Summer dynamics of the deep chlorophyll maximum in Lake Tahoe

Vertical profiles of chlorophyll and phytoplankton biomass weremeasured in Lake Tahoe from July 1976 through April 1977. Adeep chlorophyll maximum (DCM) persisted during summer and earlyautumn (July—October) near 100 m, well below the mixedlayer and at the upper surface of the nitracline. The DCM coincidedwith the phytoplankton biomass maximum as determined from cellcounts. In addition, the composition of the phytoplankton assemblagewas highly differentiated with respect to depth. Cyclotellastelligera was the predominant species in the mixed layer whilethe major species in the DCM layer included C. ocellata andseveral green ultraplanktonic species. In situ cell growth playsa substantial role in maintaining the DCM, but sinking of cellsfrom shallower depths and zooplankton grazing above the DCMmay contribute to the maintenance of the DCM. Calculations supportthe interpretation that the summer DCM persists at the boundarybetween an upper, nutrient-limited phytoplankton assemblageand a deeper, light-limited assemblage.     

Phytoplankton assemblages in the deep chlorophyll maximum layers off the Mediterranean coast of Israel

Phytoplankton assemblages in the deep chlorophyll maximum andnear-surface layers were compared at seven stations in the inshoreand offshore waters of the Mediterranean coast of Israel. Thestudy included the entire spectrum of taxonomic categories overa wide size range, comprising the nano/pico phytoplankton componentsdown to 1 µm and the larger phytoplankters consistingprimarily of diatoms and dino-flagellates. The coccolithophorids<20 µm and the monads constituted the most abundantcomponents of the phytoplankton at the deep chlorophyll maximum(DCM) and near surface layer. Certain individual species, mainlypennate diatoms and smaller dinoflagellates, seemed to adaptto the DCM to form a characteristic association.     

An attempt to simulate the subsurface chlorophyll maximum using populations of unphased oscillating cells

A model is proposed to represent the vertical distribution ofchlorophyll in tropical waters. It assumes that the phytoplanktonpopulations are composed of cells which oscillate around a meanlevel at a density _t = B (close to the nutricline), with anamplitude A; A and B are normally distributed variables; theoscillations are unphased so that the number of cells in a waterlayer does not vary with the time. This model is tested on datafrom the eastern tropical Atlantic and from the southwesterntropical Pacific. The results agree with the observations whenthe surface mixed layer is nutrient-exhausted. However, themodel seems to account only for a deep population, and is thenconsistent with the hypothesis of a distinct mixed layer floralassociation. The agreement between the results and the observationsis difficult when the surface mixed layer is not nutrient-limited,in areas of upwelling or intense vertical mixing. The modelaccounts for a splitting of the subsurface chlorophyll maximumafter the passage of a front in the Cape Lopez area (GuineaGulf). It also accounts for the relations between the subsurfacechlorophyll maximum, the pycnocline, and the nutricline at 48stations from the cruise PREFIL 2 in the southwestern tropicalPacific. The permanent subsurface chlorophyll maximum of oligotrophicareas seems to be more satisfactorily explained by the unphasedoscillations invoked by the model than by the sinking of thecells. These unphased oscillations have not yet been observed;their possible consequences concerning the primary productionand grazing are postulated.     

Phytoplankton in the surface and chlorophyll maximum off southern California in August, 1978

A synoptic sampling of 30 stations in a 25 km x40 km grid closeto the southern California coast revealed that chlorophyll andphytoplankton biomass were higher nearshore and to the south,where relatively large dinoflagellates dominated. A subsurfacechlorophyll maximum was present at each station, near the nitraclineand 10% light level. Nearshore and to the south, these maximarepresented higher levels of phytoplankton biomass than at thesurface whereas at the other stations, chlorophyll maxima couldbe attributed to physiological changes in chlorophyll contentof small flagellated phytoplankton rather than to the accumulationof phytoplankton at depth. Univariate and multivariate statisticalprocedures were used to demonstrate that phytoplankton assemblagesseparated by tens of meters in the vertical were just as differentas those separated by tens of kilometers in the horizontal.We conclude that residence time of water near the coast is ofgreat importance to the determination of the abundance and taxonomiccharacteristics of phytoplankton, and that advection of offshorewater towards the coast was the major determinant of the patternobserved during our study. ¹Present address: University of Texas, Port Aransas Marine Laboratory,Port Aransas, TX 78373, USA     

Physical-Biological Coupling in Southern Lake Michigan: Influence of Episodic Sediment Resuspension on Phytoplankton

The influence of episodic, sediment resuspension on phytoplankton abundance/volume and composition, the photosynthetic maximum rate (P^B _max) and efficiency (^B), and chlorophyll-specific growth (_Chl) was evaluated during the spring isothermal period in southern Lake Michigan (Laurentian Great Lakes, USA). Resuspension altered the nutrient and light climate of nearshore waters; light attenuation (K_d) and phosphorus concentrations corresponded (p 0.0001 and p 0.001, respectively) with concentrations of suspended particulate matter (SPM). Phytoplankton cell volume and diatom cell abundance and volume were not associated with SPM concentrations (p > 0.05). Diatom composition displayed spatial dissimilarities corresponding with resuspension (p 0.001); small centric diatoms exhibiting meroplanktonic life histories and pennate diatoms considered benthic in origin were most abundant within SPM-impacted, nearshore waters whereas taxa typically comprising assemblages in optically-clear, offshore waters and the basin-wide, spring bloom were not. Values of P^B _max and ^B corresponded (p 0.0001) with both K_d coefficients and SPM concentrations, potentially reflecting increased light harvesting/utilization within impacted assemblages. However, integral production was inversely associated with K_d coefficients and SPM concentrations (p < 0.0001) and photosynthesis was light-limited (or nearly so) for most assemblages. Although _Chl values corresponded with K_d coefficients (p 0.05), values were quite low (x ± S.E., 0.10 ± 0.004 d^-1) throughout the study. Most likely, distinct rate processes between SPM- and non-impacted assemblages reflected short-term compositional (and corresponding physiological) variations due to infusion of meroplankton and/or tributary-derived phytoplankton. Overall, resuspension appears to have little, if any, long-term impact upon the structure and function of the lakes phytoplankton.     

Population dynamics of the chydorid cladocera of a Southern Michigan Marl Lake

The seasonal population dynamics are described for 21 chydorid species in two shallow Scirpus subterminalis beds of Lawrence Lake, Michigan, USA. Six species were rare; four species were predominantly abundant. The dynamics of the populations followed no consistent pattern among the species. However, each species, with one exception, may follow the same pattern from year to year. Eight species overwintered under the ice as parthenogenetic individuals. Populations of Alonella excisa may be greatly reduced during the summer months because of competition with A. exigua.     

Phytoplankton dynamics in the stratified water column of Lake Bonney,Antarctica

Phytoplankton populations in perennially ice-covered Lake Bonney, Antarctica grow in a unique non-turbulent environment. The absence of turbulence generated by winds or major streams, combined with strong vertical gradients in temperature and nutrients, create vertically stratified environmental conditions that support three discrete phytoplankton populations in the east lobe of this lake. Phytoplankton biomass and photosynthesis were measured in the east lobe of Lake Bonney during the winter-spring transicion (September) to mid-summer (January). During this period, irradiance beneath the ice increased from 0.03 to 1.9 mol quanta m⁻² d⁻¹. Chlorophylla concentrations ranged from 0.03 to 3.8 μl⁻¹ within the trophogenic zone (just beneath the permanent ice cover to 20 m) and photosynthesis ranged from below detection to 3.2 μg Cl⁻¹ d⁻¹. Our results indicate: (1) phytoplankton photosynthesis began in late winter (before 9 September, our earliest sampling date); (2) maxima for phytoplankton biomass and production developed sequentially in time from the top to the bottom of the trophogenic zone, following the seasoral increase in irradiance; and (3) the highest photosynthetic efficiencies occurred in early spring, then decreased over the remainder of the phytoplankton growth season. The spring decrease in photosynthetic rates for shallower phytoplankton appeared to be related to nutrient availability, while photosynthesis in the deeper populations was solely lightdependent.     

Phytoplankton population dynamics in perennially ice-covered Lake Fryxell, Antarctica

Phytoplankton were collected over five austral summers (1987–88through 1991–92) to examine seasonal and annual fluctuationsin species composition and biovolume in Lake Fryxell, a perenniallyice-covered lake located in the Dry Valleys of South VictoriaLand, Antarctica. Lake Fryxell has perennial gradients of salinity,dissolved oxygen and nutrients. We found that algal speciesdiversity was low (56 taxa were collected), confirming the resultsof previous short-term studies. The phytoplankton consistedprimarily of cryptophyte and chlorophyte flagellates, and filamentouscyanobacteria. The presence of filamentous cyanobacteria, whichhave not been reported as abundant in this lake by previousworkers, may represent a significant ecological change. Eachaustral summer, one dominant species contributed >70% ofthe total biovolume; Chroomonas lacustris was dominant in 1987–88,while Cryptomonas sp. dominated the phytoplankton in the remaining4 years. No species succession occurred during the austral summer.Some common taxa were vertically stratified (Oscillatoria limnetica,Phormidium angustissimum, Pyramimonas sp., Oscillatoria sp.),while others showed no distinct vertical stratification (Chlamydomonassubcaudata, Cryptomonas sp.). The stratification of the phytoplanktonreflects the gradients of nutrients and light, and the stabilityof the water column.     

Predator-prey interactions in Lake Michigan: model predictions and recent dynamics

Synopsis Several years ago, we used a bioenergetics model to evaluate the impact of increasing salmonid stocking on the highly variable alewife forage base in Lake Michigan. At that time, we forecast an alewife population decline and the following system-wide effects: increased abundances of large zooplankton, decreased salmonid growth rates, increased diet breadth of salmonids, niche shifts among competitors of the alewife, increased alewife growth rates and increased densities of fishes suppressed by alewife. Alewives have continued to decline steadily since 1981 and are now reduced to a density similar to early outbreak levels in the early 1960s. Recent reports on fish growth rates, zooplankton size and fish community structure support our projections regarding system-wide responses to the alewife decline.     

Phytoplankton of Lake Peipsi-Pihkva: species composition, biomass and seasonal dynamics

With 33 years of phytoplankton quantitative studies carried out, a series of qualitative data with a length of over 80 years is at our disposal. About 500 algal species have been found in plankton by different researchers. In different seasons and years 35 main species (dominants and subdominants) form 68–96 % of biomass in L. Pihkva (southern, more eutrophic part) and 60–97 % in L. Peipsi (northern, less eutrophic part). L. Lämmijärv, connecting the two parts is similar to L. Pihkva in respect to phytoplankton and the trophic state. Diatoms and blue-green algae prevail in biomass, diatoms and green algae, in the species number. The oligo-mesotrophic Aulacoseira islandica (O. Müller) Sim. is characteristic of the cool period; A. granulata (Ehr.) Sim. and Stephanodiscus binderanus (Kütz.) Krieger prevail in summer and autumn, the latter being most abundant in the southern part. Gloeotrichia echinulata (J.S. Smith) P Richter and Aphanizomenon flos-aquae (L.) Ralfs dominate in summer causing water-bloom. Phytoplankton has mostly three maxima in seasonal dynamics in L. Peipsi and two in L. Pihkva. Its average biomass in spring in different years has fluctuated in the range 5.6–16 and 6–12.7 g m^–3, in summer 3.1–14.8 and 5.6–125 (10–20 in most cases); and in autumn 7–16.3 and 5.2–26 in the northern and southern parts, respectively.The dominant complex has not changed considerably since 1909; however, the distribution of dominant species in lake parts has become more even in the last decades. Periods of high biomass occurred in the first half of the 1960s and 1970s and in 1988–1994, of low biomass in 1981–1987. The first coincided, in general, with periods of low water level and high water temperature.     

Ecology of Phytoplankton of the Volta Lake

Summary Regular limnological observations at the lower reaches of the Volta Lake revealed significant changes in the transparency, colour, total iron and dissolved oxygen from 1965 to 1966 but not from 1966 to 1967. These factors also showed correlations of such a significance that many of them could be estimated from a simple measurement of the transparency alone. The density of phytoplankton did not change significantly either from 1965 to 1966 or from 1966 to 1967. The chief constituents, however, passed through a series of changes from a green alga (Actinastrum) in early 1965, through the flagellates (Crytomonas and Peridinium) during 1965–1966, to the diatoms (Nitzschia and Synedra) during 1966–1967. The density of phytoplankton correlated significantly only with the dissolved oxygen from which it could be roughly estimated.

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Résumé Les observations limnologiques régulières dans la partie Sud du Lac Volta révélaient des changements significatifs dans la transparence, la couleur, la quantité totale de fer et l'oxygène dissout pour la période de 1965 à 1966 mais rien de semblable pour celle de 1966 à 1967. Ces facteurs montraient aussi des corrélations si significatives que beaucoup d'entre eux pouvait être estimé à partie de la simple mensuration de la seule transparence. La densité de phytoplancton ne changeait pas de manière significative que ce soit de 1965 à 1966 ou de 1966 à 1967. Toutefois, les principaux constituants passaient par une série de changements allant des algues vertes (Actinatrum) au début de 1965, aux flagellates (Cryptomonas et Peridinium) de 1965 à 1966 puis aux diatomées (Nitzschia et Synedra) de 1966 à 1967. La densité de phytoplancton presentait une corrélation significative uniquement avec l'oxygène dissout duquel il pouvait être grossièrement estimé.

     

Population dynamics of Bythotrephes cederstroemii in south-east Lake Michigan 1995–1998

1. Population characteristics (density, size, reproductive patterns) of the predatory cladoceran Bythotrephes cederstroemii in south-east Lake Michigan were monitored at an offshore station (110 m) in 1995–98 and at a nearshore station (45 m) in 1997–98.
2. The mean density of B. cederstroemii at the offshore station was generally highest in July–September (145–914 m⁻²) and at the nearshore station in October–November (168–1625 m⁻²). In 1995 and 1998, density was also high at the offshore station in November (211–284 m⁻²). Fish predation may limit B. cederstroemii in nearshore regions in the summer. The maximum annual densities of B. cederstroemii for 1995–98 were generally similar to those reported from the late 1980s, when the species arrived in Lake Michigan.
3. Body size increased rapidly each year to a maximum in August. Thereafter, body size declined and converged for stage-2 and 3 individuals, suggesting food scarcity or size-selective fish predation was affecting large individuals.
4. Most reproduction occurred asexually (90%), and by stage 2 or 3 females (99%). Asexual brood size was highest when B. cederstroemii first appeared each year, and decreased in August, when larger neonates were produced. There appeared to be differences in reproductive mode for stage 2 and 3 females, with a higher percentage of stage 2 females reproducing sexually.     

Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan

The abundance, biomass, and life history traits of Mysis relicta were evaluated in the spring, summer, and autumn of 2000 at 8 nearshore (45 m) and 8 offshore (75–110 m) stations throughout central and southern Lake Michigan. Abundance was also evaluated on a smaller scale during June 2000 at 4 nearshore and 4 offshore stations in southeastern Lake Michigan. For large-scale sampling, the abundance of M. relicta did not differ among locations in the spring. In the summer and autumn the abundance of M. relicta was similar among offshore stations with the exception of one station each season; for nearshore stations, abundance was generally highest off Pentwater, Michigan. The abundance of mysids was not consistently high for central or southern basin sites, although overall biomass was higher in the southern basin each season. Abundance of Mysis was positively correlated with bottom depth, but not with bottom water temperature, surface water temperature, or mean chlorophyll concentration. Within the smaller region in southeast Lake Michigan, the abundance of M. relicta differed among locations for both nearshore and offshore stations. Brood size and size of reproductive females did not differ among lake wide locations, but the proportion of females with broods and the size distribution of M. relicta did.     

Virus morphological diversity and relationship to bacteria and chlorophyll across a freshwater trophic gradient in the Lake Michigan watershed

The effect of starvation on sexual reproduction in cyclic parthenogenetic rotifers has been studied using life history experiment. Short-time starvation of rotifers that experienced starvation immediately after hatching from resting eggs can cause high induction of sexual reproduction up to the 10th generation. However, it is not clear whether the induction of sexual reproduction can occur beyond the 10th generation. To investigate this phenomenon, we conducted a sex induction study using the monogonont rotifer Brachionus manjavacas. Newborn stem females were starved for 12 h, while controls were supplied with 7.0 × 10⁶ cells ml^?1 of Nannochloropsis oculata. In a life history experiment, the rotifers were individually cultured in 96-well microplates containing 0.2 ml of seawater (22 ppt) in each well at 25 °C with daily feeding thereafter. Mixis induction in offspring from starved stem females was significantly higher than in those from non-starved stem females up to the 40th generation. The effect of accumulative generations increased mixis induction up to the 20th generation. Effect on future generations of the rise in mixis ratio by the starvation to stem females may facilitate colonization by favoring population growth via female parthenogenesis and by decreasing food requirements for survival and reproduction.     

Phytoplankton dynamics in the Congo River

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Phytoplankton dynamics in a shallow,hypertrophic reservoir (Lake Arancio,Sicily)

Phytoplankton abundance and composition in the hypertrophic man-made Lake Arancio was analyzed, based on a programme of weekly sampling from May 1990 to November 1991 and supported by measurements of limnological parameters. The highest value of phytoplankton biomass (78 mg l^–1) was observed in October 1990, during a bloom of the desmid Closterium limneticum var. fallax, while the lowest (0.15 mg l^–1) was measured in April 1991. During spring, autumn and winter 1990, species of the genus Closterium dominated the community, in the sequence: C. aciculare, C. limneticum var. fallax, C. limneticum. The summer community was more diverse with the predominance of organisms belonging to Chlorophyceae (Chlamydomonas, Eudorina, Coelastrum) and Cyanophyceae (Microcystis, Anabaena). In spring 1991, there was a long clear-water phase during which small green algae (Ankyra, Oocystis) and cryptomonads dominated. Subsequently, the summer season was characterized by a clear sequence of dominants, drawn, in turns, from species belonging to: Bacillariophyceae, Dinophyceae, Cyanophyceae, Euglenophyceae. The physics of the reservoir and its depth, owing to filling/draining constraints in a summer-arid climate, appeared to play a key role in the dynamics of phytoplankton community.     

Depuration of PCBs in the Lake Michigan Ecosystem

A suite of physical processes controls the rate at which polychlorinated biphenyl (PCB) concentrations change in Lake Michigan fauna. To the extent that these processes are homogenous in space, predator and forage fish species are exposed to the same water column and sediment concentrations of PCB in Lake Michigan, whereas exposure due to diet varies with trophic position. We can think of observed PCB concentrations in a particular species as realizations of an unknown process consisting of many sources of variability. Because all species are utilizing the resources of the same ecosystem, a component of variability of PCB concentration is shared by all organisms in the system, whereas other components are not. Individual species were modeled with parameters that were estimated as the overall, or reference mean and a shared rate of change, plus an individual species offset from that reference mean and shared rate of change. This approach allows species-specific differences to be manifest via the offset, whereas the shared components of variability are known with greater precision than would be possible if we considered only a single species. The objective of this work is to determine the extent to which observed declines in PCB concentrations in various species in the Lake Michigan ecosystem are similar and synchronous. A high degree of similarity and synchrony between species would imply a high degree of spatial homogeneity of the physical processes controlling PCB concentrations in Lake Michigan fauna. A lack of synchrony suggests that other factors, such as food web interactions or spatial patterns, may be important. Received 9 March 1999; accepted 29 February 2000.     

Phytoplankton photosynthesis in the subsurface chlorophyll-maximum layer of the tropical North Pacific Ocean

Maximum quantum yield (φ_m^B) and maximum photosynthetic rate (P_m^B) of light-saturation curves of phytoplankton photosynthesis were determined for nannoplankton (< 20 μm) and netplankton (>20 μm) from the subsurface chlorophyll-maximum layer at 14 stations in the tropical North Pacific Ocean in the spring of 1976. The maximum quantum yield $\text{(φ}{}_{\mathrm{m}}{}^{\mathrm{B}}\text{±}\text{s.e.}\text{)}$ was significantly higher for nannoplankton (0.056 ± 0.006 moles CO₂·Einstein^?1 absorbed) than netplankton (0.039 ± 0.002 moles CO₂·Einstein^?1 absorbed). The importance of nannoplankton in the maximum photosynthetic rate (P_m^B) appears to be less consistent. At least 60% of the theoretical maximum quantum yield (0.12 moles CO₂·Einstein^?1 absorbed) was probably incorporated into the particulate fraction at the subsurface chlorophyll-maximum layer.