Aims Harmful algal blooms caused by multiple toxic or harmful algal species have globally expanded and threaten marine sustainability, and interaction among bloom species is thought to play an important role in bloom development and elimination. Our objective is to study the interactions of two causative bloom-forming species of coastal China, Heterosigma akashiwo and Karenia mikimotoi, under controlled laboratory conditions.
Methods The experiments were carried out in mono- and co-culture. Results were analyzed with the software package Sigmaplot 8.0 and SPSS 13.0, and ANOVA and Duncan’s multiple range tests were used for data analysis.
Important findings Growth of both species at different initial cell densities in mono-culture was well predicted by a logistic model. Their environmental capacity (K) decreased steadily while the intrinsic rate of increase (r) increased with the initial cell density increment, and the time for entering exponential and stationary growth phases shortened simultaneously. Both K values of the microalgae in co-culture were inhibited as compared to mono-culture (p＜0.05), and their competition changed simultaneously with the ratio of their initial biomass. Under co-culture, K. mikimotoi became dominant when the initial biomass ratio of H. akashiwo (H): K. mikimotoi (K) was set at 1:4 and 1:16; however, H. akashiwo overcame K. mikimotoi when the ratio turned to H:K = 1:1. Therefore, initial biomass played an important role in microalgal inter-specific competition in co-culture. Allelopathy is a possible reason for the observed results.