Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland

The catabolic diversity of wetland microbial communities may be a sensitive indicator of nutrient loading or changes in environmental conditions. The objectives of this study were to assess the response of periphyton and microbial communities in water conservation area-2a (WCA-2a) of the Everglades to additions of C-substrates and inorganic nutrients. Carbon dioxide and CH₄ production rates were measured using 14 days incubation for periphyton, which typifies oligotrophic areas, and detritus, which is prevalent at P-impacted areas of WCA-2a. The wetland was characterized by decreasing P levels from peripheral to interior, oligotrophic areas. Microbial biomass and N mineralization rates were higher for oligotrophic periphyton than detritus. Methane production rates were also higher for unamended periphyton (80 mg CH₄-C kg⁻¹ d⁻¹) than detritus (22 mg CH₄-C kg⁻¹ d⁻¹), even though the organic matter content was higher for detritus (80%) than periphyton (69%). Carbon dioxide production for unamended periphyton (222 mg CO₂-C kg⁻¹ d⁻¹) was significantly greater than unamended detritus (84 mg CO₂-C kg⁻¹ d⁻¹). The response of the heterotrophic microbial community to added C-substrates was related to the nutrient status of the wetland, as substrate-induced respiration (SIR) was higher for detritus than periphyton. Amides and polysaccharides stimulated SIR more than other C-substrates, and methanogenesis was greater contributor to SIR for periphyton than detritus. Inorganic P addition stimulated CO₂ and CH₄ production for periphyton but not detritus, indicating a P limitation in the interior areas of WCA-2a. Continued nutrient loading into oligotrophic areas of WCA-2a or enhanced internal nutrient cycling may stimulate organic matter decomposition and further contribute to undesirable changes to the Everglades ecosystem caused by nutrient enrichment.     

Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cadmium and lead toxicity on tropical freshwater periphyton communities under laboratory-based mesocosm experiments

Periphyton constitutes an important community that is useful for assessment of ecological conditions in lotic systems. The objective of this study was to assess the effects of different mixtures of Cd and Pb on periphyton growth as well as Cd and Pb mixtures toxicity to diatom assemblages in laboratory mesocosm experiments. A natural periphyton community sampled from the Monjolinho River (South of Brazil) was inoculated into five experimental systems containing clean glass substrates for periphyton colonization. The communities were exposed to mixtures of dissolved Cd and Pb concentrations of 0.01 and 0.1 mg l⁻¹ Cd and 0.033 and 0.1 mg l⁻¹ Pb. Periphyton ash-free dry weight, growth rate, diatom cell density and diatom community composition were analyzed on samples collected after 1, 2 and 3 weeks of colonization. High Cd concentration (0.1 mg l⁻¹) has negative effects on periphyton growth while high concentration of Pb (0.1 mg l⁻¹) decreased the toxic effects of Cd on periphyton growth. Shifts in species composition (development of more resistant species like Achnanthidium minutissimum and reduction of sensitive ones like Cymbopleura naviculiformis, Fragilaria capucina, Navicula cryptocephala, Encyonema silesiacum, Eunotia bilunaris, and Gomphonema parvulum), decreases in species diversity of diatom communities with increasing Cd and Pb concentrations and exposure duration have been demonstrated in this study making diatom communities appropriate monitors of metal mixtures in aquatic systems.     

Periphyton biomass,potential production and respiration in a shallow lake during winter and spring

Abundance, depth distribution, potential productivity and respiration of periphyton on short-time (1 month) and long-time incubated strips were followed monthly during the winter–spring (January–May) transition in a shallow eutrophic lake. A taxonomic shift occurred from dominance of diatoms under ice to chlorophyte dominance in spring communities on the long-time incubated strips, while diatoms dominated until May on the short-time incubated strips. Periphyton biomass accrual was low during the ice-covered winter months (November–January: 4 mg chl a m⁻² month⁻¹), but increased to a maximum of 112 mg chl a m⁻² month⁻¹ immediately after ice-out in February. During February–April, the biomass remained constant before declining in May. Periphyton on long-time incubated strips was equally distributed in the water column in winter (January–February), but was higher near the water surface in spring (March–May). Periphyton did not change with depth on the short-time incubated strips. The potential production to respiration ratio (P/R) was negatively correlated with periphyton biomass. Throughout the study, P/R was <1 for the short-time incubated periphyton, while this was only the case in March–April for the long-time incubations. This study showed a high productive capacity of winter periphyton, resulting in accumulation of a relatively high periphytic biomass early in the season. A massive periphyton density in eutrophic lakes already in winter–spring may potentially delay or prevent the establishment and re-occurrence of submerged macrophytes in the early oligotrophication phase following a reduction of the external nutrient loading. Handling editor: Luigi Naselli-Flores     

Effects of hydrologic and water quality drivers on periphyton dynamics in the southern Everglades

Everglades periphyton mats are tightly-coupled autotrophic (algae and cyanobacteria) and heterotrophic (eubacteria, fungi and microinvertebrates) microbial assemblages. We investigated the effect of water column total phosphorus and nitrogen concentrations, water depth and hydroperiod on periphyton of net production, respiration, nutrient content, and biomass. Our study sites were located along four transects that extended southward with freshwater sheetflow through sawgrass-dominated marsh. The water source for two of the transects were canal-driven and anchored at canal inputs. The two other transects were rain-driven (ombrotrophic) and began in sawgrass-dominated marsh. Periphyton dynamics were examined for upstream and downstream effects within and across the four transects. Although all study sites were characterized as short hydroperiod and phosphorus-limited oligotrophic, they represent gradients of hydrologic regime, water source and water quality of the southern Everglades. Average periphyton net production of 1.08 mg C AFDW⁻¹ h⁻¹ and periphyton whole system respiration of 0.38 mg C AFDW⁻¹ h⁻¹ rates were net autotrophic. Biomass was generally highest at ombrotrophic sites and sites downstream of canal inputs. Mean biomass over all our study sites was high, 1517.30 g AFDW m⁻². Periphyton was phosphorus-limited. Average periphyton total phosphorus content was 137.15 μg P g⁻¹ and average periphyton total N:P ratio was 192:1. Periphyton N:P was a sensitive indicator of water source. Even at extremely low mean water total phosphorus concentrations ( ≤ 0.21 μmol l⁻¹), we found canal source effects on periphyton dynamics at sites adjacent to canal inputs, but not downstream of inflows. These canal source effects were most pronounced at the onset of wet season with initial rewetting. Spatial and temporal variability in periphyton dynamics could not solely be ascribed to water quality, but was often associated with both hydrology and water source.     

Taxonomic Shift Over a Phosphorus Gradient Affects the Stoichiometry and Fatty Acid Composition of Stream Periphyton

Phosphorus enrichment of stream ecosystems generally increases primary production in the benthos, but the consequences of eutrophication for the nutritional quality of periphyton for grazers are less clear. On short timescales, high phosphorus inputs may lead to reduced C:P ratios and high essential fatty acid contents of periphyton, which are both considered important determinants of food quality for grazers. However, nutrient enrichment may alter the taxonomic composition of periphyton and favor the growth of less palatable algal taxa. In this study, periphyton was grown under a gradient of dissolved phosphorus availability from 5 to 100 µg P · L⁻¹, to investigate eutrophication effects on periphyton taxonomy, C:N:P stoichiometry, and fatty acid composition. After 1 month, periphyton grown under oligotrophic conditions was mainly composed of diatoms (~86%). With increasing phosphorus availability, diatoms were gradually outcompeted by chlorophytes and cyanobacteria, which were the predominant taxon under eutrophic conditions. Unexpectedly, periphyton C:P ratios increased with greater phosphorus supply, from ~280 under oligotrophic conditions up to ~790 at 100 µg · L⁻¹, reflecting a tendency of chlorophytes and cyanobacteria to produce more biomass per unit of assimilated phosphorus compared to diatoms. Periphyton content of essential polyunsaturated fatty acids relative to biomass followed a unimodal relationship with phosphorus availability and peaked at intermediate phosphorus levels, likely as a result of both taxonomic and nutrient effects. Our results demonstrate that phosphorus-driven eutrophication of freshwater ecosystems may worsen periphyton nutritional quality due to taxonomic sorting, which may further lead to lower growth and reproduction of herbivores.     

Periphyton nitrogenase activity as an indicator of wetland eutrophication: spatial patterns and response to phosphorus dosing in a northern Everglades ecosystem

The use of periphyton nitrogenase activity (biological N₂ fixation) as an indicator of wetland P impact was assessed using patterns of nutrient content (C, N, P, Ca, Mg, K, Fe, and Mn) and acetylene reduction (AR) in floating cyanobacterial periphyton mat (metaphyton) communities of a P-enriched portion of the Florida Everglades, USA (Water Conservation Area-2A, WCA-2A). Spatial patterns of nutrients indicate the enrichment of floating mat periphyton N, P, Fe, and K, and the reduction of Mn and TN:TP in enriched marsh areas. In highly enriched areas, floating mat periphyton AR was approximately threefold greater than that in less enriched, interior marsh zones. Multiple regression models indicated AR dependence on P in eutrophic WCA-2A areas while the AR of more interior marsh periphyton mats was more closely related to tissue levels of Ca and Fe. Nitrogenase activity of floating mat periphyton from P-loaded mesocosms revealed a significant enhancement of N₂ fixation in samples receiving approximately 2–3 mg P m⁻² of cumulative P dosing or with biomass TP content of 100–300 mg kg⁻¹. At P contents above the optimum, mat periphyton AR was suppressed possibly as a result of changes in species composition or increased levels of NH₄⁺. After 3 years of dosing, consistently high AR occurred only at low rates of P enrichment (0.4–0.8 g P m⁻² yr⁻¹), and the patterns appeared to be seasonal. These findings agree with the hypothesis that P availability is a key determinant of nitrogenase activity in aquatic systems, and thus, may support the use of periphyton nitrogenase to indicate P impacts in P-limited systems. These results also demonstrate the potential existence of a P threshhold for biogeochemical alteration of periphyton mat function in the Everglades, and that cumulative loading of limiting nutrients (i.e., P), rather than instantaneous concentrations, should be considered when evaluating nutrient criteria.     

Diatom fluxes in a tropical,oligotrophic lake dominated by large-sized phytoplankton

Alchichica is a warm-monomictic, oligotrophic lake whose phytoplanktonic biomass is dominated by large size (average ca. 55 μm) diatoms. The fast sinking phytoplankton leads to silica, and other nutrient exportation out of the productive zone of the lake. The aim of the present study was to identify and measure the sedimentation fluxes of the diatom species and their temporal dynamics to better understand the magnitude of silica and carbon fluxes. Sediment-traps were exposed at three different depths and collected monthly. A total of 13 diatom species were observed in the traps. The maximum diatom flux was in February (304 × 10⁶ cells m⁻² day⁻¹) related to the winter diatom bloom. The diatom silica (DSi) fluxes varied from 2.2 to 2,997 mg m⁻² day⁻¹ and the diatom carbon (DC) fluxes from 1.2 to 2,918 mg m⁻² d⁻¹. Cyclotella alchichicana was the main contributor (>98%) to the total DSi and DC fluxes. The annual diatom (15 × 10⁹ cells m⁻² year⁻¹), DSi (147 g m⁻² year⁻¹) and DC (92 g m⁻² year⁻¹) fluxes are higher than in other aquatic ecosystems of similar or even higher trophic conditions. Our findings in Alchichica are indicative of the relevance of the phytoplankton type and size in understanding the role tropical and oligotrophic lakes play regarding silica and carbon fluxes. In addition, our results support previous findings suggesting that inland aquatic ecosystems are more important than formerly thought in processing carbon, and can, therefore, affect regional carbon balances.     

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g⁻¹ vs. 107 μg g⁻¹, respectively) and soil TP (284 μg g⁻¹ vs. 145 μg g⁻¹). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m⁻²). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly promote the alternate community. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Seasonal modulation of bacterioplankton community at a temperate eutrophic shallow lake

At the present time, there is still a lack of information about environmental parameters modulating variations on bacterial diversity in temperate lakes, particularly from Portugal. Fermentelos Lake (Central Portugal) is a shallow water body that sustains an important wetland area. The strong nutrient inputs from agriculture and industrial runoffs have led to its current eutrophic status. The present work aimed to understand which factors modulate the seasonal bacterioplankton diversity at this lake using 16S rRNA PCR-denaturing gradient gel electrophoresis (DGGE) and multivariate analysis. Environmental data demonstrated eutrophic features throughout all samples with nitrate concentrations reaching 12.0 mg N (NO₃ ⁻) l⁻¹ in March 2006, while the highest conductivity (609 μS cm⁻¹), soluble reactive phosphorus (0.37 mg l⁻¹), total suspended solids (87.2 mg l⁻¹) and chlorophyll a (286.6 μg l⁻¹) levels were recorded in August 2007. Over the past two decades there was a general increase in nitrate, pH and conductivity levels at this lake, suggesting the eutrophication process is still in progress. Multivariate analysis showed that summer versus winter DGGE patterns could be established for bacterial assemblages and were mainly defined by water temperature and chlorophyll a. Actinobacteria were dominant throughout the study period although a general preference for higher temperature, pH, total suspended solids, conductivity, soluble reactive phosphorus (SRP) and chlorophyll a levels was observed. The highest concentrations of nitrogen sources were related to Bacteroidetes and phototrophic eukaryote (cryptophycean) dominance. The expansion of Betaproteobacteria, Alphaproteobacteria and Cyanobacteria phylotypes was generally associated to high temperature, pH, conductivity and SRP values.     

A comparison of Acroneuria lycorias (Plecoptera) production and growth in northern Michigan hard- and soft-water streams

1. Density, biomass, production and growth of a predaceous stonefly, Acroneuria lycorias, were compared between fourth-order hard- and soft-water streams in Michigan's upper peninsula, U.S.A. 2. Mean densities, estimated from Hess samples, were higher (100 ± 17 individuals m^?2) at the hard-water site than at the soft-water site (40 ± 9 ind. m^?2). Mean dry weight biomass was 4.9 times greater at the hard-water site. 3. Mean annual production, calculated using the size frequency method, was 5.0 times greater at the hard-water site (2.18 ± 0.44 g dry weight m^?2yr^?1) than at the soft-water site (0.43 ± 0.02g dry weight m^?2yr^?1). Annual production/mean biomass ratios were similar between sites. 4. Monthly growth rates of naturally occurring nymphs of paired cohorts were similar in both streams. Individual growth rates were similar for nymphs reared in artificial streams at high and low water hardnesses with unlimited food and space. 5. Stonefly production and growth rates were influenced more by indirect physical, biological, or habitat factors than by streamwater cation concentrations.     

Aquatic ecosystems of the Yucatán Peninsula (Mexico), Belize, and Guatemala

This study presents limnological and morphological characteristics, physical and chemical properties of waters, and geochemistry of surface sediments for 63 aquatic ecosystems located on the karst Yucatán Peninsula and surrounding areas of Belize and the Guatemalan highlands and eastern lowlands. Our principal goal was to classify the aquatic systems based on their water variables. A principal component analysis (PCA) of the surface water chemistry data showed that a large fraction of the variance (29%) in water chemistry is explained by conductivity and major ion concentrations. The broad conductivity range, from 168 to 55,300 μS cm⁻¹ reflects saline water intrusion affecting coastal aquatic environments, and the steep NW–S precipitation gradient, from ~450 to >3,200 mm year⁻¹. Coastal waterbodies Celestún and Laguna Rosada displayed the highest conductivities. Minimum surface water temperatures of 21.6°C were measured in highland lakes, and warmest temperatures, up to 31.7°C, were recorded in the lowland waterbodies. Most lakes showed thermal stratification during the sampling period, with the exception of some shallow (<10 m) systems. Lakes Chichancanab, Milagros, and Bacalar displayed sulfate-rich waters. Waters of sinkholes had relatively high conductivities (<3,670 μS cm⁻¹) and a broad range of δ¹⁸O values (−4.1 to +3.8‰). Ca, HCO₃, and SO₄ dominated the waters of the lowland lakes, whereas Na was the dominant cation in highland lakes. Coastal aquatic ecosystems were dominated by Na and Cl. Cluster analysis based on surface water variables classified aquatic environments of the lowlands and highlands into three groups: (1) lowland lakes, ponds, wetlands, and coastal waterbodies (2) highland lakes, and (3) sinkholes and rivers. A broad trophic state gradient was recorded, ranging from the eutrophic Lake Amatitlán and the Timul sinkhole to oligotrophic Laguna Ayarza, with the highest water transparency (11.4 m). We used major and trace elements in surface sediments to assess pollution of waterbodies. Lakes Amatitlán, Atescatempa, El Rosario, Cayucón, Chacan-Lara, La Misteriosa, rivers Subín and Río Dulce, the wetland Jamolún, and the sinkhole Petén de Monos showed evidence of pollution and urban development. Their surface sediments displayed high concentrations of As, Cu, Fe, Ni, Pb, Se, Zn, and Zr, which suggest moderate to strong pollution.     

Eutrophication of moderately deep Dutch lakes during the past century: flaws in the expectations of water management?

We studied the trophic development of the past 30–100 years in eight moderately deep Dutch lakes based on their sedimentary fossil diatom assemblages. The dominant diatoms indicating meso- to eutrophic conditions were Aulacoseira subarctica, Cyclotella ocellata, C. cyclopuncta, C. meneghiniana, Puncticulata bodanica, Aulacoseira granulata, Cyclostephanos dubius, C. invisitatus, Stephanodiscus hantzschii, S. medius, and S. parvus. Ordination of diatom data separated the lakes into four groups according to their total phosphorus concentrations (TP), water supply, water management, and origin. The first group consists of dike-breach lakes, which were in stable eutrophic to hypertrophic conditions throughout the past century with diatom-inferred TP (DI-TP) concentrations of between 70 and 300 μg l⁻¹. The main factors influencing these dike-breach lakes are river management, ground water supply of riverine origin, and local land use. The second group are artificial lakes of fluctuating oligo- to mesotrophic conditions and DI-TP concentrations of 10–30 μg l⁻¹. Only one of the artificial lakes showed a DI-TP increase due to changes in catchment agricultural practice. A third group includes an artificial moat and an inland dike-breach lake with DI-TP concentrations of 50–100 μg l⁻¹. The fourth group contains an individual dike-breach lake with stable mesotrophic conditions of 50 μg l⁻¹ throughout the past century. Rather than showing a regional pattern, the studied lakes behave very individualistically with regard to their trophic history, reflecting changes in the local hydrology and in their nutrient sources.     

A multi-proxy paleolimnological reconstruction of trophic state reference conditions for stratified carbonate-rich lakes in northern Germany

This study aims to identify reference conditions (nutrient status and diatom assemblages) as required by the European Water Framework Directive (WFD) for stratified, carbonate-rich lowland lakes with a large watershed area (watershed area to lake volume ratio (WV) > 1.5 km² 10⁻⁶ m⁻³) and a retention time (RT) from 0.1 to 10 years (Central Baltic Lake-Type 1, German Lake-Type 10) in European ecoregion 14. Diatoms, pollen and geochemistry were analysed from sediment cores of six lakes from northern Germany representing different subtypes of Lake-Type 10 (varying WV and RT) and covering the past 290–1,750 years. Historic total phosphorus levels were inferred using diatom-based transfer functions selected from a merged European data set and from optimised data sets identified with the moving-window approach. Pollen and geochemical proxies were used to identify occurrence and intensity of anthropogenic catchment usage. Lake trophic state reference conditions and associated diatom assemblages were identified for three of the six study lakes. In contrast, according to fossil pollen assemblages, two lakes were already strongly impacted by intensive catchment usage when the oldest investigated sediments were laid down. Thus, reference conditions of these already eutrophic lakes could not be identified. Similarly, the lowermost samples of a core from the sixth lake showed signs of impact, and it remains unclear whether the identified dystrophic conditions occurred naturally or if they were due to the drainage of wetlands in Medieval times. Lakes with a relatively small WV (1.5–5.0 km² 10⁻⁶ m⁻³) and RT > 1 year were naturally oligotrophic to low mesotrophic and a typical, representative diatom assemblage was identified. In contrast, typical reference conditions or diatom assemblages for lakes with higher WV (5–18.6 km² 10⁻⁶ m⁻³) and RT < 1 year could not be identified as chemical precipitation and upstream lakes (nutrient sinks or sources) additionally influenced natural nutrient levels. Therefore, the reference situation of both trophic state and diatom assemblages in a lake may be strongly influenced by other modifying, limnological processes in addition to WV and RT. Overall, this study helps to implement the WFD by identifying reference conditions and by discussing the level of differentiation of lake types required to set reference conditions. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

Ecological variation within Sellaphora species complexes (Bacillariophyceae): specialists or generalists?

The genus Sellaphora has become a model system for studies of the species concept, speciation and automated identification in diatoms. Three species, S. pupula, S. bacillum and S. laevissima, have proved to be complexes containing several or many species, which are difficult to distinguish morphologically but which are genetically differentiated and (where tested) reproductively isolated. Until now, however, there has been little information about the ecology of the species within this complex, except in relation to parasite sensitivity. In order to test whether semi- and pseudo-cryptic Sellaphora species are ecologically differentiated with respect to trophic status, we used tools recently developed in the UK in response to the EU Water Framework Directive (WFD). Diatom samples from three substrata (plants, rocks, sediment) were taken from 22 lakes in Scotland and England, covering a gradient from oligotrophic mountain lakes to eutrophic lowland ponds. The epilithic and epiphytic diatom assemblages were used to evaluate lake trophic status according to the UK WFD assessment system and showed that there was a strong environmental gradient in the dataset. Sellaphora species occurred in the sediment-derived epipelon and their distributions were analysed in relation to the trophic gradient. A total of 28 Sellaphora phenodemes (putative species) were found, and some of them differed in their environmental demands. Two groups were distinguished: (1) a group indicating rather oligotrophic conditions and containing several demes with linear valves and subcapitate or capitate poles (referred to here as S. [pupula] cap-A, cap-B and cap-C and (2) a group occurring in eutrophic lakes and containing the recently described species S. blackfordensis, S. capitata and S. obesa, as well as S. [pupula] Φ ‘small lanceolate’. The data obtained are also discussed with respect to Finlay’s hypothesis on microalgal cosmopolitanism. Handling editor: J. Padisak     

Assessing the efficacy of chironomid and diatom assemblages in tracking eutrophication in High Arctic sewage ponds

Eutrophication is the most common water quality issue affecting freshwaters worldwide. Paleolimnological approaches have been used in temperate regions to track eutrophication over time, placing changes in historical context. Diatoms (Bacillariophyta) have a direct physiological response to changes in nutrients and are effective indicators of lake trophic status. Chironomids (Diptera) have also been used to track nutrient conditions; however, given that nutrients and oxygen are often tightly linked, it is difficult to disentangle which variable is driving shifts in assemblages. Here, we analyze chironomid and diatom remains in sediments from sewage-impacted ponds in the High Arctic. These ponds have the unusual characteristics of elevated nutrient and oxygen concentrations, unlike those of typical eutrophic lakes where deepwater oxygen is often depleted. Our data show that while diatom assemblages responded to changing nutrients, no concomitant changes in chironomid assemblage composition were recorded. Furthermore, the dominance of oligotrophic, cold stenothermic chironomid taxa, and lack of so-called “eutrophic” species in the eutrophic sewage ponds suggests that oxygen, not nutrients, structures chironomid assemblages at these sites.     

Phosphorus Biogeochemistry and the Impact of Phosphorus Enrichment: Why Is the Everglades so Unique?

The Florida Everglades is extremely oligotrophic and sensitive to small increases in phosphorus (P) concentrations. P enrichment is one of the dominant anthropogenic impacts on the ecosystem and is therefore a main focus of restoration efforts. In this review, we synthesize research on P biogeochemistry and the impact of P enrichment on ecosystem structure and function in the Florida Everglades. There are clear patterns of increased P concentrations and altered structure and processes along nutrient-enrichment gradients in the water, periphyton, soils, macrophytes, and consumers. Periphyton, an assemblage of algae, bacteria, and associated microfauna, is abundant and has a large influence on phosphorus cycling in the Everglades. The oligotrophic Everglades is P-starved, has lower P concentrations and higher nitrogen–phosphorus (N:P) ratios, and has oxidized to only slightly reduced soil profiles compared to other freshwater wetland ecosystems. Possible general causes and indications of P limitation in the Everglades and other wetlands include geology, hydrology, and dominance of oxidative microbial nutrient cycling. The Everglades may be unique with respect to P biogeochemistry because of the multiple causes of P limitation and the resulting high degree of limitation. Received 23 August 2000; Accepted 23 March 2001.     

Influence of suspended clay on phosphorus uptake by periphyton

We investigated the effect of suspended clay upon the phosphorus uptake rate exhibited by lotic periphyton communities. Suspended inorganic clays and periphyton are common to aquatic environments, and both can strongly influence physical and chemical water conditions. We used replicated artificial stream channels to test the prediction that suspended clay particles would affect the uptake of soluble reactive phosphorus (SRP) by periphyton. Commercially available kaolinite and bentonite clays were characterized for their aqueous suspension behavior and affinities for SRP. Periphyton was grown in a recirculating stream system and subjected to simultaneous suspended clay and SRP additions. SRP removal from solution, both in the presence and absence of suspended clays, was used to quantify SRP uptake parameters by periphyton. Clay type and concentrations of 20, 80, and 200 mg l⁻¹ had no significant effect upon SRP uptake rate exhibited by periphyton during three 90-min experiments. Less than 1% of SRP removal was attributable to the suspended clay load or artificial stream construction materials, based on clay isotherm data and material sorption studies, indicating that 99% of SRP removal was attributable to biotic uptake. Removal of SRP (as KH₂PO₄) was described by a first-order equation with rate constants ranging between 0.02 and 0.14 min⁻¹. Our results suggest that high turbidity conditions caused by suspended mineral clays have little immediate effect upon SRP removal from the water column by periphyton. Handling editor: D. Ryder     

Biomanipulation as a useful water quality management tool in deep stratifying reservoirs

In six deep, soft-water reservoirs, ranging from oligotrophic to eutrophic, fishery management has been guided by the use of biomanipulation to improve water quality and opportunities for recreational fishing. As evidenced by the establishment of larger-bodied daphnids, a low level of zooplanktivory could be maintained in the newly filled Grosse Dhünn and refilled Brucher and Lingese Reservoirs only by regular stocking of piscivores (Sander lucioperca, Esox lucius, Salmo trutta lacustris, Onchorhynchus mykiss) in combination with size and bag limitations for the recreational fisheries. However, in the mesotrophic Pre-Reservoir Grosse Dhünn, Bever Reservoir and the newly filled slightly eutrophic Wupper Reservoir it took between 8 and 10 years before the predator-resistant zooplankton community responded to management with a switch to larger daphnids. Except for oligotrophic Grosse Dhünn Reservoir where angling is prohibited, the expected enhancement of piscivore biomass through stocking not least was prevented by anglers. However, growth of perch (Perca fluviatilis) benefited from the changed fishery management relying upon stocking piscivores only allowing them to reach the size of piscivory. The appearance of larger daphnids in Pre-Reservoir Grosse Dhünn and Wupper Reservoir resulted in the biomass of the Daphnia spring peak to increase and occur earlier causing prolonged clear water conditions. Although the larger daphnids increased transparency, total summer mean chlorophyll concentrations in the euphotic zone only decreased in the slightly eutrophic reservoirs due to reduced phosphorus availability following unchanged external phosphorus loadings. Reduced phosphorus availability in these reservoirs caused a real oligotrophication. Although the edible seston fraction was controlled “top down” the results did not support the trophic cascade hypotheses because total phytoplankton remained controlled “bottom up”, admittedly triggered by “top down” forces. In general, the results support the importance of indirect (non-lethal) effects as the driving forces for the successful biomanipulations, particularly in slightly eutrophic reservoirs. Not least, the delayed zooplankton response provides an indication that the underlying change of internal feedbacks was not driven by external forces to stabilize the reservoirs trophic state in these deep stratifying reservoirs. Handling editor: D. Ryder     

Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes

Nitrogen stable isotope (δ¹⁵N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of δ¹⁵N_POM in lakes at the global scale. The δ¹⁵N_POM variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as δ¹⁵N_POM maximum − δ¹⁵N_POM minimum) from 36 lakes with seasonal data did not change systematically with latitude, but was significantly lower in small lakes than in large lakes. The seasonal mean δ¹⁵N_POM increased from oligotrophic lakes to eutrophic lakes despite large variations that are attributed to the occurrences of nitrogen fixation across the trophic gradient and the differences in δ¹⁵N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean δ¹⁵N_POM was significantly correlated with DIN concentration and δ¹⁵N_DIN in two subsets of lakes. Seasonal minimum δ¹⁵N_POM in individual lakes is influenced by nitrogen fixation and δ¹⁵N_DIN while seasonal maximum δ¹⁵N_POM is influenced by lake trophic state and δ¹⁵N_DIN. As a result of the dominance of non-living POM in the unproductive surface waters, seasonal δ¹⁵N_POM amplitude was small (mean = 4.2‰) in oligotrophic lakes of all latitudes. On the other hand, seasonal δ¹⁵N_POM amplitude in eutrophic lakes was large (mean = 10.3‰), and increased from low to high latitudes, suggesting that the seasonal variability of δ¹⁵N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The δ¹⁵N_POM from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2‰ depletion compared to the lakes with seasonal data. Along with the large seasonal variability of δ¹⁵N_POM within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.