Ecological impacts of water net (Hydrodictyon reticulatum) in Lake Aniwhenua, New Zealand

The ecological impacts of Hydrodictyon reticulatum blooms (1989-94) were st udied at Lake Aniwhenua (a constructed lake) in North Island, New Zealand b y collating fish, invertebrate and macrophyte data collected towards the en d of a four year bloom period and following its decline. Hydrodictyon retic ulatum had some localised impacts on the biota of the lake. Some macrophyte beds were smothered to the extent that they collapsed and disappeared, and dense compacted accumulations of H. reticulatum caused localised anoxic co nditions while it decayed. However, fish and some invertebrates in the lake benefited from the H. reticulatum blooms. High numbers of Ceriodaphnia sp. (maximum, 5.5 x 10(4) m(-2)) were recorded amongst H. reticulatum, and gas tropods were exceptionally abundant, the most common being Potamopyrgus ant ipodarum (maximum, 1.8 x 10(5) m(-2)). Hydrodictyon reticulatum was consume d by three species of common gastropods in experimental trials, with Austro peplea tomentosa consuming up to 1.3 g dry weight H. reticulatum g(-1) live weight of snail day(-1). Gastropods comprised the major portion of the die t of Oncorhynchus mykiss in Lake Aniwhenua during and after the H. reticula tum bloom. A marked peak in sports fishing (with exceptional sizes and numb ers of fish caught) coincided with the period of H. reticulatum blooms and the abundant invertebrate food source associated with the blooms.