Irmgard Blindow

Trophic web structure in a shallow eutrophic lake during a dominance shift from phytoplankton to submerged macrophytes

In Lake Krankesjön, southern Sweden, sago pondweed (Potamogeton pectinatus L.) and a stonewort (Chara tomentosa L.) expanded spatially during the second half of the 1980s after more than a decade of phytoplankton blooms and sparse submerged vegetation. During the expansion of submerged plants the number of resting and breeding waterfowl increased. The increase was significant for herbivorous birds such as coot (Fulica atra L.) and mute swan (Cygnus olor (Gmelin)), but also for omnivorous dabbling ducks. The shift from phytoplankton to submerged macrophytes caused structural changes on higher trophic levels, and an altered trophic web developed. The density of planktonic Cladocera decreased, which is suggested to be a result of decreased phytoplankton productivity and biomass as nutrient levels dropped. The benthic macroinvertebrate assemblage changed from low diversity and biomass dominated by Chironomidae and Oligochaeta on bare sediment, to high diversity and biomass characterized by plant-associated forms like snails and isopods in areas covered by macrovegetation. The mean size of perch (Perca fluviatilis L.) increased, probably as a result of higher availability of macroinvertebrates in the vegetation. The perch reached a mean size where the species is known to shift to a fish diet, permitting an increased top down effect on the ecosystem. The results support the idea that shallow eutrophic lakes can shift between two states, each one stabilized by feed-back mechanisms including both biotic and abiotic factors. Shifts between these states are suggested to be a possible explanation for observed drastic changes in abundance of waterfowl in shallow eutrophic lakes.

The composition and density of epiphyton on several species of submerged macrophytes — the neutral substrate hypothesis tested

Abstract Epiphyton samples same taken from different macrophyte species (three chararaceans Potamogeton pectinatus L.). While some epiphyton taxa were evenly distributed, the density of others differed according to plant part, site or macrophyte species. Diatoms were classified according to host plant species by cluster analysis. Thus, the neutral substrate hypothesis is revised. Differences in epiphyton composition were larger between the closely related species Chara tomentosa L. and C. globularis Thuill. than between Chara tomentosa and Nitellopsis obtusa (Desv.) J. Groves. The latter two species were heavily marl-encrusted. Potamogeton pectinatus was separated from the other macrophyte species by its lower total density of epiphyton and the high abundance of Cocconeis placentula Ehr.

Epiphytic bacterial community composition on two common submerged macrophytes in brackish water and freshwater

BackgroundPlants and their heterotrophic bacterial biofilm communities possibly strongly interact, especially in aquatic systems. We aimed to ascertain whether different macrophytes or their habitats determine bacterial community composition. We compared the composition of epiphytic bacteria on two common aquatic macrophytes, the macroalga Chara aspera Willd. and the angiosperm Myriophyllum spicatum L., in two habitats, freshwater (Lake Constance) and brackish water (Schaproder Bodden), using fluorescence in situ hybridization. The bacterial community composition was analysed based on habitat, plant species, and plant part.ResultsThe bacterial abundance was higher on plants from brackish water [5.3 × 107 cells (g dry mass)-1] than on plants from freshwater [1.3 × 107 cells (g dry mass)-1], with older shoots having a higher abundance. The organic content of freshwater plants was lower than that of brackish water plants (35 vs. 58%), and lower in C. aspera than in M. spicatum (41 vs. 52%). The content of nutrients, chlorophyll, total phenolic compounds, and anthocyanin differed in the plants and habitats. Especially the content of total phenolic compounds and anthocyanin was higher in M. spicatum, and in general higher in the freshwater than in the brackish water habitat. Members of the Cytophaga-Flavobacteria-Bacteroidetes group were abundant in all samples (5–35% of the total cell counts) and were especially dominant in M. spicatum samples. Alphaproteobacteria were the second major group (3–17% of the total cell counts). Betaproteobacteria, gammaproteobacteria, and actinomycetes were present in all samples (5 or 10% of the total cell counts). Planctomycetes were almost absent on M. spicatum in freshwater, but present on C. aspera in freshwater and on both plants in brackish water.ConclusionBacterial biofilm communities on the surface of aquatic plants might be influenced by the host plant and environmental factors. Distinct plant species, plant part and habitat specific differences in total cell counts and two bacterial groups (CFB, planctomycetes) support the combined impact of substrate (plant) and habitat on epiphytic bacterial community composition. The presence of polyphenols might explain the distinct bacterial community on freshwater M. spicatum compared to that of M. spicatum in brackish water and of C. aspera in both habitats.

Even low to medium nitrogen deposition impacts vegetation of dry, coastal dunes around the Baltic Sea.

Coastal dunes around the Baltic Sea have received small amounts of atmospheric nitrogen and are rather pristine ecosystems in this respect. In 19 investigated dune sites the atmospheric wet nitrogen deposition is 3-8kg Nha(-1)yr(-1). The nitrogen content of Cladonia portentosa appeared to be a suitable biomonitor of these low to medium deposition levels. Comparison with EMEP-deposition data showed that Cladonia reflects the deposition history of the last 3-6 years. With increasing nitrogen load, we observed a shift from lichen-rich short grass vegetation towards species-poor vegetation dominated by the tall graminoid Carex arenaria. Plant species richness per field site, however, does not decrease directly with these low to medium N deposition loads, but with change in vegetation composition. Critical loads for acidic, dry coastal dunes might be lower than previously thought, in the range of 4-6kg Nha(-1)yr(-1) wet deposition.

Long-term patterns of shifts between clear and turbid states in Lake Krankesjon and Lake Takern

During the past century, Lake Tåkern and Lake Krankesjön, southern Sweden, have shifted repeatedly between a state of clear water and abundant submerged vegetation, and a state of turbid water and sparse vegetation. Long-term empirical data on such apparently alternative stable state dynamics are valuable as complements to modeling and experiments, although the causal mechanisms behind shifts are often difficult to identify in hindsight. Here, we summarize previous studies and discuss possible mechanisms behind the shifts. The most detailed information comes from monitoring of two recent shifts, one in each lake. In the 1980s, L. Krankesjön shifted to clear water following an expansion of sago pondweed, Potamogeton pectinatus. Water clarity increased when the pondweed was replaced by characeans. Zooplankton biomass in summer declined and the concentration of total phosphorus (TP) was reduced to half the previous level. The fish community changed over several years, including an increasing recruitment of piscivorous perch (Perca fluviatilis). An opposite directed shift to turbid water occurred in Lake Tåkern in 1995, when biomass of phytoplankton increased in spring, at the expense of submerged vegetation. Consistent with the findings in L. Krankesjön, phyto- and zooplankton biomass increased and the average concentration of TP doubled. After the shift to clear water in L. Krankesjön, TP concentration has increased during the latest decade, supporting the idea that accumulation of nutrients may lead to a long-term destabilization of the clear water state. In L. Tåkern, data on TP are inconclusive, but organic nitrogen concentrations oscillated during a 25-year period of clear water. These observations indicate that intrinsic processes cause gradual or periodic changes in system stability, although we cannot exclude the possibility that external forces are also involved. During such phases of destabilization of the clear water state, even small disturbances could possibly trigger a shift, which may explain why causes behind shifts are hard to identify even when they occur during periods of extensive monitoring.

Shifts between clear and turbid states in a shallow lake: multi-causal stress from climate, nutrients and biotic interactions

We used long-term monitoring data to assess causes behind a recent shift front a clear to a turbid water state in Lake Takern, Sweden. The lake has a previous record of shifts between clear-water and turbid states. but the causes behind these shifts are not well known. During the recent shift, which occurred in 1995-1997, Submerged vegetation Subsequently declined after a 30-year period of clear-water and abundant vegetation. Among the possible explanations we identify several processes unlikely to have contributed to the recent shift from clear to turbid conditions. including long-term changes in external input of phosphorus. fluctuations in water level. and changes in zooplankton grazing pressure. Instead, likely scenarios to have contributed to the macrophyte decline, and thereby to the shift were: (1) a series of mild winters with short ice cover and absence of winter-kills of fish, leading to high biomasses of benthivorous and planktivorous fish before the shift, and thereby increased bioturbation and internal nutrient recycling, (2) unusually cool and windy springs the years before and during, the shift, leading to unfavourable conditions during the establishing phase of submerged macrophytes. Both shorter periods of ice cover and high wind velocity in winter and spring were associated with climate. approximated by the North Atlantic Oscillation (NAO). We argue that none of these processes alone can force the lake front the clear to the turbid state, but that several stress factors in concert are necessary to initiate a shift. Hence, we conclude that climate variability is likely to have contributed to a multi-causal stress. reducing the resilience of the clear-water state and finally triggering the Shift through inter-year dependent changes; in biomass of submerged macrophytes and fish, organism groups known to have key roles in the dynamics of shallow lakes. (Less)

Impacts of succession and grass encroachment on heathland Orthoptera

For the conservation of biodiversity, heathlands present important ecosystems throughout Europe. The formerly widespread habitats are nowadays restricted to small and isolated remnants. Without land use heathland vegetation undergoes succession and, in addition, the increasing amount of atmospheric nitrogen deposition has resulted in an encroachment of grasses. In the present study we analysed the effects of succession and grass encroachment on Orthoptera in a coastal heathland on the Baltic island of Hiddensee, Germany. Vegetation, microclimate, soil humidity and Orthoptera were sampled in the five main stages of heathland succession, namely grey dunes, dwarf-shrub heath, grassy heath, heath with shrubs, and birch forest. Vegetation and environmental parameters showed strong differences among the successional stages. Orthoptera species richness was highest in transitional stages. The high proportion of grasses offer favourable habitat conditions for graminivorous, chorto- and thamnobiont species. Orthoptera density was highest in grey dunes. Threatened and specialised species were restricted to the young stages grey dunes and dwarf-shrub heath. Hence, in order to maintain a high diversity of Orthoptera in heathlands, maintaining different successional stages is of critical importance and this should be integrated into heathland management practices.

Elongation and mat formation of Chara aspera under different light and salinity conditions

Former laboratory results indicate that shoot elongation at low light intensities of Chara aspera is absent already at 10 psu which is within the physiologically optimal salinity range for brackish water populations. To investigate if similar restrictions occur in the field, density and morphology of C. aspera were compared between three freshwater and three brackish water sites along its depth range.

Alternative Stable States in Shallow Lakes: What Causes a Shift?

Lake TE5;kern and Lake Krankesjon, two shallow, moderately eutrophic, calcium-rich lakes in southern Sweden have shifted between turbid and clearwater states several times during the past decades (Fig. 26.1). Lake Krankesjon shifted from a clearwater state with abundant submerged vegetation to a turbid state with sparse vegetation during the mid-1970s (Karlsson et al., 1976) and back to a clearwater state during 1985. Today, the lake is in the clearwater state, with abundant submerged vegetation dominated by Charophyta (Blindow et al., 1993; Fig. 26.2). Both shifts coincided with deviations from the average water level. During the mid-1970s, the water level during spring and summer was about 15 cm higher than average, whereas it was about 10 cm lower than average during 1983-1985 (Blindow, 1992).

Allelopathic limitation of algal growth by macrophytes

Growth inhibition of phytoplanktonic algae by macrophyte secretions is an important aspect of the eutrophication model that serves as the working hypothesis for the present study. However, evidence for allelopathic effects in aquatic ecosystems is scarce. Often, the phenomena found can also be attributed to various other causes like competition for light and/or nutrients. Although the active secretion of substances like nutrients by macrophytes is well-documented, not much unequivocal evidence exists for algal growth suppression by exudates from intact plant populations.