Cell death in lake phytoplankton communities

1. The fraction of living and dead phytoplankton cells in seven Florida lakes was assessed by using the cell digestion assay, a non‐staining membrane permeability test. The cell digestion assay is an effective method to analyse cell viability in complex natural phytoplankton communities. 2. The lakes examined ranged widely in phytoplankton abundance and community composition. The variability in the percentage of living cells (% LC) was high among the taxonomic groups forming the different phytoplankton communities, ranging from 19.7% to 98% LC. 3. All cells within single cyanobacteria filaments were determined to be either dead or alive, suggesting physiological integration of the cells within colonies. 4. Within each lake, the dominant taxa generally exhibited the highest proportion of living cells. A high proportion of living cells was found to be a characteristic of the different taxa forming the communities of eutrophic lakes. The average value for the % LC for all groups comprising the phytoplankton communities in each of the lakes ranged from 29.9 ± 7.2 to 80.4 ± 4.0 (mean ± SE) and varied strongly and positively with chlorophyll a concentration. 5. These results suggest phytoplankton cell death to be an important process structuring phytoplankton communities in lakes, particularly in oligotrophic ones.     

Photosynthetic responses of Zostera marina L. (Eelgrass) to in situ manipulations of light intensity

Summary Photosynthetic responses of the temperate seagrass, Zostera marina L., were examined by manipulations of photon flux density in an eelgrass bed in Great Harbor, Woods Hole, MA during August 1981. Sun reflectors and light shading screens were placed at shallow (1.3 m) and deep (5.5 m) stations in the eelgrass bed to increase (+35% to +40%) and decrease (-55%) ambient photon flux densities. The portion of the day that light intensities exceeding the light compensation point for Z. marina (H _comp) and the light saturation point (H _sat) were determined to assess the impact of the reflectors and shades. The H _comp and H _sat periods at the deep station shading screen were most strongly affected; H _comp was reduced by 11% and H _sat was reduced by 52%. Light-saturated photosynthetic rates, dark respiration rates, leaf chlorophyll content, chlorophyll a/b, PSU_{O 2} size, PSU density, leaf area, specific leaf area, leaf turnover times and leaf production rates were determined at the end of three sets of 1- to 2-week experiments. None of the measured parameters were affected by the photon flux density manipulations at the shallow station; however, at the deep station leaf production rates were significantly reduced under the shading screen and chlorophyll a/b ratios were higher at the reflector. These results indicate that adjustment to short-term changes in light regime in Z. marina is largely by leaf production rates. Further, the most dramatic changes in the periods of compensating or saturating photon flux densities had the greatest impact on the measured photosynthetic responses.     

In situ quantum yield of phytoplankton in a subalpine lake

The percent of light absorbed by algae, estimated from a regressionof attenuation of downwelling photosynthetic photon flux densityon chlorophyll a concentration, was used in conjunction withmeasurements of phytoplankton photosynthesis to estimate quantumyields () of Castle Lake phytoplankton populations during the1980 ice-free season. The maximum values occurred during mid-summerin the deep hypolimnion (<1% of surface photon flux) andwere {small tilde} 78% of the theoretical maximum. However,later in the season (September) the deep-water values decreasedsignificantly to {small tilde}27% of the theoretical maximumin conjunction with a decrease in quantum flux density to thesewaters. The values for epilimnetic phytoplankton populationswere never >15% of the theoretical maximum. The presenceof relatively high concentrations of peridinin and chlorophyllc in the deep-water algae enabled these organisms to absorbthe predominantly green light in this region of the lake which(i) allowed the use of a single extinction value per unit ofchlorophyll (k_c) at all depths when calculating , and (ii) increasedthe of the deep-water algae, relative to those in the near-surfacewaters. These data indicate that the deep-water algae in CastleLake were able to utilize their ambient light field, particularlyin the early summer, and suggest that the deep-chlorophyll layerin this lake is maintained primarily by in situ algal growthrather than sinking and accumulation of organisms from nearsurface waters.     

Response kinetics of tethered Rhodobacter sphaeroides to changes in light intensity

Rhodobacter sphaeroides can swim toward a wide range of attractants (a process known as taxis), propelled by a single rotating flagellum. The reversals of motor direction that cause tumbles in Eschericia coli taxis are replaced by brief motor stops, and taxis is controlled by a complex sensory system with multiple homologues of the E. coli sensory proteins. We tethered photosynthetically grown cells of R. sphaeroides by their flagella and measured the response of the flagellar motor to changes in light intensity. The unstimulated bias (probability of not being stopped) was significantly larger than the bias of tethered E. coli but similar to the probability of not tumbling in swimming E. coli. Otherwise, the step and impulse responses were the same as those of tethered E. coli to chemical attractants. This indicates that the single motor and multiple sensory signaling pathways in R. sphaeroides generate the same swimming response as several motors and a single pathway in E. coli, and that the response of the single motor is directly observable in the swimming pattern. Photo-responses were larger in the presence of cyanide or the uncoupler carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP), consistent with the photo-response being detected via changes in the rate of electron transport.     

Species-specific phytoplankton responses to nutrients and zooplankton manipulations in enclosure experiments

1. In situ enclosure experiments were performed in the mesotrophic Bermejales reservoir to evaluate the algal response to changes in the nutrient supply and in the zooplankton size structure and density in a 2 × 2 factorial design. The experiments were conducted during the spring bloom of nanoplanktonic diatoms in 1989. 2. Nutrient enrichment promoted a great increase of phytoplankton biomass indicating a strong nutrient limitation on phytoplankton growth. Total phytoplankton biomass was significantly lower in the Daphina-added enclosures at a given nutrient level and strong direct an indirect effect of zooplankton on phytoplankton community structure and nutrient availability were observed. 3. Most of the nanoplanktonic species were effectively grazed but species with protective coverings and large size colonies were favoured by grazers and small chlorococcales were unaffected probably because of their compensatory high growth rates. The decrease in total biomass imposed by grazers is attributable mainly to the decrease of Cyclotella ocellata, the most abundant species. This taxon suffers two net effects of zooplankton: direct grazing and the indirect decrease of Si availability caused by the growth of C. ocellata which was promoted by P excretion by zooplankton. Indirect effects of grazers on Si availability should, therefore, be taken into account in explaining phytoplankton succession and community structure. 4. In this experiment grazers affected considerably the nanoplanktonic community in Bermejales reservoir. The extent which they were affected, however, depended not only on the algal size as a determinant of edibility but also greatly on the specific nutrient requirements and taxonomic features of the algal species.     

The role of spatial scale and area in determining richness-altitude gradients in Swedish lake phytoplankton communities

Lake phytoplankton richness data were analyzed to reveal the effects of area and scale on the richness-elevation relationship. Lakes provide a unique opportunity to accomplish this because of their well-defined boundaries, which clearly define local communities and their habitat area. Local phytoplankton richness (alpha diversity) was found to decrease with increasing elevation, even when the effect of lake area was accounted for. This decrease coincided with a decrease in lake productivity with increasing elevation. Additionally, pairwise dissimilarity calculations suggested that isolation by distance was less important in structuring local phytoplankton richness than isolation by either elevation or environmental distance. In contrast, phytoplankton richness calculated for elevation bands (gamma diversity) showed a hump-shaped dependence on elevation, with a mid-elevation maximum, consistent with the predictions of a mid-domain null model. Moreover, mean pairwise dissimilarity within elevation bands shows compositional dissimilarity to be highest at high elevations and low environmental distances, emphasizing the isolated character of high-elevation lakes. We suggest that the form of the phytoplankton richness-elevation relationship is scale-dependent, and that geometric constraints, differences in the possibility of horizontal and vertical dispersal, environmental heterogeneity and an underlying monotonic trend in productivity, are likely to be responsible for the patterns in alpha and gamma diversity observed along elevation gradients.     

Deep-water aquatic plant communities in an oligotrophic lake: physiological responses to variable light

1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelectric power generation and the lake levels are manipulated within an operating range of 3 m. There was concern that rapidly changing water levels adversely affected the littoral zone by decreasing light availability in two ways: local turbidity caused by shoreline erosion at low water levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer. 2. The littoral zone was dominated by native aquatic plants with vascular species to 6 m and a characean meadow below this to 16 m. The biomass and heights of the communities in the depth zone 0–6 m were reduced at a site exposed to wave action relative to those at a sheltered site. However, the community structure below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this bottom boundary remained constant throughout the year despite large seasonal changes in solar radiation and the 3 m variation in lake level. 3. There was evidence that the deep-water community consisting of Chara corallina had adapted physiologically to low-light conditions. Net light saturated photosynthesis (CO₂ exchange) per unit chlorophyll a (Chl a) was reduced to 1.7 μg C (μg Chl a)^?1 h^?1 at the lower boundary, half of that recorded at 5 m. The concentration of Chi a per gram of biomass (dry weight), was considerably greater at the lower boundary than higher in the profile [c. 7 mg Chl a (g dry wt)^?1 at 16 m vs. 4 mg Chl a (g dry wt)^?1 at 5 m]. Chl b also increased with depth and there was no change in the ratio of Chl a and Chl b with increasing depth. The saturation light intensity (I_k) of the community at the lower boundary was only 78 μmol photons m^?2 s^?1. Photosynthetic parameters (I_k and α) as well as the Chl a content remained relatively constant throughout the seasonal and short-term changes in radiation. 4. The photosynthetic characteristics of the littoral community were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the deep-water community would be sensitive to any further changes in underwater light availability.     

Composition and dispersal of riverine and lake phytoplankton communities in connected systems with different water retention times

1. Lake phytoplankton community structure may be influenced by both internal factors (predation, competition, resource constraints) and external ones, such as dispersal of materials and cells between connected habitats. However, little is known about the importance of cell dispersal for phytoplankton community structure in lakes. 2. We investigated the abundance and dispersal of phytoplankton cells between connected rivers and lakes, and analysed whether similarities in phytoplankton community composition between rivers and lakes were primarily related to cell import rates or to characteristics of the local habitat. We focused on lakes along a gradient of theoretical water retention times (TWRT). Two data sets from Swedish lakes were used; a seasonal study of two connected boreal forest lakes, differing in TWRT, and a multi‐lake study of 13 lakes with a continuous range of TWRTs. 3. Phytoplankton cells were transported and dispersed in all investigated rivers. In the seasonal study, cell import rates and similarities in phytoplankton community composition between the lake and its inlet(s) were much higher in the lake with a shorter TWRT. Phytoplankton community structure in different habitats was associated with total organic carbon (TOC). This indicates that local habitat characteristics may be important in determining lake phytoplankton community composition, even in the presence of substantial cell import. 4. The multi‐lake study also showed a negative relationship between TWRT and similarities in phytoplankton community composition between inlets and lakes. Moreover, similarity in community structure was related to both cell import rates from inlet to lake and differences in habitat characteristics between inlet and lake. However, the variable most strongly correlated with community structure was TOC, indicating that species sorting rather than a mass effect was the most important mechanism underlying the correlation between community structure and retention time. 5. Overall, our data suggest that local habitat characteristics may play a key role in determining community similarity in this set of lakes covering a large range of habitat connectedness. Due to the strong co‐variations between cell dispersal and TOC, it was hard to unequivocally disentangle the different mechanisms; hence, there is a need for further studies of the role of dispersal for phytoplankton community structures.     

Influence of changes in salinity and light intensity on growth of phytoplankton communities from the Schelde river and estuary (Belgium/The Netherlands)

In the Schelde continuum, a succession in the phytoplankton community is observed along the transition from the river to the freshwater tidal reaches of the estuary and from the freshwater to brackish reaches of the estuary. The goal of this study was to experimentally evaluate the contribution of changes in salinity and light climate to this succession. In summer 2000 and in spring 2001, phytoplankton communities from the river, the freshwater tidal reaches and the brackish reaches of the estuary were incubated under high or low light intensities and exposed to a change in salinity. HPLC analysis was used to evaluate the response of different algal groups to changes in light intensity and salinity. When incubated at a light intensity corresponding to the mean underwater light intensity of the freshwater tidal reaches, growth of phytoplankton from the river as well as from freshwater tidal reaches was significantly lower than when incubated at a light intensity corresponding to the mean underwater light intensity of the river. The phytoplankton community from the freshwater tidal reaches did not appear to be better adapted to low light intensities than the phytoplankton community from the river. Although diatoms were expected to be less sensitive to a reduction in light intensity than green algae, the opposite response was observed. Freshwater and brackish water phytoplankton were negatively affected by respectively an increase or decrease in salinity. However, the effect of salinity was not strong enough to explain the disappearance of freshwater and brackish water phytoplankton between a salinity of 0.5 and 10 psu, suggesting that other factors also play a role. In the freshwater phytoplankton communities from the river and the freshwater tidal reaches, green algae and diatoms responded in a similar way to an increase in salinity. In the brackish water phytoplankton community, fucoxanthin displayed a different response to salinity than lutein and chlorophyll a.     

Relationships between phytoplankton dynamics and the availability of light and nutrients in a shallow sub-tropical lake

The role of light and nutrient availability in controlling theabundance and structure of phytoplankton populations was studiedin Lake Okeechobee, a large eutrophic lake in south florida,USA. Measurement of selected environmental parameters at samplingsites within four ecologically distinct regions of the lakewere combined with direct experimental determinations of limitinglevels of light and nutrients for phytoplankton growth to determinespatial and temporal variations in the relative roles of theselimiting factors. Estimated mean light availability in the mixedlayer, I_m, was significantly lower in the turbid central regionof the lake than in other regions. Correlations between I_m andphytoplankton standing crops led to the conclusion that lowlight availability in the central region of the lake, and toa lesser extent in other areas, restricts phytoplankton standingcrops to levels below the potential provided by the nutrientsavailable. The results of the irradiance-growth experimentsconfirmed the conclusions of the correlation analyses that phytoplanktongrowth is restricted by the levels of light availability experiencedduring the winter and spring in the central region of the lake.Bioassays indicated that nitrogen was the most frequently limitingnutrient for phytoplankton growth. High rates of nitrogen fixationwere frequently observed in the lake, along with correspondinglyhigh abundances of nitrogen-fixing cyanobacteria and nitrogenfixation activity. Elevated concentrations of soluble inorganicnitrogen appeared to suppress both nitrogen fixation and therelative abundance of nitrogen-fixing cyanobacteria.     

Physical and chemical effects of macrograzers and micrograzers on enclosed,in situ phytoplankton in a Newfoundland lake

Grazing effects were evaluated by comparisons between phytoplankton populations in the lake water and in bicompartmental chambers, deployedin situ in a Newfoundland lake. Micrograzing effects were evaluated separately from both the physical effects of macrograzers and the chemical effects of macrograzer metabolites on phytoplankton assemblages in bicompartmental chambers. Macrograzer metabolites were able to pass through fine gauze between the compartments. Thus, the chemical effects of the macrograzers were observed in one compartment while the physical effects were the dominant treatment in the other. Micrograzer effects were investigated in chambers without added macrograzers.Densities of some of the desmid species were depressed by the physical effects of macrograzers. This is the first ecological study to demonstrate such a grazing effect on desmids. Some individual taxa, such asArthrodesmus triangularis var.rotundatus (Raciborski)comb. nov., andMesotaenium sp. Nägeli (Zygnemaphyceae), were augmented under the chemical effects of macrograzer metabolites. Evidence suggests that the desmids were not micrograzed. Grazing effects on desmids have not previously been studied in waters in which they were abundant.The Chlorophyceae, which were small species in Hogan's pond, and the microflagellates generally maintained their overall population densities in spite of the presence of macrograzers, however, abundances of some individual species within the assemblages changed in the presence of macrograzers. Micrograzers may also affect the species composition and abundance of the individual chlorophycean taxa.With the exceptions ofSynedra sp. Ehrenberg andTabellaria fenestrata var.lacustris Meister the diatoms were unaffected by the grazers. The Chrysophyceae were little affected by micrograzing and physical effects of macrograzers, in contrast to chemical effects of macrograzer metabolites that were evident for individual species.Several of the Cyanobacteria (Cyanophyceae) were augmented in the presence of micrograzers. Evidence indicates that they were not micrograzed. Some individual species, e.g.,Microcystis aeruginosa Kützing, were augmented by both physical and chemical effects of macrograzers.     

Learning by grasshoppers: association of colour/light intensity with food

ABSTRACT. Nymphs of Melanoplus sanguinipes learn to associate food with visual cues such as colour or light intensity. Such an ability in active exploratory insects may improve foraging efficiency by reducing time spent searching in unsuitable areas. Significant effects are demonstrated after a single rewarded trial, and the effects are extinguished after a variable number of unrewarded trials.     

Quantitative responses of lake phytoplankton to eutrophication in Northern Europe

Based on the currently largest available dataset of phytoplankton in lakes in northern Europe, we quantified the responses of three major phytoplankton classes to eutrophication. Responses were quantified by modelling the proportional biovolumes of a given group along the eutrophication gradient, using generalized additive models. Chlorophyll-a (Chl-a) was chosen as a proxy for eutrophication because all classes showed more consistent responses to Chl-a than to total phosphorus. Chrysophytes often dominate in (ultra-) oligotrophic lakes, and showed a clear decrease along the eutrophication gradient. Pennate diatoms were found to be most abundant at moderate eutrophication level (spring-samples). Cyanobacteria often dominate under eutrophic conditions, especially in clearwater lakes at Chl-a levels >10 μg l⁻¹ (late summer samples). We compare the relationships among types of lakes, based on the lake typology of the northern geographic intercalibration group, and among countries sharing common lake types. Significant differences were found especially between humic and clearwater lakes, and between low- and moderately alkaline lakes, but we could not identify significant differences between shallow and deep lakes. Country-specific differences in response curves were especially pronounced between lakes in Norway and Finland, while Swedish lakes showed an intermediate pattern, indicating that country-specific differences reflect large-scale geographic and climatic differences in the study area.     

The effects of continuous light and light intensity on the reproduction rates of twenty-two species of marine phytoplankton

The acclimated reproduction rates of 22 species of marine phytoplankton were measured at 0.01, 0.023, 0.1, and 0.23 ly/min in continuous light and in a 14: 10 h light: dark cycle. Three species that reproduced exponentially at all four light intensities in the 14: 10 LD regime did not reproduce at all in continuous light at any of the light intensities. One species, which reproduced at the two lowest light intensities in the 14: 10 LD regime, failed to reproduce at all in continuous light at any light intensity examined. Seven species reproduced more slowly in continuous light than in the 14: 10 LD regime at most or all light intensities. Four species reproduced at roughly the same rate in both light regimes. Five species reproduced more rapidly in continuous light.

No general phylogenetic trend could be discerned from the responses of the species to the different light intensities or to continuous light. In general, species from coastal regions can reproduce as rapidly or more rapidly in continuous light than in a 14: 10 LD cycle, while most species from oceanic regions are harmed by continuous light. A phylogenetic trend in maximum potential reproduction rate is apparent, with diatoms being the fastest, dinoflagellates the slowest, and coccolithophores somewhat intermediate.     

The influence of fluctuating light intensities on species composition and diversity of natural phytoplankton communities

The influence of fluctuating light intensities on phytoplankton composition and diversity was investigated for 49 days under semi-continuous culture conditions with sufficient nutrient supply, using phytoplankton assemblages from Lake Biwa, Japan. Light conditions were either periodically changed from high intensity (100 µmol photons m^-2 s^-1) to low intensity (20 µmol photons m^-2 s^-1) at intervals of 1, 3, 6 and 12 days, or fixed to constant intensities (permanent high and low light levels). All treatments additionally experienced a day:night cycle of 16:8 h. Phytoplankton abundance increased and reached a saturation level on day 19 of the treatment with permanent high light, but increased continuously until the end of the experiment (day 49) in the treatment with permanent low light intensity. In treatments with periodically changing light intensities, the phytoplankton abundance reached saturation levels between these dates. Under phytoplankton abundance saturation, chlorophytes predominated in the treatment with permanent high light, while either cyanophytes or diatoms were abundant under permanent low light intensity. Treatments with changing light supply had chlorophyte- and cyanobacteria-dominated replicates as well as replicates with balanced proportions of both. Furthermore, species diversity, measured by the Shannon index, was low in cultures under permanent light intensity, while slow fluctuating light at the scale of 3 -12 days resulted in an increased diversity index. These results indicate that species composition and diversity of the phytoplankton were affected by the periodically changing light regime in the order of days, and suggest that temporal changes in weather conditions are a major impediment to competitive exclusion of phytoplankton species in nature.     

Zooplankton grazing on lake bactenoplankton and phytoplankton

Grazing experiments with the calanoid Calamoecia lucasi andthe cladoceran Ceriodaphnia dubia feeding on free lake bacterioplanktonproduced artificially high clearance rates by the acridine orangedirect count method (AODC), but lower and more conservativeclearance rates by labelling bacteria with [³H]thymidine. Acomparison of zooplankton carbon requirements and bacterialbiomass suggests that even at maximal filtration rates bacterioplanktoncan only be a minor constituent of the diet. Bacterioplanktonand phytoplankton do not provide adequate carbon to satisfythe daily requirement for respiration let alone for growth andreproduction. This suggests that: (i) predation of small zooplanktonoccurs; (ii) stress depresses the clearance rates; or (iii)the values obtained are at the low end of the diel range offeeding rates.     

Biomass partitioning in Florida phytoplankton communities

Comparison of the partition of biomass among genera within phytoplanktoncommunities in 165 lakes demonstrated a shift from communitieswith a relatively even distribution of biomass among taxa tocommunity where one, or a few taxa, capitalized most of thebiomass, as communities biomass increased. This pattern suggeststhat interference among co-existing phytoplankton taxa increasesas community biomass increases, and provides additional evidencefor the biomass-dependence of phytoplankton community structure.     

Patterns in taxonomic and functional diversity of lake phytoplankton

1. Patterns in phytoplankton diversity in lakes and their relationships with environmental gradients have been traditionally based on taxonomic analyses and indices, even though measures of functional diversity (FD) might be expected to be more responsive to such gradients. 2. We assessed the influence of water column physical structure, and other components of the overall environment, on lake phytoplankton diversity using two taxonomically based indices [species richness (S) and the Shannon index (H’)] and a FD index, to determine whether these different measures respond in similar ways to habitat structure. The study encompassed 45 lakes in Eastern Canada, within two lake districts [the Eastern Townships Region (ETR) and Laurentians Region (LR)] that vary in geology and landscape and in lake morphometry and chemistry. 3. Across all lakes, S and H’ were higher in lakes having greater vertical temperature heterogeneity and higher susceptibility to wind mixing. In addition, H’ declined with total phosphorus concentration. FD was only related to maximum lake depth, a variable that integrates many other habitat features. 4. Further insight into the factors affecting phytoplankton diversity was obtained by contrasting the two regions. The taxonomically based diversity measures differed little between the regions, while FD was higher in the ETR where more trait variants were present and more evenly distributed amongst species. Whereas factors driving S did not differ between the regions, we found region‐dependent patterns in the relationships of H’ and FD with maximum lake depth: both indices decreased with maximum depth in the region with lakes more exposed to wind (ETR) but increased in the more hilly landscape where lakes are more sheltered from wind mixing (LR). 5. Our study demonstrates that, for phytoplankton communities, a FD index can show simpler and stronger responses to environmental drivers than a taxonomically based index, while shedding further light onto the functional traits that are important in particular lake categories.