Which factors affect phytoplankton biomass in shallow eutrophic lakes?

The restoration and management of shallow, pond-like systems are hindered by limitations in the applicability of the well-known models describing the relationship between nutrients and lake phytoplankton biomass in higher ranges of nutrient concentration. Trophic models for naturally eutrophic small, shallow, endorheic lakes have not yet been developed, even though these are the most frequent standing waters in continental lowlands. The aim of this study was to identify variables that can be considered as main drivers of phytoplankton biomass and to build a predictive model. The influence of potential drivers of phytoplankton biomass (nutrients, other chemical variables, land use, lake use and lake depth) from 24 shallow eutrophic lakes was tested using data in the Pannonian ecoregion (Hungary and Romania). By incorporating lake depth, TP, TN and lake use as independent and Chl-a as dependent variables into different models (multiple regression model, GLM and multilayer perception model) predictive models were built. These models explained >50% of the variance. Although phytoplankton biomass in small, shallow, enriched lakes is strongly influenced by stochastic effects, our results suggest that phytoplankton biomass can be predicted by applying a multiple stressor approach, and that the model results can be used for management purposes.     

Rehabilitation of shallow lakes: time to adjust expectations?

We investigated the relative importance of flood defenses and other environmental variables for the cladoceran and copepod communities in floodplain water bodies in Southeastern Norway. The water bodies covered gradients of size, distance to the river and water chemistry, and half of them were located behind flood defenses. The effects of environmental variables on the communities were analyzed using redundancy analysis. The set of environmental variables accounted for more of the explained variation in the cladoceran community than in the copepod community. Water quality was much more important than flooding-related variables for both communities. Although cladoceran species richness was slightly higher in water bodies outside flood defenses, total nitrogen, total organic carbon (TOC), and water body area were the most important factors for the cladocerans. Macrophyte coverage was the most important variable for both species richness and community structure of copepods. Although our results show that water quality and spatial/structural variables are more important than flood defenses in structuring the communities, the effect of TOC on the cladoceran community could likely be mediated through effects of flood defense on TOC. Prospects for a continued anthropogenic pressure on river floodplains raise concern for the future of these unique ecosystems.     

Littoral macrophyte–periphyton complexes in two Hungarian shallow waters

Periphyton developing on the surfaces of emergent and submerged aquatic plants has a significant influence on water quality. The periphyton types that form on various plant species can be characterized by their mass values, the proportion of the present organic and inorganic fractions, as well as their chlorophyll-a contents. Studies on periphyton complexes constituting integrated biomonitoring systems are useful to gain essential long-term information about the performance of shallow water bodies. The filtering and settling effect of Phragmites and other aquatic plants, as well as their periphyton was examined and clearly observable in the water areas and non-flooded aquatic habitats belonging to the second phase of Kis-Balaton Protection System, as it was indicated by the mass values and ash contents. The periphyton forming on the aquatic vegetation that annually develops in Kisköre Reservoir and yields a considerable biomass has a critical part in influencing water quality. The only difference (p<0.05) was found in the ash content of the periphyton, being lower in Kis-Balaton (48.64 ± 2.29 S.E., %) and higher in Kisköre Reservoir (57.42 ± 2.54 S.E., %). This paper presents the dry mass of the periphyton, as well as its ash and chlorophyll-a content, and the results obtained on the composition of the alga species of the periphyton.

     

Is UV-B radiation affecting charophycean algae in shallow freshwater systems?

The objective of this study was to determine the effects of UV-B radiation on charophycean algae under natural conditions, since charophytes enhance water transparency in freshwater systems and levels of UV-B radiation have increased by ozone depletion. Potential and actual UV-B effects were studied by combining a glasshouse experiment in which plants were exposed to various levels of UV-B radiation and field measurements in two freshwater systems dominated by charophytes in the Netherlands. The glasshouse experiment showed that charophytes were sensitive to UV-B radiation. UV-B radiation negatively affected growth, while it increased levels of DNA damage in Chara aspera. Moreover, the charophytes did not seem to develop UV-B screens to protect against UV-B radiation since no increase in UV-B absorbing compounds was found. At field conditions, both spectroradiometrical measurements and DNA dosimeters showed that UV-B radiation was attenuated quickly in both freshwater systems, indicating that UV-B does not reach the submerged charophyte vegetation. However, specific conditions, like fluctuating water tables, may result in UV-B exposure to charophytes for certain periods annually.     

Do fish regulate phytoplankton in shallow eutrophic Northeast Brazilian reservoirs?

SUMMARY 1. In a comparative study, we examined the potential for fish to structure planktonic food webs in shallow mesotrophic to hypereutrophic Northeast Brazilian reservoirs. The food webs were dominated by three guilds of fish (facultative piscivores, generalist planktivores and omnivores), small herbivorous zooplankton and bloom‐forming cyanobacteria, with few littoral macrophytes. 2. A principal component's analysis on data from 13 reservoirs (27 sampling dates in 1995–99) revealed that euphotic depth, the relative density of phytoplankton (i.e. the percentage of overall phytoplankton density) represented by cyanobacteria, and the relative biomass of fish (i.e. percentage of overall biomass) represented by omnivores and facultative piscivores, explained most of the variance in the data. Physico‐chemical conditions, lake morphometry and rainfall were secondary factors. 3. Phytoplankton was related to fish guild structure. Chlorophyll concentration increased with total phosphorus and the relative biomass of omnivorous fish, decreased with the relative biomass of facultative piscivores, but was unrelated to the biomass and mean body size of herbivorous zooplankton. Chlorophyll concentration and the densities of filamentous and colonial cyanobacteria decreased with the ratio of the biomass of facultative piscivores to that of omnivores (FP : OM). 4. We propose two complementary mechanisms for the observed relationships between fish and phytoplankton. At a low biomass of facultative piscivores, juvenile zooplanktivorous fishes may induce a trophic cascade on zooplankton in the littoral zone. Regardless of piscivore biomass, piscivores and omnivores may regulate phytoplankton via multichannel omnivory because of the predominance of omnivorous or detritivorous foraging behaviour. 5. Manipulative experiments are needed to explore further whether, depending on priorities in the use of the reservoir, fisheries management could alter the FP : OM ratio either to enhance fish yields or to reduce phytoplankton densities and cyanobacterial blooms.     

30 years' eutrophication in shallow brackish waters – lessons to be learned

The anthropogenically induced eutrophication of theDarss-Zingst bodden chain, a tidelessshallow brackish water, has been followed over a period ofmore than 30 years. The wholeprocess was dominated by increasing nutrient loads and irregular exchange processes withthe Baltic Sea.Dramatic changes started between 1964/72 in the western partsof the system, which ismore influenced by nutrient loads and less connected to theexchange with the Baltic Sea.The first signs of increased anthropogenic loading were amassive reduction in submergedmacrophytes.10 to 15 years later the same processes occurred in a slightlymodified form in the easternparts of the bodden chain.The whole eutrophication process could be followed in moredetail in the Barther Bodden.Since 1968 there have been:– no changes in soluble reactive phosphate– increases in the spring values of nitrate– stepwise changes in the community composition leading to aremarkable increase of theimportance of the microbial food web.The influences and consequences of eutrophication on thephytoplankton, bacterioplankton,protozooplankton and metazooplankton are generalized.The first signs for remesotrophication have been observed inthe last two years. This revised version was published online in July 2006 with corrections to the Cover Date.     

Does metazooplankton regulate the ciliate community in a shallow eutrophic lake?

1. As grazers on picoplankton and nanoplankton, planktonic ciliates form an important link in pelagic food webs. Ciliate communities may be controlled by predation by metazooplankton. In eutrophic systems, however, where the number of large crustaceans is often low, the mechanisms that regulate ciliate dynamics have rarely been described. 2. We conducted an enclosure experiment with natural and screened (145 μm) summer plankton communities to investigate the effect of the small‐sized crustacean zooplankton on ciliate community structure and the microbial loop in a shallow eutrophic lake. 3. The removal of the larger fraction of crustaceans initiated a decrease in total ciliate abundance. At the community level, we observed a substantial increase in large‐sized predacious ciliates (>100 μm) and a simultaneous decrease in the abundance of smaller ciliates (<20–40 μm) that were mostly bacterivores and bacterio‐herbivores. The compositional shift in the ciliate community, however, did not cascade down to the level of bacteria and edible phytoplankton.     

Do steady state assemblages occur in shallow lentic environments from wetlands?

The Okavango Delta has an annual flood cycle that spreads slowly from the northwestern`panhandle' into a widening delta to the southeast. Rotifers were sampled from areas where the flood was active, and from other areas not yet reached by the flood. In still waters, the samples were dominated by euplanktonic rotifers, particularly of the genera Brachionus, Keratella and Hexarthra, with about 20 species per 1000 individuals at a station. Where the water was moving the samples were dominated by periphytic rotifers, with over 100 species per 1000 individuals at a single station. The periphytic species were dominated by the genus Lecane,with over 45 species, followed by Lepadellawith 24 species, and Trichocercawith 23 species. Comparison of the Okavango with six other tropical localities reveals fairly consistent ratios of the numbers of species in the periphytic genera Lecane, Lepadellaand Trichocerca,indicating that there may be assembly rules for communities of tropical periphytic rotifers.     

Can allelopathically active submerged macrophytes stabilise clear-water states in shallow lakes?

Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is supposed to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. The relevance of this process at ecosystem level, however, is debated because in situ evidence is difficult to achieve. Our literature review indicates that allelopathically active species such as Myriophyllum, Ceratophyllum, Elodea and Najas or certain charophytes are among the most frequent submerged macrophytes in temperate shallow lakes. The most common experimental approach for allelopathic interference between macrophytes and phytoplankton has been the use of plant extracts or purified plant compounds. Final evidence, however, requires combination with more realistic in situ experiments. Such investigations have successfully been performed with selected species. In situ allelopathic activity is also influenced by the fact that phytoplankton species exhibit differential sensitivity against allelochemicals both between and within major taxonomic groups such as diatoms, cyanobacteria and chlorophytes. In general, epiphytic species apparently are less sensitive towards allelochemicals than phytoplankton despite living closely attached to the plants and being of key importance for macrophyte growth due to their shading. Light and nutrient availability potentially influence the sensitivity of target algae and cyanobacteria. Whether or not additional stressors such as nutrient limitation enhance or dampen allelopathic interactions still has to be clarified. We strongly propose allelopathy as an important mechanism in the interaction between submerged macrophytes and phytoplankton in shallow lakes based on the frequent occurrence of active species and the knowledge of potential target species. The role of allelopathy interfering with epiphyton development is less well understood. Including further levels of complexity, such as nutrient interference, grazing and climate, will extend this ecosystem-based view of in situ allelopathy.     

Chara beds acting as nutrient sinks in shallow lakes—a review

The capability of Chara beds to act as nutrient sinks in shallow lakes is reviewed. Under favorable conditions charophytes form dense meadows. Biomass and nutrient content in such beds are comparable or even higher than in beds of vascular aquatic macrophytes. As some Chara species are capable of overwintering, the nutrient storage in plant biomass may extend beyond the growing season. Some commonly observed phenomena in vascular plants (nutrient uptake and mobilization of nutrients from the sediment) appear to be unlikely or negligible in Characeae. Charophytes have been reported to decompose slower than their vascular counterparts prolonging nutrient storage in plant biomass.Charophytes may also indirectly affect nutrient cycling in lakes. Utilization of bicarbonate is accompanied by precipitation of calcite during periods of intensive photosynthesis, favoring immobilization of P by binding in the crystal structure or sorption on sedimenting mineral particles. Charophytes are able to deliver oxygen to the sediment, thus potentially enhancing nitrification/denitrification processes and preventing iron-bound sediment phosphorus from being released to the overlying water. Furthermore, dense Chara meadows restrict sediment resuspension, consequently blocking an important internal source of nutrients to planktonic algae. We conclude that Chara meadows probably are an efficient nutrient trap in shallow lakes.     

Does roach behaviour differ between shallow lakes of different environmental state?

The diel activity levels and spatial distribution of roach Rutilus rutilus differed markedly between two shallow lakes of different environmental state. The movements of roach (12–25 cm L _T), with surgically implanted mini‐radio transmitters, were monitored regularly during several 48 h tracking sessions in a clearwater and in a turbid lake. In both lakes, the roach in general were most active during dawn and dusk and least active during the night. Activity level in midsummer was lowest around noon in the clear lake and high around noon in the turbid lake. In summer, roach in the clear lake stayed passively in a restricted area of water lilies during the day and moved into the central part of the lake during the night. In the turbid lake, roach were dispersed all over the lake during the day and moved close to the shoreline at night. Predator : prey fish ratios did not differ in the two lakes, however the observed behaviour of roach in the clearwater lake may be explained by a larger predation pressure from fish and birds, both being favoured in the clear water.     

Is roach herbivory preventing re-colonization of submerged macrophytes in a shallow lake?

Hydrobiologia - Re-establishing a stable submerged vegetation is considered an important tool to restore shallow eutrophic lakes. Enhanced turbidity and sediment re-suspension as well as grazing by...     

Ediacaran–Cambrian bioturbation did not extensively oxygenate sediments in shallow marine ecosystems

The radiation of bioturbation during the Ediacaran–Cambrian transition has long been hypothesized to have oxygenated sediments, triggering an expansion of the habitable benthic zone and promoting increased infaunal tiering in early Paleozoic benthic communities. However, the effects of bioturbation on sediment oxygen are underexplored with respect to the importance of biomixing and bioirrigation, two bioturbation processes which can have opposite effects on sediment redox chemistry. We categorized trace fossils from the Ediacaran and Terreneuvian as biomixing or bioirrigation fossils and integrated sedimentological proxies for bioturbation intensity with biogeochemical modeling to simulate oxygen penetration depths through the Ediacaran–Cambrian transition. Ultimately, we find that despite dramatic increases in ichnodiversity in the Terreneuvian, biomixing remains the dominant bioturbation behavior, and in contrast to traditional assumptions, Ediacaran–Cambrian bioturbation was unlikely to have resulted in extensive oxygenation of shallow marine sediments globally.     

Isolation of 2-μm minichromosomes fromSaccharomyces cerevisiae using shallow metrizamide gradients

Unfixed 2-μm minichromosomes fromSaccharomyces cerevisiae may be isolated uncontaminated by genomic DNA by banding isopycnically in metrizamide [2-(3-acetamido-5-N-methylacetamido-2,4,6-triiodobenzamido)-2-deoxy-d-glucose[ according to the protein/nucleic acid ratio. Two density components are found in A364A cells; one apparently contains ribosomes and genomic chromatin fragments (ϱ = 1.26 g/cm³) and a lighter component (ϱ = 1.24 g/cm³) contains pure 2-μm DNA. The lighter component is not found in (cir⁰) cells.     

World-wide whale worms? A new species of Osedax from the shallow north Atlantic

We describe a new species of the remarkable whalebone-eating siboglinid worm genus, Osedax, from a whale carcass in the shallow north Atlantic, west of Sweden. Previously only recorded from deep-sea (1500-3000m) whale-falls in the northeast Pacific, this is the first species of Osedax known from a shelf-depth whale-fall, and the first from the Atlantic Ocean. The new species, Osedax mucofloris sp. n. is abundant on the bones of an experimentally implanted Minke whale carcass (Balaenoptera acutorostrata) at 125m depth in the shallow North Sea. O. mucofloris can be cultured on bones maintained in aquaria. The presence of O. mucofloris in the shallow North Sea and northeast Pacific suggests global distribution on whale-falls for the Osedax clade. Molecular evidence from mitochondrial cytochrome oxidase 1 (CO1) and 18S rRNA sequences suggests that O. mucofloris has high dispersal rates, and provides support for the idea of whale-falls acting as 'stepping-stones' for the global dispersal of siboglinid annelids over ecological and evolutionary time.     

Modelling sediment transport in shallow lakes — interactions between sediment transport and sediment composition

In shallow, wind exposed lakes, the light conditions, the cycling of nutrients, heavy metals and organic micro-pollutants and changes in the local composition of the sediment top layer can be dominated by resuspension/erosion of bottom sediment and sedimentation of suspended solids. A 2 dimensional model for Sediment Transport, Resuspension and Sedimentation in Shallow lakes (STRESS-2d), based on an existing transport model, is discussed. In the model, mass balance equations for the water compartment and the bottom sediment are solved numerically. Up to 7 sediment fractions can be taken into account, each having a specific set of resuspension/erosion and sedimentation parameter values. Several options for modelling the changes in the bottom sediment composition are available.A simulation experiment for Lake Veluwe (The Netherlands), in which model options with and without the distinction of sediment fractions were used, showed that using sediment fractions to account for the variability in the sediment composition leads to an improvement of the model results, particularly the simulated phosphorus sediment-water exchange fluxes. For Lake Ketel (The Netherlands) two options for modelling changes in the bottom sediment composition are compared. It is shown that an option in which a thin water-sediment layer on top of the more consolidated bottom sediment is simulated provides an improvement in the simulation of the suspended solids concentration.     

Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m^–2 d^–1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l^–1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m^–2. The internal P loading induced by resuspension is estimated to be 60–70 mg m^–2 d^–1, or 20–30 times greater than the release from undisturbed sediment.SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

Spatial and temporal diversity of small shallow waters in river Lužnice floodplain

To propose a concept of their mutual diversity, twenty-nine permanent shallow floodplain pools and oxbows in the river Lužnice floodplain were analysed for area, depth, shape, flooding, and shading by terrestrial vegetation, and sampled in all seasons for their water chemistry, phytoplankton composition and biomass, and zooplankton composition. The sites are regularly flooded, eutrophic, and often shaded by surrounding vegetation. Cryptophyceae, Chrysophyceae and Euglenophyceae dominated the phytoplankton, while Cyanophytes were rare. Within the rich zooplankton assemblage (63 species), cladocerans and rotifers dominated. Correlation matrices and multivariate analyses indicated that shaded and relatively deeper sites had lower oxygen saturation and higher concentrations of PO₄–P and NH₄–N. Shade and relative depth correlated negatively with phytoplankton biomass and number of phytoplankton taxa, and positively with Cryptophytes and large cladocerans—thus indicating poor mixing, poor light availability and low fish pressure on herbivores. Decomposition of leaf litter increased oxygen consumption, while shade from terrestrial vegetation restricted photosynthesis and decreased oxygen production. Larger sites were more species-rich in phytoplankton and supported Euglenophyceae, green algae and rotifers.     

Does sediment resuspension have persistent effects on phytoplankton? Experimental studies in three shallow lakes

1. The persistence of effects of sediment resuspension on chlorophyll a , phytoplankton production and plant nutrients was examined by artificially resuspending sediment in enclosures in three shallow lakes and monitoring concentrations for 4–8 days. Realism was assessed by relating initial suspended solids concentrations to those observed during natural wind events.
2. Positive effects on the phytoplankton, persisting for at least 4–6 days were detected in eight of the nine experiments, and similar effects on nutrient concentrations were also common, although suspended solids had normally returned to control levels within 24 h
3. The phytoplankton during the periods of persistence was normally dominated by planktonic rather than benthic/meroplanktonic genera.
4. Several of the positive responses appeared to be associated with relief of nitrogen deficiency in the algae.
5. Persistent effects from previous resuspension events may cause baseline concentrations of phytoplankton and nutrients in shallow lakes to be overestimated and the effects of resuspension on phytoplankton and nutrients to be underestimated