Fish-killing Prymnesium parvum blooms have occurred in south-central USA for at least ∼30 years, with the last decade experiencing recurrent blooms of large magnitude. In the systems reported here, Lakes Possum Kingdom, Granbury and Whitney (Texas), P. parvum blooms were winter phenomena developing under conditions far from the growth optimum. Bloom thresholds of 10 × 10⁶ cells L⁻¹ were observed as a function of inflow and salinity for the period 2000–2009. In Lake Possum Kingdom, blooms occurred only when 7-day accumulated inflows were 1.5 psu. For Lakes Granbury and Whitney, blooms occurred when 7-day accumulated inflows were <20 × 10⁶ m³ and 0.5 psu. Inflow to these lakes exceeded thresholds during the spring and early-summer months in 8 (Lake Possum Kingdom), 7 (Lake Granbury) and 6 (Lake Whitney) of the 10 years analyzed. Salinities typically exceeded these thresholds during the period of study prior to the spring of 2007. The spring of 2007 was a period of high precipitation, after which salinities were typically below thresholds. The linkage between incidence of P. parvum blooms, inflows and salinity is of concern because combined effects from human population increase and climate change could lead to periods of decreased inflow and increased salinity, which may then increase the frequency and magnitude of P. parvum blooms.

The symposium session, Indicators for Effects and Predictions of Harmful Algal Blooms, explored the current state of indicators used to assess the human health and ecological risks caused by harmful algal blooms, and highlighted future needs and impediments that must be overcome in order to provide a complete risk assessment of their impacts. Six recognized human poisoning syndromes resulting from algal toxins (paralytic, neurotoxic, amnesic, diarrhetic shellfish poisonings, ciguatera fish poisoning, and putative estuary associated syndrome) impact human health through consumption of contaminated seafood, direct contact with bloom water, or inhalation of aerosolized toxin. Thorough health risk assessment for the variety of algal toxins is hampered to varying degrees because either the toxin has not been identified or indicators for exposure and effects remain poorly defined. Predicting the occurrence and determining the impacts of harmful algal blooms in coastal ecosystems are the two major ecological risk assessment needs. In the former case, the hazard is the suite of conditions that trigger bloom initiation, magnify bloom intensity or support bloom longevity, whereas in the latter case, the hazard is the algal toxin. In both cases, indicators (of triggering mechanisms, exposure, and effects) are better defined for some HAB species and toxins than others, but are by no means complete.