We examined the combined effects of grazer infochemicals and nutrient status on colony development ofPhaeocystis globosa cultures grown under nitrogen and phosphorus (NP)–sufficient, P-deficient, and N-deficientconditions exposed to high and low Acartia spp. density filtrates. Changes in colony development relative tocontrols receiving no grazer signals were estimated. P. globosa colony development responded to grazerinfochemicals regardless of nutrient status, although the expression of the response varied between nutrients.Significant colony suppression (in terms of percent of cells allocated to colonies) occurred in both NP-sufficientand P-deficient experiments, with the response being dependent on the density of grazers for NP-sufficient cells.The percent of cells in colonial form in N-deficient P. globosa decreased in response to low grazer density filtratesbut increased in response to high grazer density filtrates. These opposite results for the N-deficient experiment arerelated to a high mortality of Acartia in the high grazer density filtrate treatment, which may affect theinfochemicals released from such grazers

We investigated whether or not the presence of copepods and different light conditions induced cyst ­formation in dinoflagellates. Scrippsiella trochoidea was exposed to Acartia tonsa directly and indirectly (grazer filtrate), in high light and low light conditions. The ingestion, faecal ­production and egg production of A. tonsa were compared between diets of S. trochoidea vegetative cells and temporary cysts. We found no effect of direct or indirect exposure to A. tonsa on S. ­trochoidea cyst formation in either high light or low light conditions. Controls and A. tonsa treatments kept in light displayed around 20% temporary cysts, whereas controls and A. tonsa treatments in low light were shown to have 50 to 80% temporary cysts. Thus, low light conditions had a strong effect on ­temporary cyst formation. No hypnocysts were observed in any experiment, which is probably related to the longer incubation times needed for their observation. Feeding on diets dominated by temporary cysts compared to vegetative cells decreased ingestion by a factor of 2.7, while faecal and egg production decreased by a factor of 2.2 and 2.9, respectively, suggesting that induction of temporary cysts in response to A. tonsa could be a survival strategy. However, S. trochoidea does not ­possess any grazer-induced defence in terms of temporary cyst formation, as it did not produce ­temporary cysts when exposed to A. tonsa. Rather, induction of temporary cysts seems to be controlled by decreased light intensity, which is a favorable trait for this species when driven to water depths where light is scarce.

Nodularia spumigena was exposed directly and indirectly (grazer cages) to increasing densities of Acartiacf. bifilosa to investigate if the presence of copepods influenced the morphology and/or the toxicity of thecyanobacterium. Monocultures with only N. spumigena and mixed cultures, containing N. spumigena andthe non-toxic Dunaliella tertiolecta, were included in each experiment. Following 6 days of incubation,the morphology and toxicity in grazer treatments were compared with grazer-free controls. Weobserved no effects of A. cf. bifilosa on either morphology or toxicity of N. spumigena. The lack of grazerinduced nodularin production and morphological alterations suggest that these two potential defensestrategies either has evolved as constitutive defenses or never evolved as grazer defenses. The mortalityof copepods was higher in the monoculture than in the mixed culture treatments. Gut contentobservations indicated a low level of grazing in monoculture treatments and a higher level of grazing inmixed culture treatments. This higher level of grazing most likely occurred on the alternative food D.tertiolecta. Given the indications of low grazing and the concentrations of dissolved nodularin observed,we postulate that the higher mortality was not related to toxic effects, but to starvation. This in turn mayhave resulted from bad taste, production of unknown grazer deterrents or morphological constraints;although the size of the filaments would not have imposed an absolute limit for ingestion by A. cf. bifilosa.The higher copepod mortality observed on monocultures of N. spumigena may contribute to the successand maintenance of N. spumigena blooms.