Small Chlorophytes Research Articles

20 Years of Global Change on the Limnology and Plankton of a Tropical, High-Altitude Lake

The present long-term (1993–2013) study was aimed at identifying the effects of global change on the environmental characteristics and the plankton community of the tropical, high-altitude Lake Alchichica, Puebla, Mexico. We found no statistically significant increasing trend in air temperature, but an increase from 2002 to 2013, and interannual variability in the meteorological variables. Accordingly, the water temperature rose from 1993 to 2003 and remained similar until 2013. Nonetheless, on a longer-term (1966-2018), longer than the period considered in the present study, air and water temperatures showed an increasing trend in the range considered indicative of climate change. The lake displayed a characteristic warm monomictic thermal pattern but exhibited interannual variability. The planktonic community composition and structure remained similar. The small chlorophytes (e.g., Monoraphidium minutum) dominated the phytoplankton abundance, while the large diatom Cyclotella alchichicana dominated the biomass. The calanoid copepod Leptodiaptomus garciai dominated both the zooplankton abundance and biomass. However, as the temperature increased the large-size phytoplankton (i.e., C. alchichicana) abundance and biomass increased, but the small-size phytoplankton abundance and biomass, as well as the adult copepod abundance and biomass, decreased. The increase in temperature could have favoured the large-size over the small-size phytoplankton. In addition, the temperature increase could have negatively affected the calanoid copepods in two different ways—direct association with the negative effects of higher temperatures on the physiology of the copepods, or indirect association with the negative effects of higher temperatures on the small-size phytoplankton, which diminish the copepods’ food resource.

A metabolomic approach to investigate effects of ocean acidification on a polar microalga Chlorella sp.

Ocean acidification, due to increased levels of anthropogenic carbon dioxide, is known to affect the physiology and growth of marine phytoplankton, especially in polar regions. However, the effect of acidification or carbonation on cellular metabolism in polar marine phytoplankton still remains an open question. There is some evidence that small chlorophytes may benefit more than other taxa of phytoplankton. To understand further how green polar picoplankton could acclimate to high oceanic CO2, studies were conducted on an Antarctic Chlorella sp. Chlorella sp. maintained its growth rate (∼0.180 d-1), photosynthetic quantum yield (Fv/Fm = ∼0.69) and chlorophyll a (0.145 fg cell-1) and carotenoid (0.06 fg cell-1) contents under high CO2, while maximum rates of electron transport decreased and non-photochemical quenching increased under elevated CO2. GCMS-based metabolomic analysis reveal that this polar Chlorella strain modulated the levels of metabolites associated with energy, amino acid, fatty acid and carbohydrate production, which could favour its survival in an increasingly acidified ocean.

Assessing the relevance of top-down and bottom-up effects as phytoplankton structure drivers in a subtropical hypereutrophic shallow lake

Although several studies explain the trophic cascade in water systems, we lack knowledge about top-down–bottom-up effects on phytoplankton from eutrophic lakes. In this study, we tested the importance of trophic cascades on phytoplankton structure, predicting that environmental variations are the main drivers. We performed a monthly sampling during a year to measure environmental variables, phytoplankton and zooplankton, plus two samplings (winter and summer) to assess fish structure. Furthermore, we analyzed zooplanktivorous fish stomach-gut contents and completed a hatching zooplankton resting egg experiment to assess the effect of fish on dormant populations. We ran a partial redundancy analysis (pRDA) for phytoplankton using zooplankton, nutrient availability and environmental variables as predictor variables. We finally calculated several ratios of the zooplankton:phytoplankton biovolume to assess potential predation effects. Phytoplankton was correlated with variations in temperature and conductivity plus nutrients (pRDA: 63.4%, F = 4.6, P = 0.001) and was dominated alternatively by diatoms and cyanobacteria. Zooplankton was dominated by microphagous rotifers (> 45% of the total biovolume), and only the ratio of microphagous rotifer:small chlorophytes was significant during summer and autumn (F = 10.6, P = 0.005). The fish community was dominated by insectivorous-planktivorous fish (> 65% of total density), yet a negative selection of zooplankton items (Ivlev’s index < 0) was found. Nevertheless, the zooplankton resting stage analysis showed that microphagous Rotifera were dominant (29 species emerged), suggesting a structuring effect of fish on the zooplankton size. We conclude that phytoplankton was mainly controlled by environmental variations plus nutrient availability, while top-down had a less evident effect.

Is thermal stability a factor that influences environmental heterogeneity and phytoplankton distribution in tropical lakes?

Abstract Aim To evaluate the effects of environmental heterogeneity promoted by thermal stratification on the distribution of limnologic variables and phytoplankton functional groups (FGs) in two natural lakes. Methods Monthly measurements were performed over a five-year period in the vertical profile of a warm monomitic shallow lake (Lago Carioca) and in a deep and meromictic (Lake Dom Helvécio). Results The vertical zonation generated by the high thermal stability during the stratification period promoted an increase in the spatial heterogeneity and, consequently, in the richness of functional groups of the two lakes. In the epilimnion, the dominance of small chlorophytes and desmids (NA, A and X1, ≤20μm) and larger dinophyceans (Lo, ≥20μm) were associated with high turbulence and light availability and soluble phosphorus limitation. In the metalimnion, the presence of filamentous cyanobacteria (R) and colonial chlorophyceans (F) of larger size (≥20µm) were associated with stable habitats with high concentrations of N-NH4 and P-PO4-3 and light limiting conditions. Comparatively, Lake Dom Helvécio presented a higher richness of FGs in the meta-hipolimnetic layers (SN, P) as well as a higher number of species per functional group. Seasonal changes in the climatic conditions (e.g. the decrease in air temperature with the consequent heat loss) caused the break of the water column stability, which promoted the redistribution of the dissolved nutrient forms and the increase of light limitation in the two lakes during the mixing period. Therefore, there was a drastic reduction in the richness and population biomass of FGs (≤80%). Conclusions Thermal stability and atelomixis were the main driving forces of vertical heterogeneity during the stratification, favoring the coexistence of FGs and, consequently, their increase in richness and biomass.

Phytoplankton composition of the water and gastrointestinal tract of the mussel Diplodon enno (Ortmann, 1921) from São Francisco river (Bahia, Brazil).

The knowledge on diet composition of the freshwater mussel Diplodon enno (Ortmann) would aid in its culture and propagation allowing, this way, the replacement of natural endangered populations in Brazil. Microalgae are the main food source for captive mussels and unionids have displayed an ability to sort algae based on the cellular characteristics prior to ingestion. The main objective of the present work is to analyze the phytoplankton composition of the water from and of the gastrointestinal contents of the mussel D. enno, as an initial step for development of a suitable rearing diet. Therefore, water samples and bivalve specimens were collected from S. Francisco River, city of Paulo Afonso, Bahia, Brazil. The microalgal composition found in water and stomach/gut content samples was very diverse being represented by the following divisions: Cyanophyta, Chlorophyta, Dinophyta and Heterokontophyta (Diatoms). Concerning the relative abundance of microalgae divisions, it is possible to state, for the water and gastrointestinal contents, that Cyanophyta represents 15% and 14%, Chlorophyta 54% in both, Heterokontophyta 31% and 27% and Dinophyta 0% and 5%, respectively. According to the Brazilian CETESB criteria for phytoplankton species classification, 50% of Cyanophyta and 15% of Chlorophyta species observed in the water samples were classified as "very frequent", as were 68% of Heterokontophyta and 33% of Chlorophyta species in the gut/stomach tract samples. Focusing at a species level, although in the water only Coelastrum sp. and Chroococcus sp. were observed in 100% and 75% of the samples, respectively, in the gastrointestinal tract the species Staurastrum sp., Aulacoseira sp., Scenedesmus sp. and Fragilaria crotonensis occurred in 80% to 100% of the samples. The present results showed that D. enno feeds not only on small chlorophytes microalgae, due to their convenient size that facilitates higher feeding rates, but also on large size diatoms, due to a possible nutritional advantage for the bivalves. Thus, a diet composed by large diatoms and small chlorophytes microalgae may be considered as the most reasonable for the maintenance of D. enno populations.

Phytoplankton communities in rock pools on the Åland Islands, SW Finland – environmental variables, functional groups and strategies

Studies of rock pools tend to focus on the surrounding emergent habitat. Moreover, research on phytoplankton in small, nutrient-rich water bodies is limited. Summer phytoplankton species, functional groups and life strategies, as well as physicochemical variables potentially influencing the phytoplankton, were surveyed in 34 brackish rock pools. Except from total nitrogen and total phosphorus, the pools were chemically quite similar. Phytoplankton abundance and biovolume consisted mainly of charophytes (Zygnematales), unidentified small flagellates, cyanobacteria, chlorophytes and dinoflagellates. Many of the phytoplankton functional groups were representative of shallow, nutrient-rich and light-abundant environments. The majority of the common and abundant species were classified as C-strategists (invasive opportunists). Total nitrogen, total phosphorus and phytoplankton biomass varied greatly between the rock pools, and were positively related. The rock pool depth and macrovegetation presence–absence played a central role in shaping the phytoplankton species distribution. Frequently occurring species, present in the majority of the rock pools, seldom dominated the phytoplankton. Species dominating the phytoplankton rock pool assemblages were not common components of the phytoplankton flora.

The allelopathic capacity of submerged macrophytes shapes the microalgal assemblages from a recently restored coastal wetland

We have tested the efficiency of isolated and combined submerged macrophyte cultures to inhibit, through allelopathy, the natural phytoplankton growth. Both plants and microalgae come from the same wetland, a recently restored area in Albufera de València Natural Park (Spain). The need to replant the area under restoration with submerged macrophytes makes this information essential for wetland management. The selection and culture of the submerged macrophytes used in that restoration (four charophytes: Chara hispida, Chara vulgaris, Chara baltica, Nitella hyalina, and one angiosperm: Myriophyllum spicatum) provided a good opportunity to test in the laboratory the allelopathic effect of macrophyte assemblages on environmental phytoplankton communities. Three experiments were carried out using spring communities: in Experiment 1, a diverse phytoplankton assemblage (29% of biomass Chlorophyceae, 26% Cryptophyceae, 19% diatoms and 9% cyanobacteria) was cultivated with exudates from monocultures of C. hispida, N. hyalina and M. spicatum. Experiment 2 proved the allelopathic effect of a macrophyte assemblage exudate (C. hispida, C. baltica, C. vulgaris, N. hyalina and M. spicatum in a mixed culture) on two different phytoplankton communities (one diverse: 53% biomass of diatoms, 27% cyanobacteria, 18% chlorophytes and another dominated by small chlorophytes). The response variables were Chl a concentration, phytoplankton biovolume and main algal groups’ biovolumes. When phytoplankton grew in water with exudates from monocultures, microalgal biomass was from 4 to 6 times lower than in the control after 5 days and Chl a concentration was up to 4 times lower. The inhibitory effect of C. hispida was greater than that of M. spicatum. Mixed macrophyte assemblages resulted in even stronger allelopathic effects than monocultured macrophytes; the biomass was reduced by 7 fold after 5 days using the mixed exudates and Chl a concentration was between 3 and 5 times lower. The experiments demonstrate that macrophytes are particularly effective in inhibiting the growth of both small diatoms and the least desirable phytoplankton component in these wetlands (filamentous cyanobacteria), but not chlorophytes (reduction by 37–69, 7–14 and 1–7 fold for diatoms, cyanobacteria and chlorophytes, respectively). The predictions are that spring macrophytes might enhance microalgae that are suitable for grazing (mostly small chlorophytes) and will decrease non-edible filamentous taxa. Thus, restoration managers should replant with the mixture of submerged native macrophytes that provide the most harmful allelopathic effects to promote benefits on aquatic communities in two synergistic ways: by directly reducing microalgal biomass and by indirectly enhancing grazing, which in turn, would promote a clear-water phase. We present replanted macrophytes as ecosystem engineers, i.e. organisms that directly or indirectly modulate the availability of resources for the wetland food webs, by causing changes in primary producer assemblages.

Responses of tropical plankton communities from the Mexican Caribbean to solar ultraviolet radiation exposure and increased temperature

The aim of this study was to evaluate the effects of UVR on growth and taxonomic composition of tropical plankton communities in a scenario of increased temperature and ultraviolet radiation. Water samples were collected from a reef lagoon in the Mexican Caribbean (20.5° N, 86.5° W) during July 2010 and grown for 16days in microcosms under two natural radiation treatments: a) PAB (PAR+UV-A+UV-B, 280–700nm) and, b) P, (PAR, 400–700nm) and two temperature conditions: a) ambient (28°C), and, b) increased (ambient+3°C). A differential factorial response of the studied variables among the main taxonomic groups and more frequent species was found. The biomass of dinoflagellates and colorless plankton was negatively affected by UVR while the increased temperature had negative effects on diatom biomass and cell abundance. During the experimental period there were changes in the contribution of each taxonomic group. At ambient temperature there was a shift from a flagellate- to a diatom-dominated community; whereas at increased temperature diatoms co-dominated with flagellates. UVR exposure decreased the contribution of naked dinoflagellates (>20μm) and cryptophytes. On the other hand, the most frequent diatom, Cylindrotheca closterium was negatively affected at increased temperature, while small chlorophytes (<10μm), which were one of the dominant groups of flagellates, contributed significantly to the biomass at increased temperature at the end of the experiment. Synergistic effects of UVR and temperature were only detected at the species level in large diatoms (>20μm; e.g. Leptocylindrus sp. and Amphora sp.) and in cryptophytes (>10μm). Our results suggest that planktonic assemblages from the Mexican Caribbean are generally well-adapted to the high UVR fluxes and temperature with some species being positively influenced by increased temperature. However there are exceptions with some species being negatively affected by UVR, increased temperature or the combination of both factors. Therefore, our results indicate that under the high radiation conditions of tropical oceans, changes in community structure in terms of taxonomic composition and size distribution would occur in a scenario of global climate change.

Community structure in boreal lakes with recurring blooms of the nuisance flagellate Gonyostomum semen

Blooms of the nontoxic raphidophyte Gonyostomum semen have shown a recent increase in frequency and distribution in the Fennoscandian region. Due to large cell size and several grazer-avoidance strategies, G. semen is hypothesized to be inedible for most zooplankton species and therefore may constitute a bottleneck for the transfer of energy and nutrients in pelagic food webs. Repression of other phytoplankton through increased competition and induced mortality could further exacerbate this effect. In a field study of four lakes with recurring blooms of G. semen and four lakes without blooms, we found significant differences in community structure between the two lake groups during the bloom period. Bloom-lakes had lower biovolumes of small chrysophytes and chlorophytes and zooplankton assemblages were predominated by small, potentially bacterivorous cladocerans, suggesting a limited availability of edible phytoplankton and an increased importance of microbial pathways during G. semen blooms. Low biovolumes of large cladocerans in bloom-lakes may be due to interference of G. semen with filter feeding. Moreover, high abundances of the phantom midge Chaoborus flavicans in bloom-lakes suggest that the flow of energy and nutrients is directed more towards this invertebrate predator than fish. This could have negative impacts on fish populations, especially if bloom periods are prolonged.

Phytoplankton dynamics in a shallow lake dominated by common water milfoil

Phytoplankton temporal fluctuation and vertical distribution were studied by seasonal and close interval siphon sampling (May-Sep) in a shallow lake dominated by common water milfoil (Myriophyllum sibiricum). Factors potentially regulating phytoplankton primary production were investigated in situ in 2 enrichment bioassays. The results suggest that the macrophyte vegetation provided an unfavourable habitat for large colonial chlorophytes. Small species, mainly cryptophytes but also small chlorophytes and cyanobacteria, characterised the summer phytoplankton. Phytoplankton abundance as well as primary production were considerable and remained in the mesotrophic range. The total nitrogen to total phosphorus ratio during the growth season, and the enrichment bioassays, showed that phosphorus was a significant regulator of primary production. Small-celled species had competitive advantages in the dense vegetation, and the canopy structure created by dense stands of common water milfoil allowed phytoplankton growth. Hence, although the macrophytes altered the physical and chemical conditions in the lake, coexistence of small-celled algae and macrophytes was possible.

Auto- and heterotrophic picoplankton in a lowland river (Warta River, Poland)

Abstract The structure of plankton in a lowland river was studied with a particular focus on autotrophic as well as heterotrophic picoplankton cells. Water samples were collected in the middle, meridional section of the Warta River in 2005. Heterotrophic picoplankton (HPP) were represented by bacteria varying in size and form, but mainly by cocci and rods. HPP abundance ranged from 3.5 × 106 to 9.5 × 106 cells ml-1. Autotrophic picoplankton (APP) densities were considerably lower and ranged from 1.2 × 103 to 92 × 103 cells ml-1. They were represented by prokaryotes (picocyanobacteria) and eukaryotes (small chlorophytes). Both components occurred together but eukaryotic APP usually prevailed in terms of biomass. In terms of biomass, APP seemed to play a minor role in this ecosystem in comparison with heterotrophic picoplankton.

Sustained primary production with changing phytoplankton assemblages in a semiarid wetland

This study has two main objectives, the first being the determination of net phytoplankton primary production to explain the phytoplankton’s function in a wetland carbon cycle, while the second objective is to relate this function with the phytoplankton assemblage composition. The annual variation in the phytoplankton production was monitored monthly for more than a year (2007–2008) in the semiarid eutrophic, hydrologically-perturbed “Tablas de Daimiel” National Park wetland. The phytoplankton fraction considered in this study comprised all organisms between the size 3 and 100 μm. The total biomass of phytoplankton was obtained by counting algae and calculating their volume, while net primary production and respiration were quantified by in situ incubations with the Winkler method. The respiration ranged from undetectable to 0.07 mgO2 l−1 h−1; net photosynthesis reached 0.20 mgO2 l−1 h−1. Net primary production was maximum at the end of the warm period (October 2007), and other peaks occurred at the start of summer (July 2007) or spring (March 2008). When maximum production took place, phytoplankton was mainly composed of small fast-growing chlorophytes (Tetraselmis cf. fontiana or Chlamydomonas sp.), in addition to some of the large, S-strategist algae (Peridinium umbonatum, Microcystis flos-aquae, Euglena sp.). The phytoplankton metabolism in “Tablas de Daimiel” was autotrophic as a whole due to changing contributions of algal groups. Only chlorophyte biomass was statistically related to net primary production. The conclusion reached is that this shallow eutrophic semiarid wetland possessed an annual net autotrophic production of phytoplankton fraction resulting from the small, fast-growing algae enhanced by hydrological perturbations that interrupted the autogenic course of S strategists.

Effects of cyanobacterial toxicity and morphology on the population growth of freshwater zooplankton: Meta‐analyses of laboratory experiments

We synthesized data from 66 published laboratory studies, representing 597 experimental comparisons, examining the effects of cyanobacterial toxicity and morphology on the population growth rate and survivorship of 17 genera (34 species) of freshwater, herbivorous zooplankton. Two meta‐analyses were conducted with these data. The primary analysis compared herbivore population growth rates for grazers fed treatment diets containing cyanobacteria versus control diets comprising phytoplankton that are generally considered to be nutritious for zooplankton (chlorophytes and/or flagellates). This analysis confirmed that cyanobacteria were poor foods relative to small chlorophytes and flagellates. More importantly, filamentous cyanobacteria were found to be significantly better foods for grazers than single‐celled cyanobacteria over all studies. Surprisingly, the presence or absence of commonly‐measured toxic compounds (microcystins in 70% of the cases) in the diet had no overall influence on grazer population growth relative to control diets. A secondary analysis compared survival rates for grazers fed cyanobacteria versus no food. In contrast to the primary analysis, grazer survival was more negatively affected by toxic cyanobacteria than non‐toxic cyanobacteria, relative to starvation. However, this difference was attributable to the effects of a single Microcystis strain, PCC7820. Thus, though some cyanobacterial strains appear to be toxic to some strains of zooplankton, the overall role of commonly‐assayed cyanobacterial toxins as a determinant of food quality may be less than widely assumed. We suggest that more attention be focused on nutritional deficiencies, morphology, and the toxicity of undescribed cyanobacterial compounds as mediators of the poor food quality of cyanobacteria for zooplankton.

Seasonal changes in plankton and nutrient dynamics and carbon flow in the pelagic zone of a large, glacial lake: Effects of suspended solids and physical mixing

The abundance and distribution of phytoplankton and zooplankton were followed from January 1993 to March 1994 in Lake Coleridge, a deep oligotrophic alpine lake in the Canterbury high country of New Zealand. Our data suggest physical processes associated with deep mixing and inputs of suspended sediments combined to limit planktonic biomass, diversity, and determine vertical distribution. The lake was strongly stratified in summer with a surface mixed layer of 20 m gradually extending down to 100 m by May. Over 50 taxa of phytoplankton were recorded. Changes in vertical distribution of most planktonic groups coincided with changes in the depth of vertical mixing. Phytoplankton biomass increased through summer with a diatom‐dominated peak in autumn. Cell numbers however, peaked in spring and were dominated by small chlorophytes. Phytoplankton photosynthetic production was highest in autumn but specific growth rates were highest in summer as a result of greater light availability combined with warmer temperatures. We suggest that large inputs of suspended sediments and glacial silt, may have limited the spring phytoplankton peak which occurs in a number of other temperate lakes. Bacteria and pico‐phytoplankton biomass peaked in winter following the phytoplankton peak suggesting they were relying at least in part on scenescing phytoplankton cells for dissolved organic carbon. Diversity of ciliated protozoa was low (11 taxa), Cladocera were absent, and the only crustacean zooplankton found in significant numbers was Boeckella hamata. Ciliated protozoan abundance was low (maximum 4600 litre 1) and peaked in summer (January‐March). Copepod nauplii peaked in August and peaks in abundance of copepodites and adults followed in September and November‐January respectively. Only two generations occurred during the year. The peak in copepod biomass followed phytoplankton peaks but lagged 1–2 months probably as a result of slow growth rates and the time for response to phytoplankton peaks.

Differential sensitivity to natural ultraviolet radiation among phytoplankton species in Arctic lakes (Spitsbergen, Norway)

Incubation experiments demonstrated a differential sensitivity to natural UV-radiation among the dominant phytoplankton species from three Arctic lakes, situated near Ny-Alesund, Spitsbergen (79° N). The growth of small chlorophytes, diatoms and picocyanobacteria from two oligotrophic lakes was inhibited primarily by the shorter wavelength UV components, while the growth of the larger colony-forming species (cyanobacteria, Planktothrix sp., Woronichinia sp. and the chrysophyte, Uroglena americana Calkins) apparently was stimulated. These colonies (not easily eaten by daphnids) dominated at the end of the experiment in those treatments where the short wavelength UV components were not excluded. For the two oligotrophic localities, 70 and 61%, respectively, of total phytoplankton biovolume were edible in the treatments excluding short wavelength UV, compared to only 13 and 19%, respectively, in the treatments including such radiation. For the third, more productive and less transparent lake, the percentage of edible species in the treatments with and without short wavelength UV radiation did not differ (ca. 75% for both treatments).

Temporal plankton dynamics in an oligotrophic maritime Antarctic lake

1 The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15-month period between December 1994 and February 1996. 2 In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice-covered except during a period of approximately 6 weeks in summer. 3 The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent. 4 There were marked seasonal and interannual variations in planktonic population density. 5 Chlorophyll a concentrations ranged from undetectable to 4.2 µg L-1 and the greatest rate of primary productivity measured was 4.5 mg C m-3 h-1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 103 to 1.2 × 107 L-1. 6 Bacterial densities of 3.6 × 108 to 1.9 × 1010 L-1 were recorded and bacterial productivity reached a peak of 0.36 µg C L-1 h-1. Numbers of heterotrophic nanoflagellates between 5.0 × 104 and 1.8 × 107 L-1, and of ciliates from undetectable to 1.1 × 104 L-1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 103 L-1.

In Situ Responses of Lake Okeechobee (Florida, USA) Phytoplankton to Nitrogen, Phosphorus, and Everglades Agricultural Area Canal Water

ABSTRACT Lake Okeechobee experiences blooms of cyanobacteria, perhaps due in part to the low nitrogen:phosphorus (N:P) ratios found in the water column, which may favor cyanobacteria over other phytoplankton taxa. The current strategy for managing the lake is to reduce external P loads. Some have recommended that pumping of N-rich water from the Everglades Agricultural Area (EAA), located adjacent to the south end of the lake, into Lake Okeechobee could increase N:P ratios and reduce cyanobacterial blooms until P loads are reduced. We conducted experiments to assess potential impacts of that strategy on phytoplankton in the lake's south pelagic region. Three in situ experiments were conducted during July 1994, with triplicated treatments (P, N, N+P, or EAA canal water additions) applied to 20-L clear plastic carboys, which were incubated in the lake for 3 days. In the first experiment, additions of P (50 μg · L−1) did not stimulate chlorophyll a or net primary productivity. Additions of N (500 μg · L−1) led to significant increases in both parameters. The greatest biomass and productivity increases (5-fold) occurred when the N and P were added together. In the second experiment, additions of N at 1X, 2X, and 4X the concentrations utilized in experiment 1 caused 3-fold increases in chlorophyll a and primary productivity. Nearly the same results were obtained in the third experiment, where EAA canal water was added to lake water in a 1:10 or 1:5 ratio, increasing soluble reactive P (SRP) by 17 and 34 μ · L−1, and dissolved inorganic nitrogen (DIN) by 175 and 350 μg · L−1, respectively. Although agricultural canal water often has been shown to contain herbicides which can suppress algal growth, no such effect was observed here. The yields of chlorophyll a per unit SRP or DIN in experiment 3, where canal water was added, were nearly identical to those in experiments 1 and 2, where N or N+P were added. In none of the experiments was there a shift from cyanobacteria to other algal taxa, despite the presence of small rapidly-growing chlorophytes in the lake water inoculum. The phytoplankton was dominated by cyanobacteria (70 to 81% of total phytoplankton densities), especially Lyngbya, Oscillatoria, Chroococcus, and Aphanocapsa, in all treatments, which encompassed DIN:SRP ratios from below 1:1 to over 140:1. From a management standpoint, these results indicate a potential for increased biomass of cyanobacteria in this N-limited lake if water pumping from the EAA were to be substantially increased.

The effects of nutrient addition and pH manipulation in bag experiments on the phytoplankton of a small acidic lake in West Virginia, USA

In situ bag experiments were performed during summer and autumn in a small acidic lake, Tibbs Run Lake, West Virginia, USA. The objective was to evaluate phytoplankton responses to pH manipulation and nutrient addition. Increasing the pH from below 4.5 to over 6.3 resulted in great declines in phytoplankton biovolume. There was also a succession from dinoflagellates (Peridinium inconspicuum to small chlorophytes. The trend was more rapid where phosphorus (P) additions were made along with pH enhancement. During summer, P limitation was indicated, while nitrogen (N) appeared to limit production in autumn. In both seasons, nutrient additions greatly altered the phytoplankton composition in high pH treatments, but had no discernable effects at (the natural) low pH. A low pH, P addition treatment in autumn was the single exception. When N was subsequently added, phytoplankton composition changed dramatically, probably because the proceeding P additions caused severe secondary N-limitation. In general, however, the results supported the view that phytoplankton compositional responses to nutrient additions are suppressed in low pH, relative to high pH lake water.

Changes in physico-chemical parameters and photosynthetic microorganisms in a deep wastewater self-depuration lagoon

Temporal changes in physico-chemical parameters and photosynthetic microorganisms were studied during a seasonal cycle (October-July) in an 8 m deep self-depuration wastewater lagoon in the south east of Spain. The mild climate of the area confers great potential on this type of water depuration. A first bloom of Chlamydomonas sp. dominated the system during the period of heavy organic contamination. This algal species disappeared without noticeable proliferation of the zooplankton grazer community. A second phase of water depuration was characterized by proliferation of small chlorophytes, mainly of the genera Golenkinia, Chlorella and Scenedesmus. Rotifers and ciliates grazed on this second bloom giving rise to a third one, dominated by a small cyanophyte (0.5–1.5 m) of the genus Synechococcus, when nutrient levels above the thermocline were low (2.5 mg PO 4 3−/1 and 1.0 mg NH 4 +/1). The cyanophyte population was controlled by rotifers, copepods and cladocerans up to the end of the study. Two blooms of photosynthetic bacteria were identified. The first one, occupying the whole water column, coincided in time with Chlamydomonas and an organic-rich phase of the water column. It was dominated by Thiocapsa sp. and Chromatium sp., both of them sulphur red photosynthetic bacteria. The second bloom, of the green photosynthetic bacteria Chlorobium, established at and below the thermocline when H 2S concentrations reached 95 mg/l. The detection of all these easily identifiable organisms has proved to be very useful as bioindicators of the water depuration stage in deep lagoons of the area.

Principal component analysis of interactions between fish species and the ecological conditions in fish ponds: II. Zooplankton

. Interactions between fish species and their effect on the ecological conditions in the fish pond were studied in ten ponds of 0.1 ha each. The ponds were stocked with bottom-feeding fish: common carp, Cyprinus carpio L., and male hybrid tilapia Oreochromis niioticus (L.) x O. aureus (Steindachner). and a filter-feeder: silver carp, Hypophthalmichtys molitrix (Valenciennes), in various combinations. This paper deals with the effect of different assemblages of these fishes on the phytoplankton populations in the ponds. A Principal Component Analysis of the data showed that only 20% of the phytoplanktonic variability is accounted for by the fish combination present in the pond (treatment). The first principal component (PCI) groups the species of phytoplankton which responded to treatment. This component is formed by small size species; Scenedesmus spp., small Chlorophytes (mainly Chlorella), Selenastrum minutum, Ankistrodesmus setigerus, Merismopedia minima and Diatoms of the order Pennales. The presence of silver carp led to an increase in total phytoplankton numbers, concurrently with a decrease in their dominant size. This was due to the predominance in the water of the small size species of the PCI group, which could not be retained by the gill filtering apparatus of the fish. Reduction in zooplankton abundance by silver carp also contributed to this situation. The presence of bottom-feeding fish resulted in a decrease in total phytoplankton numbers, and in the importance of the small-size species of the PCI group, and hence in the dominance of larger algae. The interactions between these two trophic types of fish and algae size are discussed.