NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

Effect of continuous nutrient enrichment on microalgae colonizing hard substrates

In order to understand the effect of changing nutrient conditions on benthic microalgae on hard substrates, in-situ experiments with artificial substrates were conducted in Kiel Fjord, Western Baltic Sea. As an extension of previous investigations, we used artificial substrates without silicate and thus were able to supply nutrient media with different Si:N ratios to porous substrates, from where they trickled out continuously. The biofilm developing on these substrates showed a significant increase in biovolume due to N + P enrichment, while Si alone had only minor effects. The stoichiometric composition of the biomass indicated nitrogen limitation during most of the year. The C:N ratios were lowered by the N + P addition. The algae were dominated by diatoms in most cases, but rhodophytes and chlorophytes also became important. The nutrient treatment affected the taxonomic composition mostly at the species level. The significance of the results with regard to coastal eutrophication is discussed.     

The importance of nutrient co‐limitation in regulating algal community composition,productivity and algal‐derived DOC in an oligotrophic marsh in interior Alaska

1. Compared to lakes and streams, we know relatively little about the factors that regulate algae in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change (i.e. increased permafrost collapse and soil weathering) are expected to increase nutrient inputs into aquatic systems. To investigate how accelerated nutrient inputs might affect algal structure and function in northern boreal wetlands, we added nitrogen, phosphorus and silica to mesocosms in an oligotrophic marsh in interior Alaska. 2. We conducted two in situ mesocosm enrichment experiments during consecutive summer growing seasons, each lasting 24 days. In 2007, we investigated the effects of +N, +P, +Si and +N+P+Si enrichment on benthic algal biomass (chlorophyll‐a, ash‐free dry mass, biovolume), chemistry (N : P ratio) and community composition. In 2008, we expanded our first experiment to investigate the effects +N+P, +N+Si, +P+Si and +N+P+Si on the same algal parameters as well as productivity (mg C m^?2 h^?1). 3. In both experiments, we measured water‐column dissolved organic carbon (DOC) inside treatment enclosures and related changes in DOC to standing algal biomass. 4. Benthic algal accrual did not increase following 24 days of enrichment with any nutrient alone or with P and Si together (+P+Si), but increased significantly with the addition of N in any combination with P and Si (+N+P, +N+Si, +N+P+Si). 5. Algal productivity (20 mg C m^?2 h^?1) increased between three‐ and seven‐fold (57–127 mg C m^?2 h^?1) with the addition of N in combination with any other nutrient (+N+P, +N+Si, +N+P+Si). Water‐column DOC concentration was significantly higher inside N‐combination treatments compared to the control during each season, and DOC increased linearly with benthic algal biomass in 2007 (r² = 0.89, P < 0.0001) and 2008 (r² = 0.74, P < 0.0001). 6. Taxonomic composition of the wetland algal community responded most strongly to N‐combination treatments in both seasons. In 2007, there was a significant shift from Euglena and Mougeotia in the control treatment to Chroococcus and Gloeocystis with +N+P+Si enrichment, and in 2008, a Mougeotia‐dominated community was replaced by Gloeocystis in the +N+P treatment and by Nitzschia in +N+Si and +N+P+Si treatments. 7. Together, these data provide several lines of evidence for co‐limitation, and the central importance of N as a co‐limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs and suggest that algae may provide a functional link between increasing nutrient inputs and altered wetland carbon cycling in this region.     

SEASONAL CYCLING OF ALGAL NUTRIENT LIMITATION IN CHAUTAUQUA LAKE,NEW YORK1

Algal nutrient enrichment bioassays were conducted between May 1975 and August 1978 using water samples collected from Chautauqua Lake, New York. Photosynthetic fixation rates of natural phytoplankton assemblages were enhanced by additions of phosphorus and nitrogen, although enrichment with other nutrients had no significant stimulatory effect on algal photosynthesis. Whereas phosphorus stimulated in spring and early summer, both nitrogen and phosphorus enhanced photosynthesis in midsummer and fall. Relative to the effect of phosphorus enrichment, enhancement of photosynthesis by nitrogen during the summer and fall was highest in the northern part of the lake. During the period of ice cover, photosynthesis did not appear to be limited by nutrients in that nutrient additions (P, N, Si, C, Fe, trace metals) did not enhance fixation rates. Observed temporal fluctuations in the response of the algae to P and N correlated with changes in the lake water N:P ratio as well as with temporal changes in dissolved orthophosphate and nitrate-nitrite nitrogen. The N:P ratio decreased drastically in the summer and remained at ca. 10 or less through mid-fall, suggesting that N concentrations were inadequate for the non-N-fixing phytoplankton. Studies over 3 yr indicate that states of P and N limitation undergo time-space fluctuations that occur in a cyclic pattern in the surface waters of Chautauqua Lake.     

Responses of benthic algae to nutrient enrichment in a shallow lake: Linking community production,biomass, and composition

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The influence of a dissolved inorganic nitrogen gradient on phytoplankton community dynamics in a chain of lakes

In a chain of lakes along which nutrient availability varies in a gradient, we performed factorial nutrient enrichment experiments to determine if nitrogen limitation was the principal factor controlling the differences in phytoplankton biomass, photosynthetic productivity, diversity, and species composition among two of the lakes in the chain. In the least productive lake, East Graham Lake, P and C enrichments (in the absence of N enrichment) had no effect on biomass and diversity, whereas within two weeks the N enrichments (alone or in any combination with P and/or C) increased the biomass and decreased the diversity of East Graham Lake phytoplankton to levels similar or identical to those in more productive Shoe Lake. Short-term ¹⁴C photosynthetic rates in East Graham Lake water also responded only to N in the third week. However, photosynthesis was stimulated by P in the first week, and a few species did increase in numbers with P enrichment, suggesting that some degree of P limitation remains in addition to the strong N limitation in East Graham Lake. A number of species responded individually to the enrichments in a manner similar to that of the overall community, and a strong overlapping of discriminant analysis scores for N-enriched East Graham Lake with those of Shoe Lake was consistent with our prediction that the community structure of N-enriched East Graham Lake water would shift toward that of Shoe Lake. However, many species did not respond consistently with these results, and the nutrients tested were clearly not a major factor in the differences in abundance of those species among the two lakes. The results support the argument that overall biomass production and diversity of the phytoplankton community in a lake can be a relatively simple function of a single most-limiting nutrient. However, many of the species responses also confirm that, while nutrient availability is an important factor in the control of the species composition of the community, other factors are likely to prevent reliable predictions of all species effects on the basis of nutrient availability alone.     

Nutrient-limited productivity of calcareous versus fleshy macroalgae in a eutrophic,carbonate-rich tropical marine environment

The results of a study of nutrient enrichment with nitrogen (N) and phosphorus (P) on productivity and calcification of fleshy and calcareous algae are reported in this study. Plants were collected from a nearshore eutrophic site in the Florida Keys (USA) and experimentally pulsed during the night with combinations of N and P. After several days of pulsing (7–10 days), net productivity, calcification, and alkaline phosphatase activity (APA), were measured. Productivity of fleshy algae were frequently enhanced by N, P, and N+P, during both summer and winter. Phosphorus limited the productivity of Hydroclathrus clathratus during winter and Ulva spp. during summer, whereas nitrogen limited the productivity of Laurencia intricata during both seasons. During summer, Dictyota cervicornis productivity was not enhanced by any nutrient enrichment. Nitrogen limited the productivity of the three calcareous species Penicillus capitatus, Penicillus dumetosus and Halimeda opuntia during winter and that of H. opuntia during summer. Neither N nor P enrichment increased calcification of calcareous species, and P enrichment greatly inhibited calcification of P. dumetosus during winter. Nutrient enrichment enhanced the productivity of the fleshy species to a greater extent than that of calcareous algae. The seawater DIN:SRP molar ratio was low at our eutrophic study site (molar ratio average of 3:1 during winter and 9:1 during summer) compared to more oligotrophic sites in the Florida Keys, suggesting that in carbonate-rich environments, eutrophication shifts nutrient regulation of productivity from P to N. APA activities of fleshy macroalage were higher than calcareous algae, and rates of all macro algae were 2- to 7-fold higher in summer compared to winter. Productivity was also about 3-fold higher in fleshy compared to calcareous species and about 2-fold higher in summer compared to winter. These results suggest that nutrient enrichment enhances productivity of fleshy algae to a greater extent than that of calcareous algae. Thus, overgrowth of calcareous algae by more opportunistic fleshy forms could reduce carbonate accretion in tropical coastlines experiencing increased eutrophication.     

Non-siliceous algae in a five meter core from Lake Kinneret (Israel)

The composition and succession of non-siliceous algae, studied in a five meter core from Lake Kinneret (Israel), are described. Only Chlorophyta species were recorded, probably due to the standard palynological sample processing which was used. In the lower part of the core, from the bottom to 300 cm (interval 5500–2500 years B.P.), Botryococcus braunii was the only common alga. Relevant changes in algal diversity and abundance occur at 300 cm. Many species of green algae were recorded for the first time (Pediastrum, Scenedesmus, Coelastrum, etc.). These changes may be related to an increase in nutrient concentration as a consequence of cultural disturbance. In the interval 300-0 cm, a succession of Pediastrum species is followed. The recovered green algae are extant in the present plankton of Lake Kinneret. They also constitute an important part of the algae found in the profundal sediments today.     

Evaluation of eutrophication control in Lake Chivero,Zimbabwe, by multivariate analysis of zooplankton

Zooplankton was a useful tool in evaluation of eutrophication control in Lake Chivero, a tropical man made impoundment. Principal component analysis revealed both spatial and temporal changes in zooplankton community structure which were related to nutrient inflows as well as changes in nutrient content of the lake. Studies on filtration rates of different algae by Bosmina longirostris and Daphnia lumholtzi demonstrated that the response of zooplankton to eutrophication was related to their inability to utilize colonial algal species that develop in a nutrient rich-environment.     

Lipid production in natural phytoplankton communities in a small freshwater Baltic lake, Lake Schöhsee, Germany

1. Seasonal patterns in lipid production and the photosynthetic parameters describing lipid production in Lake Schöhsee (a small freshwater, mono-dimictic and moderately eutrophic North German Baltic lake) were determined.
2. The mean lipid fraction of C fixation (LIP-FCF) was 13.2% (range 7.6–21%), measured bimonthly from January to December 1995. Periods of high nutrient concentration, low temperatures and particular species of algae (diatoms and/or cryptophytes), were associated with the highest LIP-FCF values.
3. None of the static indicators of nutrient status [dissolved nitrate, soluble reactive silica (SRS); C/N or N/P] are apparently related to the allocation of carbon to lipid in Lake Schöhsee.
4. The light saturation parameters ( I _k : I _k-LIP) indicated that carbon fixation into lipid was saturated at much lower light than that of total carbon fixation. This suggested that carbon fixation into lipid was more light efficient than total carbon fixation.
5. The relative allocation of carbohydrate and protein was related to daylength and temperature, respectively.     

Periphyton in the Volta Lake. I. Seasonal changes on the trunks of flooded trees

Seasonal changes in the periphyton on the trunks of flooded trees in the Pawmpawm arm of the Volta Lake are reported. Of the three algal groups forming the periphyton, the quantitatively most important species are in such genera as Navicula, Synedra and Melosira (diatoms), Spirogyra, Oedogonium and Stigeoclonium (green algae), and Oscillatoria and Anabaena (blue-green algae). The fluctuations in species diversity and cell numbers are attributed to changes in the nutrient status of the lake water and lake level.     

Plant community response to nitrogen and phosphorus enrichment varies across an alpine tundra moisture gradient

Background: The extent to which nutrient availability influences plant community composition and dynamics has been a focus of ecological enquiry for decades.

Aims: Results from a long-term nitrogen (N) and phosphorus (P) addition experiment in alpine tundra were used to evaluate the importance of the two nutrients in structuring plant communities in three communities that differed in their snow cover amounts and duration and soil moisture characteristics.

Methods: A factorial N and P experiment was established in three meadows differing in initial vegetation composition and soil moisture. Plant and soil characteristics were measured after 20 years, and the dissimilarity among meadows and treatments were measured using permutational analysis of variance.

Results: Plant species richness declined uniformly across the three meadow types and in response to N and N + P additions, while both evenness and the Shannon diversity index finding indicated that nutrient additions had the highest impact on moister habitats. Overall, N impacts overshadowed changes attributed to P additions, and the N and N + P plots in wet meadow sites were the least diverse and scored the lowest dissimilarity averages among treatments. Dissimilarity estimates indicated that the control and P plots in the dry meadow community were more distinct in composition than all other plots, and especially those in the moist or wet meadows. Above-ground biomass of grasses and sedges (graminoids) increased with N additions while forbs appeared to show responses dictated in part by the graminoid responses. The most abundant grass species of moist and wet meadow, Deschampsia cespitosa, dominated N and N + P plots of the wet sites, but did not show a N response in moist areas in spite of its general abundance in moist meadow. Competition from other plant species in the moist areas likely diminished the D. cespitosa response and contributed to the resilience of the community to nutrient enrichment.

Conclusions: Initial community composition, as influenced by the specific moisture regime, appears to control the extent to which changes in nutrient resources can alter plant community structure. Long-term fertilization tends to support most but not all findings obtained from shorter-termed efforts, and wet meadows exhibit the largest changes in plant species numbers and composition when chronically enriched with N.     

Tracing Si–N–P ecosystem-pathways: is relative uptake in riparian vegetation influenced by soil waterlogging,mowing management and species diversity?

Despite the growing concern about the importance of silicon (Si) in controlling ecological processes in aquatic ecosystems, little is known about its processing in riparian vegetation, especially compared to nitrogen (N) and phosphorus (P). We present experimental evidence that relative plant uptake of N and P compared to Si in riparian vegetation is dependent on mowing practices, water-logging and species composition. Results are obtained from a controlled and replicated mesocosm experiment, with a full-factorial design of soil water logging and mowing management. In our experiments, the Si excluding species Plantago lanceolata was dominant in the mown and non-waterlogged treatments, while Si accumulating meadow grasses and Phalaris arundinacea dominated the waterlogged treatments. Although species composition, management and soil moisture interacted strongly in their effect on relative Si:N and Si:P uptake ratios, the uptake of N to P remained virtually unchanged over the different treatments. Our study sheds new light on the impact of riparian wetland ecosystems on nutrient transport to rivers. It indicates that it is essential to include Si in future studies of the impact of riparian vegetation on nutrient transport, as these are often implemented as a measure to moderate excessive N and P inputs.     

SALINITY AND NUTRIENT LIMITATIONS ON GROWTH OF BENTHIC ALGAE FROM TWO ALKALINE SALT LAKES OF THE WESTERN GREAT BASIN (USA)1

Enrichment cultures of littoral benthic algae from Mono Lake, California, and Abert Lake, Oregon, were grown under conditions of varied salinity and nutrient content. Field-collected inocula were composed mainly of diatoms and filamentous blue-green and green algae. The yield of long-term cultures (30 days) showed tolerance over a broad salinity range (50–150 g·L^?1) for Mono Lake-derived algae. Algae from Abert Lake had a lower range of tolerance (25–100 g·L^?1) Organic content and pigment concentrations of algae from both lakes were also reduced above the tolerated salinity level. Within the range of salinity tolerance for Mono Lake algae, initial growth rates and organic content were reduced by increased salinity. The effects of macro- and micronutrient enrichment on algal growth in Mono Lake water were also tested. Only nitrogen enrichment (either as ammonium or nitrate) stimulated algal growth. Although the benthic algae cultured here had wide optima for salinity tolerance, the rates of growth and storage were limited by increased salinity within the optimum range. Although the lakes compared had similar species composition, the range and limits of tolerance of the algae were related to salinity of the lake of origin.     

Sub-ice algal assemblages of the Barents Sea: Species composition,chemical composition,and growth rates

Summary The ice algae of the Barents Sea were studied from 1986 to 1988. With a few exceptions, the ice algal assemblages were dominated by pennate diatoms. From March to early June there was a transition from a mixed population of both centric and pennate diatoms at the start into a well developed Nitzschia frigida assemblage. Nutrier ts in ice-covered regions were high in spring, and high N/C and protein/carbohydrate ratios indicated no nutrient deficiency in the ice algae. The N/P ratios were lower than 15, but comparable to ratios of three ice algae species grown in culture at -0.5 °C and various light conditions. The Si/N ratios were lower than corresponding ratios from the Canadian Arctic and the Antarctic. The chemical composition revealed that silicate limited growth cannot be excluded. The cells were heavily shade-adapted the entire spring season, with high Chl/C ratios (0.045–0.084), comparable to the cultures growing at low irradiances. The growth rates in the cultures peaked at 50 mol m^-2s^-1 with maximum rates of 0.6–0.8 div. day^-1, both for 12 and 24 h day lengths. The low growth rates for the May assemblages (max 0.20 div. day^-1) indicated strong light limitation by self-shading. Adaptation experiments showed that some ice algae are highly adaptable, while others are not able to adjust to new irradiances. Their growth rates are inhibited by high irradiances and this may affect the distribution in the field.     

Limnological characteristics of subtropical Lakes Phewa, Begnas, and Rupa in Pokhara Valley, Nepal

Limnological characteristics were studied and analyzed in the subtropical Lakes Phewa, Begnas, and Rupa of Pokhara Valley, Nepal, from 1993 to 1997. The annual water temperature ranged from 12° to 29°C in all lakes. Lake Phewa and Lake Begnas were monomictic and anoxic in the hypolimnion during thermal stratification from April to September. Dissolved oxygen was drastically depleted in April and/or May in shallow Lake Rupa when the macrophyte community began to decompose. NH₄ ⁺-N accumulated below 5 m during March–September when dissolved oxygen was depleted in Lakes Phewa and Begnas. The PC : PP ratio was higher, but the PC : PN and PN : PP ratios were close to the Redfield ratio (106C : 16N : 1P) in Lakes Phewa and Begnas, denoting that P was limited. Annual net primary production showed that the lakes were productive but will tend to become heterotrophic in the future. The seasonal variation of chlorophyll a concentration was high, but its annual variation was low. Ceratium hirundinella and Peridinium spp. in Lake Phewa, Microcystis aeruginosa and Aulacoseira granulata in Lake Begnas, and Tabellaria fenestra in Lake Rupa were the dominant species. The zooplankton population and species varied irregularly. On the basis of chlorophyll a concentration in the euphotic zone and phytoplankton species composition, the lakes seem to be oligoeutrophic and to have some characteristics of temperate lakes rather than tropical lakes. Received: April 26, 1999 / Accepted: September 20, 1999     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

杭州西湖浮游藻类变化规律与水质的关系

于2006～2007年对杭州西湖浮游藻类的种类及个体丰度进行初步研究,并依据<水和废水监测分析方法>对其水质现状进行了生物学评价.综合生物和理化指标数据,对杭州西湖的水质作一总体评估,并对西湖水中的物种多样性保护及水质的可持续利用提出了建议.共鉴定浮游藻类179种,其中蓝藻25种,隐藻1种,金藻3种,甲藻3种,黄藻5种,裸藻23种,硅藻41种和绿藻78种.分析了杭州西湖浮游藻类的组成和分布的特点,揭示出西湖浮游藻类的种类组成与西湖水质变化之间的关系.结果表明:藻类的组成和分布与水质变化规律基本吻合,西湖水质较整治前有明显的改善,一些清水藻类的物种逐步增多.     

Studies on decomposition ofCeratophyllum demersum litter under laboratory and field conditions: losses of dry mass and nutrients,qualitative changes in organic compounds and consequences for ambient water and sediments

A study was made of decomposition ofCeratophyllum demersum litter over a 17-day period under controlled conditions of temperature and oxygen (5, 10 and 18 °C; aerobic and anaerobic) and over a 169-day period in the field (Lake Vechten, The Netherlands). Litter, water and sediment were sampled on the 0, 2, 4, 7 and 17th day under controlled conditions and on the 0, 17, 49, 127 and 169th day in the field. The litter was analyzed quantitatively for dry mass, ash, carbon, nitrogen, phosphorus and qualitatively of organic composition by pyrolysis mass spectrometry. The water was analyzed for the elemental concentrations of organic carbon (total and dissolved), nitrogen (total, ammonia and particulate) and phosphorus (total and orthophosphate) and for the concentrations of photosynthetic pigments and bacteria. The sediment was analyzed for the elemental concentrations of nitrogen, carbon and phosphorus, and for bacterial numbers.The pattern of litter mass loss fitted an exponential model fairly well. Mass decreased faster under controlled aerobic than under anaerobic conditions and the decrease was stimulated by increasing temperature, relatively more in the range of 5 to 10 °C (by 20%) than in the range of 10 of 18 °C (by 2%). The residual mass ranged from 73 to 43% of initial under controlled aerobic conditions and from 84 to 65% under anaerobic conditions after 17 days. It decreased far less in the field, to 38% of initial mass in the field after 169 days.The litter initially lost a carbohydrate fraction by leaching in all treatments. The protein content decreased initially as well but increased subsequently at increasing temperature stimulated under anaerobic conditions. The changes in organic composition were correlated with those in nitrogen but not with those in carbon and phosphorus contents. The organic composition of litter incubated in the field differed from that of litter incubated in the laboratory. The field residues contained less proteinaceous material than the laboratory residues.The changes in carbon, nitrogen and phosphorus concentrations in the litter showed different patterns. The carbon concentration generally increased, the nitrogen concentration initially dropped and increased subsequently, and the phosphorus concentration initially dropped and remained relatively constant subsequently. Chemical immobilization of the decomposition process may have occurred in the laboratory, but was unlikely in the field.Carbon, nitrogen and phosphorus left the litter initially largely in particulate form and were recovered in the water. The ratio dissolved: total nutrient concentration was lower under controlled aerobic than under anaerobic conditions. Increasing temperature stimulated bacterial use of dissolved organic carbon and nitrogen. A rapid nutrient flow occurred from macrophyte litter, via water to sediment.The phytoplankton biomass in the water was greatly stimulated by substances freed from the decomposing litter. Diatoms increased generally relatively more than green algae, predominating alternatively with green algae under aerobic conditions and continuously under anaerobic conditions. Bacterial numbers in the water initially increased, partly due to transgression of bacteria from the sediment-water interface to the water and partly due to an actual increase in community biomass. The bacteria returned largely to the sediment-water interface, stimulated by increasing temperature, as most of the substrate readily usable by them had left the litter in the litter-bag and was associated with the upper sediment layers.It is feasible that the annual die-off of theC. demersum population of Lake Vechten barely affects nutrient cycling in the lake, because the contribution to the nutrient pools of the lake when fully mixed is only small. However, small particles originating from decomposingC. demersum litter may influence the lake considerably by decreasing water transparency and serving as a food source for filter-feeders and detritivorous macrofauna.     

Dredging effects on P status and phytoplankton density and composition during winter and spring in Lake Taihu, China

Phytoplankton density and composition, together with phosphorus (P) concentrations and size-fractionated alkaline phosphatase activity (APA), were investigated in dredged and undredged zones in Lake Taihu from January to April 2004. P concentrations were also determined in the corresponding interstitial water. Enzyme Labeled Fluorescence (ELF) was used for localizing extracellular phosphatase on phytoplankton cell membranes in April. The increase in phytoplankton density was paralleled by a significant increase in soluble reactive P (SRP) concentrations in the water column and interstitial water at all sites from January to April, with chlorophyte gradually becoming dominant. In February, at the undredged site, more algae dominated by chlorophyte occurred in overlying water, rather than in the surface, coinciding with higher SRP concentrations in overlying and interstitial water. Therefore, P status in the bottom is important to phytoplankton development in terms of density and composition. Undredged sites had higher SRP concentrations in interstitial water than dredged sites. Furthermore, Higher APA was observed, accompanied by higher dissolved organic P (DOP) and lower total P at the undredged site in February. Enzymatic hydrolysis of DOP may have been an additional source of P for phytoplankton. In April, Schroederia sp. was ELF labeled in surface water at the dredged site, which showed markedly lower SRP concentration, but not at the undredged site with higher SRP concentration. Thus, the dredging might regulate algal density and composition in water column by reducing P bioavailability.