Studies on the absorption spectra of plant pigments in estuaries

Summary Absorption spectra of acetone-soluble plant pigments were obtained from several sources, including the overlying waters, epibenthic layers and sediments of two New England estuaries, the remains of Zostera marina and its epiphytes, and pure cultures of marine diatoms. Comparisons of the absorption spectra and various absorbance ratios revealed no significant differences in the extracts of pigments sampled from the overlying waters and epibenthic layers; the extracts from both layers differed significantly, however, from the extracts of sedimentary pigments. Unacidified extracts of pigments from healthy diatom cultures and detritus rich in epiphytic diatoms resembled those of the overlying waters and epibenthic layers; acidified extracts from the above two sources and unacidified and acidified extract from non-living detritus resembled those of the sediments. Good correlations between field and laboratory studies are indicated, and the uses of absorbance ratios as ecological indicators are discussed.     

Growth form analysis of epiphytic diatom communities of Terra Nova Bay (Ross Sea, Antarctica)

Diatoms have been long collected from the Southern Ocean but almost no data exist for epiphytic communities, despite their high ecological significance as an important food source in Antarctic coastal food chains. Here, we present a first growth form analysis of diatoms associated with rhodophyte hosts from Terra Nova Bay, Ross Sea, Antarctica. We performed this study to gather baseline information on the species composition of epiphytic diatom communities, determine the influence of some environmental variables on the diatom distribution patterns, and assess the caveats that must be taken into account in terms of sampling design. Macroalgal material was collected during the Italian Antarctic expeditions between 1990 and 2004. Epiphytic diatoms were studied by means of scanning electron microscopy. In terms of growth forms, there were no significant differences between the diatom communities on the different macroalgal host species. Motile (mainly small-celled Navicula perminuta and other Navicula spp.) and adnate (Cocconeis spp.) diatoms dominated the community throughout the study period. Many of the macroalgal blades examined were also covered by epiphytic animals (calcareous bryozoans, hydroids) over most of their surface, with a significant effect on the associated diatom community structure. Our findings suggest that the bio-physicochemical characteristics of each sampling site affected the epiphytic diatom communities more than the substrate type provided by the macroalgal host or the sampling depth.     

Growth limitation of three Arctic sea ice algal species: effects of salinity, pH, and inorganic carbon availability

The effect of salinity, pH, and dissolved inorganic carbon (TCO₂) on growth and survival of three Arctic sea ice algal species, two diatoms (Fragilariopsis nana and Fragilariopsis sp.), and one species of chlorophyte (Chlamydomonas sp.) was assessed in controlled laboratory experiments. Our results suggest that the chlorophyte and the two diatoms have different tolerance to fluctuations in salinity and pH. The two species of diatoms exhibited maximum growth rates at a salinity of 33, and growth rates at a salinity of 100 were reduced by 50% compared to at a salinity of 33. Growth ceased at a salinity of 150. The chlorophyte species was more sensitive to high salinities than the two diatom species. Growth rate of the chlorophyte was greatly reduced already at a salinity of 50 and it could not grow at salinities above 100. At salinity 33 and constant TCO₂ concentration, all species exhibited maximal growth rate at pH 8.0 and/or 8.5. The two diatom species stopped growing at pH > 9.5, while the chlorophyte species still was able to grow at a rate which was 1/3 of its maximum growth rate at pH 10. Thus, Chlamydomonas sp. was able to grow at high pH levels in the succession experiment and therefore outcompeted the two diatom species. Complementary experiments indicated that growth was mainly limited by pH, while inorganic carbon limitation only played an important role at very high pH levels and low TCO₂ concentrations.     

南海某些表层沉积硅藻的分布及其古环境意义

本文通过对南海表层沉积物样品的硅藻分析,试图揭示南海某些表层沉积硅藻的分布规律,以便为南海古海洋学研究提供科学依据。研究发现,Thalassionema nitzshioides为南海地区表层沉积硅藻的优势种,且其百分含量无明显的区域差异,体现了南海与周围海洋水体交换相对较弱的特征。除此之外,南海表层沉积硅藻的分布主要受到海洋环流的影响,表现在黑潮暖流、印度洋暖水的入侵以及沿岸流对南海表层沉积硅藻分布的影响。其中,Nitzshia marina, Azpeitia neocrenulata , Azpeitia africana , Rhizosolenia bergonii等暖水硅藻可作为黑潮暖流及印度洋暖水入侵南海强度的指标种。而Cyclotella stylorum , Cyclotella striata , Diploneis bombus , Traychneis aspera , Tabularia tabulata等则可看作判断沿岸流对南海水体影响强度的标志种。     

A NUTRITIONAL ANALYSIS OF A SUBLITTORAL DIATOM ASSEMBLAGE EPIPHYTIC ON ENTEROMORPHA FROM A LONG ISLAND SALT MARSH1,2

The diatom population structure of a salt marsh epiphytic community growing on Enteromorpha intestinalis was studied at one station throughout the summer. A total of 218 species or varieties were recognized. Six species— Fragilaria construens, Cocconeis scutellum, Cocconeis placentula, Achnanthes hauckiana varieties, Achnanthes pinnata, and Amphora coffeaeformis (var. acutiuscula)–dominated the Enteromorpha epiphytic community during the summer months and comprised ∼ 40% of the total populations. Melosira nummuloides, Opephora martyi, Synedra fasiculata var. tabulata, S. affinis, Navicula platyventris, and N. pavillardi were also very common species (13% of the total population). The distribution of many species in the community was seasonal. A series of differential media have been developed which are effective as tools for the isolation and nutritional characterization of the algae and bacteria from the community. Many diatom species can be recognized by their colony type or growth pattern on solidified media. A key and illustrated plates aid in identification. Changes in population structure of the community were reflected by changes in the nutritional patterns as judged by differences in the growth of diatoms and bacteria on the differential media tested. The nutritional requirements, selectivity, and rank order of media for individual species is given. Almost half of the colony types (32–33) grew on either unenriched seawater from the collection site or basal synthetic seawater. Erdschreiber was a poorer medium for the isolation of algae than seawater alone. Few algal colonies grew on either marine nutrient agar or trypticase soy agar. Nitrite inhibited algal growth. Media enriched with thiamine, biotin, or nitrate, phosphate, and B₁₂ were stimulatory. Soil extract or an acetone extract of Enteromorpha tripled growth; a complex vitamin mixture, or glycerol, or mannitol, or an autoclaved extract of Enteromorpha doubled colony numbers. The greatest numbers of diatom types (43) and total colonies (655) were recovered on media enriched with acetone extract of Enteromorpha, suggesting a possible nutritional relationship between Enteromorpha and its epiphytes. The representativeness of the isolations from the natural community as a function of time was also considered. Concentrations of selected dissolved potential growth-stimulating nutrients were measured within and external to Enteromorpha communities. We conclude that the algal assemblage growing epiphytically on Enteromorpha intestinalis has a diverse auxotrophic profile which contributes to the productivity and stability within this important component of the epiphytic community and that much of the organic substrates used by individual species originate within and are recycled among the community members.     

SUGAR-CONTAINING COMPOUNDS ON THE CELL SURFACES OF MARINE DIATOMS MEASURED USING CONCANAVALIN A AND FLOW CYTOMETRY

Some diatom exudates may remain attached to the exterior cell surface, potentially altering cell stickiness and affecting important aspects of the diatom's ecology such as aggregation rates and grazing rates. We measured the accumulation of cell-surface sugar-containing compounds by labeling cultured marine diatoms with fluorescent-tagged sugar-binding lectins and measuring the fluorescence associated with each cell using flow cytometry. The binding of FITC-labeled concanavalin A (FITC-ConA), a lectin that binds to glucose and mannose residues, varied 5-fold among different diatom species in exponential growth (on a per-cell basis) and 2–3-fold within a given species in different physiological states. Although transparent exopolymer particles followed a simple accumulation curve over time in batch culture, FITC-ConA. cell^-1 did not follow the same pattern, suggesting that surface sugar accumulation is not driven simply by the accumulation of such particles in the medium. For Thalassiosira pseudonana (Hustedt) Hasle and Heimdal (3H clone), the amount of sugar-containing compounds on the cell surface increased transiently as growth rate slowed in early stationary phase under both N and Si limitation. For Chaetoceros neogracile (Schuett) van Landingham, FITC-ConA. cell^-1had a strong inverse relationship with growth rate across several Si-limited batch culture experiments. Both results suggest some biological mediation of cell-surface sugar-containing compounds. Our study reveals the great potential for using lectin binding to investigate cell-surface sugars on diatoms. Lectins allow us to investigate noninvasively the role of cell-surface sugar-containing compounds in modifying cell stickiness and aggregation, as well as the partitioning of exuded phytoplankton carbon. We suggest that cell-surface sugar accumulation may be related to diatom stickiness, based on a correlation between our FITC-ConA measurements and stickiness estimates in the literature on several of the species we studied.     

Flood disturbance effects on benthic diatom assemblage structure in a semiarid river network

Disturbances such as floods and droughts play a central role in determining the structure of riverine benthic biological assemblages. Extreme disturbances from flash floods are often restricted to part of the river network and the magnitude of the flood disturbance may lessen as floods propagate downstream. The present study aimed to characterize the impact of summer monsoonal floods on the resistance and resilience of the benthic diatom assemblage structure in nine river reaches of increasing drainage size within the Gila River in the southwestern United States. Monsoonal floods had a profound effect on the diatom assemblage in the Gila River, but the effects were not related to drainage size except for the response of algal biomass. During monsoons, algal biomass was effectively reduced in smaller and larger systems, but minor changes were observed in medium systems. Resistance and resilience of the diatom assemblage to floods were related to specific species traits, mainly to growth forms. Tightly adhered, adnate and prostrate species (Achnanthidium spp., Cocconeis spp.) exhibited high resistance to repeated scour disturbance. Loosely attached diatoms, such as Nitzschia spp. and Navicula spp., were most susceptible to drift and scour. However, recovery of the diatom assemblage was very quick indicating a high resilience, especially in terms of biomass and diversity. Regional hydroclimatic models predict greater precipitation variability, which will select for diatoms resilient to bed‐mobilizing disturbances. The results of this study may help anticipate future benthic diatom assemblage patterns in the southwestern United States resulting from a more variable climate.     

Can Hydrodynamics Change Phosphorus Strategies of Diatoms?—Nutrient Levels and Diatom Blooms in Lotic and Lentic Ecosystems

Diatom blooms occur in many water bodies worldwide, causing significant ecological and social concerns. In order to understand the mechanisms of diatom blooms formation, the effects of varying phosphorus (P) concentration and hydrodynamics on the growth of diatoms were studied by combining results from field observations and laboratory experiments. The field investigation showed that spring diatom blooms (Cyclotella meneghiniana and Stephanodiscus hantzschii) occurred in Lake Taihu and Hanjiang River with similar environmental factors such as water temperature, pH, and dissolved oxygen in 2008. Concentrations of total phosphorus (TP), total nitrogen, and ammonia nitrogen (NH₄-N) in Lake Taihu were significantly higher than the concentrations in the Hanjiang River. Laboratory experiments were conducted to evaluate growth and physiological responses of four lotic diatoms (Cyclotella atomus, Fragilaria crotonensis, Nitzschia palea, and S. hantzschii, isolated from the Hanjiang River) and three lentic diatoms (C. meneghiniana, Melosira varians, and Stephanodiscus minutulus, isolated from Lake Taihu, Lake Donghu, and Guanqiao Pond, respectively) to various P concentrations under small-scale turbulent and standing conditions. Our results showed that, with turbulence, lotic diatoms C. atomus, F. crotonensis, N. palea, and S. hantzschii demonstrated a significant increase in affinity for P compared with lentic diatoms C. meneghiniana, M. varians, and S. minutulus. Moreover, lotic diatoms C. atomus, F. crotonensis, and N. palea had higher growth rates and photosynthetic efficiencies with small-scale turbulence than with standing conditions both in P-limited and P-replete conditions. Lotic species S. hantzschii and three lentic diatoms (C. meneghiniana, M. varians, and S. minutulus) grew well under standing conditions. Our results may explain our field observation that the occurrence of diatom blooms in lakes is often associated with higher TP concentrations whereas in rivers, diatom blooms occur at a wide range of TP concentrations under flows. Therefore, different hydrodynamics and nutrient concentrations determined the dominant diatom species, according to their habitat-dependent physiological characteristics.     

广东鉴江水系底栖硅藻多样性与时空分布特征

为了解广东省鉴江水系底栖硅藻多样性和时空分布特征,对全流域进行了底栖硅藻采样调查。结果表明,从19个采样点4次采样中共检出底栖硅藻10科52属242种,其中舟形藻属(Navicula)、菱形藻属(Nitzschia)和异极藻属(Gomphonema)是优势类群,出现频次和相对丰度较高。硅藻多样性指数(丰富度、真香农多样性指数和真辛普森多样性指数)随河流等级呈现一定的空间分布特征,但它们季节变化不明显。底栖硅藻群落相异性在上游和下游河段较高,从一级到三级河流递减,四级河流又增加。底栖硅藻群落结构空间变化明显,季节变化显著。群落丰富度的稀疏曲线表明,热带河流底栖硅藻群落以400个体计数,不能完整反映底栖硅藻多样性。这些为鉴江水系河流健康监测和水生态保护奠定了基础。     

SPATIAL PARADIGMS OF LOTIC DIATOM DISTRIBUTION: A LANDSCAPE ECOLOGY PERSPECTIVE

Spatial distributional patterns of benthic diatoms and their relation to current velocity were investigated in an unshaded cobble-bottom reach of White Creek (Washington County, NY). On 27 August 1999, diatoms were sampled and current velocity and depth were measured on a regular square sampling grid with a grain size of 0.01 m², interval of 0.5 m, and extent of 16 m². The relative abundance of the 18 common diatom species enumerated in the 81 samples was subjected to detrended correspondence analysis (DCA). The first axis (DCA1) explained 51% of the variance in diatom data and separated the samples according to current regimes. The spatial autocorrelation of DCA1 sample scores in deposition and erosion regions of White Creek was determined by Moran's I statistic to indicate patch size. In White Creek the patch length of all diatom communities was more than 3.1 m, whereas the patch width was 1 m in the deposition region and 0.5 m in the erosion region. There were 5 dominant diatom taxa, Achnanthes minutissima Kütz. et vars, Fragilaria capucina Dezmazières et vars, F. crotonensis Kitt., Diatoma vulgaris Bory, and Synedra ulna (Nitz.) Ehr. et vars. The patch length of the dominant species varied from 1 to more than 4.1 m, whereas the patch width, if defined, was 0.5 m. Achnanthes minutissima and F. capucina, the two diatom species with the highest relative abundance, displayed spatially structured patches of low abundance and comparatively random patches of high abundance, suggesting broad scale abiotic control of species performance in low abundance regions and finer scale biotic control of high abundance areas. Another objective of this study was to test the hypothesis that higher current velocities, which generally impede immigration, would increase randomness and complexity (i.e. homogeneity of diatom distributional patterns). The spatial complexity in low versus high velocity transects was determined by calculating the respective fractal dimension (D) of DCA1 scores. D of DCA1 was higher in the higher current velocity transects, suggesting that spatial complexity and homogeneity of diatom communities increased in faster currents. Partial canonical correspondence analysis was conducted on diatom, environmental, and spatial data to assess how much of the variance in species distribution could be attributed to environmental (current velocity and depth) versus spatial factors. The variance of species data, explained by the environment (exclusively current velocity), was 38%; whereas space alone contributed only 10%, indicating that 1) current velocity was the major factor that controlled diatom distribution in streams and 2) there were other spatially dependent variables, most likely biotic, but their role in shaping diatom communities was minor.     

Characterization of the diatom flora in the Lower Mountain Fork (Oklahoma,USA), a novel regulated river with a disjunct population of the diatom Didymosphenia geminata (Bacillariophyta)

Regulated rivers are novel ecosystems with altered temperature and flow regimes that can be used to test distribution patterns of microscopic organisms, such as diatoms. Our objective was to describe the spatial and seasonal patterns of diatoms in a cold-water, oligotrophic river within a region of warm-water, mesotrophic rivers. The Lower Mountain Fork, in south-east Oklahoma (USA), is maintained as a year-around, stocked fishery by the release of cold, hypolimnetic water from Broken Bow Lake and is the southern-most known site of Didymosphenia geminata in North America. Epilithic diatoms were sampled six times at nine sites over a distance of 15.5 km and, within this area, 27 times at the site of the main Didymosphenia bloom. Percentage composition data were analysed for assemblage composition using multivariate analysis, nutrient specificity using a diatom-based metric, and species associations using similarity profiles. Eighty-eight taxa were found, of which 10 were unidentifiable and included local undescribed species and species clusters. Three species [Gomphonema (parvulum morph), Achnanthidium rivulare and Achnanthidium minutissimum] comprised over 60% of the diatom abundance at all sites, and downstream and seasonal patterns were evident for both these and less abundant taxa. Notably, diatom assemblages in the three sites below the dam were similar to that at the lowermost site, below a much smaller dam. The oligotraphentic diatom assemblage reflected the water chemistry of the river. The Didymosphenia bloom had been scoured by a large spate prior to the study and the species was present at two of the nine sites in low numbers but failed to bloom during the study, possibly because of a trend towards increasing phosphorus concentrations in the reservoir (Didymosphenia blooms under low P concentrations). No other species shared Didymosphenia’s distribution pattern over the study reach, highlighting the novelty of Didymosphenia’s presence in the river.     

Changes in diatom-dominated biofilms during simulated improvements in water quality: implications for diatom-based monitoring in rivers

Although benthic diatoms are used to assess river water quality, there are few data on the rate at which diatom assemblages react to changes in water quality. The aim of this study was to assess the reaction time of diatoms and to discuss the changes occurring during water quality improvement on the basis of their autecological characteristics. In order to simulate this improvement, diatom-dominated biofilms grown on artificial sandstone substrata were transferred from several polluted rivers to an unpolluted river. They were sampled three times: before transfer and 1 and 2 months after transfer. The ecology and growth-forms of the taxa explained most of the changes in species composition observed during the experiment. Adnate diatoms gradually replaced motile and stalked taxa. Gomphonema parvulum, a stalked diatom positioned vertically in the biofilm, is adapted for light and space competition in high-density algal biofilms. When transferred to an unpolluted site, this growth-form is less competitive and does not tolerate the high grazing pressure. Fistulifera saprophila is a single celled motile diatom, living in organic matrices. When the artificial substrata were transferred to the unpolluted site, this particular ecological niche disappeared quickly. On the other hand, Achnanthidium minutissimum, which is considered to be cosmopolitan and an early colonizer, increased during the first month of transfer and then decreased. It was gradually replaced by A. biasolettianum, which was the taxon best suited to this pristine stream. The changes observed differed between treatments depending on the species composition and architecture of the biofilms. In particular, biofilms dominated by stalked and motile diatoms were more quickly modified than those dominated by small motile diatoms. The diatom index reflects these changes, and its values showed that about 60 days following a water quality improvement were necessary for transferred diatom assemblages to reach diatom index values similar as those at the unpolluted river.     

Transparent exopolymer particle production and aggregation by a marine planktonic diatom (Thalassiosira weissflogii) at different growth rates

Transparent exopolymer particles (TEP) play an important role in the ocean carbon cycle as they are sticky and affect particle aggregation and the biological carbon pump. We investigated the effect of growth rate on TEP production in nitrogen limited semi‐continuous cultures of the diatom Thalassiosira weissflogii (Grunow) G. Fryxell & Hasle. Steady‐state diatom concentrations and other indicators of biomass (chl a, and total carbohydrate) were inversely related to growth rate, while individual cell volume increased with growth rate. There was no change in total TEP area with growth rate; however, individual TEP were larger at high growth rates and the number of individual TEP particles was lower. TEP concentration per cell was higher at higher growth rates. SYTOX Green staining showed that <5% of the diatom population had permeable cell membranes, with the proportion increasing at low growth rates. However, TEP production rates were greater at high growth rates, refuting our hypothesis that TEP formation is dependent on dying cells with compromised cell membranes in a diatom population. Measurements of particle size distribution in the cultures using laser scattering showed that they were most aggregated at high growth rates. These results indicate a coupling between TEP production and growth rate in diatoms under N limitation, with fast growing T. weissflogii producing more TEP and aggregates.     

The production of extracellular carbohydrates by estuarine benthic diatoms: the effects of growth phase and light and dark treatment

Epipelic diatoms are important constituents of estuarine microphytobenthic biofilms. Field‐based investigations have shown that the production of carbohydrates by such taxa is ecologically important. However, limited information exists on the dynamics of carbohydrate production by individual species of epipelic diatoms. The production of low and high molecular weight extracellular carbohydrates in axenic cultures of five species of benthic estuarine diatoms, Cylindrotheca closterium (Ehrenberg), Navicula perminuta (Grun.) in Van Heurck, Nitzschia frustulum (Kütz.) Grunow, Nitzschia sigma (Kütz.) Grunow, and Surirella ovata (Kütz.) Grunow, were investigated. All species produced colloidal (water‐soluble) carbohydrates during growth, with maximal production occurring during stationary phase. During logarithmic growth, approximately 20% of extracellular carbohydrates consisted of polymeric material (extracellular polymeric substances [EPS]), but during stationary phase, EPS content increased to 34%–50%. Pyrolysis–mass spectrophotometry analysis showed differences in the composition of EPS produced during logarithmic and stationary phase. All species synthesized glucan as a storage carbohydrate, with maximum glucan accumulation during the transition from log to stationary phase. Short‐term labeling with ¹⁴C‐bicarbonate found that between 30 and 60% of photoassimilates were released as colloidal carbohydrate, with EPS consisting of approximately 16% of this colloidal fraction. When cells were placed in darkness, EPS production increased, and between 85 and 99% of extracellular carbohydrate produced was polymeric. Glucan reserves were utilized in dark conditions, with significant negative correlations between EPS and glucan for N. perminuta and S. ovata. Under dark conditions, cells continued to produce EPS for up to 3 days, although release of low molecular weight carbohydrates rapidly ceased when cells were dark treated. Three aspects of EPS production have been identified during this investigation: (1) production during rapid growth, which differs in composition from (2) EPS directly produced as a result of photosynthetic overflow during growth limiting conditions and (3) EPS produced for up to 3 days in the dark using intracellular storage reserves (glucans). The ecological implications of these patterns of production and utilization are discussed.     

Spatial analysis of a subalpine heath woodland

The horizontal spatial patterns of heath species have implications in dynamic community modelling, fire behaviour modelling and ecological interpretation. The distribution pattern of the dominant woody species in a subalpine heath woodland near Kiandra, New South Wales was analysed as spatial point processes. The species analysed were Bossiaea foliosa, Grevillea australis, Hakea lissosperma and Oxylobium ellipticum, and both large (>2m) and small (<2 m) snowgums (Eucalyptus pauciflora) They all exhibited significant aggregation at scales ranging from 1 to 15 m. Bivariate spatial analyses of these species revealed significant negative association between G. australis and H. lissosperma, between G. australis and O. ellipticum, and between O. ellipticum and H. lissosperma, B. foliosa was independently distributed with respect to other shrub species. There was some evidence for negative association between small snowgums and shrub species, though small snowgums were positively associated with large snowgums. The joint spatial distribution of the individuals of all shrub species was also aggregated. A Poisson cluster process was developed and tested to model the joint spatial pattern of the shrub stratum.     

Observations on Copulation in Navicula pupula and Amphora ovalis in Relation to the Nature of Diatom Species

Within the same water body, morphological variants of the samediatom species often coexist. Observations of such phenodemesof Navicula pupula and Amphora ovalis show that, when sexual,they either fail to attract or fail to recognize each otherand do not interbreed. Since in both species the morphologicaland reproductive differences between demes appear to be reflectedalso in ecological differences, the present species conceptin diatoms needs re-evaluation. Amphora ovalis (Ktz.) Ktz., Navicula pupula Ktz., diatom microevolution, diatom systematics, reproductive isolation     

DIATOMS AS HOSTS FOR OTHER DIATOMS: OBSERVATIONS FROM SOUTHERN CALIFORNIA

Large epiphytic and epilithic diatom species hosting other diatoms were observed in several fresh‐ and brackish water sites in Southern California. The most commonly encountered hosts were species forming long filaments attached to rocks or macroalgae, Hydrosera whampoensis (Schwarz) Hendey, Melosira varians Agardh, Pleurosira laevis (Ehrenberg) Compere and Terpsinoe musica Ehrenberg. These large diatoms often had smaller diatoms attached, usually to the girdle bands and occasionally to the mucilage pads connecting the cells. For example, cells of T. musica were observed supporting growth of a diverse diatom assemblage composed of species of the genera Achnanthes, Achnanthidium, Amphora, Cocconeis and Tabularia; Synedra sp. was attached to M. varians and B. paxillifer; and Cocconeis placentula was seen on H. whampoensis. Thus, large epiphytic and epilithic diatoms seem to provide suitable sites for attachment of small epiphytic diatom species, and it appears that this phenomenon is more common than previously thought.     

COMPARATIVE PHYSIOLOGICAL STUDY OF MARINE DIATOMS AND DINOFLAGELLATES IN RELATION TO IRRADIANCE AND CELL SIZE. II. RELATIONSHIP BETWEEN PHOTOSYNTHESIS,GROWTH, AND CARBON/CHLOROPHYLL a RATIO1,2

Photosynthetic rates, growth rates, cell carbon, cell protein, and chlorophyll a content of two diatom and two dinoflagellate species were measured. The microalgae were chosen to have one small and one large species from each phylogenetic group; the two size categories differed from each other by 1.5 orders of magnitude in terms of cell carbon or cell protein. The cultures for the experiments were grown under continuous light at an irradiance high enough for the light-saturation of growth for all four species. The four species were found to have similar maximum photosynthetic rates per unit chlorophyll a. The diatom species showed lower carbon/chlorophyll a ratios and higher photosynthetic rates per unit carbon than the dinoflagellates. The higher growth rates of the diatoms were shown to be related to their higher photosynthetic rates per unit carbon. The ecological significance of the physiological difference between these two groups of microalgae is discussed.     

Can environmental constraints determine random patterns of plant species co-occurrence?

Plant community ecologists use the null model approach to infer assembly processes from observed patterns of species co‐occurrence. In about a third of published studies, the null hypothesis of random assembly cannot be rejected. When this occurs, plant ecologists interpret that the observed random pattern is not environmentally constrained – but probably generated by stochastic processes. The null model approach (using the C‐score and the discrepancy index) was used to test for random assembly under two simulation algorithms. Logistic regression, distance‐based redundancy analysis, and constrained ordination were used to test for environmental determinism (species segregation along environmental gradients or turnover and species aggregation). This article introduces an environmentally determined community of alpine hydrophytes that presents itself as randomly assembled. The pathway through which the random pattern arises in this community is suggested to be as follows: Two simultaneous environmental processes, one leading to species aggregation and the other leading to species segregation, concurrently generate the observed pattern, which results to be neither aggregated nor segregated – but random. A simulation study supports this suggestion. Although apparently simple, the null model approach seems to assume that a single ecological factor prevails or that if several factors decisively influence the community, then they all exert their influence in the same direction, generating either aggregation or segregation. As these assumptions are unlikely to hold in most cases and assembly processes cannot be inferred from random patterns, we would like to propose plant ecologists to investigate specifically the ecological processes responsible for observed random patterns, instead of trying to infer processes from patterns.