Phytoplankton Population Dynamics Research Articles

Phytoplankton population dynamics at the Bermuda Atlantic Time-series station in the Sargasso Sea

Abstract Phytoplankton populations were analyzed using flow cytometry in monthly samples at the Bermuda Atlantic Time-series Study (BATS) station in the Sargasso Sea from 1989–1994 for picoplankton ( Synechococcus and Prochlorococcus ) and from 1992–1994 for eukaryotic phytoplankton in order to better understand the mechanisms that dictate seasonal and inter-annual patterns in the phytoplankton community. The eukaryotic phytoplankton were dominated by populations of small nanoplankton (mostly 2–4 μm diameter), though populations of coccolithophores and sometimes pennate diatoms also could be distinguished. Flow cytometric measurements of population abundances, individual cell light scattering (which can be related to cell size), and chlorophyll fluorescence were made. Synechococcus and the eukaryotic phytoplankton reached their greatest concentrations during the spring bloom each year when the water column was deeply mixed and nutrients were detectable in surface waters. The maximum cell concentration for Prochlorococcus was in the summer and fall of each year, with a deeper sub-surface maximum than Synechococcus . Picoplankton chlorophyll fluorescence and estimated cell size were greater at depth than near the surface, and were lowest in midsummer for both Synechococcus and Prochlorococcus . In the summer and fall, Prochlorococcus cells were often smallest at mid-depth, even when fluorescence per cell and cell concentration were lower at the surface. For the eukaryotes (including coccolithophores), cell concentrations were high during the spring in both 1992 and 1993, and in fall 1992. At these times, mean cell size and fluorescence were low. Improved size and carbon estimates were made and it was found that the estimated contribution of phytoplankton carbon to total particulate organic carbon, integrated over the upper 200 m, averaged 33% (range 21–43%) with no pronounced seasonal pattern.

Incorporation of the intracellular elemental correlation pattern into simulation models of phytoplankton uptake and population dynamics

Compelling evidence of complex statisticalrelationships among various elements contained withinphytoplankton cells has traditionally been ignored inmodels of algal nutrient uptake and populationdynamics. Here we present a new approach,incorporating a phytoplankton intracellular elementalcorrelation pattern into the existing dynamicsimulation model of a freshwater lake. Within thisapproach, uptake and cycling of elements that arelikely to become limiting during the simulation periodare described by ordinary differential equations.Dynamics of nutrients that are unlikely to becomelimiting are described either by differentialequations or, when more practicable, by multipleregressions on environmental variables and cell quotasof other elements. This allows an easy simultaneousconsideration of a wide range of elements.The model adopting the described approach wastested on a data set for Rostherne Mere, Cheshire, UK.It showed a good fit between observations andsimulations for all considered variables, includingthe population dynamics of Ceratium hirundinellaand Microcystis aeruginosa, the outcome ofinterspecific competition and changes inconcentrations within algal cells and in thesurrounding lake water. The approach could easily beimplemented in models of bioreactors, chemostatexperiments and aquatic ecosystems.

The regulation of phytoplankton population dynamics in the world’s largest lakes

Abstract The world’s largest lakes, really inland seas, are characterised by long retention times, and are dominated by internal physical forcing, generally low nutrient loadings and predominantly internal recycling. Most are oligotrophic. Many are severely phosphorus deficient. Certainly few ever support large crops of phytoplankton. Horizontal heterogeneity sometimes permits enhanced production in shallow bays or behind thermal bars and occasional blooms develop under conditions of near-surface stratification. An assessment of published information on the composition and seasonality of phytoplankton in the open water habitats of these lakes confirms the oligotrophic nature of the world’s great lakes. There is a predominance of diatoms at all latitudes; chrysophytes are seasonally prominent in some lakes at high latitude; other flagellates, including dinoflagellates, and species of cyanobacteria represent increasing proportions of the pelagic biomass towards the equator. There is an indication that species composition is influenced by underwater light availability and a positive correlation between Microcystis plankton and relatively higher concentrations of total phosphorus (TP > 30 μg l-1) is suggested. Picoplankton is apparently abundant during periods of relatively high insolation of the water column. Although the carrying capacity of the nutrients available is scarcely large, the production of biomass is strongly related to seasonal variability in the intensity and extent of water-column mixing and its relation to the periodicity and underwater penetration of photosynthetically active radiation. Attainment of nutrientlimited crops generally coincides with shallow mixing whereas deep circulation suppresses production. The differential effects of latitude, local climate and salinity upon this general deduction are also evaluated. The role of grazing, its contribution to nutrient recycling, and its contribution to sustaining pelagic food webs, is also considered. The paper makes some deductions about the threats placed on large-lake ecosystems by pollution, eutrophication and acidification and upon how their ecosystem health might be monitored and conserved in the future.

The regulation of phytoplankton population dynamics in the world's largest lakes

The world's largest lakes, really inland seas, are characterised by long retention times, and are dominated by internal physical forcing, generally low nutrient loadings and predominantly internal recycling. Most are oligotrophic. Many are severely phosphorus deficient. Certainly few ever support large crops of phytoplankton. Horizontal heterogeneity sometimes permits enhanced production in shallow bays or behind thermal bars and occasional blooms develop under conditions of near-surface stratification.An assessment of published information on the composition and seasonality of phytoplankton in the open water habitats of these lakes confirms the oligotrophic nature of the world's great lakes. There is a predominance of diatoms at all latitudes; chrysophytes are seasonally prominent in some lakes at high latitude; other flagellates, including dinoflagellates, and species of cyanobacteria represent increasing proportions of the pelagic biomass towards the equator. There is an indication that species composition is influenced by underwater light availability and a positive correlation between Microcystis plankton and relatively higher concentrations of total phosphorus (TP>30μgl−1) is suggested. Picoplankton is apparently abundant during periods of relatively high insolation of the water column. Although the carrying capacity of the nutrients available is scarcely large, the production of biomass is strongly related to seasonal variability in the intensity and extent of water-column mixing and its relation to the periodicity and underwater penetration of photosynthetically active radiation. Attainment of nutrient-limited crops generally coincides with shallow mixing whereas deep circulation suppresses production.The differential effects of latitude, local climate and salinity upon this general deduction are also evaluated. The role of grazing, its contribution to nutrient recycling, and its contribution to sustaining pelagic food webs, is also considered. The paper makes some deductions about the threats placed on large-lake ecosystems by pollution, eutrophication and acidification and upon how their ecosystem health might be monitored and conserved in the future.

Interaction effects of fish, nutrients, mixing and sediments on autotrophic picoplankton and algal composition

1. We examined the effects of nutrients, turbulent mixing, mosquitofish, Gambusia affinis Baird and Girard and sediments on algal composition, algal biomass and autotrophic picoplankton (APP) abundance in a 6‐week experiment of factorial design in twenty‐four 5‐m3 outdoor mesocosms during late autumn 1995.2. Turbulent mixing decreased surface temperature and increased turbidity, which also was increased by the addition of sediments. Total algal biomass was significantly enhanced by nutrients and mixing, and decreased by the sediment treatment. In the mixing × nutrient treatment, algal biomass increased more than expected from the individual effects, while the fish × mixing and mixing × sediment treatments increased algal biomass less than expected.3. Cryptomonas (cryptomonad) blooms were observed in the unmixed, high nutrient treatment; Synedra (diatom) blooms were observed in the high nutrient, high sediment treatment; Ulothrix (green algae) blooms were observed in the mixed, high nutrient, low sediment treatment.4. Eukaryotic APP abundances were increased by sediment addition and by turbulent mixing, and increased synergistically by mixing × sediment and mixing × nutrient interactions. Prokaryotic APP abundances were decreased by nutrient enhancement and by a mixing × nutrient interaction. There were no main effects of fish on APP abundance, but fish were involved in some of the two–way interactions.5. The large number of significant interaction effects indicates that APP and other phytoplankton are regulated by a complex set of interdependent factors which should be considered simultaneously in studies of phytoplankton population dynamics and community composition.

Zooplankton effects on vertical particulate flux: Testable models and experimental results

The effects of herbivorous zooplankton on the sedimentation of particles out of the euphotic zone are examined with mathematical models, a large‐scale field experiment, and descriptive data from a eutrophic lake. The theory is rooted in the population dynamics of phytoplankton and zooplankton and so explicitly accounts for the potential effect of zooplankton grazing on primary production and the connection between the rate at which phytoplankton cells sink and sustainable zooplankton biomass. The models predict positive, negative, or unimodal zooplankton effects, depending on the values of four parameters: rate of direct phytoplankton sinking, fraction of zooplankton fecal material exiting the euphotic zone, zooplankton assimilation efficiency, and system productivity. Models were parameterized with data from a eutrophic lake to make a priori predictions about the shape and direction of zooplankton effects. Predictions were tested against the results of an independent experiment in which a gradient of Daphnia biomass was established in large enclosures. Daphnia negatively affected sedimentation rates of carbon, nitrogen, and phosphorus in the enclosure experiment, which confirmed model predictions. Daphnia had a strong negative effect on phytoplankton biomass, and phytoplankton biomass was positively correlated with sedimentation in the enclosures. Experimental results were congruent with relationships between Daphnia biomass and sedimentation rate in the lake. Successful application of the theory suggests that these models may be of utility for assessing the direction of zooplankton effects on vertical flux in other systems. More generally, the models help to identify parameters that should be measured in studies of zooplankton effects on downward particle flux.

Dynamics of a toxic cyanobacterial bloom (Cylindrospermopsis raciborskii) in a shallow reservoir in the semi-arid region of northeast Brazil

The species composition and the seasonal succession of phytoplankton were analyzed in a eutrophic drinking water reservoir located in the semi-arid region of northeast Brazil. Investigations were based on bimonthly or monthly sampling over 2 yr (1997 to 1998) conducted at 1 representative station with 2 sampled depths (0.5 and 5 m near the bottom). Limnological parameters (temperature, pH, dissolved oxygen, conductivity, light, dissolved inorganic nutrients) were simultaneously measured to determine the possible factors affecting the phytoplankton composition. We determined 30 taxa dur- ing the survey which were numerically dominated by the class Chlorophyceae. However, both in terms of abundance and biomass, Cyanobacteria dominated the phytoplankton community with Cylindros- permopsis raciborskii (Wolsz.) Seenayya et Subba Raju. This species can represent biomass close to 96-100% of total phytoplankton biomass, with values reaching 70 mg 1-I (fresh weight) between April and November 1998. Over the survey, the filaments of C. raciborskii were coiled (average of 97 %) with a mean proportion of 12.3 % of terminal heterocytes. The species toxicity was determined from bioas- say analysis and the presence of neurotoxins was revealed during the bloom. By March 1998, chloro- phyll concentration reached 135 pg 1-l at the surface level, inducing a sharp decrease of the euphotic zone depth. Favorable environmental conditions were observed for the bloom with high temperatures, high pH, low N/P ratio, and absence of efficient predators. The nutrient context seemed to play a role in the cyanobacterial bloom despite the absence of external nutrient supply. However, annual rain deficit and lack of water renewal in 1998 linked to the 1997 El Nino consequences seem to be the major factors responsible for both hypereutrophic conditions and cyanobacterial blooms in the reservoir stud- ied. Thus, global climate change can influence phytoplankton population dynamics in continental waters, as demonstrated frequently in oceanic ecosystems.

Population Dynamics of Phytoplankton and Viruses in a Phosphate-limited Mesocosm and their Effect on DMSP and DMS Production

The effect of phosphate limitation on viral abundance, phytoplankton bloom dynamics and production of dimethylsulphoniopropionate (DMSP) and dimethyl sulphide (DMS) was investigated in seawater mesocosm enclosures, in a Norwegian fjord, during June 1995. Daily estimates of viral concentrations, based on transmission electron microscope (TEM) counts, varied on an apparently random basis in each of the enclosures. A largeSynechococcusspp. bloom developed in an enclosure which was maintained at a high N:P ratio, simulating phosphate-deplete growth conditions. Following phosphate addition to this enclosure, there was a large increase in estimated virus numbers shortly before an apparent collapse of theSynechococcusbloom. It is tentatively suggested that lysogenic viruses were induced following phosphate addition to the phosphate-limited enclosures, and that these observations add to a growing body of evidence which supports the hypothesis that nutrient availability may be responsible for the switch between lysogeny and lytic production. High DMS concentrations and viral numbers were observed on the demise of the flagellate (predominantlyEmiliania huxleyi) and diatom blooms, but overall there was no significant correlation. Highest concentrations of DMSP were associated with blooms ofE. huxleyi, for which an intracellular concentration of 0·5pg cell−1(SD, 0·06) was calculated. Good correlation of DMSP withSynechococcusspp. cell numbers was observed, suggesting that these species of picoplankton may be significant producers of DMSP. No effects of phosphate limitation on DMS and/or DMSP production were evident from the data.

Primary and secondary effects of methabenzthiazuron on plankton communities in aquatic outdoor microcosms

The primary and secondary effects of methabenzthiazuron on plankton were studied in outdoor microcosms (6 m3) for five months. Macrophytes, phytoplankton, zooplankton and the emergence of insects were examined. The herbicide was added to nine of the twelve mesocosms in concentrations ranging from 10 to 3371 μg a.i. l−1 in a single application. The other three ponds were used as untreated controls. Phytoplankton population dynamics fluctuated depending on light intensity, temperature, nutrients, competition and herbicide concentrations. Primary production was temporarily inhibited at test concentrations from 89 to 3371 μg l−1. Cryptophyta (Phytoflagellata) showed also a recovery at the higher concentrations during the study period. Other primary producers did not recover at these concentrations, as expected from the disappearance curve of the active ingredient at these concentrations. This was also indicated by structural indices such as the diversity, eveness and similarity index. The lower concentrations evoked no or only short-term, weak responses. Chemical water parameters were strongly influenced by the absence of macrophytes (Elodea canadensis) at the high application rates, which also caused significant changes in zooplankton communities. No effects were observed on macro-invertebrates and benthic organisms. In conclusion, noticeable effects were detected only at concentrations clearly above the effect threshold known from standard laboratory tests with aquatic primary producers.

Disruption of grazer populations as a contributing factor to the initiation of the Texas brown tide algal bloom

Disruption of grazer populations, or mismatches between phytoplankton blooms and population dynamics of their planktonic and benthic grazers, may play an important role in the initiation of some harmful algal blooms. The Laguna Madre of Texas has experienced a persistent, nearly monospecific phytoplankton bloom since January 1990 commonly referred to as the “brown tide.” Whole‐water samples collected in the area where the bloom began reveal the pattern of bloom initiation and dispersion during the first few months of the bloom. Enumeration of microzooplankton from these same samples also reveals that protozoan grazer populations were depressed and nearly eliminated before the bloom began. Benthic biomass and species diversity also declined before the onset of the bloom. The disruption of planktonic and benthic grazers may have been due to extreme hypersaline conditions caused by an extended period of drought (salinities > 60‰). A massive fish kill caused by an abnormal period of below freezing temperatures released a pulse of nutrients into an already disturbed environment allowing this nuisance bloom to become established. Planktonic and benthic grazers have been unable to contribute to the demise of this bloom, which has persisted for >7 yr without interruption.

FROM THE CELL CYCLE TO POPULATION CYCLES IN PHYTOPLANKTON–NUTRIENT INTERACTIONS

The internal demographic structure of a population influences its dynamics and its response to the environment. Most models for phytoplankton ignore internal structure and group all cells in a single variable such as total biomass or density. However, a cell does have a life history, the cell division cycle. We investigate the significance of the cell cycle to phytoplankton population dynamics in a variable nutrient environment, using chemostat models. Following the transition point hypothesis, nutrient uptake affects cell development only within a limited segment of the cell cycle. Simulation results demonstrate oscillations in cell numbers and population structure generated by this interaction. When nutrient input is varied periodically, the population displays an aperiodic response with frequencies different from that of the forcing. These results also hold for a model that includes nutrient storage by the cells. These dynamics differ from those of traditional chemostat models and from cell cycle models driven by light cycles. Resource control of cell cycle progression may explain the time delays previously postulated to explain oscil- latory transients in chemostat experiments.

Sedimentation of phytoplankton during a diatom bloom: Rates and mechanisms

ABSTRACT Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11 -day study period, averaging ca. 0.1 d-l, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were co.07 d-t; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low ‘stickiness’ of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 3&50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms 1. Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark (email: tk@dfu.min.dk) 2. Marine Biological Laboratory, Strandpromenaden 5, DK-3000 Helsinger, Denmark. 3. Marine Sciences Institute, University of California, Santa Barbara, California, 93106, U.S.A. 4. Department of Oceanography, Texas A&M University, College Station, Texas, 77843, U.S.A. 5. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340-6097, U.S.A. 6. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02143, U.S.A. 7. Present address: School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Welling- ton, New Zealand. 8. Universidad de Cadiz, Department de Biologia Animal, Vegetal y Ecologia, Aptdo 40, E-l 1510 Puerto Real (Cadiz), Spain. 1123

Sedimentation losses of Scenedesmus in relation to mixing depth

In Lake Nieuwe Meer, artificial mixing led to replacement of cyanobacterial dominance by a dominance of non-buoyant algae. In this study, sedimentation losses of Scenedesmus in the mixed lake are compared with those in the stratified lake. Sedimentation losses were much lower in the mixed lake and this can explain the higher number of cells of non-buoyant algae. Sedimentation losses of Scenedesmus were also investigated in experimental plankton towers with mixed layers of 2 and 10 metres depth. The sedimentation losses in the deep mixed tower were much lower than in the shallow mixed tower and this can explain the higher net increase of cells, despite the lower average irradiance experienced by the plankton in the deep mixed tower. A simple model was used to relate the growth rate and loss processes to mixing depth. This allowed a reconstruction of the phytoplankton population dynamics for the summer period for the stratified and mixed lake on basis of determined Z(eu)/Z(m) ratios. These calculations illustrate the decisive impact of the mixing depth on the phytoplankton population dynamics.

Gulf of Gdansk spring bloom physical, bio-optical, biological modelling and contact data assimilation

In this study we propose a model of phytoplankton population dynamics in the marine ecosystem, which includes physical, biological and bio-optical parts. As an example we simulate the abnormal 1993 Gulf of Gdansk spring bloom, when extremely high chlorophyll concentrations were observed. For the one-dimensional model we use two different methods of contact chlorophyll observation assimilation to fit a model of “in situ” data. The results are compared with two-dimensional ecosystem modelling based on a barotropic model of wind-driven circulation without assimilation.

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 Kutz., 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.

Long-term response of phytoplankton population dynamics to oligotrophication in Mondsee, Austria

(1993). Long-term response of phytoplankton population dynamics to oligotrophication in Mondsee, Austria. SIL Proceedings, 1922-2010: Vol. 25, No. 1, pp. 657-661.

Killing of marine phytoplankton by a gliding bacterium Cytophaga sp., isolated from the coastal sea of Japan

A marine gliding bacterium Cytophaga sp. (strain J18/M01) was isolated from Harima-Nada, eastern Seto Inland Sea, Japan in 1990. This bacterium preys upon various species of marine phytoplankton. All of the five raphidophycean flagellates, all of the four diatoms, and one of the two dinoflagellates examined were killed within a few days when cultured with the bacterium. The bacterium presumably achieves this by direct attack, because the culture filtrate in which host organisms were totally destroyed had no significant effects on the growth of the same host organism (Chattonella antiqua). If one or a few bacterial cells were inoculated into C. antiqua culture, all of the host organisms were killed. The bacterium proliferated in filter-sterilized seawater, suggesting its ubiquitous existence in the coastal sea. The killing of phytoplankton by bacteria such as Cytophaga sp. J18/M01 may be a significant factor influencing the population dynamics of phytoplankton in nature and may contribute to the sudden disappearance of red tides in the coastal sea. Bacterial destruction of phytoplankton may also be a factor that regulates primary productivity in marine ecosystems.

Models on Nutrient Limitation of Uptake and Growth of Microalgae. Variability of Half-Saturation Constants.

Quantitative regulation of nutrient uptake by unicellular microalgae (phytoplankton) is the key physiological process that governs nutrient cycling and phytoplankton population dynamics in natural waters. Kinetic models are widely used in quantitative description of uptake and growth as functions of nutrient concentration. A review of both experimental and theoretical works on nitrogen and phosphorus uptake by cultured unicellular algae is presented here. At least three different values of half saturation constants are identified depending on the time scale of analysis, while a single system for ion transport is considered. Quantitative relationship is obtained theoretically and experimentally among these different values of half saturation constants.

Phytoplankton and zooplankton population dynamics and production of a recently formed African reservoir

The study provides a 2.5 year record of Rhenosterkop Dam (KwaNdebele, South Africa) plankton population dynamics and production in relation to physical and chemical changes which occurred during the trophic depression and stabilization phases of the reservoir. The mean volume of the reservoir was 4% of full storage capacity. Water temperatures ranged from 14 °C to 27 °C. Due to inorganic suspensoids, the euphotic zone averaged 2.6 m. An anaerobic zone developed each summer. The nitrogen, soluble reactive phosphorus (SRP) and silica concentrations did not displaya seasonal pattern, but the latter two nutrients declined over the study. The dominant phytoplankton group was the cryptophytes while the zooplankton population was dominated by crustaceans. Chlorophyll a concentrations ranged from 1.1 to 27 mg m−3 and were positively correlated to silica and SRP concentrations and inversely with NH4-N concentrations. Primary production ranged from 22.6 to 375 mgC m−2 h−1; changes in Amax were positively correlated to silica and SRP concentrations. Total zooplankton dry weight biomass varied from 4 mg l−1. Annual zooplankton (secondary) production was 8 to ∼ 15 gC m−3 a−1; both primary and secondary production were greatest in the first 12 months of study and remained at low levels for the remainder, similar to the trends for silica and SRP. The data indicate that the reservoir shifted from eutrophic to mesotrophic during the study, typical of events in new reservoirs, and that changes in the plankton populations were largely the result of changing nutrient concentrations.

Effects of a Diazinon formulation on unialgal growth rates and phytoplankton diversity

Diazinon and other organophosphorus insecticides are used primarily for their broad effectiveness, short persistence, and relatively low mammalian toxicity. Although these insecticides are less toxic to algae than most organochlorines, the extensive use of Diazinon and the subsequent exposure to aquatic communities may pose a serious threat to algal growth and population diversity. The significance of phytoplankton as primary producers as well as their ability to intrinsically alter the balance of aquatic ecosystems has warranted greater concern for the toxic effects of this widely accepted insecticide. Few reports are available on the effects of Diazinon on nontarget aquatic organisms and even less is known about its effects on algal growth and phytoplankton diversity. In light of the sparse information available on the effects of Diazinon on phytoplankton population dynamics, the objectives of this study were: (1) to determine the effects of a Diazinon formulation on the growth rates of three widely distributed species of freshwater algae, (2) to ascertain the effects of this formulation on the diversity of a natural phytoplankton assemblage.