Nitrogen processing by grazers in a headwater stream: riparian connections

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Benthic community responses to nutrient enrichment and predator exclusion: Influence of background nutrient concentrations and interactive effects

Potential community effects of nutrient enhancement are a topic of theoretical interest and increasing management concern in coastal marine systems. While increased nutrient levels may lead to increased microalgal production and biomass, studies have provided variable evidence regarding the existence of upward cascade effects on macrofauna. In benthic marine communities, limitation by predation or factors preventing recruitment response may contribute to weak coupling between resource availability and macrobenthos abundances. We conducted blocked nutrient addition and predator exclusion experiments in the intertidal of two estuaries that varied in background nutrient concentrations (Cape Fear and White Oak, southeastern North Carolina). Benthic community comparisons were also made among these and two other North Carolina estuaries to examine correlations in distribution patterns. Cape Fear, which had the highest background nitrogen and phosphorus concentrations, also had highest ambient benthic microalgal biomass. There was no significant response of microalgal biomass to local nutrient additions in Cape Fear and only one macrofaunal taxon during one season exhibited abundance responses to nutrient additions. White Oak, with lower background nutrient levels, was characterized by significant microalgal responses to nutrient additions and significant macrofauna abundance responses for 50% of the species examined during summer experiments. However, all of these macrofauna declined in abundance with nutrient enhancement while biomass remained constant or significantly increased with nutrient additions. This suggests a complex response of macrofauna to nutrient additions in this estuary with greater biomass per individual but a corresponding decline in abundances. Top-down/bottom-up interactive effects were observed for haustoriid amphipods, which were uncommon or absent when predators had access, but exhibited strong biomass responses to nutrient enhancement when predators were excluded. These results support a growing body of literature that indicates the importance of background conditions in regulating benthic community responses to nutrient enhancement. However, responses may be complex with biomass per individual rather than densities being the primary response variable for some taxa and predator moderation of responses occurring for some taxa but not others.     

Understorey benthic microalgae and their consumers depend on habitat complexity and light in a microtidal coastal ecosystem

Presence of habitat-forming macroalgae is supposed to mitigate effects of altered resources on benthic microalgae and their consumers. In a field experiment in a microtidal area of the Western Baltic Sea, we tested the interactive effects of nutrient enrichment, artificial shading, and habitat complexity on microalgal biomass and diversity as well as invertebrate abundance and richness. Habitat complexity comprised three levels, the presence of macroalgal canopy of Fucus vesiculosus, the presence of macroalgal propagules, and the absence of both (=control). Microalgal biomass (and richness) was significantly reduced by canopy presence (−88%, compared to control) and shading (−42%), with the highest biomass in the absence of both canopy and macroalgal propagules at ambient light. Within the microalgal assemblage, higher biomass was related to lower evenness (higher dominance). Density of two main invertebrate groups (snails and amphipods) strongly increased with canopy presence (on average from 53 to 154 individuals m⁻² stone area for snails, and from 234 to 1203 individuals m⁻² for amphipods) and so did invertebrate richness (from 4.3 to 10.3). Additionally, snail density doubled with increasing light availability. The snail responses to light and canopy were independent, the former relating to higher availability of microalgal prey, the latter to more structure. Microalgal taxon richness and biomass decreased with increasing invertebrate richness and with density of snails and amphipods. Our experiment thus showed that the presence of habitat-forming macroalgae alters biomass and diversity across trophic levels in benthic coastal communities as well as their response to resource manipulations.     

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m^–2 d^–1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.     

The seasonal periodicity of algae on Antarctic fellfield soils

Investigation of the seasonal changes in composition of an immature Antarctic fellfield cyanobacterial/microalgal community has demonstrated a repeated periodicity. The community consisted of only 14 species. Early spring growth of filamentous chlorophytes under snow and ice was followed by summer dominance of the community by cyanobacteria, particularly Phormidium autumnale. Limitation of the chlorophyte populations appeared to be a result of either dehydration of the soil or increased irradiance. The population maximum of the cyanobacteria occurred in mid-summer, although there were no obvious reasons for the cessation of growth at this time, and declined rapidly in late summer. Regrowth of the community occurred from very small inocula each spring, most of the biomass having been lost during late summer or during the washout associated with the spring thaw. This regrowth demonstrates the potential for the population to establish an immature fellfield community very rapidly foliowing exposure by glacial retreat or physical disturbance.     

Seasonal variation in light utilisation,biomass production and nutrient removal by wastewater microalgae in a full-scale high-rate algal pond

There has been renewed interest in the combined use of high-rate algal ponds (HRAP) for wastewater treatment and biofuel production. Successful wastewater treatment requires year-round efficient nutrient removal while high microalgal biomass yields are required to make biofuel production cost-effective. This paper investigates the year-round performance of microalgae in a 5-ha demonstration HRAP system treating primary settled wastewater in Christchurch, New Zealand. Microalgal performance was measured in terms of biomass production, nutrient removal efficiency, light absorption and photosynthetic potential on seasonal timescales. Retention time-corrected microalgal biomass (chlorophyll a) varied seasonally, being lowest in autumn and winter (287 and 364 mg m^?3day^?1, respectively) and highest in summer (703 mg m^?3day^?1), while the conversion efficiency of light to biomass was greatest in winter (0.39 mg Chl- a per μmol) and lowest in early summer (0.08 mg Chl- a per μmol). The percentage of ammonium (NH₄–N) removed was highest in spring (79 %) and summer (77 %) and lowest in autumn (47 %) and winter (53 %), while the efficiency of NH₄–N removal per unit biomass was highest in autumn and summer and lowest in winter and spring. Chlorophyll-specific light absorption per unit biomass decreased as total chlorophyll increased, partially due to the package effect, particularly in summer. The proportional increase in the maximum electron transport rate from winter to summer was significantly lower than the proportional increase in the mean light intensity of the water column. We concluded that microalgal growth and nutrient assimilation was constrained in spring and summer and carbon limitation may be the likely cause.     

底栖生物影响下的潮滩微地貌演化数值模拟

潮滩是海岸带湿地的主要类型之一,其中分布的底栖生物对生态环境具有重要的调节作用。潮滩底栖微藻、泥沙与水动力之间存在相互作用,影响潮滩微地貌形态,明晰底栖生物对潮滩微地貌的演化机制至关重要。以黄河三角洲潮滩湿地为研究区,通过构建潮滩微地貌动力模型,探究底栖生物对微地貌格局演化的作用机制,分析底栖生物对微地貌系统稳定性的影响。结果表明(1)底栖微藻生长与泥沙扩散、水流再分配过程交互作用驱动下,潮滩上可形成底栖微藻覆盖的高丘与积水洼地交替分布的规则性微地貌斑图;(2)微地貌斑图的形成提高了潮滩生态系统初级生产力和泥沙淤积高度;(3)底栖微藻与泥沙、水流的交互作用使得潮滩微地貌系统对侵蚀扰动呈现非线性响应行为,系统存在临界点,且在一定侵蚀率范围内存在双稳态;(4)黄河口泥螺入侵使得微地貌系统抵抗侵蚀扰动能力减小,且系统稳定性随泥螺生物量的增加而降低。     

Interactions among irradiance,nutrients, and herbivores constrain a stream algal community

Using stream-side, flow-through channels, I tested for the effects of nutrients (NU) (nitrogen plus phosphorus), irradiance (L), and snail grazing (G) on a benthic algal community in a small, forested stream. Grazed communities were-dominated by a chlorophyte (basal cells ofStigeoclonium) and a cyanophyte (Chamaesiphon investiens), whereas ungrazed communities were comprised almost entirely of diatoms, regardless of nutrient and light levels. Snails maintained low algal biomass in all grazed treatments, presumably by consuming increased algal production in treatments to which L and NU were increased. When nutrients were increased, cellular nutrient content increased under ambient conditions (shaded, grazed) and biomass and productivity increased when snails were removed and light was increased. Together, nutrients and light had positive effects and grazing had negative effects on biomass (chlorophylla, AFDM, algal biovolume) and chlorophyll-and areal-specific productivity in ANOVAs. However, in most cases, only means from treatments in which all three factors were manipulated (ungrazed, +NU&L treatments) were significantly different from controls; effects of single factors were generally undetectable. These results indicate that all three factors simultaneously limited algal biomass and productivity in this stream during the summer months. Additionally, the effects of these factors in combination were in some cases different from the effects of single factors. For example, light had slight negative effects on some biomass parameters when added at ambient snail densities and nutrient concentrations, but had strong positive effects in conjunction with nutrient addition and snail removal. This study demonstrates that algal biomass and productivity can be under multiple constraints by irradiance, nutrients, and herbivores and indicates the need to employ multifactor experiments to test for such interactive effects.     

Diurnal and seasonal variations of nitrogen fixation and photosynthesis in cyanobacterial mats

In situ measurements of nitrogenase activity and photosynthesis were performed simultaneously in cyanobacterial mats of intertidal sand flats of the Southern North Sea. Two types of cyanobacterial mats, which differed in species composition and biomass content, were investigated. The measurements were done monthly during 3 years to detect seasonal variations of nitrogen fixation and photosynthesis. Diurnal variations were investigated as well. The results showed that (i) freshly colonized sediment with the cyanobacteriumOscillatoria limosa as the dominant organism revealed the highest specific nitrogenase activities (ii) nitrogenase activities were highest in spring and summer, when mat development was initiated and (iii) diurnal fluctuations of nitrogenase activity indicated that it occurred temporally separated from oxygenic photosynthesis.     

Resource limitation,competition and the influence of life history in a freshwater snail community

Summary Previous work on a snail community occurring throughout lakes in southwestern Michigan showed that predation by molluscivorous sunfish had large impacts on only the rarest snail species. Thus, competition might play a major role in population limitation because dominant members of the snail community are relatively immune to predation. The present experiments were conducted to determine the extent to which the snail community depleted the abundance of food resources (epiphytes) and the extent to which epiphyte abundances limited snail production. An experimental gradient in snail densities showed that removal of snails increased epiphyte biomass by approximately 3-fold relative to that observed at natural snail densities. Enrichment of the environment with phosphorus fertilizer increased epiphyte biomass by approximately 20-fold and provided a test of food limitation in the snail community. All snail taxa exhibited positive numerical or growth responses to enrichment. The observations that snails depleted resources and that resources limited snail production demonstrated that snails competed exploitatively for epiphytes. The response of each snail species to increased food abundance differed depending on the timing of fertilization relative to the snails' life histories. Snails hatched before the experiment began were larger in fertilized treatments, due to increased growth, but their densities were similar among treatments. On the other hand, densities of snails born during the experiment were up to 15-fold greater in fertilized treatments, due in part to increased survival of newborn snails. Comparison of the responses of snails to food addition and to predator removals (based on prior experiments) suggested that food availability limits snail production more than predators do. Additionally, the large responses by algae and snails to fertilization demonstrated that both the producers and herbivores in this simplified food chain were strongly resource limited.     

Recruitment,growth, and vertical distribution of the endangered mud snail Cerithidea rhizophorarum A. Adams, 1855: implications for its conservation

Some fundamental natural-history traits of the endangered mud snail Cerithidea rhizophorarum were investigated for three years in an open, bare, sandy tidal flat located on the east of Shikoku Island in southwestern Japan. Our study demonstrated a highly aggregated distribution of younger snails and complicated differences in vertical migration patterns between younger and older snails that might have prevented comprehensive understanding of the precise demography of C. rhizophorarum in previous studies. From late summer, juveniles of the first-year cohorts appeared patchily at 12–16 cm lower tidal heights than snails of the oldest cohorts and reached 5.0–7.5 mm in mean shell length by the first mid-autumn. In the second year, juveniles gradually moved upward and appeared at higher tidal heights. The second-year cohorts reached 15.6–19.1 mm in mean shell length by the end of the second autumn and converged into the oldest cohorts during the third year or later. Our results suggest that C. rhizophorarum matures sexually and starts reproduction from the fourth summer at the earliest. On the other hand, older snails showed a similar seasonal migration pattern every year—downward movement in summer and upward movement in autumn—probably associated with their reproductive activity and/or a seasonal change in the maximum tidal height. For conservation of C. rhizophorarum, it is very important to preserve both the higher area as a main habitat for older snails and the lower tidal flat as the nursery ground.     

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade

Summary We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m² enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m³), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.     

Indirect effects of a parasite on a benthic community: an experiment with trematodes, snails and periphyton

1. Few studies have directly addressed the role played by parasites in the structure and function of ecosystems. Parasites influence the behaviour, reproduction and overall fitness of their hosts, but have been usually overlooked in community and ecosystem‐level studies. We investigated the effects of trematode parasites on snail–periphyton interactions. 2. Physa  acuta (Gastropoda: Pulmonata) snails infected with the trematode Posthodiplostomum minimum (often >30% of within‐shell biomass) grazed more rapidly than uninfected snails. Trematode effects on snail grazing indirectly affected the standing stock and community structure of periphyton. Populations of snails with 50% infected individuals reduced algal biomass by 20% more than populations with lesser (10% or 0%) infection rates. 3. The alga Cladophora glomerata dominated periphyton communities grazed by snail populations with 50% infection rates, whereas diatoms and blue–green algal taxa dominated when grazed by snail populations with lower infection rates. 4. Thus, trematodes indirectly affected periphyton communities by altering host snail behaviour, a trait‐mediated indirect effect. These results indicate that trematodes can indirectly influence benthic community structure beyond simple population fitness, with possible related effects on ecosystem function.     

Characterization of Microbial Mats from a Siliciclastic Tidal Flat (Bahía Blanca Estuary,Argentina)

Biofilms and microbial mats cover the tidal flats of the central zone of the Bahía Blanca estuary (Argentina), creating extensive layers. The objective of this study was to characterize the microphytobenthic communities in these biofilms and mats from sediment cores taken in March, June, September and December 2010. Microorganisms were identified and enumerated by microscopy, and their biomass (chlorophyll a, biovolume) quantified at two different stations in the lower supratidal zone, located ～210 m apart from each other (namely S1 and S2). Additionally, the colloidal carbohydrates produced by these microbial communities were quantified, together with physical parameters such as temperature, granulometry, moisture and organic matter content of the sediment layers that comprise a typical epibenthic mat. On the other hand, changes in biomass and colloidal carbohydrate content were studied through a half-tidal cycle (7 h). There were significant seasonal differences in microphytobenthic biovolume (P < 0.001) with a considerably lower biomass in summer, but no significant differences in microalgal biovolume between stations (P = 0.454). Cyanobacterial biomass (largely composed of the filamentous Microcoleus chthonoplastes) was dominant on all dates at both stations, followed by pennate diatoms. Chlorophyll a and colloidal carbohydrate contents in sediment presented a similar pattern to that of microalgal biovolume; with a 5-fold variation in chlorophyll a for S1 between consecutive sampling events on September and December. There were significant differences between sampling dates in colloidal carbohydrates (P < 0.001) with the lowest values recorded during fall and winter; conversely there were no significant differences between stations (P = 0.324). Silt was the dominant sediment fraction at S1 while sand dominated throughout the uppermost 20 mm at S2. Chlorophyll a contents did not show significant differences throughout a half-tidal cycle, likely the product of vertical migration along the section sampled. Conversely, the content of colloidal carbohydrates varied 5-fold, showing a significant (P < 0.001) and steady increase with time of exposure to air and pointing to the rapid metabolic rates of the community. In conclusion, the microphytobenthic community of the Bahía Blanca estuary presented marked seasonality in its biological parameters and overall physiognomy, also showing elevated metabolic rates when subject to tidal fluctuations.     

Effects of invasive species on plant communities: an example using submersed aquatic plants at the regional scale

Submersed aquatic plants have a key role in maintaining functioning aquatic ecosystems through their effects on the hydrological regime, sedimentation, nutrient cycling and habitat of associated fauna. Modifications of aquatic plant communities, for example through the introduction of invasive species, can alter these functions. In the Sacramento-San Joaquin River Delta, California, a major invasive submersed plant, Brazilian waterweed Egeria densa, has become widespread and greatly affected the functionality of the submersed aquatic plant community. Rapid assessments of the distribution and abundance of this species are therefore crucial to direct management actions early in the season. Given the E. densa bimodal growth pattern (late spring and fall growth peaks), summer assessments of this species may indicate which and where other submersed species may occur and fall assessments may indicate where this and other species may occur in the following spring, primarily because the Delta’s winter water temperatures are usually insufficient to kill submersed aquatic plant species. We assessed community composition and distribution in the fall of 2007 and summer of 2008 using geostatistical analysis; and measured summer biomass, temperature, pH, salinity, and turbidity. In the fall of 2007, submersed aquatic plants covered a much higher proportion of the waterways (60.7%) than in the summer of 2008 (37.4%), with a significant overlap between the seasonal distribution of native and non-native species. Most patches were monospecific, and multispecies patches had significantly higher dominance by E. densa, co-occurring especially with Ceratophyllum demersum. As species richness of non-natives increased there was a significant decrease in richness of natives, and of native biomass. Sustained E. densa summer biomass negatively affected the likelihood of presence of Myriophyllum spicatum, Potamogeton crispus, and Elodea canadensis but not their biomass within patches. Depth, temperature and salinity were associated with biomass; however, the direction of the effect was species specific. Our results suggest that despite native and invasive non-native submersed plant species sharing available niches in the Delta, E. densa affects aquatic plant community structure and composition by facilitating persistence of some species and reducing the likelihood of establishment of other species. Successful management of this species may therefore facilitate shifts in existing non-native or native plant species.     

Improvement of maize (Zea mays L.) anther culture responses by algae-derived natural substances

This is the first report on the beneficial effect of microalgal and cyanobacterial biomass on anther cultures of maize (Zea mays L.). Investigations were made on the cytokinin- and auxin-like activity and content of terrestrial and fresh-water living microalgal and cyanobacterial strains. The influence of media supplemented with biomass from four selected strains on the anther induction, the frequency of microspore-derived embryo-like structures, and regeneration capacity in anther cultures of maize was also studied. The addition of cyanobacterial and microalgal biomass to the induction and regeneration media in concentrations of 1 or 2 g/L improved the androgenic response, and was able to reduce the quantity of the synthetic auxin 2,4-dichlorophenoxy-acetic acid (2,4-D) required, or replace it completely.     

Composition,distribution and biomass of meiobenthos in the Oosterschelde estuary (SW Netherlands)

Meiofauna composition, abundance, biomass, distribution and diversity were investigated for 31 stations in summer. The sampling covered the whole Oosterschelde and comparisons between the subtidal — intertidal and between the western-central — eastern compartment were made.Meiofauna had a community density ranging between 200 and 17 500 ind 10 cm^–2, corresponding to a dry weight of 0.2 and 8.4 gm^–2. Abundance ranged between 130 and 17 200 ind 10 cm^–2 for nematodes and between 10 and 1600 ind 10 cm^–2 for copepods. Dry weight biomass of these taxa was between 0.5–7.0 gm^–2 and 0.008–0.3 gm^–2 for nematodes and copepods respectively.The meiofauna was strongly dominated by the nematodes (36–99%), who's abundance, biomass and diversity were significantly higher intertidally than subtidally and significantly higher in the eastern part than in the western part. High numbers were positively correlated with the percentage silt and negatively with the median grain size of the sand fraction. The abundance and diversity of the copepods were highest in the subtidal, but their biomass showed an inverse trend being highest on the tidal flats.The taxa diversity of the meiofauna community and species diversity of both the nematodes and the copepods were higher in subtidal stations than on tidal flats. In the subtidal, the meiofauna and copepod diversity decreased from west to east, whereas nematode diversity increased.The vertical profile clearly reflected the sediment characteristics and could be explained by local hydrodynamic conditions.Seasonal variation was pronounced for the different taxa with peak abundance in spring, summer or autumn and minimum abundance in winter.Changes in tidal amplitude and current velocity enhanced by the storm-surge barrier will alter the meiofauna community structure. As a result meiofauna will become more important in terms of density and biomass, mainly due to increasing numbers of nematodes, increasing bioturbation, nutrient mineralisation and sustaining bacterial growth. A general decrease in meiofauna diversity is predicted. The number of copepods is expected to decrease and interstitial species will be replaced by epibenthic species, the latter being more important in terms of biomass and as food for the epibenthic macrofauna and fishes.     

The response of two submerged macrophytes and periphyton to elevated temperatures in the presence and absence of snails: a microcosm approach

Global warming may affect snail–periphyton–macrophyte relationships in lakes with implications also for water clarity. We conducted a 40-day aquaria experiment to elucidate the response of submerged macrophytes and periphyton on real and artificial plants to elevated temperatures (3°C) under eutrophic conditions, with and without snails present. With snails, the biomass and length of Vallisneria spinulosa leaves increased more at the high temperature, and at both temperatures growth was higher than in absence of snails. The biomass of periphyton on V. spinulosa as well as on artificial plants was higher at the highest temperature in the absence but not in the presence of snails. The biomass of Potamogeton crispus (in a decaying state) declined in all treatments and was not affected by temperature or snails. While total snail biomass did not differ between temperatures, lower abundance of adults (size >1 cm) was observed at the high temperatures. We conclude that the effect of elevated temperature on the snail–periphyton–macrophyte relationship in summer differs among macrophyte species in active growth or senescent species in subtropical lakes and that snails, when abundant, improve the chances of maintaining actively growing macrophytes under eutrophic conditions, and more so in a warmer future with potentially denser growth of periphyton.     

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

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

Seasonal variability in the palatability of freshwater macrophytes: a case study

The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.