Seasonal dynamics of aufwuchs Naididae (Oligochaeta) on Phragmites australis in a eutrophic lake

The Oligochaeta and Aphanoneura in the aufwuchs on Phragmites australis in a eutrophic hardwater lake were studied at two sites over a period of one year, in order to elucidate the structure and dynamics of this assemblage. The naidids Chaetogaster diastrophus, Nais spp., and Stylaria lacustris dominated the assemblage at any season. At both sites these taxa showed the same distinct pattern of successive population maxima in spring and summer: Chaetogaster diastrophus reached its peak density first, followed by Nais species, and eventually by Stylaria lacustris. Differences in temporal dynamics between sites were small apart from a second Stylaria maximum which was only observed at one site. Total naidid densities reached peak values of 3.8 individuals per cm² reed stem surface area. With mean individual biomass of 2.2 µg dry mass for Chaetogaster diastrophus, 13.3 µg for Nais, and 86 µg for Stylaria lacustris, respectively, maximum total naidid biomass on reed stems was 44 µg dry mass per cm². The biomass peak occurred later than that of total naidid density because in summer larger naidids dominated the assemblage. The observed succession appears to be consistent with seasonal changes in periphytic algal communities on the reed stems.     

Effects of litter accumulation and water table on morphology and productivity of Phragmites australis

Phragmites australis (Common reed) occurs in the interface between water and land. The water depth gradient from deep water to dry land is inversely related to litter accumulation. Eutrophication can result in an excessive production of litter, which may have a large impact on the occurrence of P. australis in this gradient. In an outdoor pot experiment, it was investigated how water tables in combination with substrates containing variable amounts of litter affect morphology and productivity of P. australis. Vegetatively propagated P. australis was grown in pots filled with river sand, litter, and different mixtures of sand and litter (25, 50 and 75% by volume). Four water table treatments were applied; drained (–12 cm), waterlogged (0 cm), flooded (+12 cm), and weekly fluctuating drained and flooded conditions (–12/+12 cm of water relative to substrate level). When drained, no differences between substrate treatments were present. Waterlogging, flooding fluctuating water table treatments caused growth reduction in substrate containing litter. The plants formed short shoots and thin rhizomes. With increasing water table, allocation of dry matter to stems increased at the expense of leaves and rhizomes. At intermediate levels of litter in the substrate, allocation to leaves was lowest. In both instances a lower leaf weight ratio (LWR) was (partly) compensated for by a higher specific leaf area (SLA), resulting in less pronounced differences in leaf area ratio (LAR). Aquatic roots developed when plants were waterlogged or flooded, and increased when litter was present in the substrate. Aquatic roots were formed in the top soil layer when waterlogged. The percentage of aquatic roots increased with increasing amount of litter in the substrate when plants were flooded. It was concluded that the morphological responses of P. australis to litter strongly constrain its ability to maintain itself in deep water when the substrate contains litter. This might one of the explanations for the disappearance of P. australis along the waterward side of littoral zones.     

Plankton biomass partitioning in a eutrophic subtropical lake: comparison with results from temperate lake ecosystems

Plankton were sampled for 6 years in a subtropical eutrophiclake in FL, USA, and absolute and relative carbon biomass wasdetermined for bacteria, phytoplankton, heterotrophic and phototrophicnanoflagellates, ciliates, rotifers and crustacean zooplankton.We compared the results with findings from a comprehensive studyof carbon biomass partitioning in eutrophic German lakes withelucidate common patterns and differences. Similarities betweenthe temperate and subtropical systems included: similar seasonaldynamics, with maximal carbon biomass of nanoflagellates andmetazoan zooplankton in spring and phytoplankton in summer toautumn, yearly averaged carbon occurring mainly in the phytoplanktonand phytoplankton accounting for a much greater proportion ofcarbon than bacteria. There also were differences: the Floridalake had lower absolute and relative carbon biomass in crustaceanzooplankton, stronger dominance of protozoa in total grazercarbon biomass, a lower ratio of zooplankton to phytoplanktoncarbon and almost a monoculture of predation-resistant copepods(versus a relatively balanced distribution of carbon among cladocerans,copepods and rotifers in the temperate lakes). The subtropicallake also had 4-fold higher relative biomass of small filamentouscyanobacteria in its phytoplankton, which we attribute to lightlimitation. Although the Florida and German studies did notmeasure biomass of planktivorous fish, the differences observedhere are consistent with a recent hypothesis that fish predationexerts stronger top–down control on the pelagic food webin subtropical lakes than in temperate lakes of similar trophicstatus.     

Variation in the decomposition of Phragmites australis litter in a monomictic lake: the role of gammarids

A decomposition study has been carried out in Lake Geestmerambacht, a moderately deep (max. ca. 22 m), monomictic slightly brackish lake in The Netherlands. To assess the relative importance of biotic (benthos) and physico-chemical factors, the mass loss rate (K) of reed leaf litter was measured at 10 sites, both in winter and in summer, in the absence (`protected' litter bags) and the presence (`unprotected' litter bags) of invertebrates. The aim was to investigate the variation in mass loss rate within the habitat and between seasons, and the role of the litter-associated invertebrate community. The experiments showed high spatial variation in decomposition rates. The spatial pattern changed with season. In summer, decomposition rates were higher than in winter: 4.4 times in the presence of invertebrates, and 2.6 times in their absence. The exclusion of invertebrates (`protected' litter bags versus `unprotected' litter bags) led to significantly lower decomposition rates. In particular, the decomposition rate was strongly correlated with the number of gammarids, the dominant taxon in the litter bags, which are well known for their ability to feed on leaf litter. The abundance of gammarids was directly correlated to the level of dissolved oxygen and inversely correlated to the effective fetch in summer, when the spatial structure of the decomposition process was evident. Therefore, the results of this study indicate that there are seasonal and spatial differences in the rate of detritus decomposition, most likely due to changes in habitat characteristics that influence the distribution of gammarids, key-species responsible for the first steps of the leaf breakdown in Lake Geestmerambacht. The spatial dependency of the process leads to formation of heterogeneous ecological patches in which the probability of disturbance propagation may vary.     

Regulation of spatial and seasonal variation of macroalgal biomass in a brackish, eutrophic lake

Processes leading to biomass variation of Ulva were investigated at two contrasting sites in the eutrophic Veerse Meer (The Netherlands). Ulva species dominated at the Middelplaten site, while at the Kwistenburg site a mixture of Ulva spp. and Chaetomorpha linum dominated. Total summer macroalgal biomass was higher at Middelplaten than at Kwistenburg (282 and 79 g DW m^–2, respectively). Growth rates of Ulva spp. were high at both sites in May 1992 (cage mean 0.28–0.30 day^–1), but quickly dropped to lower values (0.05–0.10 day^–1). In May, growth rates were significantly highest at Kwistenburg, while during the rest of the season growth rates were similar for both sites. Temperature, pH, dissolved oxygen, salinity, light attenuation, phytoplankton and nutrient concentrations did not differ between sites. The relation between variation in Ulva spp. growth rates and environmental parameters was analysed using stepwise multiple regression, showing that light and temperature were the main variables regulating Ulva spp. growth rates. As Ulva growth rates were similar for both sites but biomass was much lower at Kwistenburg it was concluded that a large amount of produced biomass was lost at Kwistenburg. Although the exact reason for this remains unclear, it seems most likely that transport of macroalgae by wind and waves is a very important factor. This study shows the importance of simultaneously measuring growth rates and biomass at a high temporal resolution to reveal the mechanisms responsible for spatial variation in macroalgal biomass in shallow coastal areas. Electronic Publication     

Effects of environmental stressors and their interactions on zooplankton biomass and abundance in a large eutrophic lake

We assessed long-term impacts of multiple stressors and their interaction on the zooplankton community of the large, eutrophic, cyanobacteria-dominated Lake Peipsi (Estonia, Russia). Stressor dataset consisted in time series (1997–2018) of temperature, nutrients, pH, water transparency, phytoplankton biomass and taxonomic richness. The best predictors were selected with random forests machine-learning algorithms and the subsequent models were constructed with generalized linear modeling. We also aimed to identify graphical thresholds representing non-linear, marked responses of abundance or biomass to stressors. Temperature was the dominant stressor for explaining zooplankton abundance and biomass, followed by cyanobacteria biomass, total nitrogen concentration and water transparency. The effect of water temperature was positive, whereas the effect of cyanobacteria became negative after their biomass exceeded a threshold of?~?2 mg l^?1. However, the two stressors together had antagonistic effects on zooplankton, causing a decrease in biomass and abundance. For zooplankton, critical thresholds of total nitrogen (~?700 μg l^?1), total phosphorus (~?70 μg l^?1), and water transparency (~?1.4 m) after which zooplankton metrics changed drastically, were determined. These findings show that although lake warming alone could be positive for zooplankton, the necessity of reducing interacting stressors that influence harmful cyanobacteria growth and biomass, especially nitrogen loads, must be considered.

     

Bloom formation of Gloeotrichia echinulata and Aphanizomenon flos-aquae in a shallow,eutrophic, Danish lake

Over a period of four years, the seasonal periodicity of dominant phytoplankton species in a shallow, eutrophic Danish lake changed markedly. Cyanophytes prevailed during the summer period of all four years. In the first three years, species of Microcystis, Anabaena and Aphanothece dominated, whereas in the fourth year of investigation, these algae were replaced by Gloeotrichia echinulata (J. E. Smith) Richter and Aphanizomenon flos-aquae (L.) Ralfs. The most striking environmental differences in the fourth year as compared with the previous three years, were an increase in tranparency, from about 0.5 meter in 1989–1991 to more than 2 metres preceding the summer maximum in 1992, and a simultaneous occurrence of low oxygen concentrations. A collapse of the fish population was followed by an increased proportion of large Cladocerans in the zooplankton. Improved light conditions at the bottom and grazing pressure from large Cladocerans favoured growth of the large colony forming blue-green algae, Gloeotrichia echinulata and Aphanizomenon flos-aquae. These species germinate from resting spores in the sediment and are able to sustain some growth there before migration to the lake water. The transfer of algal biomass from the bottom sediment to the water phase was accompanied by a marked increase in concentrations of particulate phosphorus and nitrogen in the entire lake.     

Effects of genotypic diversity of Phragmites australis on primary productivity and water quality in an experimental wetland

An increasing number of studies have shown that genetic diversity within plant species can influence important ecological processes. Here, we report a two-year wetland mesocosm experiment in which genotypic richness of Phragmites australis was manipulated to examine its effects on primary productivity and nitrogen removal from water. We used six genotypes of P. australis, and compared primary productivity and nitrogen concentration in the outflow water of the mesocosms between monocultures and polycultures of all six genotypes. We also quantified the abundance of denitrifying bacteria, as denitrification is a primary mechanism of nitrogen removal in addition to the biotic uptake by P. australis. Plant productivity was significantly greater in genotypic polycultures compared to what was expected based on monocultures. This richness effect on productivity was driven by both complementary and competitive interactions among genotypes. In addition, nitrogen removal rates of mesocosms were generally greater in genotypic polycultures compared to those expected based on monocultures. This effect, particularly pronounced in autumn, may largely be attributable to the enhanced uptake of nitrogen by P. australis, as the abundance of nitrite reducers did not increase with plant genotypic diversity. Although our effect sizes were relatively small compared to previous experiments, our study emphasizes the effect of genotypic interactions in regulating multiple ecological processes.     

Climate forcing of diatom productivity in a lowland,eutrophic lake: White Lough revisited

1. In cultural landscapes, lake response to climate can be masked by land‐use change and nutrient loss from their catchments. Palaeolimnological methods were used to reconstruct the ecological response of diatoms in a eutrophic lowland lake (White Lough, Co. Tyrone, Northern Ireland) to altered nutrient P loading and precipitation variability over c. 100 years. 2. ²¹⁰Pb‐dated sediment cores were analysed to determine diatom assemblage variability, biogenic silica concentration, geochemical phosphorus concentration and accumulation rate. Manure P and agricultural N surplus data were collated from documentary sources. Long‐term trends in annual temperature and precipitation were derived from the Armagh Observatory. 3. Diatom community turnover from 1890 until c. 1960 was limited, and assemblages were dominated by Aulacoseira subarctica; after this date, changes primarily reflected a eutrophication sequence owing to increased diffuse nutrient inputs associated with intensification of land use (external P loading increased by a factor of three). 4. Diatom and biogenic Si profiles were compared with North Atlantic Oscillation (NAO) records, an index of regional weather patterns. Biogenic Si exhibited a c. 7‐year cycle, which tracked a cycle of similar timescale in the Armagh climate record for dry summers. In turn, this cycle was related to the variation in the NAO. 5. Monitoring data from 1971 to 2007 of nitrate exports from the Blackwater River showed that these too followed a roughly 7‐year cycle at least up to 2000, in which dry summers were followed by sharp increases in nitrate export. It is argued that diatom production in White Lough reflects the cyclic behaviour in nitrate loading and the constraints that nitrogen availability places on the spring diatom bloom in a lake that is dominated by cyanobacteria.     

Between- and within-population differences in Phragmites australis

Phragmites australis (common reed) is a dominant clonal species in the interface between land and water in many European wetlands. Along the land-water gradient, strong consistently different selective forces might operate to give rise to genetic substructuring. I have investigated the occurrence of genetic substructuring in European P. australis populations. The present paper examines whether seedlings, from seeds collected at both ends of the land-water gradient, showed differences in response to nutrient supply. Under controlled conditions, the relative growth rate (RGR) in the exponential growth phase, and growth characters of 10-week old seedlings were assessed. Among populations, no differences in response to nutrient supply were found. Although total dry weight was not related to the geographic origin of the populations, northern/western compared to southern/eastern European populations (1) formed more but shorter shoots, (2) formed thinner but longer rhizomes, and (3) invested more dry matter in leaves at the expense of stems. It was concluded that these trait differences are likely to originate from differences in the length of the growing season in the native habitat. Within populations, ’water-side’ seedlings had a higher RGR under sub-optimal while for ’land-side’ seedlings this was under optimal nutrient conditions. Ten-week-old ’water-side’ seedlings had a higher total dry weight than ’land-side’ ones, irrespective of nutrient loading. Differences in growth could not clearly be related to differences in single biomass allocation and morphological traits. A discriminant analysis on these traits, however, revealed that ’water-side’ seedlings showed higher plasticity in discriminant scores than ’land-side’ seedlings in response to nutrient supply. Discriminant scores also pointed towards a subtle trade-off between height versus expansion growth of seedlings, from the water to landward side. In the Romanian population, this could be related to morphological differences between ploidy levels. Overall, it was concluded that within populations, selection on growth form rather than on adaptations to the nutrient status of the habitat might have taken place. Received: 20 August 1998 / Accepted: 29 July 1999     

5'-nucleotidase activity in a eutrophic lake and an oligotrophic lake

Differences in enzymatic hydrolysis of dissolved organic phosphorus and subsequent phosphorus uptake were compared by using dual-labeled (gamma-P and 2-H) ATP in oligotrophic Lake Michigan and a moderately eutrophic lake in southeastern Michigan. More than 50% of the phosphate that was hydrolyzed was immediately taken up into bacterium-sized particles in the eutrophic lake and at a near-shore site in Lake Michigan. Less than 50% of the hydrolyzed phosphate was taken up into bacterium-sized particles at an offshore site in Lake Michigan. It is hypothesized that differences in size-fractionated uptake were the result of greater phosphorus utilization capacity in bacteria in habitats where loading of organic carbon is greater. Substantial isotope dilution of labeled phosphate uptake by unlabeled phosphate occurred, which implied that the phosphate was hydrolyzed extracellularly in both systems. Comparable nucleotidase activities were measured in the eutrophic lake and Lake Michigan, but the significance of the phosphate regenerated relative to particulate phosphorus pools was an order of magnitude greater in Lake Michigan. Seventy percent of the nucleotidase activity was inhibited by 100 muM phosphate in the eutrophic lake, which suggests that most hydrolysis was by phosphatase. Therefore, nucleotidase activity may be more important to phosphorus regeneration in oligotrophic habitats than phosphatase activity.     

The effect of mineral nutrient enrichment of lake Balaton on the common reed (Phragmites australis)

On the north shore of Lake Balaton four study areas were selected with different levels of mineral nutrient supply (two of them are at sewage inflows, one is at the mouth of a natural watercourse, and one is unaffected by water inflow). Studies were made on the development ofPhragmites australis (Cav.) Trin. exSteudel stands. Monthly levels of N, P, K, Na, Ca and Mg in the organs (leaf-blade, leaf-sheath, culm, aquatic root, rhizome and root) of reed were determined from April to October in 1980. Differences were detected between the four study areas in the shoot height, mass, growth rate and element contents. In the aboveground organs, the levels of N, P, and K showed a decrease all through the growing season, but Ca and Mg rose to a peak before declining for the rest of the season. In the underground organs the levels of N and P decreased to a minimum in June to July and thereafter the quantities of these elements gradually increased. In the most heavily contaminated area (area IV), significantly higher levels of tissue N, P and K were found.     

Zooplankton grazing on bacteria and cyanobacteria in a eutrophic lake

During in situ experiments in a cyanobacteria- and copepod-dominatedeutrophic subtropical lake, all taxa of protozoa and metazoanzooplankton grazed fluorescently labelled bacteria and all metazoanzooplankton taxa grazed large filamentous and colonial cyanobacteria.     

Rhizome dynamics and resource storage in Phragmites australis

Seasonal changes in rhizome concentrations of total nonstructural carbohydrates (TNC), water soluble carbohydrates (WSC), and mineral nutrients (N, P and K) were monitored in two Phragmites australis stands in southern Sweden. Rhizome biomass, rhizome length per unit ground area, and specific weight (weight/ length ratio) of the rhizomes were monitored in one of the stands.Rhizome biomass decreased during spring, increased during summer and decreased during winter. However, changes in spring and summer were small (< 500 g DW m-2) compared to the mean rhizome biomass (approximately 3000 g DW m^–2). Winter losses were larger, approximately 1000 g DW m-2, and to a substantial extent involved structural biomass, indicating rhizome mortality. Seasonal changes in rhizome length per unit ground area revealed a rhizome mortality of about 30% during the winter period, and also indicated that an intensive period of formation of new rhizomes occurred in June.Rhizome concentrations of TNC and WSC decreased during the spring, when carbohydrates were translocated to support shoot growth. However, rhizome standing stock of TNC remained large (> 1000 g m^–2). Concentrations and standing stocks of mineral nutrients decreased during spring/ early summer and increased during summer/ fall. Only N, however, showed a pattern consistent with a spring depletion caused by translocation to shoots. This pattern indicates sufficient root uptake of P and K to support spring growth, and supports other evidence that N is generally the limiting mineral nutrient for Phragmites.The biomass data, as well as increased rhizome specific weight and TNC concentrations, clearly suggests that reloading of rhizomes with energy reserves starts in June, not towards the end of the growing season as has been suggested previously. This resource allocation strategy of Phragmites has consequences for vegetation management.Our data indicate that carbohydrate reserves are much larger than needed to support spring growth. We propose that large stores are needed to ensure establishment of spring shoots when deep water or stochastic environmental events, such as high rhizome mortality in winter or loss of spring shoots due to late season frost, increase the demand for reserves.     

Alcian Blue-stained particles in a eutrophic lake

We used a neutral solution of Alcian Blue to stain transparentparticles in eutrophic Lake Frederiksborg Slotss0, Denmark.Alcian Blue-stained particles (ABSP) appeared to be similarto the so-called transparent exopolymer particles (TEP) identifiedwith an acidic solution of Alcian Blue. Our results on the abundance,size distribution and bacterial colonization of ABSP thereforereflect general patterns of TEP. The abundance of ABSP in thesize range 3–162 µm and retained by 3mu;m pore sizefilters averaged 3.62.4910⁵ ml^–1 (SD), which is amongthe highest concentrations reported for comparable size spectraof TEP. On average, 35 % of ABSP (by number) were colonizedby bacteria and 8.610⁵ bacteria ml^–1 lake water wereattached to ABSP, which corresponds to 7% of the total bacterialabundance.     

Ramet size equalisation in a clonal plant,Phragmites australis

The influence of shading from older generations of dead culms (standing litter) on density, growth rate and development of size structure at the ramet level was investigated in a pure stand of Phragmites australis by experimental neutral shading of plots after removal of standing litter. Initial differences in height distribution between autumn and spring cohorts disappeared in the course of shoot growth. The Gini coefficients of shoot heights and estimated shoot weights indicated that the size structure of the shoots became more equal with increasing mean size in both shaded and unshaded plots. Relative growth rate for height (RHGR) and weight of individual shoots was negatively related to shoot size during the early and presumably storage-dependent growth period, suggesting a strong support for growth of smaller shoots. No etiolation was indicated by mean or maximum height in shaded and unshaded plots, or by the relationship between shoot height and weight. Mean shoot density was significantly lower in shaded than in unshaded plots in one of two shade treatment years. A regression model indicated a small but significant effect of shoot density on the approximately linear relationship between RHGR and the logarithm of height. The growth rate of small shoots was slightly larger at low than at high shoot density. Therefore, it is suggested that the shade from standing litter in P. australis stands can decrease shoot natality in the spring cohort, and thereby increase the support to fewer small shoots.     

Comparative study of periphyton colonisation on common reed (Phragmites australis) and artificial substrate in a shallow lake,Manyas, Turkey

Hydrobiologia - Colonisation of epiphytic algae on the common reed (Phragmites australis) and on glass slides were studied during a twenty-four week exposure period in a shallow, turbid lake,...     

Coupling between benthic biomass of Microcystis and phosphorus release from the sediments of a highly eutrophic lake

Variations in microbial biomass and activity in the sediments of hypereutrophic Lake Vallentunasjön were followed during a period of 5 years. The data were compared to the calculated release of phosphorus from the sediments during the same period. A strong co-variation was found between biomass of Microcystis, heterotrophic bacterial activity in the sediments and internal phosphorus loading. These parameters exhibited mainly a declining trend during the investigation period. A pronounced stability of the sediment chemistry, including the fractional composition of the sediment phosphorus, during the studied period indicates that microbial activity affected the phosphorus release from the sediments. Calculations of the percentage of sediment bacteria that was associated to the mucilage of Microcystis colonies imply, together with the specific bacterial production, that Microcystis in the sediment stimulates bacterial production. In the highly phosphorus-saturated sediments of Lake Vallentunasjön this would ultimately lead to an increased release of phosphorus from the sediment. Lake Vallentunasjön does not follow the common pattern of recovery after reduction of external phosphorus loading. The large biomasses and long survival of Microcystis in the sediment are probably important reasons for the delayed recovery of the lake.