Response of Drinking-water Reservoir Ecosystems to Decreased Acidic Atmospheric Deposition in SE Germany: Signs of Biological Recovery

Strongly decreasing atmospheric emissions and acidic deposition during the 1990s have initiated chemical reversal from acidification in several drinking-water reservoirs of the Erzgebirge, SE Germany. We studied responses of phytoplankton, zooplankton and fish stocks in five reservoirs and at enclosure scale after experimental neutralization of 1,200 m³ of lake water. About 4 months after this treatment, diatoms and cryptomonads replaced the predominating chrysophytes and dinoflagellates. The colonization by acid-sensitive species of green algae, cryptomonads, rotifers and Cladocera (e.g. Bosmina longirostris) is explained by the occurrence of dormant stages or by survival of individuals in very low abundances. Analogous to the enclosure experiment, three reservoirs showed significantly (p < 0.01) falling trends of chlorophyll a and phytoplankton biovolume, mainly due to the decline of dinoflagellates. Picoplankton and diatoms increased slightly in two reservoirs. The zooplankton communities were dominated by rotifers and small Cladocera. Representatives of the genus Daphnia were lacking. Two reservoirs were re-colonized by zooplanktivorous fish populations of either perch (Perca fluviatilis) or sunbleak (Leucaspius delineatus). The latter exhibited extremely high fluctuating abundance and biomass and even suffered from a population crash. This natural mortality was caused by a limited food supply. Hence, severe top-down control may delay the recovery of larger zooplankton species like daphnids. Fishery management comprising the introduction of predatory fishes can help to control zooplanktivorous fish populations and to prevent their mass mortality.     

Untersuchungen zum ökotoxikologischen Schädigungspotenzial und Erosionsrisiko von kontaminierten Sedimenten in staugeregelten Flüssen

Particle-bound pollutants accumulate in river-bottom sediments, a process which results in a significant decrease in the ecotoxicological availability of toxicants for the majority of aquatic organisms. Under normal hydrologic conditions, the release of contaminants from bottom sediments is usually of minor importance. In contrast, flood events may remobilize highly contaminated sediments via in-stream erosion. The objective of this study was to develop a combined ecotoxicological and hydraulic approach to elucidate the ecotoxicological implications associated with the risk of erosion of contaminated sediments. This integrated strategy was applied to the lock-regulated Neckar river in Southern Germany. Both the bottom-sediment cores and suspended matter from two intensive flood events were investigated. Sediment samples below an erosional unconformity showed a sharp increase in the ecotoxicological load. Moreover, it was found that major flood events (HQ₅ and higher) could possibly erode even very old, well-consolidated and highlycontaminated sediments. The suspended matter of the high discharge events investigated (return periods of 15 to 20 years) exerted significantly higher cytotoxicity and mutagenicity than a moderate flood with a 1-year return period. These findings support the conclusion that the observed ecotoxicological effects during major floods may at least in part be due to the in-stream erosion of highly contaminated bottom sediments.     

The infrabuccal pellet piles of fungus-growing ants

Fungus-growing ants (Attini) live in an obligate mutualism with the fungi they cultivate for food. Because of the obligate nature of this relationship, the success of the ants is directly dependent on their ability to grow healthy fungus gardens. Attine ants have evolved complex disease management strategies to reduce their gardens exposure to potential parasitic microbes, to prevent the establishment of infection in their gardens, and to remove infected garden sections. The infrabuccal pocket, a filtering device located in the oral cavity of all ants, is an integral part of the mechanisms that leaf-cutter ants use to prevent the invasion and spread of general microbial parasites and the specific fungal-garden parasite Escovopsis. Fungus-growing ants carefully groom their garden, collecting general debris and pathogenic spores of Escovopsis in their infrabuccal pocket, the contents of which are later expelled in dump chambers inside the nest or externally. In this study we examined how a phylogenetically diverse collection of attine ants treat their infrabuccal pellets. Unlike leaf-cutters that deposit their infrabuccal pellets directly in refuse piles, ants of the more basal attine lineages stack their infrabuccal pellets in piles located close to their gardens, and a separate caste of workers is devoted to the construction, management, and eventual disposal of these piles.