The first decade of oligotrophication of Lake Constance

In Lake Constance, after several decades of cutrophication, a decrease in phosphorus loading over the last decade has lead to a partial recovery from eutrophication. Here we analyse the shift in the taxonomic composition of phytoplankton during the first decade of oligotrophication in Lake Constance. During the 1980s, spring total P concentrations decreased from ca. 130 to less than 50 ·l^–1. This decrease was reflected by an approximately proportional decrease in summer phytoplankton biomass while spring phytoplankton biomass seemed unresponsive. Major taxonomic changes occured during both growth seasons. In spring, the proportion of diatoms, green algae and Chrysophyta increased while the proportion of Cryptophyta decreased. The summer trend was very different: the relative importance of diatoms decreased and Cryptophyta and Chrysophyta increased, while Chlorophyta reached their peak around 1985. These trends are also analysed at the genus level. Comparison with taxonomic trends during the eutrophication period shows the expected reversals in most cases. Comparison with other lakes shows general similarities, with the notable exception that Planktothrix rubescens has never been important in Lake Constance. The increase of diatoms during spring is attributed to their improved competitive performance with increasing Si:P ratios. Their decrease during summer is explained by the increasing silicate removal from the epilimnion by increasing spring populations.     

Causation of reversal simultaneity for diatom biomass and density of <Emphasis Type="Italic">Phormidium tenue</Emphasis> during the warm season in eutrophic Lake Barato,Japan

The diatom biomass of Lake Barato, as measured from July to September, decreased simultaneously with an increase in filament density of Phormidium tenue after 1997. There was a high negative correlation between the diatom biomass and the densities of P. tenue (r² = 0.928). Although total nitrogen (TN) and total phosphorus (TP) concentrations decreased from 1996, TN:TP ratio increased from 1997 because the TP concentration became markedly lower. The decrease in diatom biomass might have been due to the loss in phosphorus available for algae. Because the increase in density of P. tenue might have been due to the decrease in diatom biomass, experiments using a growth inhibitor for diatoms were performed to examine whether the density of cyanobacteria increases without diatom growth. Samples of the lake water collected in three seasons (August and October 1998, May 1999) were incubated with and without germanium (Ge) as a growth inhibitor of diatoms. The increase in density of P. tenue was inhibited concurrent with the increase in diatom biomass in the first and middle stages of incubation without the addition of Ge in August 1998 and May 1999. In contrast, a higher density of P. tenue was observed in the incubation with diatom growth inhibited by Ge over the same period. These results suggest that diatoms have an effect in restraining the growth of P. tenue.     

Sensitivity and responses of diatoms to climate warming in lakes heavily influenced by humans

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Phyto- and protozooplankton biomass during austral summer in surface waters of the Weddell Sea and vicinity

Summary Phyto- and protozooplankton were sampled in the upper 10 m of the water column in austral summer during a cruise of RV Polarstern from January 6 to February 20 1985 in the eastern Bransfield Strait vicinity and in the northern, southeastern (off Vestkapp, twice: I and II) and southern Weddell Sea (Vahsel Bay across the Filchner Depression to Gould Bay). The plankton assemblages are discussed in relation to physical, chemical and biological factors in the different geographical areas in summer. Phytoplankton biomass (Phytoplankton carbon, PPC) ranged from 4–194 g carbon/l and consisted on average of 65% diatoms and 35% autotrophic flagellates. Whereas in the northwest phytoplankton assemblages were dominated by small nanoflagellates (78% of PPC), higher biomass of diatoms (54–94% of PPC) occurred at the other sampling sites. In general autotrophic flagellates and small pennate diatoms dominated at oceanic stations; in neritic areas large centric diatoms prevailed. Chlorophyll a concentrations ranged from 0.25–3.14/g chl a/l with a mean of 1.13/gmg chlorophyll a/l and an average phytoplankton carbon/chlorophyll a ratio of 39. Protozooplankton biomass (Protozooplankton carbon, PZC) ranged from 0–67 g carbon/l and consisted of 49% ciliates, 49% heterotrophic dinoflagellates and 2% tintinnids. Heterotrophic dinoflagellates were more important in the northern investigation areas (58%–84% of PZC). Ciliates dominated the protozooplankton in the southeast and south (56%–65% of PZC); higher abundances of tintinnids were observed only in the south (11% of PZC). The most remarkable feature of the surface waters was the high protozooplankton biomass: protozooplankton amounted to 25% on an average of the combined biomass of PPC plus PZC for the entire investigation period. Protozoan biomass in the southeastern and southern Weddell Sea occasionally exceeded phytoplankton biomass. Temperature, salinity, and inorganic nutrients were generally lower in the southern regions; at most of these stations a meltwater layer occurred in the upper meters of the water column. We suggest that this physical regime allows a well developed summer system with a high proportion of heterotrophic microplankton. In the eastern Bransfield Strait, in the northern Weddell Sea and close to the coast off Vestkapp (I), however, early summer conditions occurred with less protozooplankton contribution.Contribution no. 427 from the Alfred-Wegener-Institute for Polar and Marine Research     

The first decade of oligotrophication in Lake Constance

Phytoplankton biomass and species composition were measured with a relatively high temporal resolution (once or twice a week during the growing season) from 1979 to 1989 in Lake Constance/Überlingersee. Over this period soluble reactive phosphorus (SRP) concentrations during winter mixing were reduced by ca. 50% from 104 to 47 g 1^–1, which caused a prolongation and amplification of the epilimnetic P depletion during the growth period. Seasonal dynamics of phytoplankton reacted to the decrease of SRP in the following ways: (1) Algal biomass decreased at least proportionally to the winter SRP concentrations in summer, but not in spring and autumn when biomass fluctuated irregularly. (2) The peak of biomass concentration changed from summer to spring. (3) The earlier onset of epilimnetic P depletion during the season in recent years promoted a stronger growth of some pennate diatoms in spring. It caused an amplification of the silicon depletion in summer, which may cause still greater reduction of diatoms and total algal biomass in summer. (4) Reduction of algal biomass during the clear-water phase proper became shorter and less pronounced. (5) The temporal variability of algal biomass decreased in summer and autumn but not in spring. (6) Average cell sizes remained unchanged in summer and autumn but increased in spring during the beginning of oligotrophication. These results are largely in agreement with other studies on lake restoration and expectations derived from the PEG (Plankton Ecology Group) model (Sommer et al. 1986). They show that a 50% reduction of SRP concentrations during homothermy may have pronounced effects on seasonal dynamics of algal biomass in a large and deep lake. The algal response to the external change of SRP concentrations can be described by the Le Chatelier principle, implying that the internal structure of the system (e.g. species composition) changes in order to minimize the effect of the external pressure (e.g. reduction of total biomass). Suggestions are made as to how this system behaviour may emerge from local interactions.     

Dissolved oxygen dependent phosphorus release from profundal sediments of Lake Constance (Obersee)

Phosphorus release rates from profundal sediments of Lake Constance (Obersee) have been determined in D.O., pH regulated sediment-water systems. Above 10% O₂ saturation (> 1.2 ppm D. O.) and with pH as in situ, no net release could be found. Sedimentations of diatom sludge (Asterionella formosa) and carbonate-phosphate coprecipitate (CaCO₃.CaHPO₄) increased the release to 0.5 mg × m^–2 × d^–1 which, however, will not be relevant to the P balance in Obersee. The annual phosphorus accumulation in profundal Obersee and Ueberlingersee is, therefore, observed as due to sinking of phosphorus-bound detritus during the stagnation period.The experimental work was carried out at the Limnological Institute of the University of Freiburg/Breisgau (West Germany) and has been supported by the Industrieverband Körperpflege und Waschmittel e.V. and the Gernan Research Council (DFG)The experimental work was carried out at the Limnological Institute of the University of Freiburg/Breisgau (West Germany) and has been supported by the Industrieverband Körperpflege und Waschmittel e.V. and the Gernan Research Council (DFG)     

Local environment and connectivity are the main drivers of diatom species composition and trait variation in a set of tropical reservoirs

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Seasonal changes in silicon content of diatoms estimated from the ratio of particulate silicon to diatom volume under silicon sufficiency in diatom-rich Lake Barato

To clarify the changes in Si content of diatoms, the particulate silicon (PSi) concentration and total diatom volume (TDV) were determined in Lake Barato, Japan, from April to July 1998–2000. The soluble reactive silicon (SRSi) concentration decreased markedly with the rapid increase in TDV in May and June in all three years, although the value did not fall below that at which diatom growth might be limited. The proliferation of small discoid diatoms contributed to the decrease in SRSi concentration each year. The Si content of diatoms may not be constant as indicated by the changes in PSi:TDV ratio. The low PSi:TDV ratio and the fact that PSi concentration was lower than diatom PSi concentration (calculated from the volume of diatom species) accompanying the decrease in TDV suggests the possibility of a disturbance in the silicification in May and June 1999. These parameter changes accompanying the increase in TDV suggest that the silicification did not catch up with the cell division in early April 1998, early May 1999, and mid-June 2000. In addition, the PSi:TDV ratio increased rapidly and showed large fluctuations in July 1998 and 1999. This may have been caused by a change in dominant species from small discoid diatoms to Aulacoseira granulata because of the differences in Si content per unit cell volume.     

Meteorological forcing of plankton dynamics in a large and deep continental European lake

The timing of various plankton successional events in Lake Constance was tightly coupled to a large-scale meteorological phenomenon, the North Atlantic Oscillation (NAO). A causal chain of meteorological, hydrological, and ecological processes connected the NAO as well as winter and early spring meteorological conditions to planktonic events in summer leading to a remarkable memory of climatic effects lasting over almost half a year. The response of Daphnia to meteorological forcing was most probably a direct effect of altered water temperatures on daphnid growth and was not mediated by changes in phytoplankton concentrations. High spring water temperatures during ”high-NAO years” enabled high population growth rates, resulting in a high daphnid biomass as early as May. Hence, a critical Daphnia biomass to suppress phytoplankton was reached earlier in high-NAO years yielding an early and longer-lasting clear-water phase. Finally, an earlier summer decline of Daphnia produced in a negative relationship between Daphnia biomass in July and the NAO. Meteorological forcing of the seasonal plankton dynamics in Lake Constance included simple temporal shifts of processes and successional events, but also complex changes in the relative importance of different mechanisms. Since Daphnia plays an important role in plankton succession, a thorough understanding of the regulation of its population dynamics provides the key for predictions of the response of freshwater planktonic food webs to global climate change. Received: 15 February 1999 / Accepted: 23 August 1999     

Summer distribution of netphytoplankton in the Atlantic sector of the Southern Ocean

The surface distribution of netphytoplankton (>20 m) in the Atlantic sector of the Southern Ocean was investigated along two transects during early and late austral summer 1990/91. Sampling was under-taken at intervals of 60 of latitude between 34° and 70°S for the analysis of nutrients and for the identification and enumeration of netphytoplankton. Peaks in total diatom abundances were recorded at the Antarctic Polar Front (APF), in the vicinity of the South Sand wich Islands, in the marginal ice zone and in the neritic waters of the Atlantic sector of Antarctica. Cluster analysis indicates the existence of two major zones between Southern Africa and Antarctica. Diatom abundance increased south of the Antarctic Polar From along both transects, which can be partially explained by gradients of silicate concentration. Small chain-forming species (e.g. Fragilariopsis kerguelensis and Nitzschia lineata) dominated the diatom assemblages in early summer, while larger species, such as Rhizosolenia hebetata f. semispina and Corethron criophilum, dominated late summer diatom assemblages. The predominance of typically ice-associated forms in early summer suggests that the release of epontic cells during ice melt provides the initial inoculum for the netphytoplankton biomass. These small diatoms are subsequently replaced by larger species.     

Diatoms in the plankton and sediments of two lakes of different trophic type

Species composition, quantity and distribution of diatoms in both the plankton and the surface sediments (0–30 cm) of mesotrophic Lake Krasnoye and eutrophic Lake Vishnevskoye (Karelian Isthmus) were studied. In the mesotrophic lake the composition of dominant diatoms (mainly Melosira) corresponded to those in the plankton. The Araphidineae/Centrales (A/C) ratio was 0.2–0.3%, increasing in the upper layers to 7%. Alkaliphilous and alkalibiontic forms constituted 60% of the total. In the eutrophic lake diatoms with thin valves (mainly Synedra) predominated in the plankton but their quantity in the sediments was insignificant in comparison with other plankton. Nevertheless, the A/C ratio was much higher, 17–35%. Alkaliphilous and alkalibiontic forms accounted for 78–90% of the total number of valves. In both lakes the highest number of diatom valves was registered in the upper layer of the sediments. From the ratio of the total number of diatoms in the upper 5 cm layer to their annual flux to the sediment from the plankton the approximate sediment accumulation rate was calculated to be 1.9 mm a^?1 for the mesotrophic lake and 2.5 mm a^?1 for the eutrophic one.     

Succession of phytoplankton in a deep stratifying lake: Mondsee,Austria

Phytoplankton numbers, biovolume, chlorophyll-a and various physico-chemical characteristics were followed at weekly intervals in Mondsee, Austria during the year 1982. Secchi-disk transparency varied from 10 m in winter to 2 m in September. Prior to the onset of stratification phosphate-phosphorus concentration was 4 µg 1^–1 decreasing to undetectable values thereafter. Nitrate-nitrogen dropped from 590 µg 1^–1 to about 100 µg 1^–1 during the same time. The vernal bloom was dominated by Asterionella formosa Hass. which abruptly declined after silicon depletion. Spring growth ceased in early June, when Tabellaria flocculosa (Lyngb.) Kütz var. asterionelloides Grun. dominated. Oscillatoria rubescens D.C. and Microcystis aeruginosa Kütz. dominated summer and early autumn followed by the chrysophyte Dinobryon divergens Imh. and D. sociale Ehr. which formed up to 69% of total biovolume in October. Thereafter diatoms and Cryptophyceae (Rhodomonas lacustris Pascher and Ruttner, Cryptomonas pusilla Bach.) became abundant again.Maximum chlorophyll-a concentration in the epilimnion (16 µg 1^–1) was reached during spring growth of the diatoms. During summer higher chlorophyll-a levels were always associated with the metalimnetic layer of Oscillatoria.Compared with earlier studies, both the total biovolume and the share of Oscillatoria rubescens significantly decreased because of reduced nutrient loading of the lake and wash-out of Oscillatoria (theor. renewal time of the lake: 1.7 years).     

Centric diatoms from Lake Frolikha (Transbaikal area) and peculiarities of distribution of some taxa in Asia

The species composition of centric diatoms from Lake Frolikha (Transbaikal area) has been studied. The lake is located on the eastern shore of Lake Baikal and connected with it by a river. Twenty-three species of Centrophyceae from 7 genera (Aulacoseira, Cyclostephanos, Cyclotella, Discostella, Handmania, Pliocaenicus, and Stephanodiscus) have been found. The most represented genus is Aulacoseira (11 species). Fifteen species and 4 genera are new for the lake record. All revealed species are known in other lakes of the Baikal region; however Baikal endemics are absent in Lake Frolikha. The flora of centric diatoms in Lake Frolikha can be divided into two groups. The first group includes taxa common in Lake Baikal, and the second group includes taxa not typical for Lake Baikal. The level of differences between flora in Lake Baikal and Lake Frolikha is high (43%) despite the close location and connection to the river. An analysis of distribution of interesting species in Asia is presented.     

Seasonal changes of C:P ratios of seston, bacteria, phytoplankton and zooplankton in a deep, mesotrophic lake

• 1 The C:P ratios of seston, bacteria, phytoplankton and zooplankton were measured twice a week in situ in mesotrophic, large and deep Lake Constance from April to December 1995. Except for zooplankton, a strong seasonality was exhibited with low C:P ratios during P‐enriched early spring conditions and high values during P‐depleted summer conditions.
• 2 Molar C:P ratios of seston varied between 180:1 and 460:1 demonstrating moderate phosphorus limitation in spring and during the clear‐water phase, and strong limitation for the rest of the season. The sestonic C:P ratio increased significantly during two decades of re‐oligotrophication of Lake Constance, reflecting an enhanced phosphorus limitation of the plankton community in summer. Molar C:P ratios of bacteria and phytoplankton varied seasonally between 50:1 and 130:1 and 180:1 and 500:1, respectively, and indicate carbon or light limitation in winter and phosphorus limitation in summer. Zooplankton had a molar C:P ratio of about 124:115 which was nearly constant throughout the seasons.
• 3 These differences in the C:P ratios of planktonic organisms have direct implications for phosphorus recycling within the food web as C:P ratios of excreta should be highly variable.

     

Diatoms in sediment cores in Utah Lake,Utah, U.S.A.

Diatoms from sediment cores taken from Provo Bay, Geneva and midlake sites in Utah Lake, Utah County, Utah were studied. Algal communities from strata at four centimeterintervals of the cores were analyzed. This study has demonstrated that the diatom flora in Utah Lake has changed through time and that the diatom communities of Provo Bay are floristically unique from those of the main body of the lake.     

Internal seiches in Lake Constance: influence on plankton abundance at a fixed sampling site

During thermal stratification, pronounced internal seiches occurin Lake Constance with periods of 4–6 days The amplitudesreach 12 m at the sampling site in berlinger See (northwesternpart of Lake Constance). Seiches change the thickness of thewarm and generally biologically rich epilimnion and, thus, alsothe areal abundance of planktonic organisms at a fixed samplingsite. The potential impact of seiche-dnven changes of epilimneticthickness on observed plankton abundance was calculated usingcontinuously recorded temperature profiles and weekly verticalprofiles of plankton biomass. Observed plankton biomass wasrecalculated by taking into account the effect of changing layerthickness in order to give a more realistic picture of biomasschanges caused by waxing and waning of populations Our calculationsimply that changes of strata thickness do not mask the generalseasonal trend in population dynamics (e g. spring bloom andclear water phase), estimates of average yearly standing stockare not significantly affected if sampling is done at leastweekly. However, changes of strata thickness can alter arealplankton abundance at the sampling site by a factor of 2 within2 days During pronounced seiches, areal abundance of phytoplankton,which generally exhibit a stronger vertical gradient than-mostother plankton, may be changed by a factor of 3–4 withinhours Hence, at our sampling site, the impact of changes instrata thickness cannot be ignored for detailed analysis ofpopulation dynamics of autotrophs involving comparisons betweensingle observations     

Protozoan communities in chalk streams

This study assessed the individual effects of three mayflies (Paraleptophlebia sp., Ephemerella subvaria McDunnough and Epeorus sp.) and one caddisfly (Psilotreta sp.) on periphyton communities associated with clay tiles and leaves. Algal densities were estimated for leaf discs and tiles from experimental chambers (with individual grazers) and control chambers (i.e., no grazers). Scanning electron micrographs (SEM) of leaf discs and tiles also were taken for all mayfly grazing experiments. Densities of algae on leaf discs were two to five times lower than on tiles. Mouthpart morphology influenced how different insects grazed the periphyton community. Paraleptophlebia had typical collector-gatherer mouthparts and had no effect on diatom densities associated with leaves whereas diatom densities on grazed tiles were higher than densities on tiles from control chambers. Epeorus had brusher mouthparts and had little impact on diatom densities regardless of substratum type. The other two grazers had the blade-like mandibles of a scraper. Psilotreta did not reduce the numerical abundance of diatoms on either substratum, but did alter community structure by significantly reducing densities of stalked Gomphonema olivaceum and large species of Navicula and Nitzschia; densities of smaller diatoms (Achnanthes spp) increased. However, E. subvaria reduced densities of most algal species regardless of size on both substrata and also significantly altered community structure. SEMs of substrata grazed by mayflies showed reductions in fungal hyphae on all grazed leaf discs, decreases in filamentous algal forms on grazed tiles, and greatly shortened stalks of G. olivaceum (Paraleptophlebia only). Thus, periphyton communities are different on leaves versus tiles and grazers with different mouthpart morphologies have varying effects on both algal and heterotrophic microbial community structure.     

Alkaline soda Lake Velika Rusanda (Serbia): the first insight into diatom diversity of this extreme saline lake

Alkaline soda lakes are unique habitats found in specific geographic regions, usually with dry climate. The Carpathian Basin is one of those regions very important for habitat and biodiversity conservation in Europe, with natural soda lakes found in Austria, Hungary and Serbia. In comparison to other two countries from Central Europe, algal biodiversity studies of saline soda lakes in Serbia are scarce. Lake Velika Rusanda has the highest measured salinity of all saline lakes in the Carpathian Basin and there were no reports of its diatom species richness and diversity till now. We conducted 2-year investigation programme to study biodiversity and seasonal dynamics of diatoms in this lake. A total of 27 diatom taxa were found, almost all of them attached to reed and much less in benthos and plankton. Five new diatom species for Serbia were recorded, Craticula halopannonica, Navicymbula pusilla, Hantzschia weyprechtii, Nitzschia thermaloides and Navicula staffordiae. The last mentioned is new for Europe as well. Lake Velika Rusanda is inhabited mostly by alkaliphilous and halophilic diatoms. Since diatoms are used as bioindicators in soda lakes, our results will improve their further application in ecological status assessment of these fragile habitats in the Carpathian Basin.

     

Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

The plankton ecology of Lake St. Clair, 1984

Lake St. Clair phytoplankton and zooplankton abundance and composition was analyzed during the period of May to September 1984. In addition, size-fractionated primary productivity and other limnological parameters were measured. Highest phytoplankton biomass was observed during spring (May) with high values for the southern and southeastern regions of the lake. Seasonally, the mean phytoplankton biomass ranged between 0.17 and 1.18 g m^-3 with high values recorded during spring (May, June) compared to summer. In the spring the phytoplankton was dominated by Diatomeae followed by Chrysophyceae and Cryptophyceae. During the summer the diatoms showed a decreasing trend due to the relative prevalence of Chrysophyceae, Cryptophyceae, and Chlorophyta. The species composition was oligotrophic-mesotrophic with mixed occurrence of some eutrophic species. The phytoplankton size composition indicated dominance of microplankton/netplankton (> 20 µm) and ultraplankton (< 20 µm) during spring and summer respectively. On an overall basis ultraplankton contributed overwhelmingly to primary productivity, as much as 75 percent in the summer.The mean zooplankton biomass ranged from 173.0 to 1306.0 mg l^- dominated by Cladocerans (bosminids) in contrast to the other Great Lakes. Statistical evaluation of the phytoplankton — nutrient-contaminant interactions revealed positive correlations with heavy metals, suggestive of a physiological adaptation to contamination from the chemical valley. Based on low biomass, high Production/Biomass ratio, dominance of ultraplankton, characteristic species composition and plankton spectra, the lake appears to be an oligotrophic-mesotrophic perturbed ecosystem.