Control of microbial growth and of amphipod grazing by water-soluble compounds from leaves of Zostera marina

The role of minor components of the leaves of Zostera marina L. in altering the activity of micro-organisms directly (and indirectly by affecting amphipod grazers) was investigated in laboratory experiments, using plants collected at Roberts Bank (49°2N; 123°8W) on the west coast of Canada. Water-soluble extracts of eelgrass leaves inhibited the growth of a micro-alga (Platymonas sp.) and many marine bacteria at concentrations equivalent to as little as 10 mg dry leaf l^-1. The potency of leaf extracts was higher (1) in young, actively metabolizing tissue than in older leaves, and (2) in leaves collected during rapid growth in summer 1980 than during the following winter. Water-soluble inhibitors (especially phenolic acids) may explain the low biomass of epiphytes on actively growing leaves. Three phenolic acids inhibited the test micro-organisms at concentrations as low as 0.3 mg l^-1; caffeic acid was more potent than either protocatechuic or gentisic acid. Extracts of young leaves also inhibited grazing by amphipods [Eogammarus confervicolus (Stimpson)] on dead leaves. The time required for leaching of soluble inhibitors may account for the delay between the loss of leaves from the plants and the onset of grazing. Thus, interactions among the biotic components of the detritus ecosystem may be significantly modified by minor compounds in the leaves of Z. marina     

The role of isopods and amphipods in the initial fragmentation of eelgrass detritus in Nova Scotia,Canada

Daytime observations on the isopods Idotea phosphorea and I. baltica and the amphipod Gammarus oceanicus held in laboratory microcosms showed that I. phosphorea and G. oceanicus spent 45% and 30% respectively, of their active time feeding on dead, intact eelgrass leaves which had been recently released from plants. I. baltica spent 41% of its active time consuming intact green leaves. The shredding of intact dead leaves by I. phosphorea and G. oceanicus resulted in production of small detrital particles which were liberated from the faeces of the invertebrates and this type of feeding led to the breakdown of whole leaves. Field experiments which separated the effects of shredding by invertebrates and grinding by waves and ice on the loss of weight from leaf packs showed that relative to controls isopods significantly increased weight loss from dead leaves. Loss of weight from leaf packs exposed to both biotic and physical shredding forces was not significantly different from that found on those exposed only to shredding by isopods. However, trends in the data indicated that fragmentation of whole, dead leaves in the field probably is a result of the synergistic effects of shredding by invertebrates and physical factors, particularly ice grinding. The role played by invertebrates in fragmenting intact, dead leaves is discussed in the light of energy flow and nutrient cycling within seagrass systems.     

Increased macrophyte nitrate reductase activity as a consequence of groundwater input of nitrate through sandy beaches

While most marine macrophytes preferentially assimilate ammonium to meet growth demand for nitrogen, some also utilize nitrate and exhibit high nitrate reductase activity (NRA). Although nitrate concentrations are often low in coastal waters during the summer and sandy beaches are generally considered to be low nutrient-input habitats, we have observed elevated NRA in leaves of some eelgrass (Zostera marina L.) plants growing immediately adjacent to the shoreline. We postulated that nitrate may become available to eelgrass and macroalgae via groundwater inputs that enter the nearshore water column. To address this possibility, we investigated the availability of groundwater nitrate for the induction of NRA in the leaves of eelgrass and in the macroalgaeSargassum filipendula C. Agardh (Phaeophyceae) andEnteromorpha intestinalis L. Link (Chlorophyceae) collected adjacent to two sandy beaches in the vicinity of Woods Hole, Massachusetts, USA. Induction of NRA was determined in the laboratory for eelgrass collected from one of the beach sites and from an offshore site, Lackey's Bay, which is isolated from groundwater input. At the two beach locations, pore water nitrate concentrations were 100 to 400µM within a few meters inland from the waterline. Nitrate efflux into the nearshore water column was quite high and variable (2160±660µmol m^–2 h^–1) when associated with rapid percolation (37±11 1 m^–2 h^–1) of nitrate-enriched pore water. Turbulent wave mixing rapidly diluted the nitrate. Macroalgae and eelgrass growing adjacent to a beach with high nitrate efflux had NR activities three- to sevenfold higher than those of algae and eelgrass growing along a beach section with low nitrate efflux. NRA of eelgrass plants from Lackey's Bay and Great Harbor increased in response to low daily nitrate additions (10 to 25µM) in the laboratory, with higher nitrate additions (50 to 200µM) yielding less dramatic responses. The increase in NRA was roughly three times higher for Great Harbor than for Lackey's Bay eelgrass. It appears that groundwater input of nitrate is sufficient to induce NRA in marine macrophytes growing near some beaches, including those with turbulent wave mixing.     

Uptake of cadmium by the eelgrassZostera marina

The uptake of cadmium by the shoots ofZostera marina L. (eelgrass) was examined in the laboratory. Experiments were carried out in experimental chambers which allowed the separation of the leaves from the root-rhizome portions of intact shoots. Cadmium uptake by the root-rhizome portions over 24 h was directly related to substrate cadmium concentration (1.0, 5.0 and 10.0 g Cd ml^-1) and varied from 6.5 to 30.0 g Cd g^-1. Cadmium uptake by the root-rhizomes and the leaves in a substrate concentration of 1 g Cd ml^-1 was also related to exposure times (24, 48 and 72 h). Maximum uptake by the root-rhizomes and the leaves was observed after 72 h at a substrate concentration of 1 g Cd ml^-1 and was equivalent to 48 and 94 g Cd g^-1, respectively. Translocation of cadmium from the leaves to the root-rhizomes was observed after 24 h, and at the end of 72 h was equivalent to 27% of the total leaf uptake. No cadmium movement from the root-rhizomes to the leaves was detected.     

Reductions in photosynthetic carbon uptake in epiphytic diatoms by water-soluble extracts of leaves of Zostera marina

A water-soluble fraction of green leaves of Zostera marina L. decreased carbon uptake rates in diatoms found as epiphytes at Roberts Bank (Lat 49°2N; Long. 123°8W) on the west coast of Canada. Extract concentrations of 11 to 1 g dry leaf ml^-1, added immediately before ¹⁴C-uptake was measured over a 3-h period, reduced uptake to 36 to 80% of controls, respectively. A lower concentration (0.6 g dry leaf ml^-1) reduced uptake significantly in only one of four cultures. Addition of the whole extract 3 d before carbon uptake was measured or in one-third doses, 0, 1, and 2 d before, resulted in inhibition similar to that observed when the whole extract was added immediately before the ¹⁴C.     

A model for measuring leaf growth rate of vegetative shoot in the seagrass, Zostera marina

A new model for determining leaf growth in vegetative shoots of the seagrass Zostera marina (eelgrass) is described. This model requires the weights of individual mature and immature whole leaves and leaf plastochrone interval (P_L) as parameters, differing from the conventional leaf marking technique (CLM) that requires cutting and separation between new and old tissue of leaves. The techniques required for the model are the same as for the plastochrone method, but the parameters differ between both methods in use of the weight of individual immature leaves. In a mesocosm study, eelgrass growth was examined, and parameters for the new model and plastochrone method (the weights of individual mature and immature leaves and P_L) were measured. Leaf growth rate was measured using the CLM and determined by the new method and the plastochrone method. The results were then compared between the CLM, the new model, and the plastochrone method. The results obtained with the new model were similar to those obtained with the CLM. However, the results of the plastochrone method differed from those of the CLM, while the weight of immature leaves varied seasonally. The new model was also used to determine leaf growth in a natural eelgrass bed in Mikawa Bay, Japan, and revealed the growth rates in all shoots and those of different ages. This method would be advantageous as an accurate means of direct measurement in fieldwork, and should therefore be a useful tool for monitoring seagrass growth.     

Chemical resistance of gorgonian corals against fungal infections

The frequency and impact of diseases affecting corals throughout the Caribbean have been increasing but little is known about the factors promoting the emergence and outbreak of disease. A disease caused by a fungal pathogen [Aspergillus sydowii (Thom et Church)] which affects Caribbean sea fan corals provided an opportunity to examine the efficacy of coral crude extracts in disease resistance. Minimum inhibitory concentration (MIC) assays showed that of the 20 common gorgonian species in the Florida Keys, extracts from 15 species had MICs < 15 mg ml⁻¹ against A. sydowii pathogenic to sea fans. Extracts from several species in two gorgonian genera (Pseudoplexaura and Pseudopterogorgia) were among the most active, with MICs < 10 mg ml⁻¹. Gorgonia ventalina L., one of two sea fan species known to be hosts to A. sydowii in the field, had an MIC < 10 mg ml⁻¹, suggesting that complete disease resistance requires more active extracts. For the antifungal compounds to be effective in situ, they must also occur in sufficiently high concentrations in living coral tissue. For example, Pseudopterogorgia americana (Gmelin) had comparatively potent extracts but did not have sufficient concentrations in the tissue to be effective. Conversely, Plexaura homomalla Esper extracts were less potent but occurred in high enough concentrations in the tissue to be effective against A. sydowii. When potency and extract concentration are considered together (i.e. potency × concentration), several other gorgonian corals emerge as likely hosts to A. sydowii. Crude extracts from the most active gorgonian species were also effective against two geographic variants of A. sydowii pathogenic to sea fans, a non-pathogenic terrestrial strain of A. sydowii, and three strains of A. flavus Link known to be human, plant, and insect pathogens (MIC range, 7.5 to > 15 mg ml⁻¹). Although the potency in these assays did not attain a clinically significant level, the potency is comparable to a known antifungal agent, hygromycin B, which had an MIC ≤7.5 mg ml⁻¹ in our assays, highlighting the potential of these gorgonian corals for bioprospecting. Received: 29 May 1999 / Accepted: 22 May 2000     

Survival, growth and reproduction of the salt-marsh amphipod Uhlorchestia spartinophila reared on natural diets of senescent and dead Spartina alterniflora leaves

The talitrid amphipod Uhlorchestia spartinophila lives in close association with standing-dead leaves of the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the Atlantic coast of North America. This study probed the strength of the trophic link between the amphipod population and the decomposition process in this detrital-based ecosystem. We measured survival, growth and reproductive output in groups of amphipods reared for 6 wk on five diets derived from sheath and blade portions of S. alterniflora leaves just prior to (senescent) and during (dead) decomposition. In unfed treatments, the daily specific mortality rate was 0.391 and starved amphipods survived no longer than 11 d. Among the fed treatments, a diet of senescent sheaths resulted in the lowest survival (20%) and yielded no offspring. Groups fed senescent blades, dead sheaths, dead blades and unwashed dead sheaths had survival rates of 56 to 84% and produced 5.0 to 12.5 offspring replicate⁻¹. Sex ratio usually favored females, but approached unity in treatments with high overall survival, suggesting that quality of available food resources may influence sex ratio in this species. Mean specific growth rates (mm mm⁻¹ d⁻¹) ranged from 0.013 to 0.016, and matched previous estimates of growth from field populations. Overall ecological performance (survival + growth + reproduction) was similar for all food treatments, except senescent sheaths, which yielded a final mean (±SD) dry biomass (0.4 ± 0.42 mg replicate⁻¹) of amphipods significantly lower than that of other diets (1.7 ± 0.81 to 2.6 ± 0.69 mg replicate⁻¹). Natural diets derived from decomposing cordgrass leaves can fulfill the nutritional requirements of U. spartinophila populations, but variation in initial amounts of living fungal biomass among the five experimental diets only partially explained the responses of amphipods in our experiment. Structural characteristics and variation in rates of fungal occupation within different portions of cordgrass leaves may affect the amphipod's ability to access plant production made available by decomposers. Received: 12 December 1996 / Accepted: 18 December 1996     

Leaf choice by crustaceans in a mangrove forest in Queensland

The feeding behaviour of leaf eating crustaceans feeding on leaves shed by Avicennia marina, Bruguiera gymnorhiza and Rhizophora stylosa in the mangrove forest at Myora Springs, Queensland, Australia was studied between 1980 and 1984. Individual Sesarma erythrodactyla (carapace >9 mm long), one of the most abundant species of crabs in the forest, processed approximately half a leaf from any of the three species of mangroves in 24 h under laboratory conditions. of the amount of leaf material processed, 20% was lost from the mandibles due to sloppy feeding, 68% was egested as faeces and 12% converted into crab biomass. Crabs processed more leaf material at 30°C than at 20°C. S. erythrodactyla preferred leaves of A. marina to those of the other two species, probably because leaves of A. marina have a lower tannin and a higher nitrogen content. Of the other two species, B. gymnorhiza was preferred to R. stylosa. Among leaves of R. stylosa, S. erythrodactyla exerts a strong preference for aged rather than freshly fallen, and for thick rather than thin leaves. The mesofauna (carapace length <9.0 mm) which processed dead mangrove leaves comprised juveniles of S. erythrodactyla, the crab Ilyograpsus paludicola, the isopod Exosphaeroma alata and the amphipods Orchestia sp. and Melita sp. These species processed between 0.2 and 24.7 mg dry wt of a leaf per individual over a period of 4 d. Of this, 72 to 85% was egested as faeces. The significance of leaf eating crabs to nutrient cycling in mangrove forests is discussed. I conclude that leaf processing by crustaceans shortens the time span between leaf fall and consumption of leaf material by organisms. This may have the effect of conserving leaf biomass inside the mangrove forest.     

Evaluation of the copepod Tigriopus californicus as a bioassay organism for the detection of chemical feeding deterrents produced by marine phytoplankton

Marine phytoplankton have been shown to use chemical feeding deterrents to reduce or inhibit zooplankton grazing. In order to screen phytoplankton species for feeding deterrent production and to isolate and identify feeding deterrent compounds, a new, rapid, and reliable laboratory bioassay was developed. This bioassay used the laboratory-reared harpacticoid copepod Tigriopus californicus and measured inhibition of feeding by measuring the fecal pellet production rate. The bioassay was capable of detecting deterrent compounds: (1) adsorbed onto ground fish food (a normally palatable food); (2) dissolved in a mixture of seawater and live Thalassiosira pseudonana cells (a species of diatom which had no feeding deterrent activity); and (3) present in live cell cultures. Method (2) was recommended for use in bioassay-guided fractionation (isolation of chemical compounds), as it was reliable, rapid, accurate, and easy to perform with large numbers of samples. The total bioassay time was < 48 h, and data collection required only a microscope. Methanolic cell extracts of several phytoplankton species were screened for feeding deterrent activity. Extracts from the diatom Phaeodactylum tricornutum and the dinoflagellate Gonyaulax grindleyi gave feeding deterrent responses, while extracts from the diatom Thalassiosira pseudonana gave no feeding deterrent responses. Live P. tricornutum cells deterred feeding at densities of 6x10⁵ cells ml^-1. This bioassay should provide a valuable tool in screening phytoplankton for feeding deterrent compounds and determining the chemical nature of these compounds.     

Oviposition of carrot fly (Psila rosae) in response to foliage and leaf surface extracts of host plants

Summary. Foliage of twelve host and two non-host species and surrogate leaves treated with the respective leaf extracts were presented to laboratory populations of the carrot fly (Psila rosae) in oviposition choice assays. The stimulatory activity of dichloromethane surface extracts and the diethyl ether fraction of hot water extracts did not reflect accurately the differences in acceptability observed among intact leaves. A better correlation was found using hexane extracts prepared in a microwave oven. Two out of five fractions of this crude hexane extract obtained by silica gel column chromatography stimulated oviposition. The diethyl ether fraction, which contained the previously identified oviposition stimulants (propenylbenzenes, furanocoumarins, polyacetylenes), could account for only a minor part of the variation in the acceptability of host leaves. The preference hierarchy for intact leaves corresponded better to the ranking of species according to activity of the methanolic fraction, which apparently contains unknown stimulatory compounds.? The water fractions of the hot water extracts were shown to reduce egg-laying underneath surrogate leaves treated with a stimulatory extract. This oviposition-deterring effect was particularly strong with the non-preferred species Pimpinella major, which is also highly resistant in the field. Hence, unidentified inhibitory compounds may also contribute to differential accept ability of host plants. It is concluded that antixenotic (non-preference) resistance of host plants to carrot fly attack depends on complex mixtures of semiochemicals. Received 11 June 1997; accepted 26 November 1997.     

Bacterial colonization of the phyllosphere of nineteen plant species and antimicrobial activity of their leaf secondary metabolites against leaf associated bacteria

Summary. The scope of this work was to examine whether leaf constitutive secondary metabolites play a role in determining bacterial colonization of the phyllosphere. To this aim, we surveyed nineteen native or cultivated plant species that share a common bacterial pool in a North Mediterranean area, and estimated the size of total and ice nucleation active (INA) bacterial populations on their leaves. Large differences in the colonization of their phyllosphere were found; the population size of epiphytic bacteria ranged from 7.5 × 10² to 1 × 10⁶ CFU/g fresh weight, in eucalypt and celery, respectively. Species native in Mediterranean-type climate areas, particularly those belonging to the group of aromatic plants, are characterized by scarce presence of INA bacteria. The antibacterial activity of essential oils, surface phenolics and leaf tissue extracts was also estimated against the INA strains P. syringae and E. herbicola, isolated from two of these plant species. E. herbicola proved more sensitive than P. syringae. Of the species examined, oregano [Origanum vulgare L. subsp. hirtum (Link.) Ietswaart], an aromatic plant, had the highest antimicrobial activity, whereas six species showed no activity at all. Further experiments were performed with oregano and bean (Phaseolus vulgaris L.) that represent two extremes in their secondary metabolite content. Both plants were inoculated with P. syringae. By the end of incubation, the bacterial population on bean plants was about 100 times higher than that on oregano leaves. Scanning electron micrographs showed that bacterial growth on oregano leaves was confined to sites away from glandular hairs. Results from the bacterial colonization survey together with those from the toxicity tests showed that all species rich in antibacterial secondary metabolites harbored low leaf bacterial populations. These results provide substantial evidence that leaf secondary metabolites function as constitutive defense chemicals against microbial invasions. However, the fact that species with non- or moderately active leaf secondary metabolites are not always highly colonized suggests mediation of other unknown factors, the contribution of which requires further investigation.     

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m⁻²) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Effect of suspended bottom material on growth and energetics in Mytilus edulis

The influence of suspended, natural silt (0 to 20 mg l^-1) in addition to unicellular algal cells (Phaeodactylum tricornutum) (o to 20.000 cells ml^-1) on clearance, growth and energetics in Mytilus edulis has been studied. Clearance increased by 32 to 43% by the addition of 5 mg silt l^-1 as compared to clearance in a pure algal suspension. Ingestion and growth rate increased with algal concentration, and growth rate was further increased by 30 to 70% by the addition of 5 mg silt l^-1. A growth rate comparable to maximum natural growth rates was reached only at the highest algal concentration in the presence of 5 mg siltl^-1. Assimilation efficiency of P. tricornutum decreased from 77% at 5,000 cells ml^-1 to 52% at 20,000 cells ml^-1. In the experiments with silt added, some 20 to 30% of the assimilated organic matter originated from the suspended bottom material. Net growth efficiency increased with growth rate at a decelerating rate, approaching a maximum of about 70%. It is concluded that suspended bottom material, which is always present in M. edulis' natural habitats, serves as an additional food source, and that M. edulis depends on suspended bottom material to exploit fully its clearance potential, and to reach the maximum growth rates observed in nature.     

The potential of Berkheya coddii for phytoextraction of nickel,platinum, and palladium contaminated sites

The study involved assessing the potential of the native plant species (Berkheya coddii) for the phytoextraction of nickel, palladium, and platinum contaminated sites. Plant and soil samples were randomly collected from Barberton area, near Agnes mine, Mpumalanga Province, South Africa. Samples were analysed for total nickel, palladium, and platinum concentrations together with other elements found in the soil and in the plants' roots, and leaves. Soil versus leaves and soil versus roots uptake of these metals by the plant were compared. The mean concentration of nickel in the leaves/canopy was found to be 13,980?±?10,780?mg?kg^?1?dry mass, in the roots it was 2046?±?789?mg?kg^?1 dry mass, and in the soil it was 1040?±?686?mg?kg^?1?dry mass. This resulted in a mean concentration ratio in the leaves to soil of 13.44. The platinum mean concentration in the leaves was 0.22?±?0.15?mg?kg^?1?dry mass, in the roots it was 0.14?±?0.04?mg?kg^?1?dry mass, and in the soil it was 0.04?±?0.03?mg?kg^?1?dry mass. This resulted in a mean concentration ratio in the leaves to soil of 5.5. Palladium was found to have a mean soil concentration of 0.07?±?0.045?mg?kg^?1?dry mass. The mean concentrations in the roots and in the leaves were 0.18?±?0.07 dry mass and 0.71?±?0.52?mg?kg^?1?dry mass, respectively. This gave a mean concentration ratio in the leaves to soil of 10.1 for palladium. Other elements that were found to have a mean concentration ratio in the leaves to soil of around 2.5 or above are sodium, potassium, magnesium, calcium, and sulfur. Berkheya coddii was found to be most efficient in accumulating nickel, palladium, and platinum from the soil. The results for the first time revealed that the plant may have the potential to uptake platinum and palladium; both metals are in the same group of the periodic table as nickel.     

Production dynamics of the eelgrass, <Emphasis Type="Italic">Zostera marina</Emphasis> in two bay systems on the south coast of the Korean peninsula

Production dynamics of eelgrass, Zostera marina was examined in two bay systems (Koje Bay and Kosung Bay) on the south coast of the Korean peninsula, where few seagrass studies have been conducted. Dramatically reduced eelgrass biomass and growth have been observed during summer period on the coast of Korea, and we hypothesized that the summer growth reduction is due to increased water temperature and/or reduced light and nutrient availabilities. Shoot density, biomass, morphological characteristics, leaf productivities, and tissue nutrient content of eelgrass were measured monthly from June 2001 to April 2003. Water column and sediment nutrient concentrations were also measured monthly, and water temperature and underwater irradiance were monitored continuously at seagrass canopy level. Eelgrass shoot density, biomass, and leaf productivities exhibited clear seasonal variations, which were strongly correlated with water temperature. Optimal water temperature for eelgrass growth in the present study sites was about 15–20°C during spring period, and eelgrass growths were inhibited at the water temperature above 20°C during summer. Daily maximum underwater photon flux density in the study sites was usually much higher than the light saturation point of Z. marina previously reported. Densities of each terminal, lateral, and reproductive shoot showed their unique seasonal peak. Seasonal trends of shoot densities suggest that new eelgrass shoots were created through formation of lateral shoots during spring and a part of the vegetative shoots was transformed into flowering shoots from March. Senescent reproductive shoots were detached around June, and contributed to reductions of shoot density and biomass during summer period. Ambient nutrient level appeared to provide an adequate reserve of nutrient for eelgrass growth throughout the experimental period. The relationships between eelgrass growth and water temperature suggested that rapid reductions of eelgrass biomass and growth during summer period on the south coast of the Korean peninsula were caused by high temperature inhibition effects on eelgrass growth during this season.     

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations.     

Insecticidal and growth-reducing activity of foliar extracts from Meliaceae

Summary Thirty-one species in twenty genera of the plant family Meliaceae were assayed for the production of growth-inhibiting phytochemicals, using the generalist herbivorePeridroma saucia. Most species were inhibitory when methanolic extracts were incorporated into artificial diets at concentrations at or below those occurring naturally. In general members of the subfamily Melioideae were more inhibitory than members of the Swietenioideae. Extracts of deciduous species with short leaf lifetimes were significantly more inhibitory than those of evergreen species with longer leaf lifetimes. In a smaller sample of species, evergreen species showed a trend towards having tougher leaves than deciduous species. These results support the resource availability hypothesis of Coleyet al. (1985), and suggest that life history attributes may be of some value in selecting plants for phytochemical prospecting.     

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.