EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS,I. HYDROLOGY1

We examined the hypothesis that the heterogeneity of epilithic algal assemblages in streams may be partly a result of hydrologic differences created when water flows over a rough substrate. A 32-day experiment was conducted in laboratory streams that contained either 22.5 × 22.5 × 4 cm or 7.5 × 22.5 × 4 cm tile blocks. Free water velocities in the streams overaged 28 cm·s^?1. Hydrologic parameters and algal assemblages associated with surfaces on top of blocks and with recessed surfaces between blocks were compared to corresponding surfaces in streams with of relief. In streams with blocks, shear velocities averaged 1.7 cm·s^?1 on the top of blocks and 0.8 cm·s^?1 in the recessed areas. Shear velocity at corresponding surfaces in the control (no relief) streams averaged 1.9 cm·s^?1 and exhibited little variation. The hydrologic differences created by the larger blocks significantly affected the distribution of algal biomass, with recessed areas having an average of 2.6 g·m^?2 AFDW more biomass than surfaces on the top of blocks. Differences in shear velocities and biomass accumulation between top and recessed areas for the smaller blocks were less than for large blocks. Successional changes on all substrates were similar with the exception that recessed surfaces had a significantly greater abundance of the filamentous chlorophyte Stigeoclonium tenue (Ag.) Kütz after day 16. The results suggest that in cobble riffle areas of natural streams, the interaction between current flow and substrate relief has the potential to create patches of algae which are different in biomass and taxonomic composition.     

NUTRIENT AND LIGHT LIMITATION OF ALGAE IN TWO NORTHERN CALIFORNIA STREAMS1

Nutrient-diffusing subsrates were used to investigate nutrient limitation of attached algal assemblages in a shaded stream and an unshaded stream in northern California. Water from both streams contained low levels of nitrogen (< 14 μg.L^?1) and very low N:P ratios (< 2). After 31 days of colonization and growth, attached algal biomass on nitrate-diffusing substrates was significantly greater than on control substrates in the unshaded stream. Nitrate-diffusing substrates also supported larger numbers of grazing insects in the unshaded stream. The prostrate diatoms Achnanthes lanceolata Bréb. and Coconeis placentula Ehr. displayed the most consistent positive responses to nitrate enrichment. Nutrient enrichment did not increase the accrual of algal biomass in the shaded stream, but algal biomass was significantly greater at sites located under openings in the tree canopy, implicating light as a limiting factor in this stream. Several Navicula and Nitzschia species, and one unidentified Gomphonema species, were positively associated with higher light levels in the shaded stream. Shade appears to be the primary factor limiting algal growth in small northern California streams, but when its effect is reduced by logging, the inherently low levels of nitrogen in these streams can become limiting.     

DENSITY-DEPENDENT GROWTH,ECOLOGICAL STRATEGIES,AND EFFECTS OF NUTRIENTS AND SHADING ON BENTHIC DIATOM SUCCESSION IN STREAMS1

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low-nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 10⁶ and 2 × 10⁶ cells-cm^?2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop. Synedra ulna (Nitz.) Ehr. and Achnanthes minutissima Kütz. were codominant during early stages of community development in both habitats, but succession to an A. minutissima-dominated community was much faster in NP-enriched than in control conditions. Species dominating early stages tended to immigrate quickly, whereas species that increased in relative abundance during community development had either fast growth rates or fast immigration and average growth rates. Decreases in growth rates indicated resource supply became limiting during community development in control and enriched channels. Density-dependent competition was indicated because nutrient concentrations in the water column and light did not decrease during the 30-d study. Species autecologies were defined by effects of nutrient enrichment, shading, and community development on species growth rates. Differing autecologies of early and late succession species indicated that competition for nutrients was more important than competition for light. Species autecologies also indicated ecological strategies. The species most stimulated by nutrient enrichment were least able to maintain growth rates as algal abundances on substrata increased. In addition, these species that best sustained their growth rates during succession tended to have the highest immigration rates, indicating that drift and immigration may have been an important mechanism of persistence for some populations when resources become limiting within thick benthic mats.     

EFFECTS OF IRRADIANCE AND GRAZING ON LOTIC ALGAL ASSEMBLAGES1

A laboratory experiment was conducted for 75 days to examine how irradiance levels and grazing influence algal biomass and community structure. Twelve laboratory streams were used for experimental analyses, with four channels exposed to one of three irradiance levels (15, 100, or 400 μE·m^?2·s^?1). Three of the four stream at each light level were stocked with the snail Juga silicula (250·m^?2), leaving one stream at each light level without snails. Grazed stream exposed to low light levels developed low amounts of algal biomass (<2 g AFDW·m^?2) and were dominated by adnately attached diatoms. Mean algal biomass increased over time in the grazed streams exposed to intermediate light; by day 75, these streams were characterized by moderate algal biomasses (30-40 g AFDW·m^?2) and filamentous chlorophytes. Algal assemblages in high light, grazed channels had high levels of biomass at day 43 (70 g AFDW·m^?2) that declined to 30 g AFDW·m^?2at day 75 and were dominated by chlorophytes. Among ungrazed streams, algal biomass at day 75 was relatively low in the low light streams (<7g AFDW·m^?2) and was dominated by adnately attached diatoms. Ungrazed streams exposed to intermediate and high light levels had moderate biomasses (23 and 19 g AFDW·m^?2, respectively) and were dominated by chlorophytes and large diatoms. Grazing appeared both to delay and alter successional trajectories of algal assemblages, with alterations most noticeable during early seral stages at intermediate and high light levels. Grazing had the least effect on successional trajectories at low light.     

LONG-TERM EFFECTS OF MANIPULATING LIGHT INTENSITY AND NUTRIENT ENRICHMENT ON THE STRUCTURE OF A SALT MARSH DIATOM COMMUNITY1,2

The long-term effects of manipulating light intensity and nutrient enrichment on the structural characteristics of a diatom community inhabiting the sediments beneath a pure stand of dwarf Spartina alterniflora Loisel. were investigated over a yearly cycle. Clipping or shading the cord grass cover, or phosphorus enrichment caused significant decreases in both species diversity (H') and the number of diatom species, whereas nitrogen enrichment only significantly decreased the latter parameter. Of the 105 diatom taxa identified, only 10 were restricted to certain of the 12 study areas; and of these, 8 occurred exclusively in the clipped habitats. An analysis of variance (light × nutrient × collection date) involving 19 of the most abundant taxa revealed that certain experimental treatments had significant effects on the relative abundances of each and every taxon. However, attempts to group taxa with similar response patterns proved unsuccessful because of the frequent significance of the 3-way interaction term. Synthesis of these results with earlier work by the author showed that differences in structure of diatom communities inhabiting the sediments beneath the 3 dominant marsh grasses were not primarily caused by differences in reduction of light intensity by their grass canopies, and that clipping of the cord grass produced a shift in community structure towards that characteristic of a salt panne algal mat.     

VARIATION IN DIATOM COMMUNITY STRUCTURE AMONG HABITATS IN SANDY STREAMS1

Community structure of benthic diatoms in two sandy streams was studied for two summers to assess differences in abundances and species composition among habitats and between years and streams. The greatest differences in abundances (cells·cm^?2) were lower abundances on rocks than on sand, whether the sand was clean or covered with flocculent organic material. Relatively little variation in abundances occurred between years and streams. Species composition of diatom communities varied more among habitats and between streams than from year to year. Species composition was most unique in floc-covered sand when communities in clean and floc-covered sand, rock, plant, and plankton were compared. Diatom species composition in these sandy streams was most similar on sand and rocks.     

DIATOM COLONIZATION ON ARTIFICIAL SUBSTRATA IN POOL AND RIFFLE ZONES STUDIED BY LIGHT AND SCANNING ELECTRON MICROSCOPY1

Light and scanning electron microscopy were utilized to quality diatom colonization in Oak Creek, Arizona. Aluminum SEM stubs with and without plexiglass discs were anchored into rocks. Early colonization on five stub microzones was examined at hourly intervals; weekly intervals of up to 3 wk were employed to record community development in pool find riffle. Plexiglass was more suitable for microbial colonization than aluminum. Organic matter and bacteria were important surface pre-conditioning agents while fungi were instrumental in trap/ting cells during early stages of colonization in the riffle. Diatom colonization was initialed within 1 h on the upstream side of substrata in riffles, while the tap face was colonized first in pools. Colonization moved rapidly to the perimeter in each system. Early colonization of-side microzones was considerably more asymmetric in the riffle than, pool. At Idler stages (2 wk) diatoms with their associated mucilage and algal filaments contributed to the stability of the microbial communities. Horizontally positioned species (Achnanthes, Cocconeis) were early colonizers in both systems while vertically positioned species (Gomphonema, Nitzschia) were more important in later successional stages (3 wk) in the riffle. Horizontally positioned species remained dominant throughout the 3 wk period in the pool. After 3 wk, diversity was normally greater in the pool while density was higher in the riffle. Detrital microcosms containing viable microbiol assemblages frequently collected on tin-upstream face of substrata in the riffle. The random nature by which these detrital microcoms contact downstream substrata greatly contribute to the spatial variation of periphyton in streams. These detrital microcosms expedite repeated colonization in lotic systems.     

香蕉种植密度对蕉园光能利用与产量的影响

本试验研究了福建沿海地区蕉园种植密度与光能利用效率和产量之间的关系。结果表明,适当提高种植密度,可增加香蕉单位空间的光能利用,充分利用地力,改善蕉园生态条件,显著地提高香蕉亩产量。亩植160～180株是沿海地区达到高产稳产的有效措施。     

IMPORTANCE OF PHYSICAL VARIABLES ON THE SEASONAL DYNAMICS OF EPILITHIC ALGAE IN A HIGHLY SHADED CANYON STREAM1

The seasonal abundance of epilithic algae was correlated with major physico-chemical parameters in a first-order, heavily shaded stream in northern Arizona. Diatoms made up over 85%, by numerical abundance, of the epilithon community Light energy, water temperature, and stream discharge were most highly correlated with seasonal abundance of epilithic diatom taxa when analyzed with stepwise multiple regression. None of the chemical variables measured in the study (NO₃-N, O-PO₄, SiO₂, including PH) was found to be significantly correlated with the seasonal community structure of epilithic diatoms. Total diatom cell densities showed a significant negative correlation to stream bed light energy. Likewise, total diatom cell densities along a transect in the stream bed showed a negative correlation to current velocity during those months when base flow was low and stable, and current velocity was ≤25 cm·sec-¹. Most diatom taxa had highest cell densities at temperatures < 16°C and at daily mean stream bed light levels < 400 μE·m^?2·s^?1. Highest cell densities of green algae occurred at temperatures between 6–16°C and at daily mean stream bed light levels of > 400 μE·m^?2·s^?1. Blue-green algae (cyanobacteria) grew best at the highest recorded water temperatures and daily mean stream bed light energy (16–20°C and 900–1200 μE·m^?2·s^?1). Abrupt increases in NO₃-N coincided with a brief pulse of Nostoc pruniforme colonies during June, and leaf drop from Alnus oblongifolia during October.     

EFFECTS OF DISTURBANCE ON EPIPHYTIC COMMUNITY ARCHITECTURE1

Microdistributional patterns of attached algal communities on Cladophora and glass slides were compared under varying disturbance regimes in the Upper Mississippi River, using light and scanning electron microscopy. Localized physical disturbance, induced by boat traffic and wind-generated wave action in the main channel, inhibited development of complex attached algal communities and maintained an adnate flora with two-dimensional architecture. In contrast, communities protected from disturbance developed a more complex, three-dimensional architecture. A high degree of architectural similarity was observed between communities on Cladophora and glass substrates exposed to similar levels of disturbance. Communities exposed to severe disturbance resembled ones in early stages of colonization and development, whereas less disturbed communities were similar to ones in advanced stages of development. These results demonstrated that turbulence strongly influences the community structure of periphyton on both micro and macro scale levels.     

EFFECTS OF LIGHT AND WAVE DISTURBANCE ON VERTICAL ZONATION OF ATTACHED MICROALGAGE IN A LARGE RESERVOIR1

Experimental field manipulations of artificial substrata were used to examine the mechanisms controlling attached algal zonation down the face of the dam in Lake McConaughy, a large reservoir in western Nabraska. Sets of clay tiles were incubated in the upper (2.5 m depth) and lower (8 m depth) growth zones for two weeks. Five sets tiles were then switched from the upper to the lower growth zone and vice versa. Five additional sets of tiles were switched to the lower zone and artificially disturbed. Diatom cell densities increased rapidly in both the zones; however, wind-induced turbulence caused dramatic declines (up to 61%) in densities in the upper zone. Consequently, cell densities in the upper and lower growth zones were not significantly different after four weeks, despite the 17–30% higher light levels in the upper zone. Based on cell densities and relative abundances on clay tiles and naturally occurring rocks, 26 of the 32 most common diatom taxa had a significant upper (10) or lower (16) zone preference. Of these, 15 taxa exhibited a consistent response to one or both switching manipulations, confirming a growth zone preference, and two showed a clear preference for disturbed substrata. Diatom growth form appeared to play a major role in determining the vertical zonation of attached communities, since actively motile taxa exhibited a lower zone preference and stalked forms occurred primarily in the upper zone. The present study indicates that light attenuation and wave disturbance are primary mechanisms that control the vertical zonation of freshwater epilithic algae.     

PHOTOSYNTHESIS-IRRADIANCE PATTERNS IN BENTHIC MICROALGAE: VARIATIONS AS A FUNCTION OF ASSEMBLAGE THICKNESS AND COMMUNITY STRUCTURE

Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobenthos) have been considered primarily for entire assemblages. How P-I responses vary with mat thickness and with community composition has not been considered in detail. We used a combined approach of modeling, microscale determinations of photosynthetic rate and light attenuation, and whole-assemblage O₂ flux measurements to explore P-I relationships. The modeling approach suggested that the onset of photosynthetic saturation and photoinhibition will occur at higher irradiance and that whole-mat photoinhibition (decreased photosynthesis at very high irradiance), biomass-specific maximum photosynthetic rate, and initial slope of the P-I function (α) should decrease as assemblage thickness increases or light attenuation increases. Spherical light microsensor profiles for a variety of stream algae indicated a strongly compressed photic zone with attenuation coefficients of 70–1791 m^?1 for scalar photosynthetic photon fluence density. The O₂ microelectrode measurements showed little if any photoinhibition at 2 and 4 mm depths in one filamentous green algal (Ulothrix) assemblage, with a relatively low attenuation coefficient, and no photoinhibition in a second Ulothrix community. An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xanthophyte (Vaucheria) community exhibited no photoinhibition at all. The microelectrode data revealed increases in α over several millimeters of depth (photoacclimation). These data supported the model predictions with regard to the effects of mat optical thickness on whole-assemblage values for α and photoinhibition. Whole-community O₂ flux data from 15 intact assemblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or α expressed per unit area; the relationships with chlorophyll a were negative when photosynthetic rates were expressed per unit chlorophyll a. None of the whole assemblages exhibited photoinhibition. Thus, the data from the whole communities were consistent with model predictions.     

EFFECT OF TEMPERATURE,LIGHT AND pH ON GROWTH,PHOTOSYNTHESIS AND RESPIRATION OF THE DINOFLAGELLATE PERIDINIUM CINCTUM FA. WESTII IN LABORATORY CULTURES1

Optimum light, temperature, and pH conditions for growth, photosynthetic, and respiratory activities of Peridinium cinctum fa. westii (Lemm.) Lef were investigated by using axenic clones in batch cultures. The results are discussed and compared with data from Lake Kinneret (Israel) where it produces heavy blooms in spring. Highest biomass development and growth rates occurred at ca. 23° C and ≥50 μE· m^?2·s¹ of fluorescent light with energy peaks at 440–575 and 665 nm. Photosynthetic oxygen release was more efficient in filtered light of blue (BG 12) and red (RG 2) than in green (VG 9) qualities. Photosynthetic oxygen production occurred at temperatures ranging from 5° to 32° C in white fluorescent light from 10 to 105 μE·m^?2·s^?1 with a gross maximum value of 1500 × 10^?12 g·cell^?1·h^?1 at the highest irradiance. The average respiration amounted to ca. 12% of the gross production and reached a maximum value of ca. 270·10^?12 g·cell^?1·h^?1 at 31° C. A comparison of photosynthetic and respiratory Q₁₀-values showed that in the upper temperature range the increase in gross production was only a third of the corresponding increase in respiration, although the gross production was at maximum. Short intermittent periods of dark (>7 min) before high light exposures from a halogen lamp greatly increased oxygen production. Depending on the physiological status of the alga, light saturation values were reached at 500–1000 μE·m^?2·s^?1 of halogen light with compensation points at 20–40 μE·m^?2·s^?1 and I_k-values at 100–200 μE·m^?2·s^?1. The corresponding values in fluorescent light in which it was cultured and adapted, were 25 to 75% lower indicating the ability of the alga to efficiently utilize varying light conditions, if the adaptation time is sufficient. Carbon fixation was most efficient at ca. pH 7, but the growth rates and biomass development were highest at pH 8.3.     

EFFECT OF BLUE-GREEN LIGHT ON PHOTOSYNTHETIC PIGMENTS AND CHLOROPLAST STRUCTURE IN THE MARINE DIATOM STEPHANOPYXIS TURRIS1

Blue-green light increased the chlorophyll concentration and chloroplast number of cells of Stephanopyxis turris (Grev.) Ralfs, compared to white light controls. Light fields for growth were 400 μW·cm^?2 (12:12 LD cycles). Chlorophyll increased up to 100%/cell, but no change in the ratio of chlorophylls to major carotenoids occurred. The effect was, therefore, not that of complementary chromatic adaptation. At the same time, blue-green light enhanced the photosynthetic fixation of CO₂. At the ultrastructure level, an increase in, and rearrangement of, the thylakoid system occurred.     

THE EFFECTS OF LIGHT AND TEMPERATURE ON GROWTH RATES IN BOREAL-SUBARCTIC CRUSTOSE CORALLINES1

A number of boreal-subarctic crustose corallines were kept in natural seawater tanks at temperatures ranging from 0 to 19 C and, using fluorescent lamps at light intensities, ranging from 7 to 750 lux with periods of 8 and 14 hr/day. The resultant growth rates as a function of temperature and light are presented and discussed in relation to the ecology of the plants. All of the Lithothamnieae studied had growth maxima at temperatures from 9 to 15 C. Growth in these species showed little light dependence below 4–6 C, but had a strong light dependence at higher temperatures. The one Lithophyllum species examined gave a flatter growth-temperature curve than the Lithothamnieae and showed little light dependence. The effects of temperature variation, salinity, and current on growth rates were also examined and are discussed. It was found to be especially important in studying growth rates of crustose corallines to allow time for growth stabilization following temperature change. In general, growth was found to exhibit a hysteresis effect, increased rates with the raising of temperatures 5–10 C and decreased rates with lowering temperatures.     

SPATIAL AND TEMPORAL VARIATION OF EPIPSAMMIC DIATOMS IN A SPRING-FED BROOK1

Epipsammic diatom communities were sampled and quantified bimonthly from June 1983 to April 1985 at four sites in a small brook in northwest Ohio. The primary objective was to determine the extent of temporal and spatial variability in the epipsammon. The four sites, approximately 250 m apart, differed in current velocity, illumination and the amount of sand-associated detritus. Diatoms were significantly more abundant in the portion of the brook flowing through an unshaded marsh than at three heavily shaded sites located upstream. Fragilaria leptostauron (Ehr.) Hust. and Achnanthes lanceolata var. dubia Grun. dominated the epipsammic assemblage at all sites throughout the entire study period. Only Meridion circulare (Grev.) Ehr. displayed a marked seasonal distribution. Although the same species were generally found at all sites, there were sufficient differences in relative abundance that communities could be discriminated according to site. Using two canonical variate axes, all 48 samples were correctly assigned to their proper sampling site based on community composition. Data from this epipsammic assemblage support the idea developed from the river continuum concept that species comprising riverine benthic assemblages continually persist and rarely become completely absent.     

EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of many Great Basin lakes has resulted from the diversion of tributary streams for agricultural or municipal uses. At Mono Lake, an alkaline salt lake in eastern California, salinities have risen from 50 to 100 g·L^?1 in just 50 years. Experimental mesocosms were established to simulate some of the potential ecological effects that could have accompanied this change. The influence of salinity on diatom diversity, taxonomic structure, and primary production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0.5 m deep, with open tops covered by 1 mm mesh net. Five treatments (50, 75, 100, 125, and 150 g·L^?1) with four replicates per treatment were used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g·m^?2) and diversity (from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities ≥75 g·L^?1. Photosynthetic oxygen production also was significantly lower at salinities ≥75 g·L^?1. Diatom indicator taxa for these shifts included Denticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Stephanodiscus oregonicus increasing in relative abundance in higher salinity treatments, accompanied by decreases in Achnanthes minutissima, Cymbella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting dominance at moderate salinity levels (75 to 125 g·L^?1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophyte, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occurred only in salinity treatments from 50 to 100 g·L^?1. Diversion of tributary stream flow and resulting salinity increases in this lake threaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision of the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to around 75 g·L^?1 and is expected to increase the diversity and productivity of the benthic algae of this ecosystem.     

RESPONSE OF PERIPHYTON COMMUNITIES TO CLAY AND PHOSPHATE LOADING IN A SHALLOW RESERVOIR1

Enclosures in a small piedmont reservoir in the southeastern United States were used to determine the effects of loading by phosphate and two clay sediments on periphyton community structure and production during the summer growing season. The experimental design included replicated controls and the following treatments added at 2-to 3-day intervals: phosphate (PHOS), kaolinite (KAOL), montmorillonite (MONT), kaolinite with phosphate (K+P), and montmorillonite with phosphate (M+P). Periphyton were sampled from polyethylene strips of the same material as the enclosure walls, suspended at depths of 0.5 m and 2.0 m to assess treatment effects on shallow (epilimnetic) vs. deep (hypolimnetic) communities. Colonization after 40 days was sparse, dominated in biovolume by blue-green algae in all but the epilimnion of controls, in which small gelatinous green algae and chrysophyte flagellates were abundant. The biovolume of shallow periphyton after 79 days was lowest in KAOL and MONT, intermediate in controls and PHOS, and greatest in clay + P. Hence, P enrichment alleviated the deleterious effects of clay on periphyton production, and clay + P appeared to act synergistically in stimulating benthic algal growth. Periphyton biovolume was comparable among shallow and deep controls, PHOS, and treatments with the rapidly settling kaolinite. Lowest biovolume occurred at depth in treatments with montmorillonite under extreme light attenuation imposed by the suspended, finely particulate clay. The relative contribution of blue-green algae to total periphyton production was highest in clay + P treatments and lowest in controls. Heterotrophic dinoflagellates increased in the shallow periphyton under clay loading, and in the deep communities under PHOS and M + P. Periphyton contributed only 20–32% of total mesocosm productivity in controls, PHOS, and KAOL, increasing to about 40% in MONT, and to 72–91% of the total in M + P and K + P. The data indicate that benthic microalgae can represent a major proportion of the primary production in shallow reservoirs under high phosphate and sediment loading.     

STREAM PERIPHYTON PHOTOACCLIMATION RESPONSE IN FIELD CONDITIONS: EFFECT OF COMMUNITY DEVELOPMENT AND SEASONAL CHANGES1

The photochemical behavior of intact stream periphyton communities in France was evaluated in response to the time course of natural light. Intact biofilms grown on glass substrata were collected at three development stages in July and November, and structural parameters of the biofilms were investigated (diatom density and taxonomy). At each season, physiological parameters based on pigment analysis (HPLC) and pulse‐amplitude‐modulated (PAM) chl fluorescence technique were estimated periodically during a day from dawn to zenith. Regardless of the community studied, the optimal quantum yield of PSII (F_v/F_m), the effective PSII efficiency (Φ_PSII), the nonphotochemical quenching (NPQ), and the relative electron transport rate (rETR) exhibited clear dynamic patterns over the morning. Moreover, microalgae responded to the light increase by developing the photoprotective xanthophyll cycle. The analysis of P‐I parameters and pigment profiles suggests that July communities were adapted to higher light environments in comparison with November ones, which could be partly explained by a shift in the taxonomic composition. Finally, differences between development stages were significant only in July. In particular, photoinhibition was less pronounced in mature assemblages, indicating that self‐shading (in relation to algal biomass) could have influenced photosynthesis in older communities.     

冷驯化对中缅树能量代谢的影响

在 5± 1℃条件下对中缅树鼠句进行冷驯化处理 ,测定其能量代谢。冷驯化 2 8d后 ,体重比对照组显著增加 7 3 3 % ;整体能值达到 3 0 47± 0 46kJ/g (N =8) ,比对照组增加4 98% ;摄入能比对照组增加 3 6 1 7% ;同化能比对照组增加 6 6 2 % ;生长能达到 6 98±0 5 3kJ/1 0 0g (N =7)体重·天 ,是对照组的 4 85倍 ;维持能比对照组增加 6 4 0 % ,达到3 5 2 96± 2 8 3 4kJ/1 0 0g体重·天 (N =7)。以上结果表明中缅树鼠句在冷胁迫影响下 ,以增加能量摄入、能量储存和维持能和降低排泄能量的生理机制来维持能量代谢平衡 ,以此对策来提高低温环境适应能力