Taxonomic structure of phytoplankton assemblages in Crater Lake, Oregon, U.S.A.

SUMMARY. 1. A taxonomic analysis of 171 phytoplankton samples obtained from Crater Luke, Oregon, between 1985 and 1987 revealed 132 taxa in the upper 250 m of the water column. The greatest temporal variation in taxonomic structure occurred between 40 and 80 m below the water surface, a depth range which corresponded to the zone of maximum primary production.
2. Phytoplankton cell biovolume in the upper 20 m of the water column was relatively high during the summer months, a period when Nitzschia gracilis was dominant in the epilimnion. However, 72% or more of the cell biovolume between 0 and 200 m was distributed below 20 m and, during the winter and spring months, 61% was found below 80 m.
3. Cluster analysis identified a sparse, temporally ubiquitous flora which was modified to various degrees when environmental conditions became favourable for the growth of a few dominant taxa. These surges ot dominance by individual taxa accounted for 2 to 5-fold increases in cell biovolume and generated a pronounced taxonomic discontinuity between the floras in the epilimnion and hypolimnion.
4. While the taxonomic structure of the phytoplankton in the epilimnion corresponded closely with the structure found in a 1978–80 study, the flora below the metalimnion was more diverse and less predictable in species composition than the pattern reported in the earlier study.     

Nutrient limitation in Crater Lake,Oregon

Experiments were carried out to determine what nutrient (or nutrients) was primarily responsible for limiting phytoplankton productivity in ultraoligotrophic Crater Lake. The experiments included in situ and laboratory nutrient addition bioassays utilizing the natural phytoplankton community, Selenastrum capricornutum bottle assays, photosynthetic responses, photosynthetic carbon metabolism, and response of dark uptake of ¹⁴CO₂ with the addition of NH ₄ ⁺ . The results suggested that a trace metal(s) or its availability was the primary factor limiting the epilimnetic phytoplankton productivity. Nitrogen was extremely low, and quickly became limiting with the addition of trace metals and a chelator. Iron is the most likely candidate as the limiting nutrient. Trace metals and nitrogen are also both important in limiting phytoplankton at 100 m, a depth where biologically mediated turnover of nutrients seems to be more important.     

Ultraviolet radiation and bio-optics in Crater Lake,Oregon

Crater Lake, Oregon, is a mid-latitude caldera lake famous for its depth (594 m) and blue color. Recent underwater spectral measurements of solar radiation (300–800 nm) support earlier observations of unusual transparency and extend these to UV-B wavelengths. New data suggest that penetration of solar UVR into Crater Lake has a significant ecological impact. Evidence includes a correlation between water column chlorophyll-a and stratospheric ozone since 1984, the scarcity of organisms in the upper water column, and apparent UV screening pigments in phytoplankton that vary with depth. The lowest UV-B diffuse attenuation coefficients (K _d,320) were similar to those reported for the clearest natural waters elsewhere, and were lower than estimates for pure water published in 1981. Optical proxies for UVR attenuation were correlated with chlorophyll-a concentration (0–30 m) during typical dry summer months from 1984 to 2002. Using all proxies and measurements of UV transparency, decadal and longer cycles were apparent but no long-term trend since the first optical measurement in 1896.     

Evaporation and the hydrologic budget of Crater Lake, Oregon

The hydrologic budget of Crater Lake, Oregon is investigated by taking advantage of its relatively simple geometry, climatic circumstances, and the concurrent availability of many years of traditional data. Buoy data are here utilized for the first time for this purpose. The lake gains water through precipitation and delayed runoff from the caldera sides and Wizard Island. The lake loses water through evaporation and seepage. Seepage can be estimated quite well from ice-covered precipitation-free intervals in 1985, and is 127 cm/year. Evaporation has previously been determined as a residual, but is here estimated directly from the floating buoy, with an approximate value of 76 cm/year, a downward revision from previous estimates. These losses are balanced by precipitation input, nearly all in the form of snow or snowmelt runoff. Factors contributing to the uncertainty in each of the water budget components are discussed in some detail. The buoy data corroborate previous findings based on studies of stage that evaporation is greatest on the coldest days. Seasonally, the greatest evaporation occurs in the autumn and the least in spring. Proxy records are used to extend the effective length of the buoy record. Monthly estimates of evaporation are calculated for 1950–1996 and used to deduce temporal characteristics. The standard deviation of water year precipitation is 4.6 times larger than that of evaporation. Thus the water budget is controlled more by variability of precipitation than evaporation. An additional 15 years of data since earlier studies confirm that the annual lake level variations from one September 30 to the next are highly correlated (r = 0.96) with Park Headquarters water year precipitation for the 42 years from 1961–2003. The lake rises 1.4 cm for every cm of measured precipitation over equilibrium value (168.6 cm) at Park Headquarters. Sources of this “magnification” are discussed.     

Seasonal and interannual variability in the taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake,Oregon

Taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake, Oregon, were examined during time periods between 1984 and 2000. The objectives of the study were (1) to investigate spatial and temporal patterns in species composition, chlorophyll concentration, and primary productivity relative to seasonal patterns of water circulation; (2) to explore relationships between water column chemistry and the taxonomic composition of the phytoplankton; and (3) to determine effects of primary and secondary consumers on the phytoplankton assemblage. An analysis of 690 samples obtained on 50 sampling dates from 14 depths in the water column found a total of 163 phytoplankton taxa, 134 of which were identified to genus and 101 were identified to the species or variety level of classification. Dominant species by density or biovolume included Nitzschia gracilis, Stephanodiscus hantzschii, Ankistrodesmus spiralis, Mougeotia parvula, Dinobryon sertularia, Tribonema affine, Aphanocapsa delicatissima, Synechocystis sp., Gymnodinium inversum, and Peridinium inconspicuum. When the lake was thermally stratified in late summer, some of these species exhibited a stratified vertical distribution in the water column. A cluster analysis of these data also revealed a vertical stratification of the flora from the middle of the summer through the early fall. Multivariate test statistics indicated that there was a significant relationship between the species composition of the phytoplankton and a corresponding set of chemical variables measured for samples from the water column. In this case, concentrations of total phosphorus, ammonia, total Kjeldahl nitrogen, and alkalinity were associated with interannual changes in the flora; whereas pH and concentrations of dissolved oxygen, orthophosphate, nitrate, and silicon were more closely related to spatial variation and thermal stratification. The maximum chlorophyll concentration when the lake was thermally stratified in August and September was usually between depths of 100 m and 120 m. In comparison, the depth of maximum primary production ranged from 60 m to 80 m at this time of year. Regression analysis detected a weak negative relationship between chlorophyll concentration and Secchi disk depth, a measure of lake transparency. However, interannual changes in chlorophyll concentration and the species composition of the phytoplankton could not be explained by the removal of the septic field near Rim Village or by patterns of upwelling from the deep lake. An alternative trophic hypothesis proposes that the productivity of Crater Lake is controlled primarily by long-term patterns of climatic change that regulate the supply of allochthonous nutrients.     

Long-term observations of deepwater renewal in Crater Lake,Oregon

We examine observations of key limnological properties (primarily temperature, salinity, and dissolved oxygen), measured over a 14-year period in Crater Lake, Oregon, and discuss variability in the hypolimnion on time scales of days to a decade. During some years (e.g., 1994–1995), higher-than-average wintertime deep convection and ventilation led to the removal of significant amounts of heat and salt from the hypolimnion, while dissolved oxygen concentrations increase. In other years, such as the winter of 1996–1997, heat and salt concentrations increase throughout the year and dissolved oxygen levels drop, indicating conditions were dominated by the background geothermal inputs and dissolved oxygen consumption by bacteria (i.e., minimal deep convection). Over the entire 14 year period, no statistically significant trend was observed in the annual hypolimnetic heat and salt content. Measurements from several thermistors moored in the hypolimnion provide new insight into the time and space scales of the deep convection events. For some events, cool water intrusions are observed sequentially, from shallower depths to deeper depths, suggesting vertical mixing or advection from above. For other events, the cooling is observed first at the deepest sensors, suggesting a thin, cold water pulse that flows along the bottom and mixes more slowly upwards into the basin. In both cases, the source waters must originate from the epilimnion. Conditions during a strong ventilation year (1994–1995) and a weak ventilation year (1996–1997) were compared. The results suggest the major difference between these 2 years was the evolution of the stratification in the epilimnion during the first few weeks of reverse stratification such that thermobaric instabilities were easier to form during 1995 than␣1997. Thus, the details of surface cooling and wind-driven mixing during the early stages of␣reverse stratification may determine the net␣amount of ventilation possible during a particular year.     

Representative freshwater bacterioplankton isolated from Crater Lake, Oregon

High-throughput culturing (HTC) methods that rely on dilution to extinction in very-low-nutrient media were used to obtain bacterial isolates from Crater Lake, Oregon. 16S rRNA sequence determination and phylogenetic reconstruction were used to determine the potential ecological significance of isolated bacteria, both in Crater Lake and globally. Fifty-five Crater Lake isolates yielded 16 different 16S rRNA gene sequences. Thirty of 55 (55%) Crater Lake isolates had 16S rRNA gene sequences with 97% or greater similarity to sequences recovered previously from Crater Lake 16S rRNA gene clone libraries. Furthermore, 36 of 55 (65%) Crater Lake isolates were found to be members of widely distributed freshwater groups. These results confirm that HTC is a significant improvement over traditional isolation techniques that tend to enrich for microorganisms that do not predominate in their environment and rarely correlate with 16S rRNA gene clone library sequences. Although all isolates were obtained under dark, heterotrophic growth conditions, 2 of the 16 different groups showed evidence of photosynthetic capability as assessed by the presence of puf operon sequences, suggesting that photoheterotrophy may be a significant process in this oligotrophic, freshwater habitat.     

Representative Freshwater Bacterioplankton Isolated from Crater Lake, Oregon

High-throughput culturing (HTC) methods that rely on dilution to extinction in very-low-nutrient media were used to obtain bacterial isolates from Crater Lake, Oregon. 16S rRNA sequence determination and phylogenetic reconstruction were used to determine the potential ecological significance of isolated bacteria, both in Crater Lake and globally. Fifty-five Crater Lake isolates yielded 16 different 16S rRNA gene sequences. Thirty of 55 (55%) Crater Lake isolates had 16S rRNA gene sequences with 97% or greater similarity to sequences recovered previously from Crater Lake 16S rRNA gene clone libraries. Furthermore, 36 of 55 (65%) Crater Lake isolates were found to be members of widely distributed freshwater groups. These results confirm that HTC is a significant improvement over traditional isolation techniques that tend to enrich for microorganisms that do not predominate in their environment and rarely correlate with 16S rRNA gene clone library sequences. Although all isolates were obtained under dark, heterotrophic growth conditions, 2 of the 16 different groups showed evidence of photosynthetic capability as assessed by the presence of puf operon sequences, suggesting that photoheterotrophy may be a significant process in this oligotrophic, freshwater habitat.     

Distribution and abundance of zooplankton populations in Crater Lake,Oregon

The zooplankton assemblages in Crater Lake exhibited consistency in species richness and general taxonomic composition, but varied in density and biomass during the period between 1988 and 2000. Collectively, the assemblages included 2 cladoceran taxa and 10 rotifer taxa (excluding rare taxa). Vertical habitat partitioning of the water column to a depth of 200 m was observed for most species with similar food habits and/or feeding mechanisms. No congeneric replacement was observed. The dominant species in the assemblages were variable, switching primarily between periods of dominance of Polyarthra-Keratella cochlearis and Daphnia. The unexpected occurrence and dominance of Asplanchna in 1991 and 1992 resulted in a major change in this typical temporal shift between Polyarthra-K. cochlearis and Daphnia. Following a collapse of the zooplankton biomass in 1993 that was probably caused by predation from Asplanchna, Kellicottia dominated the zooplankton assemblage biomass between 1994 and 1997. The decline in biomass of Kellicottia by 1998 coincided with a dramatic increase in Daphnia biomass. When Daphnia biomass declined by 2000, Keratella biomass increased again. Thus, by 1998 the assemblage returned to the typical shift between Keratella-Polyarthra and Daphnia. Although these observations provided considerable insight about the interannual variability of the zooplankton assemblages in Crater Lake, little was discovered about mechanisms behind the variability. When abundant, kokanee salmon may have played an important role in the disappearance of Daphnia in 1990 and 2000 either through predation, inducing diapause, or both. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Thermal,chemical, and optical properties of Crater Lake,Oregon

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km² and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.     

The extent and significance of petroleum hydrocarbon contamination in Crater Lake,Oregon

In order to evaluate hydrocarbon inputs to Crater Lake from anthropogenic and natural sources, samples of water, aerosol, surface slick and sediment were collected and analyzed by gas chromatography-mass spectrometry (GC-MS) for determination of their aliphatic and aromatic hydrocarbon concentrations and compositions. Results show that hydrocarbons originate from both natural (terrestrial plant waxes and algae) and anthropogenic (petroleum use) sources and are entering the lake through direct input and atmospheric transport. The concentrations of petroleum hydrocarbons range from low to undetectable. The distributions and abundances of n-alkanes, polycyclic aromatic hydrocarbons (PAH) and unresolved complex mixture (UCM) from petroleum are similar for all surface slick sampling sites. The estimated levels of PAH in surface slicks range from 7–9 ng/m² which are low. Transport of petroleum-derived hydrocarbons from the lake surface has resulted in their presence in some sediments, particularly near the boat operations mooring (total petroleum HC = 1440 μg/kg, dry wt. compared to naturally derived n-alkanes, 240 μg/kg, dry wt.). The presence of biomarkers such as the tricyclic terpanes, hopanes and steranes in shallow sediments further confirms petroleum input from boat traffic. In the deep lake sediments, petroleum hydrocarbon concentrations were very low (16 μg/kg, dry wt.). Very low concentrations of PAH were detected in shallow sediments (17–40 μg/kg at 5 m depth near the boat operations) and deep sediments (3–15 μg/kg at 580 m depth). The individual PAH concentrations in sediments (μg/kg or ppb range) are at least three orders of magnitude less than reported threshold effects levels (mg/kg or ppm range, test amphipod Hyalella azteca). Therefore, no adverse effects are expected to occur in benthic biota exposed to these sediments. Boating activities are leaving a detectable level of petroleum in surface waters and lake sediments but these concentrations are very low.     

Physicochemical conditions of deposition of bacillariophyta in Davis Lake,Klamath and Deschutes Counties,Oregon, U.S.A.

Summary A diatom population composed of 74 taxonomic entries was found at a site on Odell Creek at the edge of Davis Lake, Klamath County, Oregon, U.S.A. The population, a mixture of planktonic and non-planktonic forms, indicates that the ecological conditions were alkaline, oligohalobous (indifferent), oligosaprobic and eutrophic, and that the current conditions were limnobiontic to indifferent.Ecological characteristics of the Davis Lake population and the Vestaburg Bog (one of the six deposits presented for comparison) are similar; moreover, as a whole the ecological spectra of the Davis Lake population denote deposition in a bog or swale that could have served as part of a drainage basin for the runoff from volcanic uplands. It was not possible to determine the age of the deposit on the basis of the diatom flora; however, it is probably Holocene or Recent.

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Zusammenfassung Eine Diatom Bevölkerung, die aus 74 taxonomischen Einträgen besteht, wurde an einer Stelle am Odell-Bach am Rande des Davis-Sees, Klamath Bezirk, Oregon, USA, gefunden. Diese Bevölkerung, eine Mischung von planktonischen und nicht-planktonischen Formen zeigt an, da die ekologischen Bedingungen alkalisch, oligohalobous (mittelmäig), oligosaprobisch und eutrophobisch waren, und da die gegenwärtigen Zustdnde (Bedingungen) limnobiontisch bis mittelmäig waren.Die ekologischen Eigenschaften der Davis-See Bevölkerung und die des Vestaburg-Bogs (Sumpfes), (wie es einer der 6 Bodensätze zum Vergleich zeigt) gleichen einander; weiterhin zeigen die ekologischen Spektra der Davis-See Bev6lkerung im Groen und Ganzen den Bodensatz in einem Sumpf (Bog), der als ein Teil des Entwässerungs-Bassins für das Abwassern von vulkanischen Bergländern dienen konnte. Es war nicht möglich, das Alter des Bodensatzes auf dem Bassin auf Grund der diatomischen Flora festzustellen. Es ist wahrscheinlich jedoch Holocene oder Recent.

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Sumario Una población de diatomeas compuesto de 74 tipos taxonómicos fué encontrado a uno sitio en Odell Arroyo (Riachuelo) al borde del Lago Davis, Contado de Klamath, Oregon, Estados Unidos (de Amca). La populación, una mixtura de formas planctonicas y no planctonicas, indica que la condiciones ecol6gicas fue alcalina, oligohalobica (indiferente), oligosaprobica, y eutropica y que las condiciones conla corriente fue limnobiotica a indiferente.Las caracteristicas ecol6gicas de la poblaci6n de Lago Davis y el Pantano Vestaburg (uno de los seis depósitos presentaron para comparación) son analógicas; además en conjunto, los espectros ecológicos de la poblaci6n del Lago Davis denota deposición en un pantano o un terreno pantanoso que hubiera servido de una porción de una. cuenca de drenaje (arroyada) por el agua de desagiie de la tierras altas volcánicas. No fue posible determinar la edad del depósito (la población) por medio de flora de diatomeas; sin embargo, es probable Holoceno o Reciento.

     

An HPLC analysis of the summer phytoplankton assemblage in Lake Baikal

1. The enormous size and spatial heterogeneity of Lake Baikal require rapid methods for large sample sets. We therefore tested the applicability of a novel, high‐performance liquid chromatography (HPLC)‐based, combination of methods for analysing phytoplankton. In July 2001, samples were collected in a transect across the lake at various depths down to 30 m. Phytoplankton (>3 μm) and autotrophic picoplankton (APP) were counted under light and epifluorescence microscopes, respectively. Pigments were analysed with HPLC. 2. The pigment data allowed the contributions of the dominant phytoplankton groups to the total chlorophyll a (Chl a) in the lake to be estimated by multiple linear regression and by the CHEMTAX matrix factorisation program. Three marker pigments, fucoxanthin, lutein and zeaxanthin, were shown to be useful indicators of the abundance and spatial distribution of certain phytoplankton groups. The relative contributions of the various phytoplankton groups to the total Chl a in the lake determined using these marker pigments were similar, but not identical, to those determined by cell counts. 3. Pigment analyses of isolated strains from Lake Baikal and some European lakes confirmed that phycoerythrin‐containing Cyanobacteria with very high amounts of zeaxanthin were responsible for the low Chl a/zeaxanthin ratios of the water samples. A picoplanktonic species of Eustigmatophyceae was isolated from the lake. Its high violaxanthin content, responsible for very low Chl a/violaxanthin ratios of some water samples, can be used to estimate the contribution of this group to total Chl a.     

Parasitology of the English Sole, Parophrys vetulus Girard in Oregon, U.S.A.

A total of 877 juvenile English sole ( Parophrys vetulus Girard) from the Yaquina Bay estuary and742 juvenileandadultsole from the Pacific Ocean off Oregon were examined forparasites. Fifteen species of parasites were found in juvenile English sole on the estuarine nursery ground. Differences in the prevalence and intensity of parasite infection between size classes of juvenile sole and between sole occupying the upper and lower estuary were determined. An additional 14 parasite species were found in offshore English sole, bringing the total observed in all fish to 29 species. Parasites acquired only in the estuary included the microsporidan Glugea stephani , the acanthocephalan Echinorhynchus lageniformis , and the nematode Philometra americana . Those acquired only in offshore areas included the trematodes Otodistomum veliporum and Zoogonus dextrocirrus , the leech Oceanobdella sp. and three species of copepods. An attempt to use parasite data to indicate the presence of distinct English sole stocks along the Oregon coast was inconclusive.     

Vertical profiles of primary productivity,biomass and physico-chemical properties in meromictic Big Soda Lake,Nevada, U.S.A.

The contribution of bacteria to total primary production was estimated in Big Soda Lake and related to vertical profiles of biomass and various physical and chemical properties. The purple sulfur bacteriaThiocapsa sp. was responsible for 25% of the total primary production. Bacteriochlorophyll (BChl) distribution and DCMU sensitivity experiments confirmed this bacterial activity. High concentrations of photosynthetically inactive phytoplankton were detected in anaerobic-low redox water and were attributed to an early season algal bloom that had settled into a zone of maximal viscosity. Nutrient enrichment bioassay experiments indicated that iron was the most important nutrient controlling early season algal growth.     

The Plecoptera community of a small stream in Oregon,U.S.A. *

The Plecoptera community of a small woodland stream in Oregon was studied by examining density and species composition. Four sites were studied using benthos sampling for larvae and emergence trap collections for adults. Sixteen genera and forty-three species were identified with most species occurring in relatively low numbers. A comparison with the species list from an earlier study suggested that the species composition has not changed markedly in 40 years but there were several changes in relative abundance. Species diversity of stoneflies decreased with distance from the headwaters. These changes were correlated with decrease in habitat complexity, decrease in stability of the substrate, and increase in water depth. Examples of longitudinal succession are given as a method enabling the large number of species to partition the habitat. Concurrent benthos and emergence sampling demonstrated differences in emergence success that were correlated with habitat suitability.     

Steady state of phytoplankton assemblage in the tropical Lake Catemaco (Mexico)

Phytoplankton of the tropical lake Catemaco (Veracruz, Mexico) showed similar species composition during samplings from 1993 to 1995. There were two small dominant cyanobacterial species Cylindrospermopsis catemaco Kom.-Legn. et Tavera and Cylindrospermopsis philippinensis (Taylor) Kom., and a group of larger algae and cyanobacteria that were always present, however in smaller numbers: Aulacoseira granulata (Ehr.) Simons morphotype curvata, A. cf. italica (Ehr.) Simons mf. curvata, Fragilaria construens (Her.) Grun., Achnanthes minutissima Kütz., Planktolyngbya circumcreta (G.S.West) Anagn. et Kom., Chroococcus microscopicus Kom.-Legn. et Cronberg. Moreover we found several other scarcely present species. The percentage of total biomass of the two dominant species of Cylindrospermopsis varied between 34 and 81%, but they accounted for 80 to 95% of abundance. Apart from geomorphological features and climate conditions, biological variables played an important role. Fish-stock was formed by filter-feeding native herbivorous species of fish Dorosoma petenense (Günther), Bramocharax caballeroi (Contreras et Rivera), Astyanax mexicanus (Filippi), and an introduced, also herbivore Oreochromis niloticus (L.). Feeding activity of fish removed large species of algae and cyanobacteria as well as detrital remnants and zooplankton from the water. Smaller, inedible cyanobacteria remained in the water and formed the stable portion of the phytoplankton, dominant both in biomass and abundance. CANOCO analysis of samples and species variability demonstrated results of competition between two species of Cylindrospermopsis: steady state during the dominance of C. catemaco lasting probably for the whole year 1993 (one dry and one wet season) and steady state during the dominance of C. philippinensis in 1994 and 1995. According to the functional classification of phytoplankton suggested by Reynolds et al. (2002), Catemaco dominant assemblage would belong to the functional group S _N.     

Fluctuations in a late devonian-early Mississippian phytoplankton flora of Ohio,U.S.A

Fossil diversity and abundance fluctuations of Late Devonian (Famennian)—Early Mississippian (Tournaisian) phytoplankton assemblages were determined in a stratigraphic sequence of samples in Barberton test core 3, from northeast Ohio. Both diversity and abundance were relatively high during deposition of the lower Chagrin Shale (Upper Devonian) and both decreased during deposition of the upper Chagrin Shale. The overlying Cleveland Shale (Upper Devonian) is characterized by sharply increased abundance with a corresponding (although less marked) increase in diversity; in contrast, both abundance and diversity were at relatively low levels during deposition of the Lower Mississippian Bedford Shale, the uppermost unit of the sequence. These trends agree with the phytoplankton periodicity model presented by Tappan, which indicates large-scale extinction at the end of the Devonian.Many of the acritarch and prasinophycean algal taxa have restricted stratigraphic occurrences within the section studied, making them useful for local and perhaps worldwide correlation. The stratigraphically restricted taxa provide a basis for the proposal of five biostratigraphic zones within the Upper Famennian (Upper Devonian) and a portion of the Lower Mississippian. In ascending order occur: the Gorgonisphaeridium ohioense Assemblage Zone, the Acriora petala Assemblage Zone, the Muraticavea enteichia Range Zone, the Cymatiosphaera labyrinthica Range Zone, and the Navifusa drosera Assemblage Zone.     

Diatoms in sediment cores in Utah Lake,Utah, U.S.A.

Diatoms from sediment cores taken from Provo Bay, Geneva and midlake sites in Utah Lake, Utah County, Utah were studied. Algal communities from strata at four centimeterintervals of the cores were analyzed. This study has demonstrated that the diatom flora in Utah Lake has changed through time and that the diatom communities of Provo Bay are floristically unique from those of the main body of the lake.     

Role of nutrients and zooplankton in regulation of phytoplankton in Flathead Lake (Montana, U.S.A.), a large oligotrophic lake

1. Increased primary production in Flathead Lake during the 1980s has been variously attributed to increased nutrient loadings and/or decreases in zooplankton abundance resulting from the introduction of Mysis relicta . In order to assess the importance of these two factors in regulating the phytoplankton community in Flathead Lake, we manipulated zooplankton abundance and nutrient availability in a series of 5‐day enclosure experiments.
 2. Chlorophyll a levels were stimulated by simultaneous addition of nitrogen and phosphorus. At ambient nutrient levels, alteration of zooplankton density had no effect on chlorophyll a levels. Top‐down control through zooplankton grazing could only be demonstrated in treatments supplemented with nutrients. Under these conditions, there was a significant negative correlation between zooplankton abundance and final chlorophyll a levels.
 3. These results suggest that the phytoplankton community in Flathead Lake is regulated primarily by bottom‐up controls. Consequently, future management activities aimed at preventing further increases in algal growth in the lake should focus on nutrient abatement. Alteration of the upper trophic levels does not appear to have significantly affected phytoplankton abundance in the lake. Should nutrient levels increase in the future, then top‐down controls may become more important.
 4. A conceptual model is presented illustrating the relative importance of top‐down and bottom‐up controls across a trophic gradient.