Phytoplankton community responses to nutrient and iron enrichment under different nitrogen to phosphorus ratios in the northern Baltic Sea

The impact of nutrient enrichment on the phytoplankton community structure, and particularly cyanobacteria, was studied in a 3-week mesocosm experiment conducted in August 2001 in the Archipelago Sea, a part of the northern Baltic Sea. The factorial design experiment included daily additions of nitrogen (N) and phosphorus (P) at two mass ratios, 1N:1P and 7N:1P, respectively, additions of iron (Fe) and a synthetic chelator, ethylenediaminetetraacetic acid (EDTA). The floating enclosures (400 l) were sampled for analyses of phytoplankton biomass and community structure, phytoplankton primary production, chlorophyll a, nutrients, and hepatotoxins. Chlorophyll a concentration, phytoplankton biomass and primary production increased most in the 7N:1P treatment. The increase was mainly due to an abundant growth of chlorophytes (Dictyosphaerium subsolitarium, Kirchneriella spp., Monoraphidium contortum, and Oocystis spp.), pennate diatoms (especially Nitzschia spp.), dinophytes and the chroococcalean cyanobacterium Synechococcus sp. The nutrient enrichments had no effect on the total biomass of N₂-fixing cyanobacteria. Nevertheless, the biomass of Anabaena spp. was highest in the enrichments with a low N/P ratio. Chlorophyll a concentration and total phytoplankton biomass were not affected by Fe or EDTA, but Fe alone had a positive effect on the chlorophyte Kirchneriella sp. The N₂-fixing cyanobacteria Aphanizomenon sp. responded positively to Fe alone and to both Fe and EDTA added together. The hepatotoxin concentration increased during the experiment, but no clear responses to nutrient enrichments were found. Our study showed species-specific responses to nutrient enrichments among the N₂-fixing cyanobacteria. Although the total phytoplankton production was not Fe-limited; the availability of Fe clearly affected the phytoplankton community structure.     

Lake Kinneret phytoplankton: Response to N and P enrichments in experiments and in nature

A series of water samples from Lake Kinneret was supplemented with 100 µM N (as NH₄ or as NO₃ and/or 10 µM orthophosphate-P. The yield of phytoplankton both as chlorophyll and in cell numbers of major species was determined after a two-week incubation. During these experiments, some of the algae present initially never multiplied (e.g. Peridinium and Peridiniopsis spp. Cryptomonas spp., Rhodomonas spp. and Crysochromulina parva); others e.g. Anomoeoneis exilis, Synedra sp., Chlamydomonas sp., Elakatothrix gelatinosa), undetected in the original sample, grew out during the incubation. Chlorophyte species (the majority of commonly observed forms in the lake) responded most readily to added nutrients.The results of these enrichment experiments were related to the long-term record of phytoplankton populations observed in the lake and suggest that through summer and fall, when ambient levels of both P and N are minimal, P was generally, but not always, the most limiting nutrient for algal growth. In the spring, after the decline of the Peridinium bloom, P appeared to be limiting the growth of Chlorophyta. Although most algal species grew equally well on NH₄ or NO₃, some species appeared to respond preferentially either to the former (Coelastrum, Chodatella) or to the latter (Chroococcus, Anomoeoneis) source of N.     

Physical-Biological Coupling in Southern Lake Michigan: Influence of Episodic Sediment Resuspension on Phytoplankton

The influence of episodic, sediment resuspension on phytoplankton abundance/volume and composition, the photosynthetic maximum rate (P^B _max) and efficiency (^B), and chlorophyll-specific growth (_Chl) was evaluated during the spring isothermal period in southern Lake Michigan (Laurentian Great Lakes, USA). Resuspension altered the nutrient and light climate of nearshore waters; light attenuation (K_d) and phosphorus concentrations corresponded (p 0.0001 and p 0.001, respectively) with concentrations of suspended particulate matter (SPM). Phytoplankton cell volume and diatom cell abundance and volume were not associated with SPM concentrations (p > 0.05). Diatom composition displayed spatial dissimilarities corresponding with resuspension (p 0.001); small centric diatoms exhibiting meroplanktonic life histories and pennate diatoms considered benthic in origin were most abundant within SPM-impacted, nearshore waters whereas taxa typically comprising assemblages in optically-clear, offshore waters and the basin-wide, spring bloom were not. Values of P^B _max and ^B corresponded (p 0.0001) with both K_d coefficients and SPM concentrations, potentially reflecting increased light harvesting/utilization within impacted assemblages. However, integral production was inversely associated with K_d coefficients and SPM concentrations (p < 0.0001) and photosynthesis was light-limited (or nearly so) for most assemblages. Although _Chl values corresponded with K_d coefficients (p 0.05), values were quite low (x ± S.E., 0.10 ± 0.004 d^-1) throughout the study. Most likely, distinct rate processes between SPM- and non-impacted assemblages reflected short-term compositional (and corresponding physiological) variations due to infusion of meroplankton and/or tributary-derived phytoplankton. Overall, resuspension appears to have little, if any, long-term impact upon the structure and function of the lakes phytoplankton.     

Early growth responses of mangroves to different rates of nitrogen and phosphorus supply

Experiments were conducted in an outdoor facility to quantify growth responses of six mangrove species to rates of dissolved inorganic nitrogen and phosphorus supply mimicking the range of N and P mineralization rates in natural soils. Growth of all six species on nitrogen was nonlinear. Stem extension rates of Rhizophora apiculata and Xylocarpus granatum were enhanced to the highest rate of N supply (50 mmol m^− 2 d^− 1); Bruguiera gymnorrhiza, Avicennia marina, and Xylocarpus moluccensis stem growth leveled off by 10 mmol m^− 2 d^− 1. Stem growth of Ceriops tagal peaked at 24-26 mmol N m^− 2 d^− 1. Except for A. marina and C. tagal, rates of biomass increase declined at the highest supply rate, indicating NH₄⁺ toxicity. At different rates of P supply, stem extension rates and rates of biomass increase of R. apiculata and C. tagal best-fit Gaussian curves and B. gymnorrhiza stem growth and biomass increase best-fit sigmoidal and Gaussian curves, respectively; X. moluccensis stem and biomass growth increased linearly, but stem and biomass growth rates of A. marina did not vary in relation to P supply. Stem growth of X. granatum was Gaussian but rates of biomass increase best-fit a quadratic equation. Changes in leaf and root N and P content mirrored the growth responses. As rates of N and P mineralization in natural mangrove soils overlap with the lowest rates of N and P supplied in these experiments, the growth responses imply that mangroves are intrinsically nutrient-limited at mineralization rates often encountered in nature. Such species specificity may have significant implications for recruitment success and the establishment of species gradients within mangrove forests.     

Spatial and temporal variability of internal and external phosphorus loads in Mona Lake,Michigan

A study was conducted in Mona Lake, a small eutrophic lake located in western Michigan (USA) to address the temporal and spatial variability of external and internal phosphorus loading. External P load varied among subbasins, which was mostly related to discharge, but also to land use. Black Creek, which drains lands with natural cover and agriculture, accounted for the majority of flow, and total phosphorus (TP) and soluble reactive phosphorus (SRP) load, to Mona Lake. However, the relative contribution of SRP load was greater in Little Black Creek, which flows through a mostly urbanized subbasin, than in Black Creek. The relative importance of internal loading was strongly related to season, as internal TP loads contributed only ∼9% of the overall P load in April 2005, but ∼68–82% of the overall P load in the summer and early fall seasons. Internal TP and SRP loading was greater under anaerobic than aerobic conditions. Mean anaerobic TP release rates ranged from 0.80 to 15.56 mg P m⁻² d⁻¹, varying with site and season. Spatial variability in both internal phosphorus loading and sediment P concentration was also evident. By taking into account the spatial and temporal variability of different loading sources, management practices can be targeted to optimize nutrient source control strategies.     

The threshold responses of phytoplankton community to nutrient gradient in a shallow eutrophic Chinese lake

Excessive nutrient loads resulted in cascading trophic effects and ecosystem responses. Aims of this study were to determine if the thresholds in nutrient gradient related to phytoplankton community composition could be identified in eutrophic lake, and further to analyze the change of phytoplankton assemblage along the nutrient concentration based on Threshold Indicator Taxa ANalysis (TITAN). The results presented the significant community thresholds estimate for negative taxa declining at 1.650 mg/L TN and 131.5 μg/L TP, as well as simultaneously increasing for positive taxa at 1.665 mg/L TN and 151.5 μg/L TP along nutrient enrichment gradient. However, there was unremarkable change point determined for TN:TP ratios in Lake Dianchi. Elevated TN and TP altered the phytoplankton assemblage, even may induce the fade of algal blooms across the threshold in the hypertrophic lake. The findings could provide implications for deeply deciphering abrupt transitions for phytoplankton assemblage and developing nutrient tactics to protect the lake ecosystems.     

Complex postglacial recolonization inferred from population genetic structure of mottled sculpin Cottus bairdii in tributaries of eastern Lake Michigan,U.S.A.

This study used analyses of the genetic structure of a non‐game fish species, the mottled sculpin Cottus bairdii to hypothesize probable recolonization routes used by cottids and possibly other Laurentian Great Lakes fishes following glacial recession. Based on samples from 16 small streams in five major Lake Michigan, U.S.A., tributary basins, significant interpopulation differentiation was documented (overall F_ST = 0·235). Differentiation was complex, however, with unexpectedly high genetic similarity among basins as well as occasionally strong differentiation within basins, despite relatively close geographic proximity of populations. Genetic dissimilarities were identified between eastern and western populations within river basins, with similarities existing between eastern and western populations across basins. Given such patterns, recolonization is hypothesized to have occurred on three occasions from more than one glacial refugium, with a secondary vicariant event resulting from reduction in the water level of ancestral Lake Michigan. By studying the phylogeography of a small, non‐game fish species, this study provides insight into recolonization dynamics of the region that could be difficult to infer from game species that are often broadly dispersed by humans.     

Relationships of nitrogen and phosphorus to a tropical phytoplankton population

Summary The relationships of nitrogen and phosphorus in the plankton of tropical Lake George (Uganda) have been investigated. The data used are from analyses of samples collected from mid-lake areas during one year, and along transects of increasing plankton concentration, at various times when conditions allowed, and also from results of nutrient uptake experiments. These data are discussed in relation to previously reported information about general limnological conditions in Lake George, and in relation to physiological effects of nutrient enrichments. The features are compared with other waters.The peculiarities of the lake make difficult the precise calculations of long term primary production using oxygen exchange techniques, so that the determination of nutrient turnover rates based upon metabolic activity measured in this way could be misleading. Nevertheless, reasonable overall assessment of nitrogen and phosphorus turnover might be obtained from the proportions of these elements in the plankton. This could be done mainly due to the low variability of the limnological conditions, the apparent limitation of both nitrogen and phosphorus nutrients and the extreme predominance of algal metabolism over the remaining biota.It was also considered that the multiplicity of growth limiting factors which the nitrogen and phosphorus conditions impart contribute to an inherantly stable biomass.

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Zusammenfassung Die Beziehungen zwischen Stickstoff und Phosphor im Plankton des tropischen Sees Lake George (Uganda) wurden untersucht. Die benutzten Daten stammen aus Analysen vom Proben, die über ein Jahr hinweg aus einem Gebiet in Seemitte entnommen wurden. Des weiteren wurden Proben entlang von Profilen mit steigender Planktonkonzentration untersucht. Ferner wurden Ergebnisse von Nährstoffaufnahme-experimenten benutzt. Die Daten wurden mit denen verglichen, die bereits über die algemeinen limnologischen Bedingungen des Sees veröffentlicht wurden. Auch wurden sie in Bezug auf die physiologischen Einflüsse einer erhöhten Nährstoffzufuhr diskutiert. Die Merkmale des Sees wurden mit denen anderer Gewässern verglichen.Die charakteristischen Besonderheiten des Sees machen eine genaue Berechnung von lang anhaltender Primärproduktion unter Anwendung der Sauerstoffaustausch-Methode schwierig, so dass die Bestimmung der Raten des Nährstoffwechsels, basiert auf gemessener metabolischer Aktivität, ein falsches Bild geben könnte. Immerhin konnten annehmbare allgemeine Werte von Stickstoff und Phosphorstoffwechsel aus der Verteilung dieser Elemente im Plankton gewonnen werden. Diese Werte konnten vor allem durch die geringen Schwankungen in den limnologischen Bedingungen, durch die auffallende Beschränkung im Auftreten von Stickstoff und Phosphor und durch das extreme Überwicht vom Algenmetabolismus über die übrige Biota, gewonnen werden.Es wird angenommen, dass die Vielfalt der wachstumbegrenzenden Faktoren, die mit dem Stickstoff- und Phosphorhaushalt zusammenhängen, zu einer von Natur aus stabilen Biomasse führen.

     

杭州西湖引流冲污前后浮游藻类变化及防治富营养化效果评价

1988年4月—1989年4月西湖各湖区浮游藻类逐月分析资料表明,引水冲污后西湖蓝藻个体数量比例从1981年的84.4％下降到69.7％,藻种数118种增至192种;以蓝藻占绝对优势的群落结构特征及多项水质指标的TSI均值表明,西湖水体依然属富营养类型,且有缓慢发展趋势,引水并未根本改变西湖水质特性。根据藻量及多样性指数的方差分析,对各湖区污染及富营养程度进行了多重比较。     

The response of zooplankton and phytoplankton from the North American Great Lakes to filtration

Filtration of ballast water was investigated as a means of minimizing the introduction of nonindigenous zooplankton and phytoplankton by ships visiting the North American Great Lakes-St. Lawrence Seaway system (GLSLSS). An automatic backwash screen filtration (ABSF) system with nominal filtration options of 25, 50 or 100 μm was mounted on the deck of an operating Seaway-sized dry bulk carrier, the MV Algonorth. Water was pumped through the ABSF with a deck mounted pump at 341 m³ hr⁻¹ during routine ship operations in the GLSLSS, and effectiveness of the various screen pore sizes at removing taxonomic categories of zooplankton and phytoplankton was measured using matched treatment and control ballast tanks. The smallest pore sizes (25 and 50 μm) performed better than the 100 μm pore size at removing biological material. There was no difference in the filtration efficiency of the 25 and 50 μm screens relative to macro- or microzooplankton in these tests, but this result was probably due to low densities of macrozooplankton, and soft-bodied (aloricate) characteristics of the microzooplankton present. The 25 and 50 μm pore sizes were subjected to more controlled tests on board a stationary barge platform equipped with triplicate 700 L catchment bins moored in Duluth Harbor of Lake Superior. In these tests, filter pore size, organism size and rigidity influenced zooplankton removal efficiency by the ABSF. The 25 μm screen reduced both macrozooplankton and microzooplankton significantly more than the 50 μm screen. Zooplankton width was more determinative of filtration performance than length, and both filters removed loricate species of rotifers significantly more efficiently than aloricate species of the same length and width size classes. The 25 and 50 μm ABSF also significantly reduced algal densities, with the exception of colonial and filamentous green algae (50 μm only). Filter efficiency relative to algal particles was influenced by filter pore size, organism morphology and structure, and intake density, while algal particle size was not determinative. This research provides compelling evidence that 25 or 50 μm filtration is a potentially powerful means of reducing densities of organisms discharged by ships operating in the Great Lakes but an additional treatment step would be necessary to effectively minimize risk and meet the International Maritime Organization's discharge standards associated with organisms of all sizes in the water column.     

Anthropogenic nutrient enrichment of seagrass and coral reef communities in the Lower Florida Keys: discrimination of local versus regional nitrogen sources

Land-based nutrient pollution represents a significant human threat to coral reefs globally. We examined this phenomenon in shallow seagrass and coral reef communities between the Content Keys (southern Florida Bay) and Looe Key (south of Big Pine Key) in the Lower Florida Keys by quantifying the role of physical forcing (rainfall, wind, tides) and water management on mainland South Florida to nutrient enrichment and blooms of phytoplankton, macroalgae, and seagrass epiphytes. Initial studies (Phase I) in 1996 involved daily water quality sampling (prior to, during, and following physical forcing events) at three stations (AJ, an inshore area directly impacted by sewage discharges; PR, a nearshore patch reef located inshore of Hawk Channel; and LK, an offshore bank reef at Looe Key) to assess the spatial and temporal patterns in advection of land-based nutrients to the offshore reefs. Concentrations of dissolved inorganic nitrogen (DIN=NH₄⁺+NO₃⁻+NO₂⁻), soluble reactive phosphorus (SRP), and chlorophyll a increased at PR and LK following a wind event (∼15 knots, northeast) in mid-February. The highest DIN (mostly NH₄⁺) and SRP concentrations of the entire study occurred at the inshore AJ during an extreme low tide in March. Following the onset of the wet season in May, mean NH₄⁺ and chlorophyll a concentrations increased significantly to maximum seasonal values at PR and LK during summer; relatively low concentrations of NO₃⁻ and a low f-ratio (NO₃⁻/NH₄⁺+NO₃⁻) at all stations during summer do not support the hypothesis that the seasonal phytoplankton blooms resulted from upwelling of NO₃⁻. A bloom of the seagrass epiphyte Cladosiphon occidentalis (phaeophyta) followed the onset of the rainy season and increased NH₄⁺ concentrations at LK, resulting in very high epiphyte:blade ratios (∼3:1) on Thalassia testudinum. Biomass of macroalgae increased at all three stations from relatively low values (<50 g dry wt m⁻²) in winter and early spring to higher values (∼100-300 g dry wt m⁻²) typical of eutrophic seagrass meadows and coral reefs following the onset of the rainy season. The mean δ¹⁵N value of Laurencia intricata (rhodophyta) during 1996 at AJ (+4.7‰) was within the range reported for macroalgae growing on sewage nitrogen; lower values at the more offshore PR (+3.1‰) and LK (+2.9‰) were at the low end of the sewage range, indicating an offshore dilution of the sewage signal during the 1996 study. However, transient increases in δ¹⁵N of Cladophora catanata (chlorophtyta) from ~+2% to +5% at LK concurrent with elevated NH₄⁺ concentrations following rain and/or wind events in May and July suggest episodic advection of sewage nitrogen to the offshore LK station. The Phase II study involved sampling of macroalgae for δ¹⁵N along a gradient from the Content Keys through Big Pine Key and offshore to LK in the summer wet season of 2000 and again in the drought of spring 2001. During the July 2000 sampling, macroalgae in nearshore waters around Big Pine Key had elevated δ¹⁵N values (~+4‰) characteristic of sewage enrichment; lower values (~+2‰) at LK were similar to values reported for macroalgae in upstream waters of western Florida Bay influenced by nitrogen-rich Everglades runoff. That pattern contrasted with the drought sampling in March 2001, when δ¹⁵N values of macroalgae were elevated (+6‰) to levels characteristic of sewage enrichment over a broad spatial scale from the Content Keys to LK. These results suggest that regional-scale agricultural runoff from the mainland Everglades watersheds as well as local sewage discharges from the Florida Keys are both significant nitrogen sources supporting eutrophication and algal blooms in seagrass and coral reef communities in the Lower Florida Keys. Hydrological and physical forcing mechanisms, including rainfall, water management on the South Florida mainland, wind, and tides, regulate the relative importance and variability of these anthropogenic nitrogen inputs over gradients extending to the offshore waters of the Florida Reef Tract.     

Population and community responses of phytoplankton to fluctuating light

Light is a major resource in aquatic ecosystems and has a complex pattern of spatio-temporal variability, yet the effects of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light supply affects the structure and dynamics of phytoplankton communities. The effect of light fluctuations was tested at two average irradiances: low, 25 μmol quanta m⁻² s⁻¹ and high, 100 μmol quanta m⁻² s⁻¹ in 2- and 18-species communities of freshwater phytoplankton. Species diversity, and abundances of individual species and higher taxa, depended significantly on both the absolute level and the degree of variability in light supply, while total density, total biomass, and species richness responded only to light level. In the two-species assemblage, fluctuations increased diversity at both low and high average irradiances and in the multispecies community fluctuations increased diversity at high irradiance but decreased diversity at low average irradiance. Species richness was higher under low average irradiance and was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, especially at low average irradiance, where they became the dominant group, while cyanobacteria and green algae dominated low constant light and all high light treatments. Within each taxonomic group, however, there was no uniform pattern in species responses to light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of light supply had a significant effect on the growth rates of individual species grown in monocultures. Species responses to the regime of light supply in monocultures qualitatively agreed with their abundances in the community experiments. The results indicate that the temporal regime of light supply may influence structure of phytoplankton communities by differentially affecting growth rates and mediating species competition. Received: 24 September 1997 / Accepted: 8 July 1998     

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.     

Spatial and temporal patterns in maternal energetic traits of yellow perch (Perca flavescens) in Lake Erie

1. For many fish species, survival during early life stages is linked to the size and energetic condition of females prior to reproduction. For example, females in good energetic condition are often more fecund and produce larger eggs and offspring than those in poor condition. 2. We measured the characteristics of female yellow perch (Perca flavescens) that may influence annual population fluctuations. From 2005 to 2007, we measured spatial variation in female reproductive traits, such as age, length, mass and energy density (J g^?1) of somatic tissues and ovaries among four spawning aggregations of yellow perch in western and central Lake Erie. 3. Maternal traits, such as somatic energy density and spawner age distribution, differed between the western and central basin, whereas reproductive traits, such as fecundity and ovarian energy density, differed across years. 4. To understand the implications of observed differences in demographic rates (growth and mortality rates) between basins, we developed a deterministic model to simulate the total egg production in the western and central basins under different scenarios of fishing mortality. 5. High growth rates and low mortality rates combined to produce higher modelled estimates of total egg production in the central than in the western basin, and a larger proportion of eggs were produced by old age classes in the central basin than in the western basin. 6. Our results demonstrate that changing harvest levels for populations with different demographic rates can influence total reproductive output through complex interactions between age‐specific mortality, growth and size‐specific fecundity, which has implications for the population dynamics of yellow perch and related species across a broad geographic range.     

Plankton ecology in an ice-covered bay of Lake Michigan: utilization of a winter phytoplankton bloom by reproducing copepods

Plankton ecology was examined during the 1986 winter in Grand Traverse Bay, a 190 m deep, fjordlike bay on Lake Michigan. Before ice cover, algal concentration was low and uniformly distributed with depth, as it is in open Lake Michigan. During ice cover (February and March), a bloom of a typical winter-spring phytoplankton community developed in the upper 40 m, resulting in a 4 to 7-fold increase in feeding rate of adult Diaptomus spp. High algal concentration and zooplankton feeding persisted after ice melt (April). During and after ice cover, lipid concentrations of Diaptomus dropped rapidly from 34% of dry weight to 17 % because of egg production. High incident photosynthetically active radiation (PAR), high (45–50%) PAR transmittance of the ice due to little snow on the ice, and water column stability were probably responsible for the bloom. High ice transparency may be a common feature of large lakes and bays, where strong winds blow snow cover off the ice, or at low latitudes where snowmelt due to occasional rains and warm temperature is common. Winter reproducing calanoid copepods use these blooms to increase their reproductive output.     

Relationship between hypolimnetic phosphorus and iron release from eleven lakes in Maine, USA

We studied five eutrophic (high phosphorus) and six mesotrophic/oligotrophic (low phosphorus) lakes in Maine, USA, all of which are dimictic and develop anoxic hypolimnia during stratification. The lakes were sampled during the stratified period from May to September 1999. Late summer hypolimnetic total phosphorus (P) concentrations in the high-P lakes ranged from 185 to 460ppb; epilimnetic total P increased up to 30ppb from the spring to the fall overturn. During the same period, the low-P lakes had hypolimnetic total P concentrations in the range of 6–19ppb.Individual high-P lakes demonstrated strong temporal correlations between aqueous hypolimnetic dissolved Fe and total P concentrations (R ²0.88) with an average molar Fe:P ratio of 11.9±4.2. For the combined data, the high-P lakes exhibited strong correlation between hypolimnetic Fe and P concentrations (R ²=0.82). The low-P lakes, however, did not show a good correlation between the hypolimnetic Fe and P concentrations. Among the low-P lakes two lakes had hypolimnetic Fe fluxes comparable to the Fe fluxes of the high-P lakes. These two lakes had considerably higher hypolimnetic Fe:P ratios than all other lakes studied here. There were no significant differences in surface sediment Fe(III) or P fractions that correlated with the differences in the relationship between aqueous concentrations of Fe and P in these two outlier low-P lakes. A model for the generation of hypolimnetic acid neutralization capacity (ANC) was developed based on microbially-catalyzed reduction of Fe(III) hydroxide, Mn(IV) oxide and sulfate. Reduction of Fe(III) hydroxide was the most important contributor to the increase in the hypolimnetic ANC in all high-P and the two outlier lakes. Assuming that all hypolimnetic P was due to the reduction of Fe(III) hydroxide by bacteria and sulfide, average summer hypolimnetic P flux for each lake was predicted using the sediment reducible Fe(III):P ratio. The observed and predicted average P fluxes in the high-P lakes corresponded reasonably, suggesting that in these lakes internal P release is closely related to the reduction of Fe(III) hydroxide. Other release or sequestration mechanisms may operate for the release and availability of P in the low-P lakes.     

Energy Density of Lake Whitefish Coregonus clupeaformis in Lakes Huron and Michigan

We collected lake whitefish Coregonus clupeaformis off Alpena and Tawas City, Michigan, USA in Lake Huron and off Muskegon, Michigan USA in Lake Michigan during 2002–2004. We determined energy density and percent dry weight for lake whitefish from both lakes and lipid content for Lake Michigan fish. Energy density increased with increasing fish weight up to 800 g, and then remained relatively constant with further increases in fish weight. Energy density, adjusted for weight, was lower in Lake Huron than in Lake Michigan for both small (≤800 g) and large fish (>800 g). Energy density did not differ seasonally for small or large lake whitefish or between adult male and female fish. Energy density was strongly correlated with percent dry weight and percent lipid content. Based on data from commercially caught lake whitefish, body condition was lower in Lake Huron than Lake Michigan during 1981–2003, indicating that the dissimilarity in body condition between the lakes could be long standing. Energy density and lipid content in 2002–2004 in Lake Michigan were lower than data for comparable sized fish collected in 1969–1971. Differences in energy density between lakes were attributed to variation in diet and prey energy content as well as factors that affect feeding rates such as lake whitefish density and prey abundance.     

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m^–3 could have occurred even though 90% or more of the biogenic silica production is recycled.     

Total inputs of phosphorus and nitrogen by wet deposition into Lake Taihu, China

Lake Taihu suffers from eutrophication caused by riverine nutrient inputs and air deposition. To characterize wet deposition of phosphorus (P) and nitrogen (N) to the lake, precipitation collection and measurements of total phosphorus (TP) and total nitrogen (TN) and other components at five cities around Lake Taihu were made from July 2002 to June 2003. TP and TN concentrations and deposition rates exhibited strong spatial variation in the whole catchment. An inverse correlation between station-averaged TP and TN concentrations and precipitation amount was found. Maximal TP concentration in rainfall was found in Suzhou, and maximal TN in Wuxi. However, highest wet deposition rates of TP and TN were found in Suzhou, which suggests that atmospheric nutrients are mostly from the east and northwest area of Lake Taihu. Mean TP and TN deposition rates were 0.03 and 2.0 t km⁻² year⁻¹ respectively in Lake Taihu, which are greater than reported values in other areas by comparision. Total N and P contributed to the lake by wet deposition were 75 and 4720 t per year, respectively, which represent about 7.3% and 16.5% of total annual N and P inputs via inflow rivers. Wet deposition, especially N, could have significant effects on eutrophication in the lake, which shows that air deposition should be taken into account while reducing the external nutrients in the lake.     

The effects of predation and unionid burrowing on bivalve communities in a Laurentian Great Lake coastal wetland

Unionid (Mollusca: Unionidae) densities have declined dramatically throughout the Laurentian Great Lakes after the introduction of dreissenid mussels (Mollusca: Dreissenidae). Recent surveys in some Great Lake coastal wetlands have found abundant unionid populations, but the factors that reduce zebra mussels on unionids in these habitats are not well understood. In 2001–2002, we tested effects of predation and unionid burrowing on corbiculids, sphaeriids and dreissenids in a Great Lake coastal wetland in western Lake Erie. In one experiment, we reduced access by molluscivores using exclosures with two mesh sizes (1.3 cm × 1.3 cm; 5 cm × 10 cm) and sampled bivalves after 15 months. Small mesh exclosures had higher numbers of dreissenids, Corbicula fluminea and sphaeriids (54.9, 3.8, 22.6 individuals/m², respectively) than large mesh exclosures (0.0, 1.13, 0.13 individuals/m², respectively) or open controls (0.3, 1.0, 0.1 individuals/m², respectively). Numbers of dreissenids on C. fluminea were higher in small mesh exclosures (3.8 dreissenids/Corbicula) than in large mesh exclosures (0.1 dreissenids/Corbicula) or cageless controls (0␣dreissenids/Corbicula). In a second experiment, we held two species of live unionids (Leptodea fragilis, Quadrula quadrula) and immobile Pyganodon grandis shells in exclosures (2.5 cm × 2.5 cm mesh) with either 5 cm, 10 cm, or 20 cm deep sediments and sampled bivalves after 2 months. There were fewer dreissenids on L. fragilis than P. grandis shells, but there was no difference in the number of dreissenids on Q. quadrula and P. grandis shells. Numbers of attached dreissenids were higher inside (189–494 dreissenids/unionid) than outside (8–11 dreissenids/unionid) exclosures, and densities of sphaeriid and C. fluminea clams were also higher inside (21.8, 4.7 individuals/m², respectively) than outside (0.4, 0.9 individuals/m², respectively) exclosures. Numbers of attached dreissenids were higher on unionids that could burrow below the sediments (20 cm depth) than unionids in shallow sediments (5 cm depth) for unexplained reasons. Our data suggest that molluscivores can play a pivotal role in limiting numbers of bivalves including dreissenids in coastal wetlands.