Regulation of phytoplankton dynamics in a Laurentian Great Lakes estuary

The composition and dynamics of phytoplankton populations were examined in Old Woman Creek estuary, Lake Erie (USA). The centric bacillariophytes,Cyclotella atomus Hust.,Cyclotella meneghiniana Kütz., andAulacoseira alpigena (Grun.) Krammer, and the cryptophytes,Cryptomonas erosa Ehren. andRhodomonas minuta var. nannoplanctonica Skuja, dominated the phytoplankton most of the year. Chlorophytes, euglenophytes, and cyanophytes were observed less frequently. Estuarine and Lake Erie phytoplankton were considered distinct populations; lake taxa were largely confined to the estuary mouth and present only in low biomass. Maxima and minima of estuarine phytoplankton coincided with meteorological and hydrological forcing in the form of rainfall and subsequent storm-water inflows, respectively. Distinct population dynamics between the upper and lower estuary following storm events were attributed to the presence/absence of refugia serving as a source for repopulation by opportunistic taxa, fluctuating light conditions in the water column resulting from influx of particulate matter and resuspension of bottom sediments, and nutrient inputs associated with surface runoff and sub-surface interflow. Additionally, agricultural herbicides introduced by storm-water inflows potentially may affect and/or control the growth and physiological responses of individual taxa.     

Effects of mixing and silica enrichment on phytoplankton seasonal succession

During the summer of 1983, cryptophytes, diatoms, cyanophytes, and the dinoflagellate, Ceratium hirundinella were most prominant among the phytoplankton of Eau Galle Reservoir. In the open water, cryptophytes and diatoms peaked in the spring, cyanophytes were most successful in the early summer, and Ceratium was dominant from mid-July until early August. In general, the sequence of events corresponded quite closely to the model of seasonal succession developed by the Plankton Ecology Group of the International Society of Limnology. To a large extent, the same pattern held in four experimental water columns. Departures from the model involved the roles of specific nutrients in diatom and cyanophyte periodicity. Diatoms began to yield to cyanophytes in late spring despite intermittent mixing and silica enrichment. Although capable of buoyancy regulation and thus well adapted to stable water columns, cyanophytes had greater increases in biomass in mixed columns, and in those columns, were most successful during a period of intermittent mixing. Cyanophyte success varied inversely with TN : TP ratios during the period of intermittent mixing, but not subsequently. By mid-July, Ceratium dominated the phytoplankton of every column except that of a mixed column in which conditions favored cyanophytes and large diatom species.     

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.     

An Incubator for the Simulation of a Fluctuating Light Climate in Studies of Planktonic Primary Productivity

A laboratory system for the quantification of phytoplankton photosynthesis under fluctuating light climate conditions is described. It consists of 2 temperature-controlled incubators with a variable light supply, an algal batch culture in incubation bottles with appropriate stirrers and a set of oxygen electrodes to monitor algal photosynthesis. By the rotation of special grey filters between the incubator and the light source, a regular up and down movement in the water column is simulated in up to 7 parallel bottles. Different ratios of euphotic depth to mixing depth and different velocities can be applied. Simultaneously, 8 bottles can be incubated under constant light. The system is demonstrated in experiments with Chlamydomonas sp. Further possibilities of application are proposed.     

How do sinking phytoplankton species manage to persist?

Phytoplankton require light for photosynthesis. Yet, most phytoplankton species are heavier than water and therefore sink. How can these sinking species persist? Somehow, the answer should lie in the turbulent motion that redisperses sinking phytoplankton over the vertical water column. Here, we show, using a reaction-advection-diffusion equation of light-limited phytoplankton, that there is a turbulence window sustaining sinking phytoplankton species in deep waters. If turbulent diffusion is too high, phytoplankton are mixed to great depths, and the depth-averaged light conditions are too low to allow net positive population growth. Conversely, if turbulent diffusion is too low, sinking phytoplankton populations end up at the ocean floor and succumb in the dark. At intermediate levels of turbulent diffusion, however, phytoplankton populations can outgrow both mixing rates and sinking rates. In this way, the reproducing population as a whole can maintain a position in the well-lit zone near the top of the water column, even if all individuals within the population have a tendency to sink. This theory unites earlier classic results by Sverdrup and Riley as well as our own recent findings and provides a new conceptual framework for the understanding of phytoplankton dynamics under the influence of mixing processes.     

Planktic cyanophytes and their ecology in the shallow Lake Mikri Prespa, Greece

Cyanophytes dominated the phytoplankton of shallow Lake Mikri Prespa during the period May 1990 to September 1992 (76.5 and 52.0% of the total phytoplankton biomass in 1990–1991 and 1991–1992, respectively). Biomass peaks were observed in autumn (from 5.2 to 34.5 g m^-3) when low dissolved inorganic nitrogen, high phosphate phosphorus and low Z_cu to Z_mix ratio prevailed. The dominant species were Microcystis aeruginosa and M. wesenbergii. These represented 64 and 86% of the cyanophyte biomass in 1990–1991 and 1991–1992, respectively and revealed similar patterns of seasonality forming biomass peaks in late summer - autumn. Small chroococcalean cyanophytes (< 2 μm) showed also similar temporal distributions. Of the filamentous cyanophytes, the most important species was Anabaena lemmermannii var. minor which formed peaks in late summer and autumn (2.6 and 1.1 g m^-3 in 1990 and 1992, respectively). Rainfall and the N:P ratio were probably the main factors influencing the seasonality of all of the filamentous cyanophytes in the lake. Cyanophytes, tended to increase at temperatures higher than 16 °C and at inorganic nitrogen concentrations lower than 100 μg 1^--¹. The frequent mixing of the water column did not seem to prohibit the substantial increase of the group.     

Variation in species light acquisition traits under fluctuating light regimes: implications for non‐equilibrium coexistence

Resource distribution heterogeneity offers niche opportunities for species with different functional traits to develop and potentially coexist. Available light (photosynthetically active radiation or PAR) for suspended algae (phytoplankton) may fluctuate greatly over time and space. Species‐specific light acquisition traits capture important aspects of the ecophysiology of phytoplankton and characterize species growth at either limiting or saturating daily PAR supply. Efforts have been made to explain phytoplankton coexistence using species‐specific light acquisition traits under constant light conditions, but not under fluctuating light regimes that should facilitate non‐equilibrium coexistence. In the well‐mixed, hypertrophic Lake TaiHu (China), we incubated the phytoplankton community in bottles placed either at fixed depths or moved vertically through the water column to mimic vertical mixing. Incubations at constant depths received only the diurnal changes in light, while the moving bottles received rapidly fluctuating light. Species‐specific light acquisition traits of dominant cyanobacteria (Anabaena flos‐aquae, Microcystis spp.) and diatom (Aulacoseira granulata, Cyclotella pseudostelligera) species were characterized from their growth–light relationships that could explain relative biomasses along the daily PAR gradient under both constant and fluctuating light. Our study demonstrates the importance of interspecific differences in affinities to limiting and saturating light for the coexistence of phytoplankton species in spatially heterogeneous light conditions. Furthermore, we observed strong intraspecific differences in light acquisition traits between incubation under constant and fluctuating light – leading to the reversal of light utilization strategies of species. This increased the niche space for acclimated species, precluding competitive exclusion. These observations could enhance our understanding of the mechanisms behind the Paradox of the Plankton.     

Top-down control of phytoplankton in a shallow hypertrophic lake: Little Mere (England)

Top-down control of phytoplankton by zooplankton is possible through reductions in density of zooplanktivorous fish. Little Mere is a shallow lake where the effects of sewage effluent caused such a reduction. This allowed the large-bodied cladoceran, Daphnia magna Straus, to develop huge populations, preventing potentially large algal crops from developing.Subsequent diversion of the effluent is anticipated to lead to recovery of the fish community, reduced numbers of large-bodied grazers, and increased phytoplankton biomass. Whether the aquatic plant community, present in Little Mere, is resilient to such changes may depend upon whether cyanophytes are favoured, or not.     

Population and community responses of phytoplankton to fluctuating light

Light is a major resource in aquatic ecosystems and has a complex pattern of spatio-temporal variability, yet the effects of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light supply affects the structure and dynamics of phytoplankton communities. The effect of light fluctuations was tested at two average irradiances: low, 25 μmol quanta m⁻² s⁻¹ and high, 100 μmol quanta m⁻² s⁻¹ in 2- and 18-species communities of freshwater phytoplankton. Species diversity, and abundances of individual species and higher taxa, depended significantly on both the absolute level and the degree of variability in light supply, while total density, total biomass, and species richness responded only to light level. In the two-species assemblage, fluctuations increased diversity at both low and high average irradiances and in the multispecies community fluctuations increased diversity at high irradiance but decreased diversity at low average irradiance. Species richness was higher under low average irradiance and was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, especially at low average irradiance, where they became the dominant group, while cyanobacteria and green algae dominated low constant light and all high light treatments. Within each taxonomic group, however, there was no uniform pattern in species responses to light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of light supply had a significant effect on the growth rates of individual species grown in monocultures. Species responses to the regime of light supply in monocultures qualitatively agreed with their abundances in the community experiments. The results indicate that the temporal regime of light supply may influence structure of phytoplankton communities by differentially affecting growth rates and mediating species competition. Received: 24 September 1997 / Accepted: 8 July 1998     

The effects of a change in withdrawal operations on phytoplankton and nutrient dynamics in eau galle reservoir,Wisconsin (USA)

Selective withdrawal through the surface outlet was employed in Eau Galle Reservoir to reduce phytoplankton populations by 1) strengthening thermal stability, thereby decreasing vertical entrainment of hypolimnetic phosphorus (P); and 2) increasing epilimnetic flushing rate and discharge of algae. In spite of substantial release of sediment P over the growing season, epilimnentic P concentrations were overwhelming dominated by external sources, and thus insensitive to changes in withdrawal depth. Nor did surface withdrawal increase flushing rate sufficiently to affect phytoplankton biomass. In one year, an increase in thermal stability, due in part to surface withdrawal, permitted the development of a metalimnetic chlorophyll maximum in a summer that also experienced relatively low populations of ‘nuisance’ cyanophytes, and can be regarded as a positive effect of the change in operations.     

Environmental control of phytoplankton productivity in turbulent turbid systems

Factors affecting phytoplankton productivity are analysed in turbid systems, such as shallow lakes and rivers. When resuspension from the sediment or loading from the catchment significantly increases inorganic (non-algal) turbidity and hence light attenuation potentials for high production are not realised. Energy available for phytoplankton growth is strongly regulated by underwater light availability which depends on the critical mixing depth, fluctuating light intensities and algal circulation patterns. Higher production rates in shallow waters are often compensated by greater algal respiration due to higher water temperatures when compared to deeper lakes.Total daily integral production of turbulent, turbid environments can be predicted from a combination of easily measured variables such as maximum photosynthetic rates, algal biomass, surface irradiance and some measure of underwater light attenuation.     

ENHANCED GROWTH RATES OF THE GERMLINGS OF CYANOBACTERIAL RESTING SPORES (AKINETES)

In experiments with the toxic cyanobacterium, Anabaena circinalis , we investigated the nature of cyst formation and germination under different environmental conditions (variation in light quality and quantity, temperature, nutrient limitation). We consistently found that germlings up to about 5 days old had greater growth rates than those of vegetative cells grown under similar saturating irradiance and excess nutrient conditions in batch culture. Averaging the results from 4 experiments, the specific growth rate (K') fell from 1.9 d-1, 24 hours after germination to 1.17, 1.04, 0.8, and to 0.45 d⁻¹ after 48, 72, 120 and 144 hours, respectively. This equates to an increase in generation time from 0.37 to 1.54 days over this 6 day period. To investigate the implications of these results we modelled two hypothetical populations, A & B, with an initial akinete or resting cyst concentration of 10/mL each, but with population A having a standard growth rate of 0.45 and B with the 6 day early enhanced growth rate as determined in our laboratory experiments. The model indicates that population B quickly establishes dominance over population A. In this modeling exercise we have assumed a germination frequency of 100% for both populations but our laboratory results indicate that even given optimum germination conditions this factor may vary between 5% and 90% depending on the age of akinetes and the conditions experienced during dormancy (senescence). Some conditions that determine germination frequency were also investigated.     

Oxygen Productivity of Planktonic Algae under Fluctuating and Constant Light Conditions

Net oxygen productivity in cultures of Monoraphidium minutum, Cryptomonas sp. and Planktothrix agardhii exposed to fluctuating and constant light conditions was measured in a laboratory incubator. The fluctuating light climate simulated a linear up and down movement in a 2 m water column at 4 different ratios of euphotic depth to mixing depth. In addition, cultures were kept at a constant light climate simulating static incubation at 0, 0.5, 1 and 2 m depth and at the depth of the mean irradiance, respectively. Integral productivity in the simulated water column was lowest when algae were incubated at constant light in different depths, highest when the algae were incubated at constant mean photon flux density (PFD) and intermediate under fluctuating light. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Population dynamics in variable environments. II. Correlated environments,sensitivity analysis and dynamics

Mean and mean square number are studied for age-structured populations with serially correlated temporally fluctuating vital rates. Results are that (1) Moments of population number can be used effectively to analyse growth rates of the coefficient of variation and an approximate median population number. (2) Analytical approximations to the growth rates of moments reveal dynamic consequences of covarying phenotypic traits and of temporal correlation along environmental sequences. (3) Dynamic properties can be explicitly related to the static sensitivity of an average vital rate matrix. (4) The use of (1), (2) and (3) allows an extension of many applications of static vital rate theory to dynamics with fluctuating rates.     

The effects of barley straw (Hordeum vulgare) on the growth of freshwater algae

Bioassays were conducted to determine the efficacy of barley straw liquor in controlling algal growth of 12 freshwater species of algae representing three divisions. Barley straw liquor inhibited the growth of three nuisance algae common in freshwater: Synura petersenii, Dinobyron sp., and Microcystis aeruginosa. However, Selenastrum capricornutum, Spirogyra sp., Oscillatoria lutea var. contorta, and Navicula sp. had significantly increased growth in the presence of straw liquor. The growth of the remainder, Ulothrix fimbriata, Scenedesmus quadricauda, Chlorella vulgaris, Anabaena flos-aquae, and Synedra sp. showed no significant difference from controls. In a related field study, we treated four of six ponds with barley straw and monitored their chlorophyll a levels for one growing season. While phytoplankton populations in all ponds decreased in midsummer, the phytoplankton biomass in treated ponds did not differ significantly from that of control ponds, suggesting that the application of barley straw had no effect on algal growth in these systems.     

Water temperature and stratification depth independently shift cardinal events during plankton spring succession

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification‐dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.     

Vertical Disentrainment of Anabaena circinalis in the Turbid, Freshwater Darling River, Australia: Quantifying Potential Benefits from Buoyancy

The migration patterns of phytoplankton through time and depthwere measure in the Darling River at Revallyn, New South Wales,Australia during a bloom of Anabaena circinalis. Anabaena circinaliswas able to disentrain and maintain position witin surface watersduring the early morning, coinciding with the diel period ofleast wind speeds and a state of no detectable thermocline (0.1°Cdetection limit). Anabaena circinalis concentrations were upto 10 times higher in the surface waters than in the bottomwaters during the morning sampling periods. Afternoon and midnightsampling periods revealed either a small amount of surface accumulationor none. All other phytoplankton were found to have a relativelyeven distribution throughout the water column at all time periodsmeasured (except Aulacoseira on one occasion). These verticaldistribution data were used to determine the potential benefitbuoyant A. circinalis could gain over an evenly distributedpopulation using a quantitative estimate of primary productivity.The buoyant population was found to have a daily integral ofphotosynthetic O₂ production of 3.63 mol m ^–2, five timesgreater than that for the evenly distributed population. Lossesdue to respiration were greater for the evenly distributed population(29.5%) than the buoyant population (4.8%), probably due tothe amount of time cells spent outside the euphotic zone. Itis suggested that buoyancy may offer considerable advantageto A.circinalis in gaining dominance in turbid freshwater rivers.Further, buoyancy may offer some advantage even without strongthermal gradients.     

低平均光强下波动光对铜绿微囊藻生长的影响

为了研究波动光对藻类的影响, 以典型水华藻种铜绿微囊藻Microcystis aeruginos为研究对象, 运用了基于单片机系统的光强控制实验装置, 开展了不同光照条件下铜绿微囊藻的生长研究。共设置了四种光照条件, 分别为不同周期波动光强FL(Fluctuating Light)组(10min FL、1h FL和6h FL)和平均光强AL(Average Light)组。实验结果表明, 在低平均光强下, 6h FL、1h FL和10min FL组铜绿微囊藻藻密度相对于AL组分别增加了28.3%(P<0.05)、18.2%(P<0.05)和7.7(P>0.05)。三组波动光强下铜绿微囊藻的比增长速率、F_v/F_m和rETR均显著大于平均光强组(P<0.05), 且随着波动光周期的增大, 各指标也会显著增加(P<0.05), 而热耗散NPQ平均值、单个细胞类胡萝卜素含量等指标与上述指标呈相反的规律并且差异显著(P<0.05)。结果也表明在低平均光强下, 相比于恒定光照, 铜绿微囊藻在波动光下能更好地调节自身光合作用机制去利用光能, 且波动周期越大, 铜绿微囊藻对光能利用效率越高。这暗示了低强度波动光可以作为提高藻类产量的一种手段。     

Effects of algicide (copper sulfate) application on short-term fluctuations of phytoplankton in Lake Paranoá, central Brazil.

Short-term fluctuations and structure of phytoplankton were examined for two months in the dry season (August-September/1997) in a eutrophic reservoir of central Brazil. Algicide treatment for the control of Microcystis aeruginosa bloom influenced the short-term variation pattern of the phytoplankton community. Algicide treatment was effective in controlling M. aeruginosa bloom, but it also influenced the Cylindrospermopsis raciborskii population. These species retained low densities, showing small colonies and trichomes, respectively. Drops in Cyanobacteria populations following algicide treatment were succeeded by progressive increase in Chlorophyta. The shifts in environmental conditions may have allowed the development of this group. In this study correspondence analysis of abundance data for phytoplankton assemblage in lake Paranoa revealed that available light, rather than chlorophyll-a, total suspended material, and water temperature, accounted for most of the short-term fluctuation in phytoplankton structure during algicide application. Canonical correspondence analysis (CCA) showed the primary importance of water transparency changes in abundance of taxa in the community. Algicide treatment for the control of nuisance blooms is discussed as a contribution to improved efficiency in reservoir management.     

Photophysiology in two major southern ocean phytoplankton taxa: photosynthesis and growth of Phaeocystis antarctica and Fragilariopsis cylindrus under different irradiance levels

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125 μmol quanta/m2/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antarctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.