Abstract: Silt particles smaller than 100 μm in diameter were used to make the waters with a turbidity of 30 NTU, 60 NTU, and 90 NTU. Hydrilla verticillata seedlings were planted in the turbid waters, and their branch length, branch number, and fresh mass were measured at definite periods of time. In the meanwhile, the leaf chlorophyll fluorescence parameters were determined in situ by a submersible pulse-amplitude modulated (PAM) fluorometer (Walz GmbH, Effeltrich, Germany). With the increase of water turbidity, the branch number of the seedlings decreased remarkably, biomass also decreased, but branch length increased significantly. In turbid waters, the F_v/F_m value decreased with time, but was still higher than that in the control waters. Under the actinic light of 17 μmol·m^-2·s^-1 PPFD, the effective quantum yield (△F_v′/F_m′) of seedling leaves on the 60th day in the waters with turbidity of 30 NTU, 60 NTU, and 90 NTU increased by 48.9%, 36.8%, and 17.2% (P<0.01), and the relative electron transport rate (rETR) increased by 56.7%, 42.2%, and 21.4% (P<0.01), respectively, compared with those on the 30th day. However, under the actinic light of 104 μmol·m^-2·s^-1 PPFD, the △F_v′/F_m′,q_P, and rETR on the 60th day decreased significantly, and the heat dissipation capability (q_N) also reduced evidently. All the results suggested that the H. verticillata seedlings in turbid waters could adapt to low light environment, but their leaves were easy to be damaged under high light intensity. Therefore, it would be possible to introduce H. verticillata seedlings in shallow turbid waters.

Key words: suspended silt, Hydrilla verticillata, chlorophyll fluorescence characteristics, stand spatial structure, distance sampling, density estimation, spatial pattern, uniform angle index.