Abundance and composition of the summer phytoplankton community along a transect from the Barguzin River to the central basin of Lake Baikal

Abstract

The abundance and composition of phytoplank- ton were investigated at six stations along a transect from the Barguzin River inflow to the central basin of Lake Baikal in August 2002 to clarify the effect of the river inflow on the phytoplankton community in the lake. The water temperature in the epilimnion was high near the shore at Station 1 (17.3C), probably due to the higher temperature of the river water, and gradually decreased offshore at Station 6 (14.5C). Thermal stratification developed at Stations 2-6, and a ther- mocline was observed at a 17-22-m depth at Stations 2-4 and an 8-12-m depth at Stations 5 and 6. The concentrations of nitrogen and phosphorus nutrients in the epilimnion at all stations were \1.0 lmol N l -1 and \0.16 lmol P l -1 , respectively. Relatively high concentrations of nutrients (0.56-7.38 lmol N l -1 and 0.03-0.28 lmol P l -1 )w ere detected in the deeper parts of the euphotic zone. Silicate was not exhausted at all stations ((20 l mol Si l -1 ). The chloro- phyll a (chl. a) concentration was high ((10 l gl -1 )n ear the shore at Station 1 and low (\3 l gl -1 ) at five other stations. The\2 l mf raction of chl.a in Stations 2-6 ranged between 0.80 and 1.85 l gl -1 , and its contribution to total chl. a was high ((60%). In this fraction, picocyanobacteria were abundant at all stations and ranged between 5 9 10 4 and 5 9 10 5 cells ml -1 . In contrast, chl. a in the(2 l mf raction varied significantly (0.14-11.17 l gl -1 ), and the highest value was observed at Station 1. In this fraction, the dominant phytoplankton was Aulacoseira and centric diatoms at Station 1 and Cryptomonas, Ankistrodesmus, Asterionella, and Nitzschia at Stations 2-6. The present study demon- strated the dominance of picophytoplankton in the pelagic zone, while higher abundance of phytoplankton dominated by diatoms was observed in the shallower littoral zone. These