Nutrient limitation of periphyton growth in arctic lakes in south-west Greenland

Many arctic lakes are oligotrophic systems where phototrophic growth is controlled by nutrient supply. Recent anthropogenic nutrient loading is associated with biological and/or physico-chemical change in several lakes across the arctic. Shifts in nutrient limitation (nitrogen (N), phosphorus (P), or N + P) and associated effects on the growth and composition of algal communities are commonly reported. The Kangerlussuaq region of south-west Greenland forms a major lake district which is considered to receive little direct anthropogenic disturbance. However, long-range transport of pollutant N is now reaching Greenland, and it was hypothesised that a precipitation gradient from the inland ice sheet margin to the coast might also deliver increased N deposition. In situ nutrient bioassays were deployed in three lakes across the region: ice sheet margin, inland (close to Kangerlussuaq) and the coast (near Sisimiut), to determine nutrient limitation of lakes and investigate any effects of nutrients on periphyton growth and community composition. Nutrient limitation differed amongst lakes: N limitation (ice sheet margin), N and P limitation (inland) and N + P co-limitation (coast). Factors including variation in N supply, ice phenology, seasonal algal succession, community structure and physical limnology are explored as mechanisms to explain differences amongst lakes. Nutrient limitation of arctic lakes and associated ecological impacts are highly variable, even across small geographic areas. In this highly sensitive region, future environmental change scenarios carry a strong risk of significantly altering nutrient limitation; in turn, potentially severely impacting lake structure and function.     

Quantifying uncertainties in biologically-based water quality assessment: A pan-European analysis of lake phytoplankton community metrics

Lake phytoplankton are adopted world-wide as a sensitive indicator of water quality. European environmental legislation, the EU Water Framework Directive (WFD), formalises this, requiring the use of phytoplankton to assess the ecological status of lakes and coastal waters. Here we provide a rigorous assessment of a number of proposed phytoplankton metrics for assessing the ecological quality of European lakes, specifically in response to nutrient enrichment, or eutrophication, the most widespread pressure affecting lakes. To be useful indicators, metrics must have a small measurement error relative to the eutrophication signal we want them to represent among lakes of different nutrient status. An understanding of variability in metric scores among different locations around a lake, or due to sampling and analytical variability can also identify how best this measurement error is minimised.To quantify metric variability, we analyse data from a multi-scale field campaign of 32 European lakes, resolving the extent to which seven phytoplankton metrics (including chlorophyll a, the most widely used metric of lake quality) vary among lakes, among sampling locations within a lake and through sample replication and processing. We also relate these metrics to environmental variables, including total phosphorus concentration as an indicator of eutrophication.For all seven metrics, 65–96% of the variance in metric scores was among lakes, much higher than variability occurring due to sampling/sample processing. Using multi-model inference, there was strong support for relationships between among-lake variation in three metrics and differences in total phosphorus concentrations. Three of the metrics were also related to mean lake depth. Variability among locations within a lake was minimal (<4%), with sub-samples and analysts accounting for much of the within-lake metric variance. This indicates that a single sampling location is representative and suggests that sub-sample replication and standardisation of analyst procedures should result in increased precision of ecological assessments based upon these metrics.For three phytoplankton metrics being used in the WFD: chlorophyll a concentration, the Phytoplankton Trophic Index (PTI) and cyanobacterial biovolume, >85% of the variance in metric scores was among-lakes and total phosphorus concentration was well supported as a predictor of this variation. Based upon this study, we can recommend that these three proposed metrics can be considered sufficiently robust for the ecological status assessment of European lakes in WFD monitoring schemes.     

Shallow lake restoration by nutrient loading reduction—some recent findings and challenges ahead

Shallow lakes respond to nutrient loading reductions. Major findings in a recent multi-lake comparison of data from lakes with long time series revealed: that a new state of equilibrium was typically reached for phosphorus (P) after 10–15 years and for nitrogen (N) after <5–10 years; that the in-lake Total N:Total P and inorganic N:P ratios increased; that the phytoplankton and fish biomass often decreased; that the percentage of piscivores often increased as did the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass, and cladoceran size. This indicates that enhanced resource and predator control often interact during recovery from eutrophication. So far, focus has been directed at reducing external loading of P. However, one experimental study and cross-system analyses of data from many lakes in north temperate lakes indicate that nitrogen may play a more significant role for abundance and species richness of submerged plants than usually anticipated when total phosphorus is moderate high. According to the alternative states hypothesis we should expect ecological resistance to nutrient loading reduction and P hysteresis. We present results suggesting that the two alternative states are less stable than originally anticipated. How global warming affects the water clarity of shallow lakes is debatable. We suggest that water clarity often will decrease due to either enhanced growth of phytoplankton or, if submerged macrophytes are stimulated, by reduced capacity of these plants to maintain clear-water conditions. The latter is supported by a cross-system comparison of lakes in Florida and Denmark. The proportion of small fish might increase and we might see higher aggregation of fish within the vegetation (leading to loss of zooplankton refuges), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. Moreover, lakes may have prolonged growth seasons with a higher risk of long-lasting algal blooms and at places dense floating plant communities. The effects of global warming need to be taken into consideration by lake managers when setting future targets for critical loading, as these may well have to be adjusted in the future. Finally, we highlight some of the future challenges we see in lake restoration research.     

Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes,with target nitrogen and phosphorus concentrations for good ecological status

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Turning the Table: Plants Consume Microbes as a Source of Nutrients

Interactions between plants and microbes in soil, the final frontier of ecology, determine the availability of nutrients to plants and thereby primary production of terrestrial ecosystems. Nutrient cycling in soils is considered a battle between autotrophs and heterotrophs in which the latter usually outcompete the former, although recent studies have questioned the unconditional reign of microbes on nutrient cycles and the plants'' dependence on microbes for breakdown of organic matter. Here we present evidence indicative of a more active role of plants in nutrient cycling than currently considered. Using fluorescent-labeled non-pathogenic and non-symbiotic strains of a bacterium and a fungus (Escherichia coli and Saccharomyces cerevisiae, respectively), we demonstrate that microbes enter root cells and are subsequently digested to release nitrogen that is used in shoots. Extensive modifications of root cell walls, as substantiated by cell wall outgrowth and induction of genes encoding cell wall synthesizing, loosening and degrading enzymes, may facilitate the uptake of microbes into root cells. Our study provides further evidence that the autotrophy of plants has a heterotrophic constituent which could explain the presence of root-inhabiting microbes of unknown ecological function. Our discovery has implications for soil ecology and applications including future sustainable agriculture with efficient nutrient cycles.     

Strength and uncertainty of phytoplankton metrics for assessing eutrophication impacts in lakes

Phytoplankton constitutes a diverse array of short-lived organisms which derive their nutrients from the water column of lakes. These features make this community the most direct and earliest indicator of the impacts of changing nutrient conditions on lake ecosystems. It also makes them particularly suitable for measuring the success of restoration measures following reductions in nutrient loads. This paper integrates a large volume of work on a number of measures, or metrics, developed for using phytoplankton to assess the ecological status of European lakes, as required for the Water Framework Directive. It assesses the indicator strength of these metrics, specifically in relation to representing the impacts of eutrophication. It also examines how these measures vary naturally at different locations within a lake, as well as between lakes, and how much variability is associated with different replicate samples, different months within a year and between years. On the basis of this analysis, three of the strongest metrics (chlorophyll-a, phytoplankton trophic index (PTI), and cyanobacterial biovolume) are recommended for use as robust measures for assessing the ecological quality of lakes in relation to nutrient-enrichment pressures and a minimum recommended sampling frequency is provided for these three metrics.     

Effect of chlorophyll sampling design on water quality assessment in thermally stratified lakes

In order to adequately assess the ecological status of thermally stratified lakes based on chlorophyll, the sampling must cover all productive layers of the water column. Missing the deep chlorophyll maxima (DCM) that often occur in the meta- or hypolimnion of transparent lakes supported by sufficient illumination and good nutrient availability may cause serious underestimation of the productivity and lead to misclassification of the lake ecological status. There is no commonly accepted sampling design for stratified lakes, and various monitoring guides suggest controversial designs. Our aim was to find some robust criteria to assess the probability of occurrence of a DCM and estimate the differences in measured mean chlorophyll concentrations caused by various sampling designs. Our theoretical model showed that the probability of occurrence of a DCM increases with increasing water transparency and decreasing lake size. Empirical data from Italian and Estonian stratified lakes confirmed the results. Testing of different sampling designs on lakes with full measured chlorophyll profiles available showed that taking only surface layer samples will lead with a high probability to an underestimation of the chlorophyll concentration in the trophogenic layer. In order not to miss the Chl peak in stratified lakes, in most cases it would be more precautious not to limit the sampling to the well-mixed epilimnion but to extend it to the whole euphotic layer. Sampling the epilimnion instead of the euphotic zone could cause up to a 70% underestimation of the chlorophyll concentration, an error that would cause a misclassification of the lake by one or even two status classes in a 5-class assessment system. In most cases, the 2.5 * Secchi depths proved a suitable criterion of the sampling depth and only in the case of surface scums, would sampling of a 3 * Secchi depth layer be recommended in order not to miss the deep chlorophyll maximum.     

Growth rate of alpine phytoplankton assemblages from contrasting watersheds and N‐deposition regimes exposed to nitrogen and phosphorus enrichments

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Biological indicators track differential responses of pelagic and littoral areas to nutrient load reductions in German lakes

External nutrient loading was reduced over the past decades as a measure for improving the water quality of eutrophic lakes in western Europe, and has since been accelerated by the adoption of the European Water Framework Directive (WFD) in 2000 (EC, 2000). A variety of eutrophication-related metrics have indicated that the response of biological communities to this decreased nutrient loading has been diverse. Phytoplankton, a major component of the pelagic community, often responded rapidly, whereas a significant delay was observed for submerged macrophytes colonizing littoral areas. In this study we tested whether assessment methods developed for phytoplankton and macrophytes in lakes during Germany's implementation of the WFD reflect this differential response. An assessment of 263 German lakes confirmed that a lower ecological state was recorded when based on the biological quality element (BQE) for macrophytes than the BQE for phytoplankton during the investigated period (2003–2012). On average, lakes had a moderate ecological status for both phytoplankton and macrophyte BQEs, but differences of up to three classes were observed in single cases. Long-term data were available for five lowland lakes subject to strong reductions in phosphorus loading. Their phytoplankton-based assessments indicated a constant improvement of the ecological status in parallel to decreasing water phosphorus concentrations. In contrast, macrophyte-based assessments indicated a 10–20 year delay in their ecological recovery following nutrient load reduction. This delay was confirmed by detailed data on the temporal development of macrophyte species diversity and maximum colonization depths of two lakes after nutrient load reduction. We conclude that the available WFD assessment methods for phytoplankton and macrophyte BQEs are suitable to track the differential response of pelagic and littoral areas to nutrient load reductions in German lakes.     

Macrofaunal community types in ponds and small lakes (Overijssel,The Netherlands)

A survey was carried out at 157 sites, situated in pools and small lakes in the province of Overijssel (The Netherlands), to describe the macro-invertebrate community and their environment. A total of 61 environmental viriables was measured at each sampling site. The main aim was to described a regional ecological typology of ponds and small lakes. Multivariate analysis techniques are appropriate in data analysis for typological purposes. Different multivariate analysis techniques (FLEXCLUS, NODES, DCCA, PCA) were used in combination with ecological information on individual taxa to derive and describe site groups in terms of taxon composition and mean environmental conditions. The resulting site groups were termed cenotypes. Nine cenotypes were distinguished among the ponds and small lakes. The main differences between the cenotypes were related to duration of drought, acidity, morphology and nutrient load. In particular, the four cenotypes within the group of stagnant, pH-neutral ponds/lakes showed an overlap in taxon composition. These cenotypes represent a web-shaped continuum dominated by dimensions (relation of width to depth), nutrient load, and bottom composition (especially mesotrophic peat). The most important anthropogenic processes are acidification, eutrophication, and changes in the original hydrology.     

OPINION Endless summer: internal loading processes dominate nutrient cycling in tropical lakes

SUMMARY. 1. Fossil diatom assemblages deposited in more than a dozen African lakes roughly 9500 years BP were dominated by a single planktonic species, Stephanodiscus astraea (Ehrcnb.) Grun. (although realistically this is likely to be a species complex). These diatoms flourished when lake-levels were maximal. Data are included from many of (he large African lakes, and others extending from Lake Abhé0, Ethiopia, to Lake Cheshi, Zambia.
2. Because the ecological physiology of Stephanodiscus species is well known one can predict the nutrient regime that must have existed when Stephanodiscus bloomed. Owing to competition for resources Stephano-discus species dominate when the supply ratio of silicon to phosphorus (in moles) in the epilimnion is relatively low (Si:P∼1). Consequently, lakes dominated by S. astraea are often hypereutrophic.
3. We propose a series of hypotheses to explain why tropical lakes have decreasing Si:P ratios as lake-levels increase, primarily owing to internal P-loading processes in the epilimnia. These observations appear to contradict present conceptions of the fundamental relationships governing nutrient loadings to and within lakes. Tropical lakes appear to have had increasing epilimnetic phosphorus loading as lake-levels increased. In contrast, large, deep lakes in the temperate zone are usually oligotrophic, with high Si:P ratios.
4. Our major conclusion is that regeneration rates are greater than removal rates for phosphorus in tropical lakes as compared to temperate lakes, especially where epilimnelic mixing exceeds 50 m. Biological control of the elemental cycles dominate in tropical lakes, whereas nutrient cycles in temperate lakes are dominated by physical processes for a large part of the year. This results in major differences in the fundamental mechanisms of nutrient regeneration and their relationships to morphometric features of lakes in the two regions.     

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of ¹⁴C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of ¹⁴C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.     

Opportunities and challenges with transitioning to non-lethal sampling of wild fish for microbiome research

The microbial communities of fish are considered an integral part of maintaining the overall health and fitness of their host. Research has shown that resident microbes reside on various mucosal surfaces, such as the gills, skin, and gastrointestinal tract, and play a key role in various host functions, including digestion, immunity, and disease resistance. A second, more transient group of microbes reside in the digesta, or feces, and are primarily influenced by environmental factors such as the host diet. The vast majority of fish microbiome research currently uses lethal sampling to analyse any one of these mucosal and/or digesta microbial communities. The present paper discusses the various opportunities that non-lethal microbiome sampling offers, as well as some inherent challenges, with the ultimate goal of creating a sound argument for future researchers to transition to non-lethal sampling of wild fish in microbiome research. Doing so will reduce animal welfare and population impacts on fish while creating novel opportunities to link host microbial communities to an individual's behavior and survival across space and time (e.g., life-stages, seasons). Current lethal sampling efforts constrain our ability to understand the mechanistic ecological consequences of variation in microbiome communities in the wild. Transitioning to non-lethal sampling will open new frontiers in ecological and microbial research.     

Ecological classification of lakes: Uncertainty and the influence of year-to-year variability

Regular monitoring of lakes is important to determine their ecological state and development and of key significance when deciding whether action should be taken to improve their quality, for instance by reducing the external loading of nutrients. Imprecise or inadequate knowledge of the ecological state increases the risk of misclassification and of wrong management decisions. Based on Danish lake data, we aimed to determine temporal variations, in particular natural year-to-year differences, and to describe the uncertainty in assessing the ecological state of lakes. We analysed environmental data from ca. 350 Danish lakes (1100 lake years), including three case studies, with long-term data series (up to 24 years), with no significant changes in external nutrient loading. We used summer means of selected water chemical variables, phytoplankton and submerged macrophytes as indicators of ecological state and found considerable variations in all indicators, which could not be ascribed alone to meteorological variation. In shallow lakes, chlorophyll a concentrations exhibited large year-to-year variations, especially at TP ranging between 0.05 and 0.15 mg L⁻¹ where the lakes may shift between a macrophyte- and a phytoplankton-dominated state. For example, chlorophyll a varied by a factor 5–10 between years and was particularly low when submerged macrophyte coverage exceeded 20% compared with lakes without macrophytes. Use of a multimetric index including four phytoplankton indicators reduced the coefficient of variation. Generally, the 95% confidence interval of ecological classification was approximately 50% lower when the assessment of ecological state was based on 4–5 years’ measurements than if based on only one year's measurements. Knowledge and awareness of the uncertainty of indicators used in ecological classification are highly relevant for lake managers and policy makers when defining efficient monitoring and restoration strategies.     

Age and area predict patterns of species richness in pumice rafts contingent on oceanic climatic zone encountered

The theory of island biogeography predicts that area and age explain species richness patterns (or alpha diversity) in insular habitats. Using a unique natural phenomenon, pumice rafting, we measured the influence of area, age, and oceanic climate on patterns of species richness. Pumice rafts are formed simultaneously when submarine volcanoes erupt, the pumice clasts breakup irregularly, forming irregularly shaped pumice stones which while floating through the ocean are colonized by marine biota. We analyze two eruption events and more than 5,000 pumice clasts collected from 29 sites and three climatic zones. Overall, the older and larger pumice clasts held more species. Pumice clasts arriving in tropical and subtropical climates showed this same trend, where in temperate locations species richness (alpha diversity) increased with area but decreased with age. Beta diversity analysis of the communities forming on pumice clasts that arrived in different climatic zones showed that tropical and subtropical clasts transported similar communities, while species composition on temperate clasts differed significantly from both tropical and subtropical arrivals. Using these thousands of insular habitats, we find strong evidence that area and age but also climatic conditions predict the fundamental dynamics of species richness colonizing pumice clasts.     

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with ¹³C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.     

Addressing the trophic status in urban ponds: An evaluation of current trophic state indexes

Ponds are generally understudied. Quality problems in urban freshwaters can arise from eutrophication, and trophic status classification gives information related to their ecological situation. Ponds may strongly differ from larger lakes for which most current trophic status classification methods have been developed. This paper seeks to compare and contrast the suitability of six trophic status indexes (TSI) and OECD trophic classifications system currently used to evaluate the eutrophication level of urban ponds in the Subtropical region. The trophic status was evaluated based on total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN) and phytoplankton chlorophyll a (Chl a) in 12 ponds placed in different cities from the Pampean region (Argentina), in the warm and cold seasons. Our results demonstrate that there was no relationship between pond size (0.08–2.45 ha) and trophic status. TSIs estimated with Chl a showed significant differences between seasons and the range oligotrophic-hypertrophic of trophism was encountered. Conversely, TSIs estimated with nutrient concentrations (TP, SRP, TN) did not. The pond's classification in trophic levels fell in the eutrophic-hypertrophic extreme, and only considering TN/TP broadened the trophic range. No relationship was encountered between Chl a and P; however, Chl a and TN/TP were positively correlated during the warm season. Lower trophic levels regarding TSI (Chl a) were generally associated with a dense floating-macrophyte cover. We suggest that the TSI considered should correspond to the latitudinal region in which the ponds are located. Our results indicate that it appears restrictive to consider only P contents in ponds to assess eutrophication. Under the proposed framework, the key issues for the study of pond trophic classification will be to include not only P but N concentrations and TN/TP. Also, phytoplankton Chl a concentration jointly with floating macrophytes biomass/coverage should be considered as diagnosis parameters.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

Patterns in freshwater diatom taxonomic distinctness along an eutrophication gradient

1. A variety of species richness measures have been used to assess the effects of environmental degradation on biodiversity. Such measures can be highly influenced by sample size, sampling effort, habitat type or complexity, however, and typically do not show monotonic responses to human impact. In addition to being independent of the degree of sampling effort involved in data acquisition, effective measures of biodiversity should reflect the degree of taxonomical relatedness among species within ecological assemblages and provide a basis for understanding observed diversity for a particular habitat type. Taxonomic diversity or distinctness indices emphasize the average taxonomic relatedness (i.e. degree of taxonomical closeness) between species in a community. 2. Eutrophication of freshwater ecosystems, mainly due to the increased availability of nutrients, notably phosphorus, has become a major environmental problem. Two measures of taxonomic distinctness (Average Taxonomic Distinctness and Variation in Taxonomic Distinctness) were applied to surface sediment diatoms from 45 lakes across the island of Ireland to examine whether taxonomic distinctness and nutrient enrichment were significantly related at a regional scale. The lakes span a range of concentrations of epilimnic total phosphorus (TP) and were grouped into six different types, based on depth and alkalinity levels, and three different categories according to trophic state (ultra‐oligotrophic and oligotrophic; mesotrophic; and eutrophic and hyper‐eutrophic). 3. The taxonomic distinctness measures revealed significant differences among lakes in the three different classes of trophic state, with nutrient‐rich lakes generally more taxonomically diverse than nutrient‐poor lakes. This implies that enrichment of oligotrophic lakes does not necessarily lead to a reduction in taxonomic diversity, at least as expressed by the indices used here. Furthermore, taxonomic distinctness was highly variable across the six different lake types regardless of nutrient level. 4. Results indicate that habitat availability and physical structure within the study lakes also exert a strong influence on the pattern of taxonomic diversity. Overall the results highlight problems with the use of taxonomic diversity measures for detecting impacts of freshwater eutrophication based on diatom assemblages.     

Detritivory and the stoichiometry of nutrient cycling by a dominant fish species in lakes of varying productivity

Little is known about the stoichiometry of nutrient cycling by detritivores. Therefore, we explored stoichiometric relationships in an omnivorous/detritivorous fish (gizzard shad, Dorosoma cepedianum) in three lakes that differed in productivity. Gizzard shad can feed on plankton and sediment detritus, but in all three lakes adult gizzard shad derived >98% of carbon (C) and phosphorus (P), and >90% of nitrogen (N) from sediment detritus, and the remainder from zooplankton.
Gizzard shad selectively consumed detritus with higher C, N and P concentrations than ambient lake sediments. Selective detritivory (i.e. the nutrient content of consumed detritus divided by the nutrient content of ambient detritus) was most pronounced in the lake with the lowest detrital nutrient concentrations. N and P cycling rates per fish were also consistently higher in this lake, in agreement with the prediction of stoichiometry theory that excretion rates should increase with food nutrient content. Among-lake differences in nutrient cycling rates were unrelated to inter-lake variation in fish body nutrient contents, which was minimal. The N:P ratio excreted was near Redfield (∼14:1) in all three lakes.
Stoichiometric analyses showed that the C:N and C:P ratios of sediment detritus were much higher (∼2.8×) than ratios of gizzard shad bodies, revealing substantial N and P imbalances between consumers and their food source. Gizzard shad alleviate N imbalance by selectively feeding on high N detritus (low C:N, high N:P), and apparently alleviate P imbalance by excreting nutrients at a higher N:P than that of their food or their bodies. Thus, this detritivore apparently regulates nutrient acquisition and allocation via both pre-absorption processes (selective feeding) and post-absorptive processes (differential N and P excretion).