Abstract. Zebra mussels (Dreissena) have expanded rapidly throughout most of the Laurentian Great I~akes since their inadvertent release in 1986. These exotic molluscs now occur in great numbers on the bottom of western Lake Erie where they are found increasingly in deeper areas of the basin (average depth: 10 m), on sott, muddy substrates. '[-his study is aimed at quantifying the density and the distribution patterns of mussel colonization in the basin as a first step in investigating the effect on sediment properties of such an abrupt change in benthic community structure. Underwater video imager T and diver-collected samples taken from representative o~shore areas (seven sites) in western Lake Erie showed colonization levels of up to 20,000 live mussels per m in soft sediments (adult.,; with shells '10 mm comprised 47 %). Digital side-scan sonar records confirmed that colonization patterns were not random, but showed distinctive spatial signatures ranging from 30-m-long parallel stripes, to large ovate masses. Broad irregular mats were found in association with bard bottoms (bedrock, boulders, or wrecks and large debris). Mussel densities were averaged from the sites, assuming consistent relationships with substrate type ,and were combined with digitized percentage &areal coverage of major bottom types in western Lake Erie. This resulted in the first population figure of 10 ~3 in the basin. "['his figure includes molluscs of all sizes 9 0.84 mm.

The zebra mussel (Dreissena), inadvertently introduced to the Great Lakes in 1986, has since expanded to cover most of the shallow-water, hard substrates in Lakes Erie and Ontario. Colony densities exceed 300,000 per m2 in some bedrock areas of western Lake Erie. The objective of this study was to investigate the spread of zebra mussel onto soft sediment areas of the western basin of Lake Erie and to identify natural controls on the large-scale colonization of such sediments. Combined side scan sonar, underwater video imagery, and direct diver observation showed three modes of viable zebra mussel colonies in soft substrates: 1) attachment to zebra mussel shell debris deposited in linear troughs (stripes); 2) attachment to shells built up over hard substrates intermittently covered by soft sediments (footballs); and 3) as isolated clumps (druses) attached to dropstones, unionid clams, or their shells. Their spatial distribution suggests that zebra mussel expansion onto soft sediments is supplied primarily by nearby hard substrate areas. Zebra mussel populations living on soft sediment have the normal size distribution as those found on hard surfaces, often with a bimodal or trimodal character, representing cohorts of different ages. At the sites studied, there was a large proportion of dead shells, suggesting colonization over an extended period, as well as a relatively high mortality rate due to burial by periodic catastrophic sedimentation after storms. This vulnerability and the need to be near to source areas make it unlikely that the zebra mussel (D. polymorpha) will continue to expand into areas of soft sediment remote from hard substrate areas. The impact of the zebra mussel colonies on important textural properties of the substrate was not dramatic, but median grain diameter was significantly finer, organic carbon significantly higher, and sediment consistency more clayey below zebra mussel mats. Metal content was generally higher in the samples below zebra mussel mats, but the differences were statistically significant only in the case of iron and maganese. However, the sediment concentrations of metals at all sites were much greater than those at the remote Mid-basin site that was barren of zebra mussels, suggesting an overall metal enrichment near zebra mussel colonies.

[1] The cathodoluminescence (CL) of quartz from ore, stockwork, veins, and interstitial fillings between lava pillows from the $2.7 Ga Noranda, Ben Nevis and Matagami volcanogenic massive sulfide (VMS) districts, Abitibi greenstone belt, has been investigated using the ''hot cathode'' technique (HC1-LM system) to assess the potential of these various sample types to host primary, seafloor VMS-related fluids trapped as inclusions in minerals with primary depositional morphologies. The CL responses indicate that the various quartz types are of hydrothermal origin, and are therefore a potential host for primary hydrothermal fluid inclusions. Most notable is a transient (t < 120 s) blue CL, characteristic of hydrothermal quartz, observed in most samples. CL characteristics are similar over $250 km indicating coherent, nonrandom behavior. Furthermore, in ore and stockwork material from the Matagami and Noranda districts respectively, CL reveals primary concentric growth zoned quartz that predates pulses of sulfide deposition-clear evidence that the quartz is undeformed and directly related to VMS mineralization. These growth zones are not apparent in transmitted light. In addition, ore and stockwork quartz commonly show a very unstable (t < 30 s) yellow CL coincident with microfractures and grain boundaries, defining areas affected by secondary hydrothermal activity. In ore from Matagami, local zones of nontransient brown CL may reflect strain zones associated with the deformation and recrystallization of the massive sulfide mound and indicate that such modifications can be recognized and are minor in the investigated cases. CL clearly reveals pseudo-hexagonal, apparently zoned structures in sulfidemineralized breccia pipe quartz from the Ben Nevis area. These structures and their host quartz, characterized by a very unstable (t < 20 s) bright yellow CL, are interpreted as recrystallized quartz that has undergone rapid growth from a strongly supersaturated solution and noncrystalline precursor. The CL also clearly reveals colloform/crustiform textures indicative of open-space filling; these textures are not visible optically.

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The variation of the smaller size fraction of zooplankton was investigated during a two-year period in a brackish deep and anoxic coastal lake of western Greece (Aitoliko), along with the specific environmental characteristics of this ecosystem. The zooplanktonic community comprised a relatively small number of taxa and it was dominated by brackish-water calanoid copepods (Paracartia latisetosa, Calanipeda aquaedulcis) and in certain periods by rotifers and tintinnids. The zooplankton abundance showed an increase in the warmer period starting from late spring and reached maximum values in July. In the well oxygenated surface layer, temperature was the most important parameter influencing the seasonal cycles of all groups. In contrast, the oxygen depletion a few meters under the surface affected the vertical distribution of most of the zooplankton groups, which were found restricted in the surface layer, especially from spring until autumn. Only the meroplanktonic larvae of polychaetes presented increased proportions in the deeper layers. Salinity has not significantly influenced the zooplanktonic assemblages. The results reveal the degraded status of the Aitoliko basin, where the hypoxic/anoxic layers resulted in a high portion of dead organic material identified as copepod carcasses; they also underline the necessity of monitoring this ecosystem.

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