Karl Donabaum

Integrated management to restore macrophyte domination

Recentchanges which have been observed at Alte Donau, a shallow urban lake within the city of Vienna, have been interpreted as a shift to a new stable state. The former macrophyte-dominated state changed to a turbid state dominated by high biomass of filamentous cyanobacteria, associated with a significant reduction in Secchi-depth. Phytoplankton was dominated by the filamentous cyanobacterial species Cylindrospermopsis raciborskii (Wolosz.), Seenayya et Subba Raju and Limnothrix redekei (Van Goor) Meffert. Integrated restoration plans included internal and external measures. Improvements in the catchment aim to minimize the input of nutrients from contaminated groundwater and from storm water and to reduce large numbers of water fowl. Internal restoration measures included water exchange, chemical flocculation and nitrate oxidation of the sediments. Additionally, macrophyte re-colonisation was enhanced through planting. A pelagic predator (Aspius aspius L.) was stocked to reduce bleak (Alburnus alburnus L.), the dominant cyprinid planktivore. Results from the period after water exchange and chemical treatment, showed significant reduction of nutrient and chlorophyll a concentrations. A shift in the phytoplankton species from cyanobacteria towards diatoms and greens was observed. Secchi depth greatly increased. Macrophyte growth became apparent both through re-colonisation, as well as from the planting.

The zoobenthic community of shallow salt pans in Austria — preliminary results on phenology and the impact of salinity on benthic invertebrates

A three-year project on the benthic community of salt pans (shallow saline lakes) in the Seewinkel area of Eastern Austria has been carried out since 1996. Most of the salt pans investigated are very shallow (mean depth mostly 50 g 1−1) during the dry summer of 1998. A comparison of the benthic community in 20 salt pans at two sampling dates in 1997 revealed a negative relationship between salinity and species richness of Oligochaeta and benthic Crustacea. However, the decline of diversity was observed at salinity values much lower that those reported in the literature for most benthic invertebrates. It is thus doubtful whether or not salinity has in fact directly affected the benthic community in the salt pans of Seewinkel. Rather, it is suggested that biotic interactions controlled by changes in the hydrochemical situation are responsible for the reduction in diversity of the benthic community. Actual experiments or more detailed data on changes over a full gradient of salinity will be necessary to attribute changes in the species composition of benthic invertebrates to any environmental impact with certainty. The phenology of benthic invertebrates in Unterstinker, a sub-to hyposaline (0.5–20 g 1−1) salt pan studied in greater detail, was distinctly influenced by the development of submerged macrophytes. Abundances of chironomids and crustaceans, the two dominant major benthic groups, were high only in June and July, when Charophyceae had stabilized the lake bottom and prevented further erosion of fine sediments. Substrate characteristics and the abundance of macrophytes appeared to determine the seasonal development of the benthic community as long as the ion concentration remains low (< 3 g 1–1).

Modifications in phytoplankton size structure by environmental constraints induced by regime shifts in an urban lake

Size related changes of phytoplankton biovolume and species composition have been analysed for forward and reverse regime shifts in a shallow, urban, seepage lake. As a consequence of changes in the hydrology, the pelagic switched from a clear water, macrophyte dominated state to a turbid stable phase with an abundant cyanobacterial population. Experimental nutrient reduction forced the system into a recovery phase. Each state change was associated with significant changes in total biovolume, species composition and size structure affecting surface to volume ratios (S/V). Chlorophyll-a content and S/V drastically increased during the early recovering phase due to small cell sized species developing. As expected, state transitions were associated with significant alterations in size structure and composition.

Alternative stable states in floodplain ecosystems

Abstract Floodplain and ox-bow lakes form excellent examples of alternative stable states. Clear water, macrophyte dominated stages can alternate with turbid conditions characterised by high algal concentrations. Stable states can switch from one to the other domination through alterations of natural factors such as changes in water level, reduction in through-flow etc. Forward switches are often associated with anthropogenic pressure. In such cases, restoration to the original, macrophyte dominated stage is difficult and needs careful planning to establish sustainability. A case study from a shallow, urban, seepage lake, the ‘Alte Donau’ within the city proper of Vienna is presented. Results on switches between stable states, causes, consequences and restoration measures will be detailed. The analysis is augmented with data from floodplain lakes east of Vienna which have different levels of connectivity to the main river.

Successful Restoration of a Shallow Lake: A Case Study Based on Bistable Theory

Eutrophication of shallow lakes provides an excellent example of alternative stable states. Clear water, macrophyte-dominated stages can alternate with turbid conditions characterized by high algal concentrations. Stable states can switch from one to the other domination through alterations of natural factors such as changes in water level and reduction in throughflow. Forward switches are often associated with anthropogenic pressure. In such cases, backward shift to the original, macrophyte-dominated stage may be difficult. Return times are often prolonged due to hysteresis as a result of resilience. The theory is exemplified with results from a shallow, urban, seepage lake ‘Old Danube,’ which is within the city limits of Vienna. Causes and consequences of switches between stable states including resilience and hysteresis are discussed. The remediation measures are explained and the success of the restoration is explained in detail.

Water-quality management in a vulnerable large river: the Nile in Egypt

We review the severe water-management problems of the Nile Basin, where physical water scarcity is associated with high demographic growth, leading to a sharply rising demand for competing water uses such as hydropower and large-scale irrigation. Rapid economic growth is perceived as the means to emerge from the poverty trap that afflicts livelihoods in the Upper Basin and vital wetland ecosystem services such as fish biomass, freshwater biodiversity, groundwater recharge, flow regulation and local climate moderation are threatened by the water-development schemes and pollution that follow from this policy. Their cumulative impacts remain unaddressed. The High Aswan Dams impacts on freshwater biodiversity are incompletely understood; a significant number of species may have become threatened as a result of its construction. Today the reservoir water quality is high, it is thought to support 47 fish species, its local human activities are restricted by central government regulations and recent estimates indicate that eutrophication threats are unlikely. Sediment and nutrient inputs coming into it from upstream will, however, continue to decrease in the near future as a result of newly built and planned dams in the upper basin. The dams will also reduce discharge and cause further loss of connectivity between the river and its floodplain, exacerbated by the possible completion of the Jonglei Canal bypassing the Sudd swamps. These impacts will affect the Niles vulnerable aquatic biodiversity and regulatory services that are likely to affect local climate conditions. Under the current geopolitical scenario, management decisions that could favour participatory and sustainable options are over-ruled by high-level political trade-offs between the numerous riparian states. The financing of major hydropower developments by vested interests creates a scenario that is unlikely to favour sustainable resource management and conflict resolution.