Carbon metabolism and strobilation in Cassiopea andromedea (Cnidaria: Scyphozoa): Significance of endosymbiotic dinoflagellates

Scyphopolyps and scyphomedusae of Cassiopea andromeda Forskål (Cnidaria, Scyphozoa) containing dinoflagellate endosymbionts (zooxanthellae) were investigated for rates and pathways of carbon fixation. Photosynthesis by the algae, accounting for 80 and 15 mol C h^-1 on a dry weight basis in medusae and polyps, respectively, by far exceeds dark incorporation of inorganic carbon by the intact association. Photosynthetic carbon fixation is operated via C₃ pathway of carbon reduction. DCMU-treatment (1×10^-6 M and 1×10^-5 M) completely inhibits light-dependent carbon assimilation. Major photosynthates presumably involved in a metabolite flow from algal symbionts to animal tissue are glycerol and glucose. A total of 5–10% net algal photosynthate appears to be seleased in vivo to the host. This is probably less than the energy supply ultimately required for the nutrition of the polyps and medusae. The presence of zooxanthellae proved to be indispensable for strobilation in the scyphopolyps. However, photosynthesis by algal symbionts as well as photosynthate release is obviously not essential for the initiation of ephyrae as is shown by DCMU-treatment, culture in continous darkness, and aposymbiotic controls. It is therefore concluded that strobilation is supported, but not triggered by algal photosynthetic activity. The induction of strobilation thus seems to depend on a more complex system of regulation.     

A multi-mutualist simulation: Applying biological market models to diverse mycorrhizal communities

We present a cellular automaton that simulates the interaction between a host tree and multiple potential mycorrhizal symbionts and generates testable hypotheses of how processes at the scale of individual root tips may explain mycorrhizal community composition. Existing theoretical biological market models imply that a single host is able to interact with and select from multiple symbionts to organize an optimal symbiont community. When evaluating the tree–symbiont interaction, two scales must be considered simultaneously: the scale of the entire host plant at which carbon utilization and nutrient demands operate, and the scale of the individual root tip, at which colonization and carbon-nutrient trade occurs. Three strategies that may be employed by the host tree for optimizing carbon use and nutrient acquisition through mycorrhizal symbiont communities are simulated: (1) carbon pool adjustment, in which the plant controls only the total amount of carbon to be distributed uniformly throughout the root system, (2) symbiont selection, wherein the plant opts either for or against the interaction at each fine root tip, and (3) selective carbon allocation, wherein the plant adjusts the amount of carbon allocated to each root tip based on the cost of nutrients. Strategies were tested over various nutrient availabilities (the amount of inorganically and organically bound nutrients). Success was defined on the basis of minimizing carbon expended for nutrient acquisition because this would allow more carbon to be utilized for growth and reproduction. In all cases, the symbiont selection and selective carbon allocation strategies were able to meet the nutritional requirements of the plant, but did not necessarily optimize carbon use. The carbon pool adjustment strategy is the only strategy that does not operate at the individual root tip scale, and the strategy was not successful when inorganic nutrients were scarce since there is no mechanism to exclude suboptimal symbionts. The combination of the symbiont selection strategy and the carbon pool adjustment resulted in optimal carbon use and nutrient acquisition under all environmental conditions but result in monospecific symbiont assemblages. On the other hand, the selective carbon allocation strategy is the only strategy that maintained successful, multi-symbiont communities. The simulations presented here thus imply clear hypotheses about the effect of nutrient availability on symbiont selection and mycorrhizal community richness and composition.     

Seasonal variations of organic-carbon and nutrient transport through a tropical estuary (Tsengwen) in southwestern Taiwan

This paper reports the fluvial fluxes and estuarine transport of organic carbon and nutrients from a tropical river (Tsengwen River), southwestern Taiwan. Riverine fluxes of organic carbon and nutrients were highly variable temporally, due primarily to temporal variations in river discharge and suspended load. The sediment yield of the drainage basin during the study period (1995–1996, 616 tonne km^–2 year^–1) was ca. 15 times lower than that of the long-term (1960–1998) average (9379 tonne km² year^–1), resulting mainly from the damming effect and historically low record of river water discharge (5.02 m³ s^–1) in 1995. The flushing time of river water in the estuary varied from 5 months in the dry season to >4.5 days in the wet season and about 1 day in the flood period. Consequently, distributions of nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC) were of highly seasonal variability in the estuary. Nutrients and POC behaved nonconservatively but DOC behaved conservatively in the estuary. DOC fluxes were generally greater than POC fluxes with the exception that POC fluxes considerably exceeded DOC fluxes during the flood period. Degradation of DOC and POC within the span of flushing time was insignificant and may contribute little amount of CO₂ to the estuary during the wet season and flood period. Net estuarine fluxes of nutrients were determined by riverine fluxes and estuarine removals (or additions) of nutrients. The magnitude of estuarine removal or addition for a nutrient was also seasonally variable, and these processes must be considered for net flux estimates from the river to the sea. As a result, nonconservative fluxes of dissolved inorganic phosphorus (DIP) from the estuary are –0.002, –0.09 and –0.59 mmol m^–2 day^–1, respectively, for dry season, wet season and flood period, indicating internal sinks of DIP during all seasons. Due to high turbidity and short flushing time of estuarine water, DIP in the flood period may be derived largely from geochemical processes rather than biological removal, and this DIP should not be included in an annual estimate of carbon budget. The internal sink of phosphorus corresponds to a net organic carbon production (photosynthesis–respiration, p–r) during dry (0.21 mmol m^–2 day^–1) and wet (9.5 mmol m^–2 day^–1) seasons. The magnitude of net production (p–r) is 1.5 mol m^–2 year^–1, indicating that the estuary is autotrophic in 1995. However, there is a net nitrogen loss (nitrogen fixation–denitrification < 0) in 1995, but the magnitude is small (–0.17 mol m^–2 year^–1).     

Nutritional and related experiments on laboratory maintenance of three species of symbiont-bearing,large foraminifera

Factors were examined that affect survival and growth of two common species of large foraminifera from the Red Sea,Amphisorus hemprichii Ehrenberg andAmphistegina lobifera Larsen, 1976. The former is host for dinoflagellate and the latter for diatom zooxanthellae. Experimental conditions were modeled on conditions at 25 m during spring at Wadi Taba, Gulf of Elat, Israel, the season and site where the experimental organisms were collected between 1983 and 1988. The two species responded quite differently in nutritional experiments.A. hemprichii grew, on average, 0.270 mm in diameter in 3 mo on a diet ofNitzschia subcommunis Hustedt,Chlorella sp. (clone AT) orCylindrotheca closterium Rabenhorst isolated from their native habitat. Unfed controls did not grow. In contrast, unfed populations ofA. lobifera grew as well or better than those that were fed unialgal diets. Growth of both species was enhanced on particular mixed algal diets. Both species required photosynthetically active symbionts. Even when fed weekly and supplied with nutrients, neither species survived in the dark. All individuals ofA. hemprichii died after 8 wk incubation in the dark;A. lobifera survived longer, but all were dead by 13 wk. The highest growth rate ofA. hemprichii (0.037 mm wk^–1) was obtained when they were fed, the medium was enriched, and the medium was changed weekly. All other conditions being the same, growth rate dropped to 0.009 mm wk^–1 when the medium was changed every 3 wk. In contrast,A. lobifera grew fastest when the medium was changed every 3 wk. Food or enrichment with nitrate or phosphate did not stimulate growth (0.03 mm wk^–1) over that of the controls. Specimens ofMarginopora kudakajimensis Gudmundsson from Japan, another dinoflagellate-bearing species, were also tested. They grew best (0.02 mm wk^–1) when cultured in light, in media enriched with nitrate and phosphate changed weekly, and fed. All three species withdrew nitrate and phosphate from the medium in chemostat experiments.     

The effects of feeding or addition of dissolved inorganic nutrients in maintaining the symbiosis between dinoflagellates and a tropical marine cnidarian

The availability of solid food (Artemia nauplii) and dissolved inorganic nutrients (ammonium, nitrate, phosphate) to the shallow-water marine hydroid Myrionema amboinense was manipulated for 1-8 days in order to investigate their role in the growth of intracellular symbiotic dinoflagellates (zooxanthellae) of the genus Symbiodinium. Symbionts from hydroids collected from the field or maintained under laboratory conditions (25°C, 12 h:12 h light:dark cycle, 80 µE m^-2 s^-1 fluorescent lighting) always exhibited a single peak in mitotic index (MI) at dawn. Symbionts in freshly collected field animals had an MI peak of about 15%. Symbiotic dinoflagellates in hydroids fed Artemia nauplii twice daily in the laboratory maintained this dawn peak of MI between 10% and 15%, but in the absence of feeding or added inorganic nutrients, this peak declined to less than 1% within 2-4 days. In contrast, when hydroids were placed in solutions containing ammonium (20 µM NH₄Cl), nitrate (10 µM NaNO₃), and a combination of ammonium and phosphate (2 µM Na₂HPO₄) immediately after collection, the algal MI remained between 5% and 15% for 4-7 days; the addition of 2 µM phosphate did not increase MI relative to unfed rates. When unfed animals were placed in dissolved nitrogen or fed Artemia, the symbiont MI increased from <1% to 10-17% within 2-3 days; P alone had no effect. However, the increase resulting from added inorganic nutrients was temporary, lasting only 5-7 days. These observations suggest that algal division in the host is maintained indefinitely in the field or by feeding particulate foods twice daily in the laboratory, but the addition of inorganic nutrients alone (ammonium, nitrate and ammonium/phosphate) appeared to support the completion of a maximum of one additional round of cell division. Nutrients required for continued growth and division of symbiotic dinoflagellates are linked to host feeding and host growth; without external food, neither host nor symbiont continue to grow. The same phenomenon is seen in zooxanthellate anemones, clams and corals, where total numbers of symbionts appear to be linked to changes in host-tissue biomass (protein), achieving relatively stable densities in M. amboinense, corals and other cnidarian symbioses, depending on their local environmental conditions. The results of the present study help explain the cellular responses of algal symbionts in reef-dwelling invertebrates to additions of dissolved inorganic nutrients to coral-reef ecosystems.     

博罗县下村农场土壤养分概况

本文探讨了博罗县下村农场土壤养分含量概况，为今后长期试验研究提供基本理论依据。分析结果表明：下村场土镶有机质含量低，土壤酸性强，不利于土壤肥力的发挥，土壤全量养分Ｎ、Ｐ、Ｋ含量不平衡，速效养分虽然有些较高以至很丰富，但其含量范围大，有相当部分土壤仍很低，所以其供应水平亦很不平衡。而坡地赤红壤则全氮低，全磷不高，全钾则高低差异很大．至于果园土壤（即耕地），养分含量亦很不一致，相差极为悬殊，没有一定的规律性．土壤养分的高低水平，对作物的养分供应起很大的作用。由于下村场土壤养分的不均衡性，在生产管理上，应因地制宜，协调养分含量，以提高施肥效果。根据下村场土壤养分含量的特点，应增施有机肥，改良土壤，并适施石灰以改善土壤酸性，以利于作物的正常生长．     

The size and function of the internal inorganic carbon pool of the foraminifer Amphistegina lobifera

The existence of an internal inorganic carbon pool in the perforate foraminifer Amphistegina lobifera, as suggested recently (ter Kuile and Erez 1987), has been established by direct measurements using a new ¹⁴C tracer method. The imperforate species Amphisorus hemprichii does not contain such a pool. The size of the pool in A. lobifera is proportional to its calcification rate and approximately equals the amount of carbon incorporated into the skeleton during 24 h. Time course experiments show that inorganic carbon (Ci) is photoassimilated at constant rates by the algal symbionts, that the pool is filled to maximum capacity in ca. 24 h, and that Ci incorporation into the skeleton starts only after the pool is filled up. During the chase phase of pulsechase experiments, all ¹⁴C initially residing in the pool is transferred to the skeleton, indicating that the pool serves for calcification and not for photosynthesis. Uptake of Ci into the pool occurs only in the light, indicating that energy may be required for this process. Furthermore, calculations of the Ci concentration inside the pool suggest that it is higher by 2 to 3 orders of magnitude compared to seawater concentration, suggesting that its accumulation is an energy dependent process.     

Sexual reproduction of the photosymbiotic ascidian <Emphasis Type="Italic">Diplosoma virens</Emphasis> in the Ryukyu Archipelago,Japan: vertical transmission,seasonal change,and possible impact of parasitic copepods

Diplosoma virens is a colonial ascidian hosting prokaryotic algae Prochloron sp. in the common cloacal cavity of the colonies and is sometimes parasitized by notodelphyid copepods. In ascidian–Prochloron symbiosis, it is generally known that the host larvae acquire the algal symbionts from their mother colonies to maintain the symbiosis. A histological study of the sexually mature colonies of D. virens showed that the algal symbionts attach to pre-hatching larvae on the rastrum (plant rake) projected from the postero-dorsal part of the larval trunk, and then the rastrum is packed in the posterior half of the larval trunk that will become a cloacal cavity after metamorphosis. This process is the same as that of D. simile. Monthly sampling of D. virens colonies showed that they have embryos in summer in Ryukyus, situated near the northern-most limit of the coral reefs in the West Pacific. While the frequencies of copepod parasitism were variable among the populations, the colonies from a highly parasitized population had a significantly smaller number of eggs/embryos per zooid than the colonies from the less parasitized populations. The parasites probably have an inhibitory impact on the sexual reproduction of the host colonies.Communicated by T. Ikeda, Hakodate     

Sensitivity to biologically relevant odours may exceed the sum of component thresholds

Summary In four fruit-eating bats sniff-rate analysis revealed the threshold value for natural banana odour to be 1 × 10^–4 vol% of head-space while the threshold values for its major components were found to be 3–6 orders of magnitude higher; even the total of molecules present at the threshold dilution of banana odour was 2 orders of magnitude below the lowest threshold values obtained for single components. Thus, the responsiveness of the bats to threshold concentration of natural banana odour is discussed as an example of synergism of stimuli.     

Release of ultraviolet-absorbing compounds by the red-tide dinoflagellateLingulodinium polyedra

We tested the hypothesis that ultraviolet-absorbing compounds known as mycosporine-like amino acids (MAAs) are not only synthesized but also excreted by marine phytoplankton. An experiment was performed with cultures of the marine dinoflagellateLingulodinium polyedra (previously known asGonyaulax polyedra) exposed to visible (photosynthetically available, PAR, 400 to 700 nm) and ultraviolet (UV, 290 to 400 nm) radiation. Absorption properties of both particulate and dissolved organic matter pools (POM and DOM, respectively) showed maxima in ultraviolet absorption at 360 nm. Chromatographic analysis confirmed the presence of MAAs in both pools. Release of organic matter byL. polyedra, as measured spectrophotometrically by changes in UV absorption in the surrounding medium, showed a differential increase at 360 nm in cultures exposed to UV-B + PAR radiation. The changes in absorption in the DOM fraction were inversely proportional to intracellular UV absorption. Photodegradation experiments in which the DOM fraction was exposed to visible and UV-B radiation showed a decrease in absorption with dose. First-order photooxidation decay rates varied between – 0.005 and – 0.26 m² (mol quanta)^–1 and were also a function of the initial optical density (OD). These results indicate that UV-absorbing compounds synthesized by phytoplankton, such as certain dinoflagellates, may be a component of the DOM pool in surface waters of the ocean and contribute to the attenuation of UV radiation in the water column. Photooxidation consumes only 3 to 10% of the daily production of the DOM absorbing between 280 and 390 nm (including MAAs). This suggests that MAAs dissolved in seawater may contribute to the decrease of UV transmission through the water column on a time scale representative of phytoplankton growth (days) and bloom development (weeks).     

Detritus-Bacteria-Meiofauna interactions in a seagrass bed (Posidonia oceanica) of the NW Mediterranean

The biochemical composition of the sediment organic matter, and bacterial and meiofaunal dynamics, were monitored over an annual cycle in aPosidonia oceanica bed of the NW Mediterranean to test the response of the meiofauna assemblage to fluctuations in food availability. Primary production cycles of the seagrass and its epiphytes were responsible for relatively high (compared to other Mediterranean systems) standing stocks of organic carbon in sediments (from 1.98 to 6.16 mg Cg^–1 sediment dry weight). The biopolymeric fraction of the organic matter (measured as lipids, carbohydrates, and proteins) accounted for only a small fraction (18%) of the total sedimentary organic carbon. About 25% of the biopolymeric fraction was of microphytobenthic origin. Sedimentary organic carbon was mostly refractory (56 to 84%) and probably largely not utilizable for benthic consumers. The biopolymeric fraction of the organic matter was characterized by high carbohydrate concentrations (from 0.27 to 5.31 mg g^–1 sediment dry weight in the top 2 cm) and a very low protein content (from 0.07 to 0.80 mg g^–1 sediment dry weight), which may be a limiting factor for heterotrophic metabolism in seagrass sediments. RNA and DNA concentrations of the Sediments varied significantly during the year. High RNA and DNA values occurred during the microphytobenthic bloom and in correspondence with peaks of bacterial abundance. Bacteria accounted for a small fraction of the total organic carbon (0.65%) and of the biopolymeric organic carbon (4.64%), whilst microphytobenthos accounted for 3.79% of total organic carbon and for 25.08% of the biopolymeric carbon. Bacterial abundance (from 0.8 to 5.8 × 10⁸ g^–1 sediment dry weight) responded significantly to seasonal changes of organic matter content and composition and was significantly correlated with carbohydrate concentrations. Bacteria might be, in the seagrass system, an important N storage for higher trophic levels as il accounted for 25% of the easily soluble protein. pool and contributed significantly to the total DNA pool (on average 12%). Total meiofaunal density ranged from 236 to 1858 ind. 10 cm^–2 and was significantly related, with a time lag, to changes in bacterial standing stocks indicating that microbes might represent an important resource. Bacterial abundance and biomass were also significantly related to nematode abundance. These results indicate that bacteria may play a key role in the benthic trophic     

Influence of human-induced disturbance on benthic microbial metabolism in the Pichavaram mangroves,Vellar–Coleroon estuarine complex,India

Large areas of mangroves in India are heavily disturbed by cattle grazing, hypersalinity, and other human-induced impacts. In two disturbed Avicennia marina forests and two undisturbed A. marina and Rhizophora apiculata forests in the Pichavaram mangroves of the Vellar–Coleroon estuarine complex, southeast India, we measured the rates and pathways of microbial decomposition of soil organic matter to determine if human impact is altering biogeochemical activity within these stands. Rates of total carbon oxidation (T_COX) were higher in the undisturbed A. marina forest (mean 199 mol C m^–2 year^–1) than in the two impacted stands (43 and 79 mol C m^–2 year^–1); rates of total carbon oxidation in the R. apiculata forest averaged 75 mol C m^–2 year^–1. Sulphate reduction (range 21–319 mmol S m^–2 day^–1) was the major decomposition pathway (65–85% of T_COX), except at the most disturbed forest (30% of T_COX). Rates of sulphate reduction at all sites peaked in sub-surface soils to a depth of about 1 m, leading to little carbon burial (3–5% of total C input). There was some evidence of measurable iron and manganese reduction in association with tree roots. Rates of microbial activity were rapid in comparison with rates measured in other mangrove soils, reflecting high rates of phytoplankton production and organic matter retention in this lagoon. Human-induced disturbance creates a sharp zonation of dry, hypersaline soil overlying less saline, wetter soil, suppressing surface microbial and root growth. We conclude that this vertical alteration of soil characteristics and biogeochemistry shifts the cycling of nutrients between trees and microbes to a disequilibrium state, partly explaining why mangroves are stunted in these declining forests.Communicated by G. F. Humphrey, Sydney     

Using structural equation modeling to investigate relationships among ecological variables

Structural equation modeling is an advanced multivariate statistical process with which a researcher can construct theoretical concepts, test their measurement reliability, hypothesize and test a theory about their relationships, take into account measurement errors, and consider both direct and indirect effects of variables on one another. Latent variables are theoretical concepts that unite phenomena under a single term, e.g., ecosystem health, environmental condition, and pollution (Bollen, 1989). Latent variables are not measured directly but can be expressed in terms of one or more directly measurable variables called indicators. For some researchers, defining, constructing, and examining the validity of latent variables may be the end task of itself. For others, testing hypothesized relationships of latent variables may be of interest. We analyzed the correlation matrix of eleven environmental variables from the U.S. Environmental Protection Agency's (USEPA) Environmental Monitoring and Assessment Program for Estuaries (EMAP-E) using methods of structural equation modeling. We hypothesized and tested a conceptual model to characterize the interdependencies between four latent variables-sediment contamination, natural variability, biodiversity, and growth potential. In particular, we were interested in measuring the direct, indirect, and total effects of sediment contamination and natural variability on biodiversity and growth potential. The model fit the data well and accounted for 81% of the variability in biodiversity and 69% of the variability in growth potential. It revealed a positive total effect of natural variability on growth potential that otherwise would have been judged negative had we not considered indirect effects. That is, natural variability had a negative direct effect on growth potential of magnitude –0.3251 and a positive indirect effect mediated through biodiversity of magnitude 0.4509, yielding a net positive total effect of 0.1258. Natural variability had a positive direct effect on biodiversity of magnitude 0.5347 and a negative indirect effect mediated through growth potential of magnitude –0.1105 yielding a positive total effects of magnitude 0.4242. Sediment contamination had a negative direct effect on biodiversity of magnitude –0.1956 and a negative indirect effect on growth potential via biodiversity of magnitude –0.067. Biodiversity had a positive effect on growth potential of magnitude 0.8432, and growth potential had a positive effect on biodiversity of magnitude 0.3398. The correlation between biodiversity and growth potential was estimated at 0.7658 and that between sediment contamination and natural variability at –0.3769.     

An autoradiographic examination of carbon fixation, transfer and utilization in the Riftia pachyptila symbiosis

Riftia pachyptila, the giant vestimentiferan tubeworm from the East Pacific Rise, harbors abundant chemolithoautotrophic, sulfide-oxidizing bacteria in an internal organ, the trophosome. Several facts, such as the lack of a digestive system in the host, stable carbon isotope values and net carbon dioxide uptake all suggest that the tubeworms obtain the bulk of their nutrition from their symbionts. Using tissue autoradiography, we investigated the mode of nutritional transfer between symbionts and host, and the site of early incorporation of symbiont fixed-carbon in the host. Fast labeling in the trophosome clearly demonstrates that the symbionts are the primary site of carbon fixation. Appearance of label in some symbiont-free host tissues in as little as 15 min indicates that the symbionts release a significant amount of organic carbon immediately after fixation. The organic carbon is largely incorporated into specific, metabolically active host tissues such as fast-growing body regions in the trunk and plume, and into tube-secreting glands. In addition to immediate release of fixed carbon by the symbionts, there is evidence of a second possible nutritional mode, digestion of the symbionts, which is consistent with previous suggestions based on trophosome ultrastructure. Results suggest that symbiont-containing host cells migrate in a predictable pattern within trophosome lobules and that symbiont division occurs predominately in the center of a lobule, followed eventually by autolysis/digestion at the periphery of the lobule. Received: 1 July 1999 / Accepted: 30 December 1999     

Dynamic environ analysis of compartmental systems: A computational approach

Ecosystems are often modeled as stocks of matter or energy connected by flows. Network environ analysis (NEA) is a set of mathematical methods for using powers of matrices to trace energy and material flows through such models. NEA has revealed several interesting properties of flow–storage networks, including dominance of indirect effects and the tendency for networks to create mutually positive interactions between species. However, the applicability of NEA is greatly limited by the fact that it can only be applied to models at constant steady states. In this paper, we present a new, computationally oriented approach to environ analysis called dynamic environ approximation (DEA). As a test of DEA, we use it to compute compartment throughflow in two implementations of a model of energy flow through an oyster reef ecosystem. We use a newly derived equation to compute model throughflow and compare its output to that of DEA. We find that DEA approximates the exact results given by this equation quite closely – in this particular case, with a mean Euclidean error ranging between 0.0008 and 0.21 – which gives a sense of how closely it reproduces other NEA-related quantities that cannot be exactly computed and discuss how to reduce this error. An application to calculating indirect flows in ecosystems is also discussed and dominance of indirect effects in a nonlinear model is demonstrated.     

Variability of particulate organic carbon and nitrogen in the Namibian upwelling system

Particulate organic carbon (POC) and nitrogen (PON) were analyzed during cruises undertaken in September 1985 and April 1986 along the Namibian upwelling system. The main objectives were to provide estimates of standing stocks of particulate organic matter (POM) and analyze its temporal and spatial variability. Average estimates of total carbon standing stock (0 to 100 m depth) accounted for 1.2 g-at C m^–2 during active and 1.32 g-at C m^–2 during abated upwelling. Zooplankton carbon was estimated as 0.22 and 0.27 g-at C m^–2, respectively, for both periods. POM was generally concentrated near the surface, especially during abated upwelling. During abated upwelling, POM was not only abundant inshore but also over the shelf, as a response of a more diffuse frontal system and a more strongly stratified water column. Cross-shelf gradients were more significant during active upwelling, while alongshore gradients accounted for most of the variance of particulate organic matter during abated periods. This result was a consequence of the seasonal intrusion of warm, Angolan water from the north during the period of minimum upwelling, and resulted in poorer POM concentrations and higher consumer: producer ratios (24.4%). Nevertheless, this last conclusion should be regarded with caution due to the lack of comparative interannual variability. A 48 h study at a fixed station permitted analysis of the daily variability in POM during the intrusion process. Changes in the thickness of the surface mixed layer due to irregular time-spaced pulses of non-homogeneous water masses resulted in sudden enrichments and renewals of phytoplankton and zooplankton populations in a matter of hours.     

Temporal variation in community interfaces: kelp-bed boundary dynamics adjacent to persistent urchin barrens

We examined, over 2 years, factors affecting the temporal stability of the lower limit of kelp beds (Alaria esculenta) at five subtidal sites in the Mingan Islands, northern Gulf of St. Lawrence. The position of the lower limit of the beds varied markedly among sites and over time and was largely controlled by the green sea urchin, Strongylocentrotus droebachiensis, which formed dense (up to 500 individuals m^–2) feeding fronts at the lower edge of the beds. These aggregations advanced over the kelp most rapidly during the summer (at rates as high as 2.5 m month^–1), and there appeared to be a threshold urchin biomass of ~5 kg m^–2 below which the fronts could not substantially reduce the limit of the beds. The fronts consisted mainly of large individuals, whereas smaller urchins predominated in the barrens zone below the kelp beds. At one site, we recorded large seasonal shifts in overall urchin densities, with large increases and decreases during the summer and winter, respectively. An urchin exclusion experiment indicated that algal recruitment in the barrens was two orders of magnitude greater in the absence than in the presence of urchins. The kelp Agarum cribrosum greatly restricted urchin movements, and the greater temporal stability of the kelp bed at one site appeared related to the gradual replacement of Alaria esculenta in the lower kelp bed by a large stand of Agarum cribrosum. We propose that perturbations by abiotic factors (e.g., ice scouring and water motion) trigger important but localized changes in urchin densities that, in turn, largely determine the limits of kelp bed distribution in this region of the Atlantic where urchin barrens are a persistent community state.Communicated by R.J. Thompson, St. Johns     

怀来盆地弃耕地自然恢复过程中土壤养分动态

研究了怀来盆地丘陵区弃耕地自然恢复过程中土壤养分的变化规律。采取空间代替时间的方法,确定了代表弃耕地初期阶段、草地阶段和灌丛地阶段的3种样地,通过分层取样测定了土壤有机质、全量养分和速效养分含量。结果表明,全量养分对生态系统恢复的响应与其相应的速效养分明显不同。随着弃耕地的恢复,土壤有机质有明显增加的趋势,P循环和K循环由物质控制向生物控制转换,但速效P在恢复初期增加缓慢。速效N由于耕种期施肥的影响表现出先降低而后增加的特点。全量养分中,只有全N在弃耕地自然恢复中有增加的趋势。灌丛可以在更深的层次上影响土壤养分。植被的恢复与土壤肥力的提高密切相关,这种作用随着植被的恢复而不断增强。     

Temporal and spatial infection dynamics indicate recognition events in the early hours of a dinoflagellate/coral symbiosis

The obligate symbiotic relationship between dinoflagellates, Symbiodinium spp. and reef building corals is re-established each host generation. The solitary coral Fungia scutaria Lamarck 1801 harbors a single algal strain, Symbiodinium ITS2 type C1f (homologous strain) during adulthood. Previous studies have shown that distinct algal ITS2 types in clade C correlate with F. scutaria—Symbiodinium specificity during the onset of symbiosis in the larval stage. The present study examined the early specificity events in the onset of symbiosis between F. scutaria larvae and Symbiodinium spp., by looking at the temporal and spatial infection dynamics of larvae challenged with different symbiont types. The results show that specificity at the onset of symbiosis was mediated by recognition events during the initial symbiont—host physical contact before phagocytosis, and by subsequent cellular events after the symbionts were incorporated into host cells. Moreover, homologous and heterologous Symbiodinium sp. strains did not exhibit the same pattern of localization within larvae. When larvae were infected with homologous symbionts (C1f), ~70% of the total acquired algae were found in the equatorial area of the larvae, between the oral and aboral ends, 21 h after inoculation. In contrast, no spatial difference in algal localization was observed in larvae infected with heterologous symbionts. This result provides evidence of functional differences among gastrodermal cells, during development of the larvae. The cells in the larval equator function as nutritive phagocytes, and also appear to function as a region of enhanced symbiont acquisition in F. scutaria.     

Energy budgest,growth and filtration rates in Mytilus edulis at different algal concentrations

Growth of Mytilus edulis L. was measured in aquaria with through-flowing sea water at different levels of constant algal concentrations. The amount of food and oxygen consumed by the mussels were measured over given periods as well as the changes in dry organic weight during the same periods. From these parameters it was possible to make simple energy budgets and to compare the estimated growth with actual growth, and, further, to determine growth efficiences at different food levels. Energy budgets were made for mussels grown at algal concentrations of 0, 1.6×10³, 3.0×10³ and 26.0×10³ Phaeodactylum tricornutum cells x ml^-1. The estimated growth was found to be close to actual growth at algal concentrations above maintenance level and the net growth efficiency was found to be between 18% (3.0×10³ cells x ml^-1) and 61% (26×10³ cells x ml^-1). It has been shown that the filtration rate is independent of algal concentrations between about 1.5×10³ to 30×10³ P. tricornutum cells x ml^-1. Outside this range a decrease in filtration rate was noticed.