranding dynamics.     

Oceanic transport of calcified eggs: taphonomic and biogeographic implications

Estimates of the temporal and spatial resolution of fossil egg assemblages are required to constrain the inferences that can logically be drawn during assemblage analysis. Consequently, understanding egg transport mechanisms is required before conclusions are developed. Bird eggs are buoyant during part of development and can float from near shore nests during high tides, storm surges, or flooding. Complete and unbroken eggs have survived transoceanic transport. Evidence of these transport processes operating in the past is suggested by the recovery of fossil eggshell in marine deposits. Transport of eggs can potentially confound fossil assemblage spatial analysis since evidence of oceanic transport may not be present on an eggshell when collected. Bird embryos exposed to cold temperatures and immersed in both fresh and salt water can survive from hours to two days, suggesting that oceanic transport of viable eggs is possible, a conclusion further supported by field observations. Eggs of other taxa show similar environmental survival trends and may also be capable of surviving oceanic transport through floatation or rafting. In addition, if viable eggs are transported across seas and hatch, emigration or gene flow of terrestrial egg-laying vertebrates may be possible between landmasses.     

Optimising the duration of floating row cover period to minimise the incidence of tomato yellow leaf curl disease and maximise yield of tomato

The influence of polyester cover on tomato crops was evaluated to determine its effectiveness in reducing the incidence of tomato yellow leaf curl disease and on tomato yield. Uncovered plots were compared in a randomised block design with 14 treatments having different durations of cover. The identification of begomovirus was carried out by polymerase chain reaction, cloning and complete nucleotide sequencing of viruses from infected plants. Disease incidence was high in plots covered for less than 3 weeks with no significant differences among these treatments and those that were left uncovered. In the three treatments with the longest duration of cover, no disease was recorded but yield was significantly decreased. The optimum covering period was 6–7 weeks, providing protection against disease (10% incidence) and maintaining high yields (3 kg per plant). Floating row covers can play a major role in reducing begomovirus‐induced diseases by reducing exposure to whiteflies which transmit these viruses. The current work appears to be the first attempt to optimise the cover period by sequentially removing covers from crops to analyse the consequences in terms of crop biomass, yield and disease and to explore the interactions between these parameters.