Predicting Epipelic Diatom Exopolymer Concentrations in Intertidal Sediments from Sediment Chlorophyll a

Abstract In many intertidal cohesive—sediment habitats, epipelic diatoms are the dominant microphytobenthic organisms. In such sediments, concentrations of colloidal carbohydrate [including the exopolymeric substances (EPS) produced by diatoms during motility] are closely correlated with the biomass (chlorophyll a) of epipelic diatoms. A model describing this relationship (log (conc. coll. carbo. + 1) = 1.40 + 1.02(log (chl. a conc. + 1)) was derived from published data. It was validated against published and unpublished data from 6 different estuaries, and accounted for 64.6% of the variation in sediment colloidal carbohydrate concentrations. The model was valid for intertidal habitats with cohesive sediments where epipelic diatoms constituted >50% of the microphytobenthic assemblage. In sites with noncohesive sediments, or where the microphytobenthic assemblage was dominated by other algal groups, the model was not applicable. The mean percentage of EPS in colloidal carbohydrate extracts varied between 11 and 37% for axenic cultures of epipelic diatoms (with higher values obtained during stationary phase), and between 22.7% and 24.3% for natural sediments dominated by epipelic diatoms. Assuming an EPS percentage of 25% in colloidal extracts yielded an EPS chl. a ratio of 2.62:1. Maximum rates of EPS production in diatom cultures occurred at the beginning of stationary phase (1.6–5.09 μg EPS μg⁻¹ chl a d⁻¹), with Nitzschia sigma having a significantly (P < 0.05) higher rate of production than N. frustulum, Navicula perminuta and Surirella ovata. Similar rates of EPS production were measured in the field. The dynamics of EPS production and loss on mudflats is discussed, with reference to the model and these production rates. Received: 25 February 1997; Accepted: 23 May 1997     

Chlorophyllase in citrus leaves. Kinetic aspects of the reaction

The reaction: Chlorophyll + Enzyme → Chlorophyllide + Phytol follows a first order kinetics with regard to the quantity of enzyme, when it is saturated by substrate. K_m and V_m were determined from the average reaction rates for the substrates: Chlorophylla andb, pheophytina andb, methylchlorophyllidea and methylpheophorbide a: The lowest K_m corresponded to chlorophylla and the highest V_m to methylpheophorbidea. For a substrate of chlorophyll (a + b), K_m and V_m were determined also using the initial reaction rates. “Enzyme efficiency” was calculated using both methods of determination. The reaction products partially inhibit the hydrolytic process.     

Predicting the summer chlorophylla concentration in a reservoir based on the environmental conditions of the preceding spring

From a monthly survey over a ten-year period (1983–1992) of the Ishitegawa Reservoir, Japan, two multiple regression equations describing the mean chlorophylla (Chl-a) concentration at 0.5 m depth during July, August and September (C _S ) and the maximum Chl-a concentration in the photic zone, including its subsurface maximum during this period (C _M ), were obtained. The conductivity at 0.5 m depth in May and the mean air temperature during May or during May and June were used as independent variables. These equations were calculated using seven years of data (1983–1988 and 1992). From 1989 to 1991 two impermeable plastic-coated sheet fences were installed across the upper end of the reservoir along the top 5 m of water column. The equations were used to determine the effectiveness of this flow regulation on the summer Chl-a concentration. In 1989 and 1991, when the fences were in place from June to August, no effects were found on the observed C _S and C _M values. In 1990, when the fences had been in place since October 1989, the observed values were lower than the predicted values.     

Porphyrin metabolism in lead and mercury treated bajra (Pennisetum typhoideum) seedlings

Lead and mercury inhibited porphyrin biosynthesis significantly in the germinating seeds of bajra (Pennisetum typhoideum). Both 5-aminolevulinic acid dehydratase and porphobilinogen deaminase activities were inhibited by these metals. A comparative study of the inhibition of these two enzymes under invivo andin vitro conditions showed that 5-aminolevulinic acid dehydratase is the major site of action of heavy metals in porphyrin biosynthesis. Further, over-all production of porphyrinsviz., protoporphyrin-IX, Mg-protoporphyrin (ester) and protochlorophyllide was repressed by lead and mercury in both light and dark grown seedlings. Similarly, chlorophylla and chlorophyllb and total chlorophyll contents in dark-grown seedlings were also significantly decreased, suggesting the impairment of chlorophyll biosynthesis by lead and mercury in germinating seedlings.     

Conversion of chlorophyll b to chlorophyll a precedes magnesium dechelation for protection against necrosis in Arabidopsis

Chlorophyll is a deleterious molecule that generates reactive oxygen species and must be converted to non‐toxic molecules during plant senescence. The degradation pathway of chlorophyll a has been determined; however, that of chlorophyll b is poorly understood, and multiple pathways of chlorophyll b degradation have been proposed. In this study, we found that chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the chlorophyll degradation pathway, we first examined the substrate specificity of 7‐hydroxymethyl chlorophyll a reductase. 7‐hydroxymethyl chlorophyll a reductase reduces 7‐hydroxymethyl chlorophyll a but not 7‐hydroxymethyl pheophytin a or 7‐hydroxymethyl pheophorbide a. These results indicate that the first step of chlorophyll b degradation is its conversion to 7‐hydroxymethyl chlorophyll a by chlorophyll b reductase, although chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that chlorophyll b reductase converted all of the chlorophyll b in the light‐harvesting chlorophyll a/b protein complex to 7‐hydroxymethyl chlorophyll a, but did not completely convert chlorophyll b in the core antenna complexes. When plants whose core antennae contained chlorophyll b were incubated in the dark, chlorophyll b was not properly degraded, and the accumulation of 7‐hydroxymethyl pheophorbide a and pheophorbide b resulted in cell death. This result indicates that chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of chlorophyll b.     

A solvent partitioning procedure for the separation of chlorophylls from their degradation products and carotenoid pigments

A simple liquid/liquid partitioning procedure was developed which employed aqueous acetonitrile and hexane, for the isolation of chlorophyll and pheophytin. This procedure separated these pigments from other interfering pigmented compounds. The efficacy of this solvent separation method was evaluated using commercially available chlorophylla, b, pheophytina, b, carotenoids, and algal pigment extracts. The recovery efficiencies of this solvent partitioning process for chlorophyll a and pheophytina have been shown to be 95–98% and 93–96%, respectively, furthermore, the chlorophylla fraction was practically free of any contaminating pigments. It appears that a more accurate assessment of chlorophylla and pheophytina can be accomplished employing liquid/liquid partitioning than with the present standard method.     

NONDESTRUCTIVE TRACING OF MIGRATORY RHYTHMS OF INTERTIDAL BENTHIC MICROALGAE USING IN VIVO CHLOROPHYLL A FLUORESCENCE1,2

In vivo chlorophyll (Chl) a fluorescence was measured in undisturbed intertidal sediments with the purpose of tracing the vertical migratory rhythms of benthic microalgae. A pulse amplitude fluorometer, an instrument which does not require physical contact with the sample, was used, thus allowing successive measurements to be taken on the same sample without causing any type of disturbance to the sediment structure. The basis of the method is the possibility to detect changes in the Chl a concentration near the sediment surface caused by the vertical movement of the microalgae. This requires the verification of two conditions: the possibility to follow changes in the sediment Chl a content from fluorescence intensity, and a sediment photic depth smaller than the vertical distances covered by the moving microalgae. Both conditions were experimentally verified in intertidal muddy sediments of the Tagus estuary, Portugal. In vivo fluorescence was shown to vary linearly with the sediment Chl a content, and the sediment photic depth was estimated to reach 0.27 mm, a value clearly smaller than the reported depths for microalgal migrations. Sediment samples kept under in situ conditions exhibited large hourly Variations (over 400%) in the Chl a fluorescence intensity, which were closely synchronized with the daytime periods of emersion. The rhythmic fluctuations in Chl a fluorescence were confirmed further to represent microalgal migration by (1) its endogenous nature (fluorescence continued to follow diurnal and tidal cycles after removal of environmental stimuli), (2) its dependence on the vertical distribution of the microalgal population within the sediment (vertically homogenized samples failed to display fluorescence variations), and (3) the lack of significant temperature and light effects on the fluorescence emission under in situ conditions (tested in three species representative of the main groups found in the studied microphytobenthic communities—the diatom Phaeodactylum tricornutum (Böhlin), the cyanobacterium Spirulina maxima (Setch. et Gard.), and the euglenophyte Euglena granulata (Klebs) Lemm.). The results obtained indicate that, in spite of the potential concurrent effects of factors other than the Chl a concentration on the fluorescence intensity, in vivo Chl a fluorescence can be used to trace nondestructively the migratory behavior of benthic microalgae.     

Microphytobenthic dynamics in a Wadden Sea intertidal flat – Part II: Seasonal and spatial variability of non-diatom community components in relation to abiotic parameters

The microphytobenthos colonizing the intertidal flats forms an important component of the Wadden Sea. Ten sampling points along a 1-km transect were studied in a fringe area of the Solthörn tidal flat, southern North Sea, in order to determine seasonal differences in the microphytobenthos. An accompanying paper deals with the major component of the flora, the diatoms; here we, focus on the minor taxonomic groups. From May 2008 to May 2009 surface sediments were collected during low tide. Variation of environmental factors as well as microphytobenthic density (abundance and chlorophyll a) were monitored. The area investigated was a mixed-sediment mudflat, with a gradient from coarse to fine. Highest biomass was obtained in summer 2008 with 215.9?±?12.6?mg chlorophyll a m^–2. In late autumn the chlorophyll a concentration decreased continuously at all investigated stations. Lowest values were detected in December 2008. Species abundances varied considerably both along the transect and seasonally, depending on species-specific requirements as well as hydrodynamic conditions (tidal currents). Higher densities of benthic pro- and eukaryotic microalgae were observed in sites characterized by fine sediments. Apart from the diatoms, the most abundant microphytobenthic group was the cyanophytes. Coccoid cyanophytes, mainly Merismopedia sp., were most abundant during summer, with cell numbers up to 5.72?×?10⁶ cells cm^?2, while diatoms dominated in winter, spring and autumn. Filamentous cyanophytes, particularly Microcoleus chthonoplastes, were most abundant during autumn, while coccoid chlorophytes (spring: Chlorococcum submarinum, Crucigenia tetrapedia, Tetraselmis suecica), euglenophytes (summer: Euglena obtusa), dinophytes (autumn: Amphidinium operculatum, A. herdmanii) and cryptophytes (autumn: Hillea marina, Hemiselmis virescens) contributed to the microphytobenthos during warmer seasons. The statistical analysis confirmed that the composition of the microphytobenthos was related to sediment features and to characteristics of particular seasons.     

Extraction of chlorophyll and carotenoid pigments from Antarctic benthic mats for analysis by HPLC

The efficiency of 9:1 acetone-water, DMSO and boiling 9:1 ethanol-water in extracting chlorophyll and carotenoid pigments from benthic cyanobacterial mats from Antarctica for HPLC (high performance liquid chromatography) analysis was examined. Considerable breakdown of chlorophylla was observed after 5 min extraction in boiling ethanol and 2 h extraction in DMSO. Over 50% of the chlorophylla was degraded to chlorophyllidea and there was substantial loss of carotenoids after a 15 h exposure of ground cells to cold 9:1 acetone-water.Mild sonication of ground mat material in 9:1 acetone-water followed by a 4 h extraction at 4 ° C was found to minimise chlorophylla breakdown and dramatically improved the extraction efficiency of chlorophylla, myxoxanthophyll and -carotene.     

Membrane potentials and intracellular Cl− activity of toad skin epithelium in relation to activation and deactivation of the transepithelial Cl− conductance

Summary The potential dependence of unidirectional³⁶Cl fluxes through toad skin revealed activation of a conductive pathway in the physiological region of transepithelial potentials. Activation of the conductance was dependent on the presence of Cl^– or Br^– in the external bathing solution, but was independent of whether the external bath was NaCl-Ringer's, NaCl-Ringer's with amiloride, KCl-Ringer's or choline Cl-Ringer's To partition the routes of the conductive Cl^– ion flow, we measured in the isolated epithelium with double-barrelled microelectrodes apical membrane potentialV _a , and intracellular Cl^– activity,a _Cl ^c , of the principal cells indentified by differential interference contrast microscopy. Under short-circuit conditionsI _sc=27.0±2.0 A/cm², with NaCl-Ringer's bathing both surfaces,V _a was –67.9±3.8mV (mean ±se,n=24, six preparations) anda _Cl ^c was 18.0±0.9mM in skins from animals adapted to distilled water. BothV _a anda _Cl ^a were found to be positively correlated withI _sc (r=0.66 andr=0.70, respectively). In eight epithelia from animals adapted to dry milieu/tap waterV _a anda _Cl ^c were measured with KCl Ringer's on the outside during activation and deactivation of the transepithelial Cl^– conductance (G _Cl) by voltage clamping the transepithelial potential (V) at 40 mV (mucosa positive) and –100 mV. AtV=40 mV; i.e. whenG _Cl was deactivated,V _a was –70.1±5.0 mV (n=15, eight preparations) anda _Cl ^c was 40.0±3.8mm. The fractional apical membrane resistance (fR _a) was 0.69±0.03. Clamping toV=–100 mV led to an instantaneous change ofV _a to 31.3±5.6 mV (cell interior positive with respect to the mucosal bath), whereas neithera _Cl ^c norfR _a changed significantly within a 2 to 5-min period during whichG _Cl increased by 1.19±0.10 mS/cm². WhenV was stepped back to 40 mV,V _a instantaneously shifted to –67.8±3.9 mV whilea _Cl ^c andfR _a remained constant during deactivation ofG _Cl. Similar results were obtained in epithelia impaled from the serosal side. In 12 skins from animals adapted to either tap water or distilled water the density of mitochondria-rich (D _MRC) cells was estimated and correlated with the Cl current (I _Cl though the fully activated (V=–100mV) Cl^– conductance). A highly significant correlation was revealed (r=–0.96) with a slope of –2.6 nA/m.r. (mitochondria-rich cell and an I-axis intercept not significantly different from zero. In summary, the voltage-dependent Cl currents were not reflected infR _a anda _Cl ^a of the principal cells but showed a correlation with the m.r. cell density. We conclude that the pricipal cells do not contribute significantly to the voltage-dependent Cl conductance.     

Adaptations of tropical marine microphytobenthic assemblages along a gradient of light and nutrient availability in Suva Lagoon,Fiji

How are microphytobenthic biofilms adapted to the high incident irradiances and temperatures, low inorganic nutrient concentrations and high desiccation stresses on intertidal flats present in tropical environments? This study investigated biofilms subject to different environmental conditions in a range of tropical sites in Suva lagoon, Fiji. PAM fluorescence was used to measure photophysiological responses to the light climate. Biofilm colloidal carbohydrate, extracellular polymeric substances (EPS) and low molecular weight (MW) carbohydrate concentrations and diel carbohydrate production patterns were measured. Average biomass (Chl a) ranged from 15 to 36?mg?m^?2, and was highest in seagrass bed sediments, but biomass was not correlated with water column or sediment porewater nutrient concentrations. Biofilm photophysiology differed significantly along a combined gradient of light and nutrient availability, with F _v/F _m, relative ETR_max and E _k of biofilms highest in mangrove and intertidal main island sites and lowest in subtidal coral reef flats. Subtidal biofilms showed photoinhibition at irradiances > 1000?µmol?m^?2. Significant correlations between Chl a and colloidal carbohydrate concentrations were present (except on intertidal sandflats), and tropical biofilms had higher ratios of colloidal carbohydrate and EPS to Chl a than temperate estuarine biofilms, probably due to a combination of high irradiance and low nutrient availability leading to the production of excess photoassimilates. The percentage of EPS present in the colloidal fraction was highest in coral sand biofilms (42%), which had the lowest nutrient concentrations, compared with other sites (25–32%). Intertidal biofilms predominantly consisted of large motile taxa and showed strong rhythms of vertical migration. During tidal emersion, high sediment temperatures (41?°C), irradiance (>2300?µmol?m^?2?s^?1) and salinity (49‰) stimulated downward migration. In silty sediments, migration resulted in a reduction in photosynthetic activity during the midday period but, in sands with high light penetration (to a depth of > 1700?µm), high production rates of EPS (18.2?µg carbo. µg Chl a^?1 h^?1) and low MW carbohydrate exudates (40.2?µg carbo. µg Chl a^?1 h^?1) occurred. Vertical migration, high E _k and high rates of photoassimilate dumping are all adaptations to living in the tropical intertidal zone. Seagrass and reef flat biofilms consisted of a diverse non-migratory flora of motile and non-motile taxa that were not subject to such extreme temperature and irradiance conditions. Low values of photosynthetic parameters and high colloidal and EPS content indicated that these biofilms were nutrient-limited.     

Seasonal changes in zooplankton abundance in the lower Rhine during 1987–1991

Zooplankton in the River Rhine was surveyed for five years at the Dutch sampling stations, Lobith (German/Dutch border) and Maassluis (at the point of discharge of the river into the North Sea). The zooplankton abundance showed an apparent seasonal pattern at both stations, characterized by low densities during the winter period, and higher densities during the summer period, with a spring peak. Zooplankton was dominated by rotifers at both stations, although during the winter periods the contribution of copepods was considerable. The rotifers were dominated byBrachionus angularis, B. calyciflorus, Keratella cochlearis andK. quadrata; the copepods by cyclopoid nauplii; the cladocerans by small-sized species mainly belonging toBosmina. At Maassluis the relative contribution of copepods was higher than at Lobith. Furthermore, the zooplankton at Maassluis included the speciesEurytemora affinis, characteristic for estuarine conditions. In spring, the rotifer density and water temperature and rotifer density and chlorophylla concentration were positively correlated. Furthermore, both rotifer density and chlorophylla were inversely correlated with discharge. The possible role of environmental factors (water temperature, chlorophyll content, discharge and biotic factors) controlling the river zooplankton dynamics is indicated.     

A geomorphological model of intertidal cove marshes with application to wetlands management

Detailed topographic and hydrologic surveys were conducted in five intertidal cove marshes in an outer coastal plain landscape to test the hypothesis that the equilibrium geologic state of intertidal habitats residing in similar landscape situations conforms to a consistent geometric form. The equation V=1571.84A^1.70 (R²=96.2%) describes the relationship between hectares of marsh (A) and cubic meter volume at mean high tide (V). An empirical relationship between tide height and volume was found to obey the power series V_p=L^2.38 (R²=99.6%), where V_p is volume as a percent of full pool and L is water height as a percent of mean high tide. A dimensionless index describing the relationship between area and volume is consistent for each marsh and approaches 0.10. A channel form parameter describing width to channel depth ratios is of consistent value for four of the five marshes. These provide evidence of deterministic rather than stochastic geologic development. The benefits of applying natural basin shape patterns in the design and engineering of created/restored intertidal marshes are highlighted and a generic basin is modeled (based on the geometrical section of a paraboloid retained by simple integration) as an example of the potential applicability of the study.     

Chlorophyll-type compounds in soil

Summary The degradation of chlorophyll-type compounds (chlorophyll and its derivatives) in soil were followed by spectrophotometric and chromatographic techniques to find how closely they represent the bulk of plant material in soil. Tissue enzymes rapidly decomposed chlorophyll in chopped plant material mixed with soil, and decomposition was much slower in material in which the enzymes had been inactivated. This slow decomposition is by micro-organisms which seem to be the important cause of chlorophyll degradation in soil.Micro-organisms decomposed both chlorophyllsa andb in two to four months in field soils; chlorophylla was attacked most. Of the chlorophyll-type compounds, pheophytin, the most closely related derivative of chlorophyll, resisted decomposition largest; chlorophyllide and pehophorbide were found rarely in soil and never in large amounts.Microbiological decomposition increased with increasing moisture content of soil, was fastest at 50 to 60 per cent of the water-holding capacity. Decomposition slowed with increasing soil acidity and was very slow at pH below 4.0. Raising the temperature from 5° to 25°C increased the rate of degradation Neither the species nor quantity of plant material had much effect on the rate of decomposition.     

Studies on the light-induced loss of the D1 protein in photosystem-II membrane fragments

PS II membrane fragments produced from higher plant thylakoids by Triton X-100 treatment exhibit strong photoinhibition and concomitant fast degradation of the D1 protein. Involvement of (molecular) oxygen is necessary for degradation of the D1 protein.The herbicides atrazine and diuron, but not ioxynil, partly protect the D1 protein against degradation. Binding of atrazine to the D1 protein is necessary to protect the D1 polypeptide, as shown with PS II membrane fragments from an atrazine-resistant biotype of Chenopodium album which are protected by diuron not by atrazine.Abbreviations atrazine 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine - Chl chlorophyll, diuron - (DCMU) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMBQ 2,5-dimethyl-p-benzoquinone - DCIP 2,6-dichlorophenol indophenol - DPC diphenylcarbazide - ioxynil 4-cyano-2,6-diiodophenol - k_b binding constant - Mes 4-morpholinoethanesulfonic acid - P-680 reaction-center chlorophyll a of photosystem-II - PAGE polyacrylamide gel electrophoresis - PS II photosystem-II - Q_A and Q_B primary and secondary quinone electron acceptors - Z electron donor to the photosystem-II reaction center - SDS sodium dodecylsulfate - Tricine N-2-hydroxy-1,1-bis(hydroxymethyl)ethylglycine     

小叶锦鸡儿幼苗对沙埋的生态适应和生理响应

以中国半干旱地区固定、半固定沙地分布最广泛的灌木种之一小叶锦鸡儿(Caragana microphylla)为对象,于2010～2011年在内蒙古科尔沁沙地测定了不同深度沙埋下其幼苗的存活率、株高、渗透调节物质含量、保护酶活性和膜透性变化,探讨小叶锦鸡儿对沙埋的生理响应特征。结果显示:(1)与对照相比,当沙埋深度为小叶锦鸡儿幼苗株高的25%～75%时,存活率差异不显著;当沙埋深度为50%～75%时其存活率增加,但株高明显下降,沙埋对其生长具有抑制作用;当沙埋达到其株高100%时幼苗全部死亡。(2)不同沙埋深度对小叶锦鸡儿幼苗叶片细胞膜透性无显著影响,对沙埋深度为其株高25%时,叶片含水量增加,MDA含量下降,POD活性增强,细胞膜未受到伤害;沙埋深度为其株高50%～75%时,叶片含水量下降,MDA含量增加,但SOD和POD对细胞膜起到了协同保护作用,细胞膜也未受到损伤;沙埋深度为株高25%～75%情况下,细胞膜未受到损伤,可溶性糖和脯氨酸含量变化不明显。研究认为:小叶锦鸡儿幼苗只能耐受部分沙埋,完全沙埋则会导致其幼苗全部死亡,沙埋胁迫下可溶性糖和脯氨酸等渗透调节物质含量反应迟缓可能是其耐沙埋能力较弱的主要生理原因之一。     

Microphytic standing stocks at Kerguelen Islands (Subantarctic,Indian Ocean), annual variations in relation to environmental factors: II,intertidal and subtidal microphytobenthos

The microphytobenthic standing crop in marine coastal sediments from Kerguelen main island (from surface to 8 cm deep) varied in relation to the tidal position, grain size, and shelter of sediments. A time-series analysis was performed at three distinct sites: a sheltered fjord (PRAY), a moderately exposed embayment (PAF) and a deep fjord with higher open-ocean influence (PN). Temporal variations on all studied parameters were observed on intertidal, as well as on subtidal sediments, but with a great range in variations and pattern. Compared to phytoplankton blooms at the same location and stations, the microphytobenthos productive periods were not so marked, especially on intertidal sediments. Nevertheless, high productivity periods were in most cases linked to austral spring and summer (up to 30–50 g Chl a g^–1 dw, in intertidal sheltered sands; >170 g Chl a g^–1 dw, in sheltered subtidal muds). On subtidal muddy sites, the high phaeopigments concentrations (up to 195 g Phaeo a g^–1 dw at PRAY site) were attributed to kelp and epiphyte degradation and sedimentation, and also to macrofauna trophic influence, whereas at Portes-Noires fjord (PN) a Phaeophytine a late spring increase in 1991 was suspected to be of plankton origin. The PN site exhibited a lower productivity than at the other two sites, regardless of the tidal position of the sediment, due to a delayed annual cycle in surficial temperature and standing crop. Compared to the phytoplankton blooms, the microphytobenthos productivity at Kerguelen main island was less restricted in time and may occur throughout the year, not just to the benefit of the benthos food-chain, but also to the pelagic one via resuspension, as well as to filter feeders (i.e. mussels).     

Germination responses ofRubus palmatus var.coptophyllus andRubus parvifolius seeds with different burial durations to a variable light and temperature regime

To clarify the adaptive significance of seed dormancy, the effects of burial duration were examined for two deciduousRubus species:Rubus palmatus var.coptophyllus andRubus parvifolius, which are found mainly in relatively stable, shaded sites and disturbed sites, respectively. In early summer, newly ripened seeds were buried under litter on the soil surface in a pine forest, and germination tests were carried out for the seeds retrieved from the soil litter after 0 (not buried), 1, 2, 3, 5 and 8 or 9 months of burial. In general, the germination percentages increased and light requirements for germination decreased with increased burial duration. The percentage of seeds germinated with alternating temperatures in darkness also increased with increasing burial duration for both species. After 8 or 9 months of burial (corresponding to the next germination season in the field), the percentage of non-dormant seeds (including germination under alternating temperatures in the dark) was about 80% and 40% forR. palmatus var.coptophyllus andR. parvifolius, respectively. These seed dormancy traits of the twoRubus species may explain the differences in germination strategy in their habitats:R. palmatus var.coptophyllus seems to have adapted to the seasonal occurrence of favorable growing conditions after the dormancy breakage, whileR. parvifolius seems to have adapted to favorable conditions created by temporally unpredictable disturbances.     

Light limitation of phytoplankton development in an oligotrophic lake - Loch Ness, Scotland

• 1 The underwater light climate in Loch Ness is described in terms of mixing depth (Z_m) and depth of the euphoric zone (Z_eu). During periods of complete mixing, Z_m equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient Z_m/Z_eu is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth.
• 2 Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m^-3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch.
• 3 The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light-saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm^-3) and maximum photosynthetic rate (1–4 mg C mg Chla^-1 h^-1) are within the range reported from other lakes.
• 4 Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited.
• 5 Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m^-3) and the probable annual specific areal loading of total phosphorus (0.4–1.7 g TP m^-2 yr^-1) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate.
• 6 These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability.

     

Deepwater sediments and trophic conditions in Florida lakes

Sediment cores were taken from near maximum depth in 15 Florida lakes representing a wide range of trophic conditions. Chemical analyses of surface sediments showed Al, Fe, and Ca to be the most abundant elements in all samples, and the ratio of Al to Ca to be smaller for eutrophic lakes. Sediment organic matter increased with trophic state, as did the degree to which it was enriched in nitrogen. Corresponding sediment C/N ratios decreased with increasing lake trophic state and showed significant negative correlation with chlorophylla, total N, and total P in the water column. Concentrations of sedimentary chlorophyll derivatives showed some relation to trophic state but differences in basin morphometry hinder its use as an inter-lake index of chlorophyll production.