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Zooplankton grazers are capable of influencing food-web dynamics by exerting topdown control over phytoplankton prey populations. Certain toxic or unpalatable algal species have evolved mechanisms to disrupt grazer control, thereby facilitating the formation of massive, monospecific blooms. The harmful algal bloom (HAB)-forming dinoflagellate Karenia brevis has been associated with lethal and sublethal effects on zooplankton that may offer both direct and indirect support of bloom formation and maintenance. Reductions in copepod grazing on K. brevis have been attributed to acute physiological incapacitation and nutritional inadequacy. To evaluate the potential toxicity or nutritional inadequacy of K. brevis, food removal and egg production experiments were conducted using the copepod Acartia tonsa and K. brevis strains CCMP 2228, Wilson, and SP-1, characterized using liquid chromatography-mass spectrometry (LC-MS) as having high, low, and no brevetoxin levels, respectively. Variable grazing rates were found in experiments involving mixtures of toxic CCMP 2228 and Wilson strains. However, in experiments with toxic CCMP 2228 and non-toxic SP-1 strains, A. tonsa grazed SP-1 at significantly higher rates than the toxic alternative. Additionally, A. tonsa experienced significantly greater mortality when exposed to toxic K. brevis strains, particularly after prolonged exposure. Egg production rates of copepods fed toxic K. brevis strains were similar to those of starved copepods, while those of copepods fed non-toxic SP-1 and the nutritious Rhodomonas salina were significantly higher. Analysis indicates that K. brevis impacts grazer populations via multiple synergistic mechanisms: (1) decreased ingestion rates, (2) decreased egg production, and (3) increased mortality of copepods through a combination of toxicity and nutritional inadequacy.

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Studies of adult LD,, concentrations of Dimilin, an insect growth regulator which acts to inhibit chitin synthesis, are misleading when applied to arthropods which experience a terminal molt. The calanoid copepod Acartia tonsa Dana has a determinate number of molts after which no growth occurs and the necessity of chitin production is restricted. Survival of adult A. tonsa held for 5 d in concentrations as high as 1000 ppb Dimilin is not significantly (P < 0.05) different from the controls Fecundity of A. tonsa was not significantly (P < 0.05) altered by 1 to 4 d treatment with l , l 0 or 100 ppb Dimilin but the hatch of viable nauplii was reduced from 90+ % to less than 50 % in l ppb after 12 h of treatment and to less than 5 % in 10 ppb after 24 h of treatment. No viable n a u p l~i were produced by females held in either 1 or 10 ppb Dimilin for more than 36 h. When Dimil~n treatment ceased and the females were placed in seawater, the effect of Dimilin was not readily reversed No viable nauplii were produced by these females for at least the next 30 h after treatment ended

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