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

The ecological impacts of Hydrodictyon reticulatum blooms (1989-94) were studied at Lake Aniwhenua (a constructed lake) in North Island, New Zealand by collating fish, invertebrate and macrophyte data collected towards the end of a four year bloom period and following its decline. Hydrodictyon reticulatum 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 conditions 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 10⁴ per m) were recorded amongst H. reticulatum, and gastropods were exceptionally abundant, the most common being Potamopyrgus antipodarum (maximum, 1.8 x 10⁵ per m). Hydrodictyon reticulatum was consumed by three species of common gastropods in experimental trials, with Austropeplea tomentosa consuming up to 1.3 g dry weight H. reticulatum per g live weight of snail day(-1). Gastropods comprised the major portion of the diet of Oncorhynchus mykiss in Lake Aniwhenua during and after the H. reticulatum bloom. A marked peak in sports fishing (with exceptional sizes and numbers of fish caught) coincided with the period of H. reticulatum blooms and the abundant invertebrate food source associated with the blooms.