Stir-up effect of wind on a more-or-less stratified shallow lake phytoplankton community,Lake Balaton,Hungary

Microstratification of phytoplankton in the large shallow Lake Balaton (Hungary) was studied during a 24 h period. Dissolved O₂ showed biological stratification; flagellates exhibited a definite circadian rhythm. In the middle of the investigation a heavy storm broke out which was followed by the disappearance of differences between different layers of water. Storm-induced destratification is described by cluster-analysis. Abundances of dominant species changed differently in connection with the storm. Numbers of Nitzschia sp. increased due to stirring up from the sediment surface. Numbers of single-celled or colony-forming species (Cyclotella comta, Crucigenia quadrata, Coelosphaerium kuetzingianum) practically did not change. Numbers of all the three dominant filamentous species (Aphanizomenon fos-aquae f. klebahnii, Lyngbya limnetica, Planctonema lauterbornii) significantly decreased, which might be attributed to an unknown loss process and was followed by a competitive displacement by algae of small cell size.     

An attempt to trace eutrophication in a shallow lake (Balaton,Hungary) using chironomids

Lake Balaton, the largest shallow lake in Central Europe, is about 20 000 years old. An enormous increase in tourism and the disproportionate building development of the last few decades has resulted in the acceleration of eutrophication in the lake. Widespread research to reveal the causes of water-quality deterioration and possible ways of protection against it have recently started. The investigation of the larvae of non-biting midges (Diptera: Chironomidae) in the sediment of the open-water zone has also begun. The contemporary faunal composition strongly correlates with the trophic gradient along the longitudinal axis of the lake. We therefore supposed that the eutrophication process should be identifiable from the analysis of subfossil chironomid head capsules from the upper (15 cm thick) layer of the sediment. We found that quantitative results could only be obtained when fragments as well as relatively intact head capsules are considered. Our data verify that the originally oligo-mesotrophic community has been gradually replaced by eutrophic species in a west to east direction. Large-bodied larvae belonging to the Chironomus plumosus group mix the sediment down to 15 cm as they build their tubes and consequently alter the original proportions of head capsules at the different levels. So the sequence of communities through the sediment-layers is not quite reliable.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production, it was found that the periphyte biomass increased with the nutrient concentrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photosynthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors, such as fish farming, storm induced waves and mechanical destruction, and high water levels could be the perturbations during the system shifts, but these are not the key factors. Instead, the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production,it was found that the periphyte biomass increased with the nutrient con-centrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photo-synthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors,such as fish farming,storm induced waves and mechanical destruction,and high water levels could be the perturbations during the system shifts,but these are not the key factors. Instead,the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

Microbial activity under the ice cover of the shallow Neusiedler See (Austria, Central Europe)

In an attempt to assess bacterioplankton production and growth yieldunder low temperature conditions and to compare bacterioplankton withphytoplankton production in the ice-covered water column of the shallowNeusiedler See, outdoor measurements under near in situ conditions wereperformed during the winter of 1995/96. During the investigation period,mean chlorophyll (Chl) a concentration was 21.03 ± 14.95 µg Chla l^-1. Phytoplankton primary production integrated over thewater column ranged from 1.35 to production integrated over the water columnranged from 1.35 to 4.23 mg C m^-2 d^-1 (mean± SD = 2.46 ± 1.06 mg C m^-2d^-1). Bacterial abundance varied from 20 to 40×10⁵ ml^-1 for most of the investigationperiod and increased by the end of March concomitantly with the increase intemperature from 1.3 to 6.3 °C within 5 days. Mean bacterial productionwas 15.3 ± 12.8 µg C l^-1 d^-1(range: 3.0 to 41.7 µg C l^-1 d^-1) and meanbacterial growth rate 0.23 ± 0.16 d^-1 following closelythe pattern in bacterial production. DOC concentration declined linearlyfrom 20.7 mg C l^-1 to 16.45 mg C l^-1 over the 4months period of ice cover. The contribution of humic substances to thetotal DOC pool declined from 43.6% at the end of November to37.3% at the end of March. Calculated on an area basis, phytoplanktonproduction amounted to only 16% of bacterial production which makesit unlikely that phytoplankton supply substrate for bacterioplankton growthin significant quantities when the lake is ice covered. From the observeddecline in DOC over the investigation period and assuming only negligibleinput of DOC from other sources we calculated an average DOC uptake by thebacterioplankton community of 47.5 µg C l^-1d^-1 resulting in a bacterial growth efficiency of 15.9%for the ice covered conditions. Based on the growth efficiency we estimatethat pelagic primary production amounts to 2.8% of the bacterialcarbon demand. This might indicate that the bacterioplankton in NeusiedlerSee sustain their high growth rates at low temperatures (<2°C formost of the investigation period) by using probably the DOC originating fromthe previous season. This DOM stems most likely from the decay of the reedPhragmites australis and its epiphytes and, probably of minor importance,from phytoplankton leachates.     

Dynamics of phytoplankton detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

A study was made of the mortality and aerobic decomposition of light- and phosphorus-limited cultures of Oscillatoria limnetica, a dominant phytoplankton species in shallow, eutrophic Lake Loosdrecht (The Netherlands). When placed in the dark at 20 °C, most cells died and lysed within twelve days. The labile organic matter was completely decomposed within three weeks. Absorbance spectra indicated that blue green algae may contributed significantly to the refractory dissolved substances in the lake. Refractory particulate matter constituted from 7 to 24% of the biomass of O. limnetica, depending on the growth rate before incubation in the dark. The decomposition rate of this fraction was 0.005 d^–1. On a basis of a steady-state model of the dynamics of phytoplankton detritus, the areal organic dry weight concentration of the detritus in the lake is ca. 60 g m^–2. This means the quantities of detritus in the seston and epipelon are about equal.     

Long-term compositional response of phytoplankton in a shallow,turbid environment,Neusiedlersee (Austria/Hungary)

Data on phytoplankton biomass and on nutrient concentrations from Neusiedler See (mean depth 1.3 m) covering more than two decades are presented. The lake underwent strong eutrophication during this period. The response of annual average phytoplankton biomass and chlorophyll-a to the increase of phosphorus concentration from 10 to > 100 µg l^-1 was moderate (7-fold increase). This is caused by light limitation of the system because of the high inorganic turbidity of the lake. Analyses of the spring, summer and autumn seasons at the generic and higher taxonomic levels show significant changes in composition of the phytoplankton community. Diatoms were more important during the pre-eutrophication phase while Chlorophyceae became most prominent during the peak of the eutrophication process. Blue-green algae, including Microcystis, became more apparent after this period. The abundance of some groups or genera, e.g. Euglena, was linked to the decline and re-appearance of submerged macrophytes in the lake. Abiotic and biotic interactions as causes for the observed changes are discussed.     

Spatial distribution and short-term changes of benthic macrofauna in Lake Balaton (Hungary)

Spatial and short-term changes in the composition and density of the macrobenthic fauna were studied in Lake Balaton, a large shallow lake in Central Europe (Hungary). Spatial differences were examined along five transects and short-term changes at two stations of different trophic state. The macrobenthos consisted almost exclusively of Tubificidae and Chironomidae of the species Chironomus gr. plumosus, Procladius choreus, Tanypus punctipennis, Microchironomus tener and Cladotanytarsus sp. The Oligochaeta biomass showed a significant short-term decrease from 1995 to 1998, and their average biomass proved to be 0.86 and 0.79 g WFW (wet formalin weight) m-2 in 1996 and 1997, respectively. Chironomidae showed significant spatial and short-term differences following variations in primary production. Higher primary production resulted in higher biomass (up to 153.4 g WFW m-2 and 9785 ind. m-2 in 1995) of Chironomus-Procladius community, while lower primary production resulted in a very low biomass and abundance of the predatory Procladius-Tanypus-Microchironomus community (average: 3.16 g WFW m-2 and 1311 ind. m-2 in 1997). As compared to other lakes of similar climatic conditions, the total production of oligohaetes and chironomids proved to be very low in 1996 and 1997 (49.2 and 54.1 KJ m-2 yr-1, respectively). The 90% of the average chironomid production of Lake Balaton in 1996 and 1997 belonged to Ch. gr. plumosus (5.63 and 5.13 g m-2 yr-1), P. choreus (4.3 and 3.47 g m-2 yr-1) and T. punctipennis (0.27 and 2.21 g m-2 yr-1). This revised version was published online in July 2006 with corrections to the Cover Date.     

Distribution of submerged macrophytes along environmental gradients in large,shallow Lake Balaton (Hungary)

Distribution of submerged macrophytes was assessed at 15 littoral sites in large, shallow Lake Balaton using the echo sounding technique. Sites represented the conspicuous longitudinal and transverse environmental gradients. Absence/presence of plants, water depth, bottom consistency and distance from the shore have been derived from echo sounding data. Turbidity and meteorological variables were recorded at a single site from 2004 to 2006. Site-specific variability in underwater light conditions (an indicator of stress) and exposure to breaking waves (an indicator of disturbance) were estimated by simple models.     

Phytoplankton nitrogen demand and the significance of internal and external nitrogen sources in a large shallow lake (Lake Balaton,Hungary)

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N₂-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the ¹⁵N-technique. Ammonium was determined by an improved microdiffusion assay. N₂ fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm^?3 day^?1 in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm^?3 day^?1) and peaked in late summer (0.82 and 1.29 μg N cm^?3 day^?1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N₂ fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly.     

Coupling of phytoplankton and detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

An oscillating steady state is described of phytoplankton, dominated by Prochlorothrix hollandica and Oscillatoria limnetica, and sestonic detritus in shallow, eutrophic Lake Loosdrecht (The Netherlands). A steady-state model for the coupling of the phytoplankton and detritus is discussed in relation to field and experimental data on phytoplankton growth and decomposition. According to model predictions, the phytoplankton to detritus ratio decreases hyperbolically at increasing phytoplankton growth rate and is independent of a lake's trophic state. The seston in L. Loosdrecht contains more detritus than phytoplankton as will apply to many other lakes. The model provides a basis for estimating the loss rate of the detritus, including decomposition, sedimentation and hydraulic loss. In a shallow lake like L. Loosdrecht detritus will continue to influence the water quality for years.     

Composition,density and feeding of crustacean zooplankton community in a shallow,temperate lake (Lake Balaton,Hungary)

Composition density and filtering rates of crustacean zooplankton were studied in the open water and among the macrophytes of the oligo-mesotrophic part of Lake Balaton from 1981 to 1983. From the individual filtering rates of the different populations and the densities community grazing rates were derived.Copepoda made up 79–90% of crustacean plankton community in the open lake and 95–97% of it in littoral zone. Among them the nauplii dominated. At the end of summer 1982, when Anabaenopsis was in bloom, the filter-feeding species (Eudiaptomus, Daphnia) practically disappeared, being replaced by cyclopoids. Daphnia had the highest filtering rates followed by those of Eudiaptomus and Diaphanosoma. Among copepods, the filtering rates in decreasing order were: ovigerous > all adults > copepodites > nauplii. > . The filtering rates of the different species varied both seasonally as well as from year to year. In 1983, when the concentration of organic seston decreased, filtering rate increased compared with those in the earlier years. During the water bloom in 1982, the rates decreased by 70% on the average.The community grazing rate was very low (3% per day) in the open lake and among macrophytes, both in 1981 and 1982; also the share of crustacean zooplankton in grazing was very low. In 1983, together with the improving of water quality, the community grazing rate increased 4-fold. In 1981 and 1983 the rates were influenced by water temperature but in 1982 by seston concentration.     

Role of phosphorus and nitrogen for bacteria and phytopankton development in a large shallow lake

Nutrient (P and N) enrichment experiments in small enclosures (20 l) were carried out to determine P and/or N limitation of bacterioplankton in Lake Võrtsjärv. The specific interest of the study was to test if it is possible to detect nutrient `physiological' or growth (rate) limitation of bacterioplankton and competition for nutrients (N and P) with phytoplankton in generally nutrient rich lake. Thymidine and leucine incorporation; leucine aminopeptidase, -D-glucosidase and alkaline phosphatase activity, total count of bacteria, chlorophyll a concentration and primary production as well as the concentrations of different chemical forms of N and P were followed during 4–5 days of the experiment. To address the question of the interactions between nutrients, bacterio- and phytoplankton, experimental and seasonal data sets were included in the analyses. Phosphorus (P) had a positive effect on bacterioplankton in enclosure experiments in June 1997; no effects of nutrients were found in September 1996, while in May 1996, P affected mainly the phytoplankton. On the seasonal scale, the development of bacterioplankton was connected to primary production, total phosphorus and temperature. In enrichment experiments, bacterioplankton was mainly related with primary productivity but the possible importance of bacterial grazers could be presumed. Thus, no evidence was found for nutrient growth limitation and/or competition for N and/or P, rather bacterioplankton depended on organic food supply originating from phytoplankton.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Diversity patterns of trait-based phytoplankton functional groups in two basins of a large, shallow lake (Lake Balaton, Hungary) with different trophic state

The application of functional approaches in understanding phytoplankton community-level responses to changes in the environment has become increasingly widespread in recent years. Eutrophication is known to cause profound modifications in ecosystem processes; however, the underlying relationships between environmental drivers and phytoplankton diversity and functioning are complex and largely unknown. Therefore, in the present study, we investigated and compared the temporal diversity patterns of phytoplankton functional groups in the mesotrophic eastern and eutrophic western basin of the shallow Lake Balaton situated in Hungary. Diversity data were derived from taxonomic composition and biomass data corresponding to the years 2005–2006 and 2008–2009. With the use of cluster analysis, phytoplankton species were classified into eight distinct groups representing different combinations of functionally relevant traits including greatest axial linear dimension; surface-to-volume ratio; photosynthetic pigment composition; N₂ fixation; phagotrophic potential; growth form/complexity (unicell, filamentous, colony-, or coenobium-forming); and motility. Our results have revealed that there is a significant inverse relationship between the functional group diversity used in our study and trophic state (total phytoplankton biomass) as opposed to species diversity, where no correlation was observed. In addition, group evenness showed an even stronger negative correlation with trophic state, while species evenness yielded only a weak relationship. The observed variability in functional group diversity suggests that such an approach could provide an efficient means of revealing structural changes in phytoplankton communities, establishing new hypotheses and highlighting fundamental points in ecosystem functioning.     

Long-term phytoplankton changes in a shallow hypertrophic lake,Albufera of Valencia (Spain)

A long-term phytoplankton study was carried out in the Albufera of Valencia, a shallow hypertrophic lake (surface area 21 km², mean depth 1 m, total inorganic nitrogen load 155 g m^-2 y^-1, total inorganic phosphate load 15 g m^-2 y^-1) from 1980 to 1988. The lake functions as a reservoir for the surrounding rice cultivation. From 1940's to 1988, its phytoplankton assemblage has been altered from a mesotrophic to a hypertrophic character, as consequence of the increasing pollution. For 1980–88, annual variations in the phytoplankton were less pronounced than seasonal changes. The hypertrophic and morphometric features of the lake favoured the stability of the phytoplankton assemblage and chlorophyll a levels during the study period. Seasonal and horizontal distribution of the total phytoplankton abundance and biomass were highly influenced by the hydrological cycle of the lagoon. Compared with other shallow nutrient rich lakes, the Albufera of Valencia is similar to the shallow hypertrophic lakes of the Netherlands.     

Pelagic food web processes in an oligotrophic lake

Major pelagic carbon pathways, including primary production, release of extracellular products (EOC), bacterial production and zooplankton grazing were measured in oligotrophic Lake Almind (Denmark) and in enclosures (7 m³) subjected to artificial eutrophication. Simultaneous measurements at three days interval of carbon exchange rates and pools allowed the construction of carbon flow scenarios over a nineteen day experimental period.The flow of organic carbon was dominated by phytoplankton EOC release, which amounted from 44 to 58% of the net fixation of inorganic carbon. Gross bacterial production accounted for 33 to 75% of the primary production. The lower values of EOC release (44%) and bacterial production (33%) were found in the enclosures with added nutrients. The release of recently fixed photosynthetic products was the most important source of organic carbon to the bacterioplankton. Uptake of dissolved free amino acids was responsible for 52 to 62% of the gross bacterial production. Thus, amino acids constituted a significant proportion of the EOC. Zooplankton (< 50 µm) grazing on algae and bacteria accounted only for a minor proportion of the particulate production in May. Circumstantial evidence is presented that suggests the chrysophycean alga Dinobryon was the most important bacterial remover.The results clearly demonstrated EOC release and bacterial metabolism to be key processes in pelagic carbon cycling in this oligotrophic lake.     

The development and persistence of alternative ecosystem states in a large,shallow lake

1. Palaeolimnological data were used to investigate drivers of the community of primary producers in Lake Mattamuskeet, North Carolina, U.S.A. This is a large, shallow lake with two basins currently dominated by phytoplankton and macrophytes. The two basins were divided in 1940 by the building of a roadway across the lake, which also corresponded with the divergence in their ecosystem state. 2. Photosynthetic pigments, organic matter and nutrients (P, N, C, S) were analysed in sediment cores from each basin to reconstruct the primary producer community over the past c. 100 years. We sought to answer two questions. First, what changes to the ecosystem resulting from the building of the roadway caused the development of different primary producer communities in the two basins? Second, why have the alternative ecosystem states persisted despite a variety of human perturbations since 1940? 3. K‐means cluster analysis and principal component analysis were applied to identify three sediment types based on photosynthetic pigment data: sediments indicating low productivity (low pigment concentrations), sediments associated with macrophytes (chlorophyll a and b) and with phytoplankton (alloxanthin and aphanizophyll). In addition, other palaeolimnological proxies measured, such as loss on ignition, total phosphorus, total organic carbon/total nitrogen and other nutrients, were different in post‐1940 sediments within the two basins. 4. These differences suggest characteristics, such as nutrient cycling, water depth and other physical changes resulting from roadway construction, combined to establish and maintain the differing communities of primary producers in the two basins. Furthermore, Fe/S dynamics and waterfowl herbivory probably contributed to the development of the two ecosystem states.     

MIXOTROPHY IN THE ANTARCTIC PHYTOFLAGELLATE,PYRAMIMONAS GELIDICOLA (CHLOROPHYTA: PRASINOPHYCEAE)1

Grazing by the planktonic phytoflagellate, Pyramimonas gelidicola McFadden (Chlorophyta: Prasinophyta), and heterotrophic nanoflagellates (HNAN) in meromictic saline Ace Lake in the Vestfold Hills, eastern Antarctica was investigated in the austral summers of 1997 and 1999. Up to 47% of the P. gelidicola population ingested fluorescently labeled prey (FLP). Ingestion rates varied with depth. In January 1997 and November 1999, maximum P. gelidicola ingestion rates of 6.95 and 0.79 FLP·cell^?1·h^?1, respectively, were measured at the chemocline (6–8 m) where a deep chl maximum composed of phototrophic nanoflagellates (PNAN DCM), predominantly P. gelidicola, persisted all year. During the summers of 1997 and 1999, the grazing P. gelidicola community removed between 0.4% and approximately 16% of in situ bacterial biomass, equivalent to between 4% and>100% of in situ bacterial production. Because of their higher abundance, the community clearance rates of HNAN in Ace Lake generally exceeded those of P. gelidicola, but HNAN removed approximately only 3%–4% of bacterial biomass, equivalent to between 28% and 32% of bacterial production. Pyramimonas gelidicola growth rates were highest at the PNAN DCM concomitant with the highest ingestion rates. It is estimated that during the summer P. gelidicola can derive up to 30% of their daily carbon requirements from bacterivory at the PNAN DCM. This study confirms mixotrophy as an important strategy by which planktonic organisms can survive in extreme, polar, lacustrine ecosystems.     

The role of storms in the summer succession of the phytoplankton community in a shallow lake (Lake Balaton, Hungary)

The authors carried out day-to-day studies on the phytoplanktonof the largest shallow lake of Central Europe in the summerof 1976, 1977 and 1978. Data were analysed with diversity andcluster analyses. (i) Wind-induced stirring-up of the sedimentplays important role in the summer phytoplankton successionof the shallow lake. (ii) After storms algae with very smallcell sizes show synchronous development. In long calm periodsalgae with larger cell sizes exhibit organised population dynamics.(iii) In calm periods a shift from r-selection to predominantK-selection can be suspected. After the next storm importanceof r-selectionists again increases. (iv) Effect of changinggrowth- and loss rates on the population dynamics of the mostabundant species is discussed based on measured generation timesand calculated elimination. The phenomena discussed in the papercan appear to different extents in the summer phytoplanktonsuccession of other shallow, polimictic lakes.