Blooms of the water net Hydrodictyon reticulatum (Chlorococcales, Chlorophyta) in a coastal lake in the British Isles: Their cause, seasonality and impact

Hydrodictyon reticulatum, a fast-spreading nuisance green alga, has a uniquely structured net-like coenobia. In recent years it has invaded numerous ponds and lakes (including many trout fisheries) in the United Kingdom and is known from several major river systems (Wye, Tamar, Tweed, Tyne). It has regularly formed extensive growths in a eutrophic lake in south-west England (Loe Pool) since first observed in 1989. Its small colonies are usually evident when its water is about 15 °C, a temperature reached towards the end of May in 1995 and 2-3 weeks later in 1996. These floating, air-filled colonies act as a dispersive-phase and within days of first detection become distributed throughout the lake, often accumulating along downwind shores. As they expand and loose entrapped air, the colonies sink and many eventually come to blanket the lake floor. These colonies form dense entangled mats that exhibit diurnal movement when healthy, often sinking at night or during unstable weather and rising to the lake surface on calm sunny days. By August, most subsurface mats of Hydrodictyon had disappeared from all but a few small bays and a dense Microcystis aeruginosa bloom had developed. In 1993 this cyanobacterium was not present in any significant quantity and massive growths of Hydrodictyon persisted through August and most of September. It is hypothesised that some form of interaction between Microcystis and Hydrodictyon accounts for the latters rapid decline in July. There is no convincing evidence to indicate that Hydrodictyon has any significant impact upon the phytoplankton since these exhibit a similar seasonal succession compared to other small eutrophic lakes in the UK. Some Zooplankton (nauplii, calanoids) show abundance peaks during the main growth phase of Hydrodictyon (June, July). There is no direct evidence to indicate that its mats provide a refuge for plankton grazers although molluscs and leeches are particularly common in Hydrodictyon mats.