Primary production and respiration of the phytoplankton of the Rivers Thames and Kennet at Reading

During the period April 1967-ApriI 1968 the phytoplankton production and respiration of the River Thames and its tributary, the River Kennet, were measured at approxi-mately 2-week intervals using the light and dark bottle technique. Concentrations of chlorophyll and pheopigment were determined weekly. On fourteen occasions sets of light and dark bottles were rotated in a specially designed apparatus, and production and respiration values obtained were found to be 1·38 ± 0·31 times higher than in stationary bottles at identical depths over the same period. There was little horizontal, vertical or diurnal variation in chlorophyll concentration showing that the water was well mixed. Peaks of chlorophyll were found in spring, summer and autumn in the Thames (max. 219 mg/m³) but there was very little variation in the Kennet (max. 38·2 mg/m³). In both rivers lowest concentrations were found during winter. Pheo-pigment concentration was low in both rivers for most of the period although in the Kennet this represented on average 50% of the pigments present. In the Thames a peak of pheopigments(1·33–5 mg/m³) was associated with the autumnal bloom and repre-sented 61 % of the total pigments. No pheopigments were detected during the spring bloom. The average concentration of suspended organic matter was identical in both rivers but in the Thames over 25 % was due to phytoplankton and in the Kennet almost 95 % was non-algal. In the Thames, net oxygen production reached a peak in May (10·81 gO₂/m²/day) and was negative from November to February (min. ?0·45 gO₂/m²/day). In the Kennet, maximum production also occurred in May (0·85 gO₂/m2) but was negative from the middle of May until the following March. The average annual net production was 1250 and ?78 g O₂/m² in the Thames and Kennet respectively. Respiration rates showed similar fluctuations being 4·59 g O₂/m²/day in spring in the Thames to 0·09 g O₂/m²/day in November. The Kennet was almost always lower (1·05–0·34 g O₂/m2/day. The average annual respiration was almost three times higher in the Thames than in the Kennet (641–228 g O₂/m²). Various factors which might influence production are discussed. The average net efficiency ofthe Thames phytoplankton fell within ranges described from other rivers. Net efficiencies ofthe Kennet were almost always negative. In the Thames it appeared that net production could be explained as a function of solar radiation, chlorophyll concentration and euphotic depth.     

Phytoplankton dynamics in the Congo River

相似文献   

The influence of floodplain habitat on the quantity and quality of riverine phytoplankton carbon produced during the flood season in San Francisco Estuary

Primary productivity, community respiration, chlorophyll a concentration, phytoplankton species composition, and environmental factors were compared in the Yolo Bypass floodplain and adjacent Sacramento River in order to determine if passage of Sacramento River through floodplain habitat enhanced the quantity and quality of phytoplankton carbon available to the aquatic food web and how primary productivity and phytoplankton species composition in these habitats were affected by environmental conditions during the flood season. Greater net primary productivity of Sacramento River water in the floodplain than the main river channel was associated with more frequent autotrophy and a higher P:R ratio, chlorophyll a concentration, and phytoplankton growth efficiency (α^B). Total irradiance and water temperature in the euphotic zone were positively correlated with net primary productivity in winter and early spring but negatively correlated with net primary productivity in the late spring and early summer in the floodplain. In contrast, net primary productivity was correlated with chlorophyll a concentration and streamflow in the Sacramento River. The flood pulse cycle was important for floodplain production because it facilitated the accumulation of chlorophyll a and wide diameter diatom and green algal cells during the drain phase. High chlorophyll a concentration and diatom and green algal biomass enabled the floodplain to export 14–37% of the combined floodplain plus river load of total, diatom and green algal biomass and wide diameter cells to the estuary downstream, even though it had only 3% of the river streamflow. The study suggested the quantity and quality of riverine phytoplankton biomass available to the aquatic food web could be enhanced by passing river water through a floodplain during the flood season.     

At the extreme of physical gradients: phytoplankton in highly flushed,large rivers

Phytoplankton and associated environmental factors were collected fortnightly during a 1-year cycle in the upper and lower reaches of the River Adige (northeastern Italy). The river has a typical Alpine flow, with the period of high flow and flooding occurring in the spring and summer months. Phytoplankton biomass was constrained by physical variables, mainly water discharge and associated variables directly linked to water fluxes. These factors acted negatively and synchronously by diluting phytoplankton cells and worsening the light regime. Nutrient concentrations did not appear to limit phytoplankton growth. Compared to many other central European rivers, the very low maximum algal biomasses supported by River Adige (Chl a < 7 μg l⁻¹) are due to the Alpine flow regime, which is characterised by higher flow during the warmer months, when conditions for algal development are more favourable. Hydrology and flow regime, along with the channelisation of the river, caused development of a simplified phytoplankton community, which was almost exclusively composed of diatoms. Moreover, these factors contributed significantly to the lack of ordered and cyclic temporal patterns in phytoplankton dynamics. In fact, the gradient of species composition showed a strong association with hydrological factors. If the scenarios predicting increase of atmospheric temperatures and decrease of atmospheric precipitations and water availability in the regions south of the Alps are realistic, algal biomasses may rise and be associated with an increase of groups other than diatoms.     

Responses of phytoplankton diversity to physical disturbance under manual operation in a large reservoir, China

In aquatic ecosystems, physical disturbances have been suggested to be one of the main factors influencing phytoplankton structure and diversity. To elucidate whether large-scale artificial operation of a hydroelectric reservoir has potential impacts on phytoplankton diversity, the impact on phytoplankton biodiversity of physical disturbances under artificial operation from May 2007 to April 2008 in tributaries of the Three Gorges Reservoir (TGR), China, was analysed. Two disturbance parameters, i.e. the absolute incremental rates of discharge (R _d,i ) and precipitation (R _p,i ), were created in this study for evaluating physical disturbance intensities during low and high water level periods of the TGR. Results showed that river discharge seemed to be the main factor controlling the phytoplankton diversity in low water level periods (≤151 m), and that precipitation was a potential promoter of the physical disturbance. During the 156-m impoundment process, the species diversity clearly decreased due to the high dilution effect on the phytoplankton communities. At high water level periods (>151 m), the low levels of disturbance eventually allowed the phytoplankton community to approach competitive exclusion in late February 2008. Sharply declining diversity values appeared when the Dinophyta blooms occurred in late March and late April 2008 (Peridinium and Ceratium, respectively).     

Phytoplankton succession affects the composition of Polynucleobacter subtypes in humic lakes

Phytoplankton influence the composition of bacterial communities, but the taxonomic specificity of algal–bacterial interactions is unclear due to the aggregation of ecologically distinct bacterial populations by community characterization methods. Here we examine whether phytoplankton seasonal succession affects the composition of subtypes within the cosmopolitan freshwater bacterial genus Polynucleobacter. Changes in the composition of Polynucleobacter subtypes were characterized in samples collected weekly from May to August in 2003 and 2008 from three humic lakes using terminal restriction fragment length polymorphism fingerprinting of the protein‐encoding cytochrome c oxidase ccoN gene. Changes in phytoplankton population abundances explained, on average, 30% of temporal variation in the composition of Polynucleobacter subtypes and the interaction between phytoplankton and the environment explained an additional 18% of temporal variation. The effect of phytoplankton on specific Polynucleobacter subtypes was experimentally confirmed by changes in Polynucleobacter subtype composition following incubation with different phytoplankton assemblages or a no‐phytoplankton control. Phytoplankton‐associated subtypes and differentiation in substrate use among subtypes likely contribute to the effects of phytoplankton on Polynucleobacter subtype composition. Interactions between unique Polynucleobacter populations and phytoplankton highlight the ecological significance and specificity of species interactions in freshwater communities.     

Post‐Impoundment Biomass and Composition of Phytoplankton in the Yangtze River

Damming, and thus alteration of stream flow, promotes higher phytoplankton populations and encourages algal blooms (density >10⁶ cells L^–1) in the Three Gorges Reservoir (TGR). Phytoplankton composition and biomass were studied in the Yangtze River from March 2004 to May 2005. 107 taxa were identified. Diatoms were the dominant group, followed by Chlorophyta and Cyanobacteria. In the Yangtze River, algal abundance varied from 3.13 × 10³ to 3.83 × 10⁶ cells L^–1, and algal biomass was in the range of 0.06 to 659 mg C m^–3. Levels of nitrogen, phosphorus and silica did not show consistent longitudinal changes along the river and were not correlated with phytoplankton parameters. Phytoplankton abundance was negatively correlated with main channel discharge (Spearman r = –1.000, P < 0.01). Phytoplankton abundance and biomass in the Yangtze River are mainly determined by the hydrological conditions rather than by nutrient concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observations on the relation between phytoplankton diversity and environmental factors in the Vaal River at Balkfontein,South Africa

The relationship between specific environmental factors as independent variables and temporal changes in phytoplankton community structure in the Vaal River (a turbid system) during 1984 was investigated by employing different diversity indices. Temporal changes in community structure reflected temporal changes in certain environmental factors. Phytoplankton diversity, measured with Shannon-Wie H' and Hurlbert PIE indices, was related firstly to discharge and discharge derived variables (such as SO₄, Si, N and P loading) and secondly to turbidity derived variables (such as euphotic zone depth). Discharge appears to be of prime importance in affecting diversity. Observations were made that shed new light on conditions contributing to the development of an August peak (dominated by Stephanodiscus hantzschii fo. tenuis and Micractinium pusillum) in phytoplankton concentration. Increased environmental stress may reduce the number of sensitive species, thus reducing interspecific competition between tolerant species which could then exploit the — for them — more favourable conditions resulting in an increase in their numbers to peak concentrations.     

Phytoplankton communities in navigation pool no. 7 of the Upper Mississippi River

A study of the phytoplankton community dynamics in Navigation Pool No. 7 of the Upper Mississippi River was conducted from May through October, 1982. The objectives of this study were to estimate total standing crops, determine the taxonomic composition and examine the seasonal succession of the phytoplankton community. Four sampling sites were established: two in Lake Onalaska, a large backwater lake on the Wisconsin side of the main channel; one in the main channel near Dakota, Minnesota; and one in the main channel just upstream from Lock and Dam No. 7.The phytoplankton communities at all sampling sites were dominated by diatoms except during July and August when a bloom of blue-green algae was observed. The dominant diatoms from May through mid-July were Melosira italica, Stephanodiscus niagarae, Stephanodiscus hantzschii, Stephanodiscus astrea, and Synedra ulna. Aphanizomenon flos-aquae and Microcystis aeruginosa were the most prevalent blue-green algae during the mid-summer bloom. The diatoms Melosira italica and Melosira granulata were dominant in September and October. Lesser amounts of green algae, cryptomonads and euglenoids were also observed at various times of the sampling period.Total standing crops based on cell volume were usually greatest at the Lock and Dam No. 7 site. The maximum standing crop (10.4 mm³ 1^–1) was observed at the Lock and Dam No. 7 site on 4 September; the minimum standing crop (0.4 mm³ 1^–1) was observed at the eastern Lake Onalaska site on the same date. Concentrations of nitrogen, phosphorus, and silica remained at high levels throughout the study period and did not appear to limit phytoplankton standing crops.     

Downstream variations of phytoplankton in the St.Lawrence River (Québec, Canada)

Longitudinal variations of phytoplankton biomass and composition were assessed in a 250 km-long section of the St.Lawrence River, which alternately runs through narrow (< 2 km) river cross sections and wide (up to 10 km) fluvial lakes. In the main river stem, concentrations of suspended matter and total phosphorus increased with distance downstream, whereas light penetration decreased. Seasonal changes in plankton composition and biomass were more important than those resulting from differences in water mass (tributary) of origin. Sampling at three cross river sections and in two fluvial lakes showed a progressive downstream decrease in phytoplankton biomass and changes in size structure and taxonomic composition. River plankton was primarily composed of small (< 10 µm equivalent spherical diameter), truly planktonic cells belonging to Cryptophyceae and diatoms, with Chlorophyceae in summer. Plankton sampled in summer among rooted macrophytes in fluvial lakes exhibited a higher biomass of resuspended periphytic algae than in the main river stem, which contributed slightly to downstream phytoplankton biomass.Successive river cross sections always shared about 50% of their taxa, indicating a rapid downstream transport of algae within the main water mass. However, the proportion of species common to all cross sections was highest during the spring freshet, and lowest during summer low discharge, likely resulting from the development of a distinct flora in fluvial lakes during summer. Conversely, about 30% of the identified taxa were exclusive to a cross section and were replaced by others occurring downstream. Overall, phytoplankton composition along the St.Lawrence River is primarily controlled by advective forces, which result in a homogeneous flora in the main river stem, with a local contribution of resuspended periphyton from fluvial lakes.     

Temporal and spatial variability in the phytoplankton community of Myall Lakes,Australia, and influences of salinity

The variability in the phytoplankton communities of the Myall Lakes, a series of four interconnected coastal lakes on the lower north coast of New South Wales, was studied between 1999 and 2002. There was considerable spatial variability across the lake system. Bombah Broadwater experienced blooms of Anabaena in 1999 and early 2000, but these were replaced from late 2000 onwards by Chroococcus and a variety of eukaryotic taxa, particularly flagellates and diatoms. In comparison, the phytoplankton community of Myall Lake was dominated for much of the study period by Chroococcus, Merismopedia and chlorophyte taxa. The sites located midway between these two main lakes represent an ecotone, with elements of the phytoplanktonic flora of both main lakes being present. Changes in phytoplankton community composition in Bombah Broadwater occurred fairly frequently. In contrast, the phytoplankton community in Myall Lake changed little during the course of the study and can be considered as being at long-term equilibrium. The reasons for this lie in the morphology and hydrology of the lake system, which in turn create gradients in a number of physico-chemical water quality attributes. Bombah Broadwater is influenced by episodic and stochastic freshwater inflows from the upper Myall River catchment, and in times of drought by saline marine incursions via the lower Myall River. Myall Lake however represents a cul-de-sac, with only a small hydraulic connection to the remainder of the lake system. As it has little input from its small catchment, the limnological conditions within this lake remain relatively constant for long periods of time. Although no patterns of seasonal succession were discernable in any of the lakes, some longer-term (annual) changes did occur, and certain taxa displayed enhanced growth in summer. Salinity was found to be an important factor in determining phytoplankton community composition and abundance. Canonical Correspondence Analysis of phytoplankton and environmental data for all sites combined, showed ammonia, total nitrogen and salinity (measured as electrical conductivity) to have the most influence on the phytoplankton community composition and abundance. Anabaena growth was positively related to ammonia concentration and negatively related to conductivity.     

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.     

Selective effects of phytoplankton entrainment at the Surry Power Plant,James River,Virginia

Loss of cryptophyte cells entrained in the Surry Power Plant cooling water was significantly correlated with discharge water temperature in the range 27.2–37.5 °C. Entrained Skeletonema costatum and benthic diatom populations experienced losses of 25–80% in the summer, but correlations between % loss and discharge temperature were insignificant. Cropping by benthic filter feeders in the intake and discharge canals could account for the summer removal of diatoms. Shortening of entrained S. costatum chains was detected in both winter and summer, indicating a mechanical effect of turbulence.Benthic diatoms were vulnerable to entrainment only during daylight hours, when they migrated to the sediment surface at low tide. Skeletonema costatum was most vulnerable in the summer, when elevated salinities permitted it to range upstream to the intake area. Cryptophyte populations peaked in the summer when entrainment loss was greatest.The composition of the entrained phytoplankton community was altered by the species specific interactions of factors affecting vulnerability and entrainment loss. At Surry the discharged cooling water mixes rapidly with the main stem James River, and the selective effects of entrainment are not detectable in phytoplankton samples taken beyond the immediate discharge zone. More persistent modifications of the phytoplankton could be expected at sites where power plants discharge into creeks or embayments.Contribution No. 1106, Virginia Institute of Marine Science.Contribution No. 1106, Virginia Institute of Marine Science.     

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.     

Grazing rates and behaviors of Neocalanus plumchrus: implications for phytoplankton control in the subarctic Pacific

Grazing rates and behaviors of the copepod Neocalanus plumchrus were investigated in shipboard experiments during the first SUPER Program cruise (May, 1984). N. plumchrus can exploit cells in the 2 to 30 m size range with equal clearance efficiency but displays considerable flexibility in responding to changes in concentration and size composition. Its functional response helps to stabilize phytoplankton at low densities. In 60-liter microcosms, a density of one copepod liter^–1 was sufficient to maintain the ambient abundance and structure of the phytoplankton community for a week. In the absence of the copepod, phytoplankton bloomed to unnaturally high levels, and the community composition was dramatically altered. Despite its grazing potential, N. plumchrus was not present in sufficient density to control phytoplankton blooms in the subarctic Pacific. However, the copepod may have an important role in regulating the abundance of smaller grazers and the size structure of the phytoplankton community.Contribution No. 2002 from Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822     

Controlling the Growth of Microcystis Using Surged Artificial Aeration

This paper investigates the effect of lake mixing, due to artificial hypolimnetic aeration, on the succession and ecological composition of selected phytoplankton species in Tegeler See, Berlin. Special attention is given to Microcystis, a nuisance bloom-forming blue-green algae and its competitor (in this lake) Ceratium, a dinoflagellate. Due to the installation configuration, the aerators frequently destratified the lake, especially when the stratification was weak. Five years representing three different operating schemes of the 15 aerators (continuous aeration, non-aeration and surge aeration) in this lake were used to determine possible aeration strategies in controlling Microsystis. In order to ease correlation of the mixing status in the lake with its yearly successions of phytoplankton growth a non-dimensional parameter called the Lake number was utilised. This parameter represents a balance between: i) forcing due to wind and aerator discharge and ii) potential energy acquired from the stratification of the lake. The results indicate that surged operation of the aerators is effective in suppressing massive growth of Microcystis. Due to their ability to adapt to the mixing/stability state of the water column, prolonged aeration or prolonged stable stratified conditions have little effect in controlling this algae.     

Changes in the phytoplankton community of a coastal,hyposaline lake in western Anatolia,Turkey

The phytoplankton composition of coastal, hyposaline (≅12 g l⁻¹) Lake Bafa in western Turkey, was investigated after the alteration of salinity levels in past decades. Lake Bafa, formerly a bay of the Aegean Sea, was separated from the sea as a result of alluvium deposits of the Meander River. After the construction of a sett for flood control in 1985, freshwater inflow was reduced and salinity increased, which caused some ecological changes. Phytoplankton samples were taken in August and November 2000, and February and May 2001, from the surface and at 5-m intervals in the water column at three stations. The phytoplankton community contained a mixture of freshwater and marine origin species. A total of 45 species from Bacillariophyceae, Chlorophyceae, Cyanophyceae, Chrysophyceae, and Dinophyceae were identified. Marineoriginated species such as Chaetoceros spp. (November and May), Thalassionema nitzschioides and Prorocentrum micans (August and November), and P. minimum (May) were dominant on the basis of numerical abundance. The occurrence of salinity-tolerant species such as T. nitzschioides, P. micans, and P. minimum has not been recorded in previous studies.     

Euglenoid morphospecies replacement along a hydraulic gradient of the Lower Paraná Basin (Argentina)

1. This study deals with the distribution of euglenoid morphospecies along the length of the Lower Paraná River floodplain. Six sites located in the Paraná River, Estevez stream and Montiel shallow lake were surveyed monthly from May 1995 to April 1996. 2. Eighty‐eight infrageneric taxa were registered though the total density percentage of euglenoids seldom exceeded 15% of total phytoplankton. The increasing species richness towards the lake is mostly the result of a high contribution of species of Trachelomonas and naked infrageneric taxa of Euglena, Lepocinclis and Phacus, while the distribution of Strombomonas species along the transect is fairly constant. 3. The richness of euglenoid morphospecies follows a pattern opposite to that found for the entire phytoplankton community. The distribution of euglenoids differs among the three habitats defined in this study: river, stream, shallow lake. These differences depend on the hydraulic condition of each habitat, as revealed by canonical correspondence analysis. 4. The degree of association with each type of aquatic habitat varied among the species and differed between water phases. Strombomonas fluviatilis was equally constant along the length of the ecotone, while S. girardiana and S. scabra were particularly associated with the Paraná River and P. longicauda, L. truncata, T. hispida and T. similis with the lake. 5. The flood pulse establishes a clear pattern both in species richness and abundance of euglenoids. The number of species recorded during high water was more than twice that recorded during low water.     

The phytoplankton community of the River Meuse,Belgium: seasonal dynamics (year 1992) and the possible incidence of zooplankton grazing

Qualitative and quantitative aspects of the phytoplankton of the River Meuse were studied during 1992, at a point 537 km from the source. The phytoplankton was dominated by diatoms and green algae. The Stephanodiscus hantzschii-group was especially prominent. Other important taxa were Cyclotella meneghiniana, small Cyclotella and Thalassiosira, Aulacoseira ambigua and Nitzschia acicularis. Cell abundances varied from less than 1000 units ml^–1 to about 25 000 – 30 000 units ml^–1 during the blooms. The Stephanodiscus hantzchii-group constituted almost entirely the first spring bloom. During the summer period, small Thalassiosiraceae developed markedly and large Thalassiosira weissflogii appeared. During this period, green algae dominated diatoms as expressed in cell abundances. The main Chlorococcales were Scenedesmus quadricauda, Scenedesmus div. sp., Dictyosphaerium ehrenbergianum and Pediastrum duplex. Dinophyceae contributed a significant biomass during the summer period. Total biomass varied between 100 and 3 650 µg Cl^–1. As previously observed (Descy, 1987), the factors regulating the phytoplankton growth were clearly physical variables: discharge, temperature and irradiance. However, in the summer period, low abundances might indicate a regulation by biotic factors. The impact of grazing by zooplankton is discussed, on the basis of observations of zooplankton development in the River Meuse and on the basis of simulation by a mathematical model. A comparison is carried out with recent data of phytoplankton in large European rivers.     

Factors controlling the seasonal development and distribution of the phytoplankton community in the lowland course of a large river in Northern Italy (River Adige)

The principal environmental factors influencing the seasonal dynamics of phytoplankton were examined from September 1997 to July 1998 in three stations along a 26-km stretch of the lowland course of River Adige (northeast Italy). Nutrient concentrations did not appear to be limiting for the phytoplankton growth. Annual minimum concentrations of reactive and total phosphorus, and dissolved inorganic nitrogen were 22 μg P l⁻¹, 63 μg P l⁻¹ and 0.9 mg N l⁻¹, respectively. The most critical forcing factors were physical variables, mainly water discharge and other variables related to hydrology, i.e. suspended solids and turbidity, which acted negatively and synchronously by diluting phytoplankton cells and decreasing light availability. Higher algal biomass was recorded in early spring, in conditions of lower flow velocity and increasing water temperature. In late spring and summer, higher water discharge caused a decrease in phytoplankton biomass. Conversely, low algal biomass in late autumn and winter, during low discharge, was mainly related to low water temperatures and shorter photoperiod. Physical constraints had a significant and measurable effect not only on the development of total biomass, but also on the temporal dynamics of the phytoplankton community. Abiotic and biotic variables showed a comparable temporal development in the three sampling stations. The small number of instances of spatial differences in phytoplankton abundance during the period of lower flow velocity were related to the increasing importance of biological processes and accumulation of phytoplankton biomass.