Seasonal Phytoplankton Dynamics Research Articles

Seasonal Dynamics of Phytoplankton of the Kiev Section of the Kanev Reservoir

Seasonal Dynamics of Phytoplankton of the Kiev Section of the Kanev Reservoir

Seasonal phytoplankton dynamics in the coastal waters of the north-eastern Adriatic Sea

This study describes the dynamics of phytoplankton in relation to environmental factors in coastal waters of the north-eastern Adriatic Sea. The distant Po River's influence is rarely noted on the eastern coastal part of the northern Adriatic, but it does trigger the crucial alternations in the community, e.g. mass development of diatoms in July 2009. A highly variable ecosystem, like the northern Adriatic, sustains high species diversity. Even though quite a few species were present for more than several months, we identified the seminal species of the seasonal succession. During spring Prorocentrum micans, Leptocylindrus minimus, Chaetoceros throndsenii, Ceratium furca and Ceratium fusus were most characteristic. These species thrive in low salinity and mixed waters. Typical summer diatom species were Chaetoceros vixvisibilis and Proboscia alata, while Rhabdosphaera clavigera and Syracosphaera pulchra were identified as distinctive summer coccolithophorids. All the summer species preferred warmer waters with low nitrate and low silica content. The autumn community was characterised by species such as Calciosolenia murrayi, Chaetoceros socialis, Asterionellopsis glacialis, and Lioloma pacificum. These species were related not only to high nutrient conditions, but also to low Po River influence. In winter a prevalence of large diatoms Thalassiosira rotula, Neocalyptrella robusta and Pseudosolenia calcar-avis was recorded. The winter assemblage was characterised by species of cold water preference able to grow in well mixed conditions.

Space-borne study of seasonal, multi-year, and decadal phytoplankton dynamics in the Bay of Biscay

Seasonal and inter-annual variations in phytoplankton community abundance in the Bay of Biscay are studied. Preliminarily processed by the National Aeronautics and Space Administration (NASA) to yield normalized water-leaving radiance and the top-of-the-atmosphere solar radiance, Sea-viewing Wide Field-of-View Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Coastal Zone Color Scanner (CZCS) data are further supplied to our dedicated retrieval algorithms to infer the sought for parameters. By applying the National Oceanic and Atmospheric Administration's (NOAA's) Advanced Very High Resolution Radiometer (AVHRR) data, the surface reflection coefficient in the only band in the visible spectrum is derived and employed for analysis. Decadal bridged time series of variations of diatom-dominated phytoplankton and green dinoflagellate Lepidodinium chlorophorum within the shelf zone and the coccolithophore Emiliania huxleyi in the pelagic area of the Bay are documented and analysed in terms of impacts of some biogeochemical and geophysical forcing factors. It is shown that in the shelf zone of the Bay, the diatom-dominated phytoplankton community variations are predominantly controlled by river discharge variations, by water column stratification conditions (forming in winter–early spring), and by wind action (resulting in such phenomena as up-wellings and sediment re-suspension). Satellite data indicate that while in river deltas and adjoining waters the L. chlorophorum blooming events occur annually, in the Iroise Sea and near the Bailiwick of Guernsey, they happen irregularly. It is thought that such an irregular pattern, possibly, arises from L. chlorophorum competing with other phytoplankton species for nutrients. E miliania huxleyi blooms are found to occur nearly every year in the northern part of the Bay, whereas in the central area, this phenomenon occurs very irregularly. Satellite data indicate that variations in the water chemistry (variations in the nitrogen : phosphorus ratio due to preceding blooms of diatoms), and the incident irradiance level (degree of cloudiness), are important factors controlling the occurrence of E. huxleyi blooming in the central part of the Bay. Covering a 30 year period, the bridged data from CZCS, AVHRR, SeaWiFS, and MODIS imply that climate change might be responsible for the observed increase in E. huxleyi blooming events in the Bay since 1979.

Large-scale patterns of river inputs in southwestern Europe: seasonal and interannual variations and potential eutrophication effects at the coastal zone

We provide data on nutrient export for 28 rivers in southwestern Europe and analyze long-term changes in the context of anthropogenic pressures and regulation policies. Special attention is given to seasonal variations, because the integrated annual values that are usually provided do not allow us to establish comparisons with seasonal phytoplankton dynamics. The eutrophication risk associated with river inputs is addressed by means of an indicator (Index of Coastal Eutrophication Potential, ICEP, Billen and Garnier, Mar Chem 106:148–160, 2007). An overview of the temporal evolution and the intra-annual variability of the ICEP is discussed for specific rivers and integrated coastal regions. The annual dynamics of the eutrophication indicator is analyzed to delimit those periods when the risk of eutrophication is particularly high. The trends in nutrient fluxes and coastal phytoplankton are compared by means of a case study (Seine Bay). The decrease in phosphorus matches a general decrease in phytoplankton biomass in the summer. However, sustained high values of nitrogen still foster the emergence of harmful algal blooms, and we found an increase in the summer abundance of dinoflagellates. The abatement of phosphorus alone is not enough to shortcut harmful blooms and toxic outbreaks in the Seine Bay. A reduction in nitrogen inputs may be necessary to effectively minimize eutrophication problems.

Seasonal dynamics of phytoplankton and its relationship with the environmental factors in Dongping Lake, China

Dongping Lake is the final adjusting and storing lake in the east route of the South-to-North Water Diversion Project in China, and there has been serious concern regarding the water quality. Understanding the process of phytoplankton variation can be particularly useful in water quality improvement and management decisions. In this study, the phytoplankton taxonomic composition, abundance, temporal variations, spatial distribution, and diversity were studied based on a monthly sampling campaign from three sampling stations between May 2010 and May 2011. A total of 132 species (8 phyla, 72 genera), including 64 species of Chlorophyta, 26 species of Bacillariophyta, 21 species of Cyanophyta, 12 species of Euglenophyta, 3 species of Cryptophyta, 2 species of Xanthophyta, 1 species of Pyrrophyta, and 3 species of Chrysophyta were identified. Average phytoplankton diversity index and evenness values were 3.83 and 0.77, respectively, revealing a high biodiversity of phytoplankton community. The phytoplankton abundance averaged 5.11 × 10(6)cells/L, with Bacillariophyta dominant in winter and spring, but Cyanophyta in summer and autumn. There were 14 predominant species including Pseudanabaena limnetica, Chlamydomonas simplex, Cyclotella stelligera, and Chroomonas acuta. Phytoplankton community structure and water quality variables changed substantially over the survey period; redundancy analysis, Pearson correlations, and regression analysis as an integrated approach were applied to analyze the relationships among them. Total phosphorus and ammonium played governing roles in the phytoplankton dynamics of Dongping Lake during all periods investigated.

Analysis of the seasonal dynamics of river phytoplankton based on succession rate indices for key event identification

The seasonal dynamics of river phytoplankton was analyzed using succession rate indices based on data collected from year-round observations of two small plain rivers in the Upper Ob Basin (Western Siberia). The study revealed a generally clear seasonal pattern of structural changes in the phytoplankton of the lower reaches of the studied rivers. The dynamics of succession rate indices reflects the key events in the life of phytoplankton in the Bolshaya Losikha and Barnaulka Rivers, showing mainly changes in the dominant species during the main phases of the hydrological cycle. The most significant changes in phytoplankton structure tend to occur in the period between the spring flood decline and the beginning of summer–autumn low water. These changes coincide with the most drastic changes in both environmental conditions and phytoplankton successional stages. Use of succession rate indices to analyze the seasonal dynamics of phytoplankton allowed us to distinguish between periods of abrupt change and periods of comparatively low-intensity changes in plankton composition in small lowland temperate rivers.

Diversity and dynamics of free-living and particle-associatedBetaproteobacteriaandActinobacteriain relation to phytoplankton and zooplankton communities

The diversity of attached and free-living Actinobacteria and Betaproteobacteria, based on 16S rRNA gene sequences, was investigated in a mesotrophic lake during two periods of contrasting phytoplankton dominance. Comparison analyses showed a phylogenetic difference between attached and free-living communities for the two bacterial groups. For Betaproteobacteria, the betaI clade was detected at all sampling dates in free-living and attached bacterial communities and was the dominant clade contributing to 57.8% of the total retrieved operational taxonomic units (OTUs). For Actinobacteria, the acIV cluster was found to be dominant, followed by acI contributing to 45% and 25% of the total retrieved OTUs, respectively. This study allows the determination of eight new putative clades among the Betaproteobacteria termed lbI-lbVIII and a new putative clade named acLBI belonging to the Actinobacteria. The seasonal dynamics of phytoplankton and zooplankton communities have been reflected as changes in distinct bacterial phylotypes for both attached and free-living communities. For attached communities, relationships were observed between Actinobacteria and Chrysophyceae, and between Betaproteobacteria and Dinophyceae and Chlorophyceae biomass. On the other hand, within free-living communities, few actinobacterial clades were found to be dependent on either nutrients or phytoplankton communities, whereas Betaproteobacteria were mainly associated with biological parameters (i.e. phytoplankton and copepod communities).

Seasonal phytoplankton dynamics in extreme southern South America (Beagle Channel, Argentina)

The phytoplankton biomass and composition dynamics in relation with environmental factors were explored during an annual cycle (July 2006–2007) for the first time in the Beagle Channel. Samples were examined by light and scanning electron microscopy and carbon biomass was estimated using measured cell biovolumes. The annual cycle was characterized by a sharp contrast between low density and biomass during the autumn–winter period and a significant increase during spring and summer. The maximum biomass peak was observed during early spring (141 μg C l − 1 and 8.7 μg Chl a l − 1 ) and was associated with a bloom of the diatom genus Chaetoceros. In spite of a strong nutrient depletion that lead to the subsequent bloom decline, a second peak of biomass was observed after 2 weeks, mainly represented by the diatoms T. mendiolana and T. anguste-lineata. Towards the end of spring and during summer diatom density decreased and assemblages were enriched by unidentified tiny phytoflagellates and dinoflagellates. A total of 138 taxa were identified during the study period. From these, 10 are known as potentially toxic species: Alexandrium catenella, A. ostenfeldii, A. tamarense, Dinophysis acuminata, D. rotundata, Prorocentrum cordatum, Pseudo-nitzschia australis, P. calliantha, P. fraudulenta and P. cf. seriata. Their presence is noteworthy since the area sustains a growing aquiculture production.

Modeling the long-term dynamics of nutrients and phytoplankton in the Gulf of Riga

The long term dynamics of nitrogen, phosphorus, and phytoplankton in the Gulf of Riga were simulated with a biogeochemical box model that resolved seasonal cycles. The model was calibrated using a numerical optimization procedure that adjusted 37 parameters to maximize the model data fit for field observations from 1973 to 2000 and validated with an independent dataset covering 2001–2007. Both the long-term increase and subsequent decline in winter nitrogen concentrations, as well as the continuous increase in winter phosphate levels were well reproduced by the model, which also gave reasonable representations of the seasonal dynamics of nutrients and phytoplankton. Starting from the mid-1990s, the model simulated an increase in cyanobacteria growth sustained by internal phosphorus loading. While nitrogen was efficiently removed by denitrification from the Gulf of Riga, comparatively slow export to the Baltic Proper was the main removal pathway of phosphorus. Modeled residence times were 5.4 years for nitrogen and 38 years for phosphorus. Scenario simulations indicated that the Gulf of Riga responds to phosphorus load reductions with a gradual decrease in primary production and cyanobacteria growth, while the effect of nitrogen load reductions is largely offset by nitrogen fixation.

Application of a 3D hydrodynamic–biological model for seasonal and spatial dynamics of water quality and phytoplankton in Lake Erie

In large lakes, temporal variability is compounded by strong spatial variability associated with mesoscale physical processes such as upwelling and basin-scale circulation. Here we explore the ability of a three dimensional model (ELCOM–CAEDYM) to capture temporal and spatial variability of phytoplankton and nutrients in Lake Erie. We emphasized the east basin of the lake, where an invasion by dreissenid mussels has given special importance to the question of spatial (particularly nearshore–offshore) variability and many comparative observations were available. We found that the model, which did not include any simulation of the mussels or of smaller diffuse nutrient sources, could capture the major features of the temperature, nutrient and phytoplankton variations. Within basin variability was large compared to among-basin variability, especially but not exclusively in the western regions. Consistent with observations in years prior to, but not after, the mussel invasion the model predicted generally higher phytoplankton concentrations in the nearshore than the offshore zones. The results suggest that the elevated phytoplankton abundance commonly observed in the nearshore of large lakes in the absence of dreissenid mussels does not have to depend on localized nutrient inputs but can be explained by the favourable light, temperature and nutrient environment in the shallower and energetic nearshore zone. The model is currently being extended to allow simulation of the effects of dreissenid mussels.

Patterns of seasonal phytoplankton dynamics as the element of ecological successional changes proceeding in a lake (Lake Kortowskie, northern Poland)

Patterns of seasonal phytoplankton dynamics as the element of ecological successional changes proceeding in a lake (Lake Kortowskie, northern Poland)The aim of the research was to determine the successional changes proceeding in the phytoplankton of a lake. The basis of the study was the analysis of long-term seasonal changes in the taxonomic structure and the estimation of the intensity of algal community development in phytoplankton of Lake Kortowskie. Seasonal changes in the phytoplankton taxonomic structure manifested themselves as a decreasing share of Bacillariophyceae in spring and autumn and Chlorophyta in summer phytoplankton. The retreating chlorophytes and the diatoms were replaced by blue-green algae, the increasing share of which caused a reduction in the stability of the remaining phytoplankton taxonomic groups, and consequently, taxonomic shifts and progressive development of the overall phytoplankton community in all phenological seasons. Intensive algal growth was observed in spring and lasted until late autumn. The highest phytoplankton biomass was always recorded in summer. Summer phytoplankton development was related to increased bluegreen algae domination, the massive growth of which was observed from spring to autumn. The development of the Cyanoprokaryota community had a negative impact on the diatoms, since the latter earlier disappeared from the biocenosis in spring and re-developed to a lesser degree in autumn.

In situ evaluation of the initiation of the North Atlantic phytoplankton bloom

Two years of continuous physical and optical measurements from a profiling float in the western subarctic North Atlantic are used to analyze seasonal phytoplankton dynamics. The observed annual cycle challenges the traditional view that initiation of spring accumulations of phytoplankton in the upper water column requires a critical stratification threshold (known as the ‘Gran effect’ or the ‘Sverdrup Hypothesis’). Instead, we find that biomass accumulation begins in mid‐winter when light levels are minimal and near‐surface mixing is deepest. These observations are consistent with the recently proposed dilution–recoupling hypothesis which states that deep winter mixing in the North Atlantic is essential for bloom formation as it decouples phytoplankton growth from grazing losses, thereby allowing net biomass accumulation despite low‐light conditions.

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

Summary1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales.2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance.3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L−1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes.4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.

Assessment of water quality in a large lowland river (Narva, Estonia/Russia) using a new Hungarian potamoplanktic method

Phytoplankton is one of the four biological elements used for the assessment of the water quality of surface water bodies. In rivers phytoplankton-based assessment of water quality has not been conducted in Estonia up to now. The aim of the present study was to test a new Hungarian potamoplanktic method for the assessment of the water quality of the Narva River, a large river on the Estonian-Russian border. For testing the method, algal species in the phytoplankton of the Narva River were classified into functional groups. Then the Ecological Quality Ratio (EQR) was calculated and its value was compared with the corresponding values for different water quality classes given in the literature. The mean value of the EQR for the Narva River revealed seasonal variation: in most cases it indicated 'good' or 'excellent' quality classes in spring and summer and 'very bad' quality class in autumn. The median value for the vegetation period of 2001-2008 indicated 'moderate' quality class. Variation in the functional groups of phytoplankton and in the EQR values reflected the seasonal dynamics of phytoplankton and the impact of Lake Peipsi. Comparison of the results of the assessments made by using the phytoplankton EQR and benthic diatom indices revealed agreement between the two metrics in the summer period: both were sensitive to the water quality and indicated at least 'good' quality class. The new Hungarian method appears to be suitable for the assessment of water quality in this Estonian large river. However, the numerical boundaries of the EQR for different water quality classes should be specified in the future on the basis of a larger Estonian phytoplankton database.

Trophic state and seasonal dynamics of phytoplankton communities in two sand-pit lakes at different successional stages

The seasonal and inter-annual dynamics of phytoplankton in two Italian sand-pit lakes have been analysed over three years (2004-2006). The studied lakes, Ca' Morta (CM) and Ca' Stanga (CS), have a common origin and are contiguous, but are of different ages and have been managed in different ways. CM was created in 2002 and was still being dredged during the study period, while sand quarrying in CS began in 1998 and ended at the beginning of the study period. These conditions were thought to be of particular interest in assessing the influence of dredging activities on algal assemblages. Physical and chemical water parameters were also analysed to characterise the two lakes and investigate the effects of hydro-chemical features on phytoplankton. CS was stratified from April to October each year during the study period, while CM showed weaker thermal stratification. Conductivity, dissolved nitrate and dissolved reactive silica concentrations were greater in CM than in CS. The mixing depth/euphotic depth ratio was also higher in CM, probably because of water column mixing induced by hydraulic dredging. Overall, 185 phytoplankton taxa were found, of which ca 50% were present in both lakes. No significant differences in species richness were observed between lakes, but the decreasing trend of Simpson index values measured in CS showed a reduction in biodiversity after the excavation phase. Phytoplankton biomass was higher in CS than in CM, the latter being oligo-mesotrophic and the former mesotrophic. Diatoms and dinoflagellates were the dominant taxa in both lakes. The main differences in phytoplankton assemblages were related to the persistence of chryptophytes and chlorophytes in CM. Multivariate analyses support the hypothesis that different thermal patterns induced by dredging activities, along with certain physico-chemical parameters of the water, were the major factors shaping phytoplankton communities.

Satellite-detected fluorescence reveals global physiology of ocean phytoplankton

Abstract. Phytoplankton photosynthesis links global ocean biology and climate-driven fluctuations in the physical environment. These interactions are largely expressed through changes in phytoplankton physiology, but physiological status has proven extremely challenging to characterize globally. Phytoplankton fluorescence does provide a rich source of physiological information long exploited in laboratory and field studies, and is now observed from space. Here we evaluate the physiological underpinnings of global variations in satellite-based phytoplankton chlorophyll fluorescence. The three dominant factors influencing fluorescence distributions are chlorophyll concentration, pigment packaging effects on light absorption, and light-dependent energy-quenching processes. After accounting for these three factors, resultant global distributions of quenching-corrected fluorescence quantum yields reveal a striking consistency with anticipated patterns of iron availability. High fluorescence quantum yields are typically found in low iron waters, while low quantum yields dominate regions where other environmental factors are most limiting to phytoplankton growth. Specific properties of photosynthetic membranes are discussed that provide a mechanistic view linking iron stress to satellite-detected fluorescence. Our results present satellite-based fluorescence as a valuable tool for evaluating nutrient stress predictions in ocean ecosystem models and give the first synoptic observational evidence that iron plays an important role in seasonal phytoplankton dynamics of the Indian Ocean. Satellite fluorescence may also provide a path for monitoring climate-phytoplankton physiology interactions and improving descriptions of phytoplankton light use efficiencies in ocean productivity models.

Seasonal Dynamics of Phytoplankton in Relation to Key Aquatic Habitat Factors in a Polluted Urban Small Water Body in Tianjin, China

To evaluate the seasonal dynamics of phytoplankton and its relationships with aquatic habitat factors in polluted urban landscape lakes, annual investigations have been carried out in the West zone of Xinkai Lake. The results showed that the lake belongs to the eutrophic-supertrophic type in terms of key habitat factors measurement. The seasonal succession of phytoplankton community was determined. The dominant cyanobacterial species Oscillatoria tenuis Ag. bloomed in autumn, although the other species with higher degrees of dominance never bloomed in the year investigated. Significant correlations between some habitat factors and phytoplankton biomass were observed.

Seasonal dynamics of phytoplankton in two Amazon flood plain lakes of varying hydrologic connectivity to the main river channel

Seasonal dynamics of phytoplankton were evaluated in Lakes Amapa and Pirapora, small oxbows in the floodplain of Rio Acre, a tributary of the Amazon River in western Brazil. The two lakes differ primarily in their degree of connectivity to the main river channel, with the younger Lake Pirapora maintaining a connection year-round and displaying physical and chemical characteristics responding to rising river water levels, versus the older Lake Amapa connecting to the river only during a 2-3 month flooding phase and having a distinct period of isolation at the end of the dry season. The phytoplankton of Lake Pirapora has a low numeric density year-round and is dominated by small C-strategists (cryptomonads). The phytoplankton of Lake Amapa has a similar low density when the lake is connected to the river. However, once that lake is isolated and turbidity declines, density increases an order of magnitude and the assemblage becomes dominated by filamentous N-fixing Aphanizomenon tropicalis. Coincidentally concentrations of dissolved oxygen increase and levels of TN decline, similar to changes during autogenic succession in temperate shallow lakes. The seasonal changes in phytoplankton of Lake Amapa, from the time it is connected to the time when it is isolated from the river, generally conform to the C-S-R conceptual model of succession. This is the first study to directly compare lakes of different developmental stages, in terms of connectivity with the river channel. There remains a need for more comprehensive long-term comparisons of these unique lake ecosystems.

Seasonal variation in the community and size structure of nano- and microzooplankton in Gyeonggi Bay, Yellow Sea

To investigate the seasonal variation and community structure of nano- and microzooplankton in Gyeonggi Bay of the Yellow Sea, the abundance and carbon biomass of nano- and microzooplankton were evaluated at 10-day intervals from January 1997 to December 1999. Four major groups of nano- and microzooplankton communities were classified: heterotrophic ciliates, heterotrophic dinoflagellates (HDF), heterotrophic nanoflagellates (HNF), and copepod nauplii. The total carbon biomass of nano- and microzooplankton ranged from 10.2 to 168.8 μg C L −1 and was highest during or after phytoplankton blooms. Nano- and microzooplankton communities were composed of heterotrophic ciliates (7.4–81.4%; average 41.7% of total biomass), HDF (0.1–70.3%; average 26.1% of total biomass), copepod nauplii (1.6–70.6%; average 20.7% of total biomass), and HNF (0.8–59.5%; average 11.5% of total biomass). The relative contribution of individual components in the nano- and microzooplankton communities appeared to differ by seasons. Ciliates accounted for the most major component of nano- and microzooplankton communities, except during summer and phytoplankton blooming seasons, whereas HDF were more dominant during the phytoplankton blooming seasons. The abundance and biomass of nano- and microzooplankton generally followed the seasonal dynamics of phytoplankton. The size and community distribution of nano- and microzooplankton was positively correlated with size-fractionated phytoplankton. The carbon requirement of microzooplankton ranged from 60 to 83% of daily primary production, and was relatively high when phytoplankton biomass was high. Therefore, our result suggests that the seasonal variation in the community and size composition of nano- and microzooplankton appears to be primarily governed by phytoplankton size and concentration as a food source, and their abundance may greatly affect trophic dynamics by controlling the seasonal abundance of phytoplankton.

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.