Functional diversity of phytoplankton highlights long‐term gradual regime shift in the middle section of the Danube River due to global warming,human impacts and oligotrophication

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Seasonal dynamics and the structure of colonies of Proterospongia skujae Bourrelly

The seasonal dynamics, the structure of colonies and the abundance of cells in the colonies of the monad Proterospongia skujae have been simultaneously studied in the river Danube and in a backwater of the Danube. Significant differences have been found in both the dynamics and the abundance of P. skujae in selected localities. Colonies are formed of 2 up to 16 cells, most frequency of 4 and 8 cells.     

Development of the bacterial compartment along the Danube River: a continuum despite local influences

Microbial food webs dominate heterotrophic food webs in large rivers with bacterial metabolism being a key component of carbon processing. Thus, analysis of bacterial population dynamics is critical to understanding patterns and mechanisms of material cycling and energy fluxes in large rivers. Within the frame of the Joint Danube Survey (JDS) 2007, the longitudinal development of the natural bacterial community in the Danube in terms of bacterial numbers, morphotype composition, and heterotrophic production of the suspended and particle-attached fractions was followed at a fine spatial resolution of approximately 30 km for the first time in such a large river along a 2,600-km stretch. Twenty-one major tributaries and branches were also included. This allowed us to investigate whether bacterial standing stock and production undergo continuous, linear changes or whether discontinuities and local processes like the merging of tributaries or the potential impact of sewage input drive the bacterial population in the Danube. The presented investigation revealed surprising continuous patterns of changes of bacterial parameters along the Danube River. Despite the presence of impoundments or hydropower plants, large municipalities, and the discharge of large tributaries, most bacterial parameters (standing stock, morphotype succession, and attached bacterial production) developed gradually, indicating that mainly broad-scale drivers and not local conditions shape and control the bacterial community in the midstream of this large river. As most important broad-scale drivers, nutrients (inorganic and organic) and changes in particle concentrations were identified. These data are also in remarkable accordance with the patterns of changes of the genetic bacterial community composition, observed during the first JDS (2001) 6 years before. In contrast, bacterial activity did not follow a continuous trend and was mainly controlled by the input of sewage from large cities in the middle section, leading to a bloom of phytoplankton. The observed patterns and the comparison between the Danube, its tributaries and other large rivers worldwide indicate that the bacterial community in rivers has a powerful indicator function for estimating the ecological status of large river ecosystems once enough information has been collected at various temporal and spatial scales.     

Spatial and temporal dynamics of phytoplankton communities along a regulated river system, the Nakdong River, Korea

The longitudinal distribution and seasonal fluctuation of phytoplankton communities was studied along the middle to lower part of a regulated river system (Nakdong River, Korea). Phytoplankton biomass decreased sharply in the middle part of the river (182 km upward the estuary dam), and then increased downstream reaching a maximum at the last sampling station (27 km upward the estuary dam). In contrast, there was little downstream fluctuation in species composition, irrespective of pronounced differences in nutrient concentrations (TN, TP, NO₃, NH₄, PO₄) as well as in algal biomass. In the main river channel, small centric diatoms (Stephanodiscus hantzschii, Cyclotella meneghiniana) and pennate diatoms (Synedra, Fragilaria, Nitzschia) were dominant from winter to early spring (November–April). A mixed community of cryptomonads, centric and pennate diatoms, and coenobial greens (Pediastrum, Scenedesmus) was dominant in late spring (May–June). Blue-green algae (Anabaena, Microcystis, Oscillatoria) were dominant in the summer (July–September). A mid-summer Microcystis bloom occurred at all study sites during the dry season, when discharge was low, though the nutrient concentration varied in each study site. Nutrients appeared everywhere to be in excess of algal requirement and apparently did not influence markedly the downstream and seasonal phytoplankton compositional differences in this river.     

The effects of northern italian rivers and Eastern Mediterranean ingressions on the phytoplankton of the Adriatic Sea

An analysis of the distribution of the phytoplankton standing crop during a period of high stability characterized the relative influence of northern Italian rivers and Mediterranean waters on the Adriatic Sea. The region influenced by northern Italian rivers was marked by low community diversities, relatively high chlorophyll a standing crops and cell densities, high microplankton to nannoplankton community volume ratios, and the dominance of Nitzschia seriata. To the contrary, the region influenced by Mediterranean waters exhibited high community diversities with prominent oceanic and warm water elements, low chlorophyll a standing crops and microplankton cell densities, low microplankton to nannoplankton ratios, and a relatively conspicuous coccolithophore flora.Nannoplankton populations remained relatively constant throughout the Adriatic. However eutrophication resulted in a marked increase in the microplankton component of the community, with the diatom Nitzschia seriata serving as a prime indicator of the process, even in regions several hundred kilometers downstream from nutrient sources.Even under relatively low rates of river discharge, the influence of northern Italian rivers could be traced along the entire western sode of the sea to the Otranto Strait, with low north to south and high west to east gradients resulting from the effect of a cyclonic surface circulation pattern on inflowing Mediterranean waters in the south and river discharge in the north.     

Shallows of the lower Danube as additional sources of potamoplankton

This paper presents studies on the algal flora of the Bulgarian Danube section during three summer periods (1980–1982) and during the four seasons of 1987/1988. The following variables were investigated: 1) species composition of the river phytoplankton and the phytobenthos; 2) frequency quotients of the species; 3) number of species at the investigated sites; 4) phytoplankton numbers and biomass; 5) floristic similarity between the sites. The data show the existence of a dynamic connection between phytoplankton and phytobenthos. The shallows in the midstream and river arms play an important role as potamoplankton sources during the periods of low waters. It is comparable with the role of the adjacent wetlands to which previously priority was given. This is especially so below 597th river kilometer.Academia Naturalis Heidelberg     

How dams on the River Danube might have caused hybridization and influenced the appearance of a new cyprinid taxon

Synopsis When a new species is found, or a known one collected outside its recognized distribution, most people either assume that the species has been overlooked in the past or that it has recently invaded the area in question from elsewhere. While other, more evolutionary explanations are possible, these are nearly always ignored, in spite of the common perception that animals are constantly in states of change. Recent findings in the Danube River are used as examples for such alternative interpretations. The case of two forms of Vimba — short and long snouted — which live in sympatry in the upper Danube is dispelled, as one of them is identified as a hybrid between female V. vimba and male Blicca bjoerkna. Frequent occurrences of this hybrid (and a few of its backcrosses) in recent times are explained by man-induced alteration of reproductive isolation between their parent species, caused by the impoundment of the river by dams built as part of the Rhein-Main-Donau Kanal. These impoundments, however, might be responsible not only for the creation of hybrids but also for direct speciation, as the case of Gobio albipinnatus may represent. The ability for epigenetic creation of two alternative life-history states (as part of the theory of alprehost) and the survival of an alternative state when environmental conditions change might be ultimately responsible for the appearance of a new taxon.     

Sturgeon re‐introduction in the Upper and Middle Danube River Basin

All six sturgeon species naturally occurring in the Danube River from the Black Sea (BS) to the upstream reaches in Germany today are under severe pressure and classified as either vulnerable, critically endangered or already extinct in the Danube River Basin (DRB). This sharp decline of populations is the result of two major drivers: habitat alteration and unsustainable harvest. The Danube River Basin can be divided in three functionally separated sections by its hydromorphology and character: The Lower Danube (LD) and BS, now also separated ecologically from the Middle Danube (MD) by two hydropower stations at the Iron Gate gorge, 800 km from the sea, currently restricting the remaining anadromous sturgeon species (A. gueldenstaedtii, A. stellatus, Huso huso) to the LD and BS also restraining migrations of potamodromous species. The Upper Danube (UD) is nowadays disconnected from the approx. 850 km long MD by Gab?ikovo Dam. The UD itself, as well as many tributaries along the Danube, are fragmented by numerous barriers, leaving no more than a total of 300 km free flowing river out of 700 km. Overharvest led to disappearance of most sturgeon species in the UD already in the Middle Ages, with populations in the MD following in the subsequent centuries. Today the potamodromous sterlet (A. ruthenus) still exists in small decreasing populations in the UD and MD basins, where its occurrence is supported by stocking. Most probably the potamodromous ship sturgeon (A. nudiventris) is extirpated in the whole basin. The Danube basin comprises 19 countries, being the most international river basin in the world, rendering coordinated conservation efforts a challenge. Strategic guidelines for the conservation of the Danube sturgeons are available in form of several action plans, but implementation remains insufficient as of yet. This paper attempts an analysis of the potential for sturgeon restoration in the UD and MD basin and highlights the need for concerted approaches when it comes to ex situ actions and reintroductions.     

Modelling the effects of changing retention time on abundance and composition of phytoplankton species in a small lake

1. The phytoplankton community model, PROTECH, was used to model the algal response to changing annual mean retention time in a small lake. 2. Simulations of short retention time with a fixed nutrient load resulted in a reduced chlorophyll concentration. A similar relationship was observed when the simulations were repeated but with inflowing nutrients increased in proportion to river discharge. 3. Longer retention time caused the spring bloom to start earlier and the autumn bloom to persist longer. 4. Changes in discharge of the inflowing river also caused a change in the thermal structure of the lake. This change in thermal structure, in turn, influenced the magnitude and composition of the phytoplankton population, particularly those in the CS‐functional group, such as Aphanizomenon.     

Hydrologic variability, water chemistry, and phytoplankton biomass in a large floodplain of the Sacramento River, CA, U.S.A.

The Yolo Bypass, a large, managed floodplain that discharges to the headwaters of the San Francisco Estuary, was studied before, during, and after a single, month-long inundation by the Sacramento River in winter and spring 2000. The primary objective was to identify hydrologic conditions and other factors that enhance production of phytoplankton biomass in the floodplain waters. Recent reductions in phytoplankton have limited secondary production in the river and estuary, and increased phytoplankton biomass is a restoration objective for this system. Chlorophyll a was used as a measure of phytoplankton biomass in this study. Chlorophyll a concentrations were low (<4 μg l^?1) during inundation by the river when flow through the floodplain was high, but concentrations rapidly increased as river inflow decreased and the floodplain drained. Therefore, hydrologic conditions in the weeks following inundation by river inflow appeared most important for producing phytoplankton biomass in the floodplain. Discharges from local streams were important sources of water to the floodplain before and after inundation by the river, and they supplied dissolved inorganic nutrients while chlorophyll a was increasing. Discharge from the floodplain was enriched in chlorophyll a relative to downstream locations in the river and estuary during the initial draining and later when local stream inflows produced brief discharge pulses. Based on the observation that phytoplankton biomass peaks during drainage events, we suggest that phytoplankton production in the floodplain and biomass transport to downstream locations would be higher in years with multiple inundation and draining sequences.     

Using Phytoplankton to Assess the Trophic Status of German Rivers

We introduce the German multi‐metric assessment method PhytoFluss for large rivers, based on phytoplankton, which can be used to implement the European Water Framework Directive (WFD). First, we examined the response of chlorophyll a concentration to total phosphorus (TP) at 220 German river sites. Rivers could be assigned to one of four groups, based on the response curves: a) streams with high specific run‐off (Rhine, Danube) had the lowest yield of chlorophyll a per unit of TP, followed by b) low yield in small lowland rivers (Trave, Stepenitz), c) medium‐sized lowland rivers (Spree) and large central mountain rivers (Main) had intermediate yields of chlorophyll a, and d) streams with low specific run‐off (Elbe, Weser) and Baltic tributaries (Penne, Warnow) had the highest yield of chlorophyll a per unit of TP. To define the five trophic status classes of phytoplankton biomass according the WFD, the chlorophyll a boundaries were derived from these response curves at the 75% percentile level. All selected sites were pre‐classified with the combined trophic boundaries for TP and chlorophyll a to create the new four taxa composition metrics: The proportions of Chlorophytes, Cyanobacteria, or Pennales and of indicator taxa were selected as indicative parameters. When applying the final PhytoFluss method to 418 years of investigation from 150 German river sites, the percentages in each status according to the bio‐ component phytoplankton were: high 3%, good 19%, moderate 42%, poor 30%, and bad 5%. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Population dynamics and estimation of production in Bosmina longirostris (O. F. Müller) in the River Danube (Danubialia Hungarica,CVIII)

Analysis of the population dynamics of Bosmina longirostris (O. F. Müller) was carried out in the River Danube in the years 1981 and 1982. An estimation method of secondary production elaborated far standing waters was adapted to river conditions. In the River Danube, compared to lakes, standing crop and production values were smaller, while fecundity, P/B values were larger and turnover time was shorter. Possible reasons for this are discussed.     

Water temperature behaviour in the River Danube during the twentieth century

Monthly mean water temperatures in the River Danube at Linz, Austria during the period 1901–1990 have been investigated in relation to equivalent information on air temperature and river discharge. Statistical analysis revealed a significant increase in monthly mean water temperatures of 0.8 °C and showed strongest rises in mean values for autumn and early winter months. No statistically significant trends were evident for air temperature or river discharge, and rising water temperatures are likely to reflect increasing human modification of the river system. A strong overall correlation between monthly mean water and air temperatures at Linz was made up of a series of more scattered and less steep water/air temperature relationships for individual months, while the influence of snowmelt runoff depressed average water temperatures in the spring and early summer period by 1.5 °C. Multiple regression relationships developed for individual months from data on air temperature, river discharge and time trend during the study period were able to predict monthly mean water temperatures in 1991 and 1992 with a root mean square error of 0.5 °C. These regression equations, when combined with scenarios of future changes in air temperature and river flow as a consequence of global warming, suggest that only modest rises in monthly mean water temperature will be experienced in the River Danube by the end of the present century, but that increases of > 1 °C for all months, and > 2 °C for the autumn period of low flows, can be anticipated by the year 2030.     

Origin of phytoplankton and the environmental factors governing the structure of microalgal communities in lowland streams

Information on the structure of microalgal assemblages in the epiphyton and epilithon is necessary to understand the origin of phytoplankton in lowland rivers. To this end, we carried out concurrent investigations on phytoplankton, epiphyton and epilithon in 18 reaches of three Estonian rivers during the midsummers of 2002 and 2003. A total of 251 taxa was recorded, of which 192 were epiphyton species, 158 were epilithon species and 150 were phytoplankton species. Canonical correspondence analysis (CCA), based on the 31 most abundant taxa, indicated differences in the structure of the algal assemblages between the different biotopes (phytoplankton, epiphyton and epilithon) as well as between the studied rivers. The composition of the phytoplankton clearly differed from that of the other biotopes, with prevailing small flagellates, a chrysophyte (Synura uvella) and cryptophytes (Rhodomonas lacustris and Cryptomonas erosa). The epiphyton was characterized by a large number of diatoms, while the epilithic community contained filamentous cyanobacteria (Phormidium tergestinum and Planktolyngya sp.) and a green alga (Stigeoclonium tenue) in addition to diatoms. Based on redundancy analysis (RDA), phosphorous was the most relevant parameter determining the distribution of species in the phytoplankton assemblages. Shading by trees on the river bank, dissolved oxygen concentration and water temperature as well as river width determined the distribution of species in the epiphyton. The data set on the epilithon did not reveal any significant relationships between species distribution and the measured environmental parameters.     

Abiotic drivers of interannual phytoplankton variability and a 1999–2000 regime shift in the North Sea examined by multivariate statistics

The Dutch coastal zone is a region of the North Sea with a marked interannual and long‐term abiotic and phytoplankton variability. To investigate the relationship between abiotic variability and phytoplankton composition, two routine water monitoring data sets (1991–2005) were examined. Multivariate statistics revealed two significant partitions in the data. The first consisted of interannual abiotic fluctuations that were correlated to Rhine discharge that affected the abundance of summer and autumn diatom species. The second partition was caused by a shift in the abiotic data from 1998 to 1999 that was followed by a shift in phytoplankton composition from 1999 to 2000. Important factors in the abiotic shift were decreases in suspended matter (SPM) and phosphate (DIP) concentrations, as well as in pH. The decrease in SPM was caused by a reduction in wind speed. The increase in water column daily irradiance from the decrease in SPM led to increases in the abundance of winter–spring species, notably the prymnesiophyte Phaeocystis globosa. Because wind speed is related to the North Atlantic Oscillation (NAO) index it was possible to correlate NAO index and P. globosa abundance. Only five abiotic variables representing interannual and long‐term variability, including Rhine discharge and NAO index, were needed to model the observed partitions in phytoplankton composition. It was concluded that interannual variability in the coastal phytoplankton composition was related to year‐to‐year changes in river discharge while the long‐term shift was caused by an alternating large‐scale meteorological phenomenon.     

Phaeocystis colony distribution in the North Atlantic Ocean since 1948, and interpretation of long-term changes in the Phaeocystis hotspot in the North Sea

Monitoring of Phaeocystis since 1948 during the Continuous Plankton Recorder survey indicates that over the last 5.5 decades the distribution of its colonies in the North Atlantic Ocean was not restricted to neritic waters: occurrence was also recorded in the open Atlantic regions sampled, most frequently in the spring. Apparently, environmental conditions in open ocean waters, also those far offshore, are suitable for complete lifecycle development of colonies (the only stage recorded in the survey). In the North Sea the frequency of occurrence was also highest in spring. Its southeastern part was the Phaeocystis abundance hotspot of the whole area covered by the survey. Frequency was especially high before the 1960s and after the 1980s, i.e., in the periods when anthropogenic nutrient enrichment was relatively low. Changes in eutrophication have obviously not been a major cause of long-term Phaeocystis variation in the southeastern North Sea, where total phytoplankton biomass was related significantly to river discharge. Evidence is presented for the suggestion that Phaeocystis abundance in the southern North Sea is to a large extent determined by the amount of Atlantic Ocean water flushed in through the Dover Strait. Since Phaeocystis plays a key role in element fluxes relevant to climate the results presented here have implications for biogeochemical models of cycling of carbon and sulphur. Sea-to-air exchange of CO₂ and dimethyl sulphide (DMS) has been calculated on the basis of measurements during single-year cruises. The considerable annual variation in phytoplankton and in its Phaeocystis component reported here does not warrant extrapolation of such figures.     

The estimation of production and mortality ofBosmina longirostris (O. F. Müller) in the River Danube (Danubialia Hungarica,CIX)

Produced and eliminated biomass ofBosmina longirostris in the River Danube (Hungary) was estimated in 1982. An attempt was made to calculate the rate of immigration and mortality, taking into consideration the hydrological characteristics of the river. Production and immigration influence population change to about the same extent.     

Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.     

Discharge-driven flood and seasonal patterns of phytoplankton biomass and composition of an Australian tropical savannah river

Discharge is a primary determinant of river phytoplankton, but its influence can be mediated by climate, water quality and catchment development. The relationship between discharge and phytoplankton over seasonal and flood temporal scales was examined for the Daly River in the Australian tropical savannah. Phytoplankton, water quality and hydrographic data were collected over the high-discharge wet season and lower-discharge dry season. Wet season main channel river conditions were unfavourable for phytoplankton growth. Floods, however, connected the main channel to off-channel water bodies that supplied phytoplankton, predominately Cryptomonas, to the river. Wet season phytoplankton biovolume and cell concentrations were higher than in the dry season and comprised the majority of the annual phytoplankton biomass load. High discharges served to both temporally connect the river to off-channel phytoplankton sources as well as dilute these sources. In the dry season, the Daly River was groundwater-fed and disconnected from off-channel phytoplankton sources and dominated by a potamoplanktonic population of Cryptomonas and Peridinium. River phytoplankton were determined by the seasonal discharge regime which drove water quality and underpinned a shift from highly productive, spatially extensive off-channel sources in the wet season to less productive more spatially confined in-channel sources in the dry season.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.