Density-dependent nutritional value of the dinoflagellate Cochlodinium polykrikoides to the copepod Acartia tonsa

The nutritional value of the dinoflagellate Cochlodinium polykrikoides to the copepod Acartia tonsa was evaluated over a range of ecologically relevant cell densities. Based on egg production rate, egg hatching success, and naupliar recruitment rate of A. tonsa, mixed-diet experiments indicated C. polykrikoides was nutritionally insufficient or had no nutritional value to A. tonsa at 600 μg C L^-1 (330 cells mL^-1), and was toxic at 1000 μg C L^-1 (550 cells mL^-1) when compared with the nontoxic flagellate Rhodomonas lens. However, the nutritional value of C. polykrikoides to A. tonsa at 100 and 200 μg C L^-1 (55 and 110 cells mL ^-1) was greater than or equal to that of R. lens. The density-dependent nutritional value of C. polykrikoides to A. tonsa was also demonstrated in the long-term survival experiments. Survivorship of A. tonsa fed C. polykrikoides was lower than those fed R. lens at 900 and 1800 μg C L^-1. In contrast, C. polykrikoides supported higher survivorship of A. tonsa than R. lens at 180 and 540 μg C L^-1. The nutritional value of C. polykrikoides to A. tonsa decreased from beneficial to deleterious with increasing cell density. A putatively "harmful" alga is not always deleterious to grazers, and its ecological effects may be distinctly different during bloom and non-bloom periods.