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 Unite d Kingdom and is known from several major river systems (Wye, Tamar, T weed, Tyne). It has regularly formed extensive growths in a eutrophic lake in south-west England (Loe Pool) since first observed in 1989. It s small colonies are usually evident when its water is about 15 degree s C, a temperature reached towards the end of May in 1995 and 2-3 week s later in 1996. These floating, air-filled colonies act as a dispersi ve-phase and within days of first detection become distributed through out the lake, often accumulating along downwind shores. As they expand and loose entrapped air, the colonies sink and many eventually come t o blanket the lake floor. These colonies form dense entangled mats tha t exhibit diurnal movement when healthy, often sinking at night or dur ing 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 signifi cant quantity and massive growths of Hydrodictyon persisted through Au gust and most of September. It is hypothesised that some form of inter action between Microcystis and Hydrodictyon accounts for the latters r apid 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 eu trophic lakes in the UK. Some zooplankton (nauplii, calanoids) show ab undance peaks during the main growth phase of Hydrodictyon (June, July ). There is no direct evidence to indicate that its mats provide a ref uge for plankton grazers although molluscs and leeches are particularl y common in Hydrodictyon mats.