No abstract

The species composition and standing crop of the algal epiphytes on the walls of experimental cages in a small lake were greatly altered following the emergence of frog tadpoles from eggs laid in the cages. The results support the hypothesis that tadpoles may be a major cause of the massive spring reduction in standing crop of filamentous green algae in the lake.

No abstract

Primary production in Marion Lake is inversely related to the rate at which water enters the lake when light intensity is corrected to a standard level. Increased flushing rates reduce the phytoplankton standing crop thereby lowering the total primary productivity in the lake. Thus seasonal variations in rainfall in southwestern British Columbia exert an appreciable influence on the annual productivity pattern of the lake’s phytoplankton. Lake water artificially enclosed within small areas of the lake produced algal blooms while phytoplankton standing crop in the rest of the lake remained low. Nannoplankton appear to have a selective advantage over larger, more slowly reproducing forms in Marion Lake. The production: biomass ratio for lake phytoplankton was used as an indication of the general type of limiting factor affecting the instantaneous rate of productivity in the lake.

ABSTRACT. Our study of 34 ships (20 direct from Oakland, California, USA, and 14 more after open ocean exchange) is the first year-long seasonal study to assess the effectiveness of open ocean exchange of ballast waters. The highest number of harmful species occurred in April and February when water temperatures in Hong Kong (China) and Oakland were low. The periods with the greatest species richness of harmful species did not correspond to periods with the highest abundance of harmful species. The latter occurred in early September and mid-August, when Skeletonema costatwn (14 000 cells 1-') dominated the plankton in the ballast water from Oakland Harbor. From April 1996 to April 1997, ballast water samples were collected from 34 Orient Overseas Container Lines Ltd ships whch took 16 d to travel from Oakland to Hong Kong. Of the 34 ships. 14 exchanged their Oakland Harbor ballast water for open ocean water (referred to here as reballasting). Open ocean was defined as waters with a depth >2000 m. Once reballasting was completed, the contents of ballast tank no. 1 were not discharged until the ship reached Hong Kong. Six harmful diatoms and 9 harmful dinoflagellate species were observed in the ballast water from Oakland Harbor. These included the diatom Chaetoceros concavicornis and the PSP (paralytic shellfish poisoning) toxin producing dinoflagellate Alexandrium catenella. Of the 15 harmful species found in the Oakland Harbor ballast water, 8 (53%) were also found in the ballast water of ships that had exchanged coastal for open ocean ballast water. The abundance of harmful species in open ocean ballast water was 87 % lower than those In the ballast water from Oakland Harbor. The reason that mid-ocean exchange failed to eliminate all harmful diatoms and dlnoflagellates was probably because the ballast tank was never completely emptied before it was reballasted with mid-ocean water.

In a small meromictic lake near Toronto, Canada, a mass mortality of photosynthetic bacteria followed the ventilation of the chemocline during fall (autumn) and resulted in 3–8 g m‐2 day‐1 of organic matter being deposited as a dark layer in sediment traps which were suspended in the permanently anaerobic zone. This mass mortality of photosynthetic bacteria occurred in late autumn following the annual thermal destratification of the lake's mixolimnion. Wind mixing during this period of homeothermy resulted in the introduction of low levels of dissolved oxygen into the lake's chemocline. The ensuing mass mortality of photosynthetic bacteria resulted in the release of elemental sulphur as the sulphur‐rich bacteria decomposed and sank to the bottom of the lake. The ferrous ions in the water below a depth of 15 m in Crawford Lake reacted with this sulphur to form black ferrous sulphides and pyrite which formed a dark microlamina on the lake floor. Each dark microlamina was overlain by a light coloured (calcite‐rich) layer which was deposited each spring and summer during the 3 yr period of this study. The mechanism of microlamina formation elucidated here has been based on the examination of bi‐weekly sediment trap information. This approach has permitted an explanation of the mechanisms by which specific events such as calcite precipitation and phytoplankton seasonal succession are transcribed into the sediment record.

No abstract

No abstract