PARENT SESSION
Oral Session # 51: Limnology II: Plankton; Carbon.
Presiding: K Forshay
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 101.

Influence of drought on the phytoplankton bloom formations in a river-reservoir system.

Jeong, Kwang-Seuk^*,1, Ha, Kyong¹, Kim, Myoung-Chul¹, Joo, Gea-Jae¹, ¹ Pusan National Univ., Busan, S. Korea

ABSTRACT- This study presents the influence of drought on the phytoplankton proliferations in a river-reservoir system. In the case of freshwater ecology, reduction of discharge caused by droughts can cause diverse changes of ecosystem functions and structures at the point of time. When considering flow regulation, the river systems may show a different pattern responding to droughts. The lower Nakdong River in South Korea is a good example of a eutrophic river-reservoir system which contains multi-purpose dams and an estuarine barrage at the river mouth (chlorophyll a concentrations, 43.6±76.7 L^-1, n=385). Due to the monsoon and typhoons, the river had concentrated rainfall during summer (> 60% of total rainfall in June to August; average annual rainfall, 1,250 mm). The flow regulation can shuffle the lentic and lotic characteristics in this river. Ten years (1993-2002) of weekly study exhibited that Microcystis aeruginosa and Stephanodiscus hantzschii blooms occurred in summer and winter respectively. Seasonally, winters are dry (5% of total annual rainfall), and S. hantzschii proliferated in every year. Summer blooms of M. aeruginosa showed inter-annual variability and dominated about 90% of algal species during dry years (1994-1996, < 800 mm of annual rainfall). A non-linear clustering by Self-Organizing Map on the algal proliferation in this river revealed that the increased heat energy, accumulated nutrients and decreased flow were responsible for both phytoplankton blooms. Furthermore, in the case of S. hantzschii, larger magnitude of the cell density could be observed when summer drought occurred (r²=0.61). It might be due to the regulated discharge from upstream dams. This study could suggest that drought can give favorable conditions on the algal bloom formation, and the inter-annual climate changes can have time-delayed influence on the algal species succession in river-reservoir systems.

Key words: river-reservoir systems, droughts, phytoplankton blooms, self-organizing map