HABs Don't kill Calanus finmarchicus but is their fitness effected?

Harmful algal blooms (HABs) are natural phenomena. They result from the rapid growth of microscopic algae or cyanobacteria in water and negatively impact other organisms. In the Gulf of Maine, in both inshore and offshore waters, the dinoflagellate Alexandrium fundyense is responsible for the formation of annual HABs. They occur during Spring/Summer when nutrient levels are high and grazing pressures are low. Large blooms of A. fundyense can be problematic because they produce various toxins, one of which is the potent neurotoxin, saxitoxin. The toxin accumulates up food chains and is responsible for paralytic shellfish poisoning (PSP) which can be fatal in humans.

 

The copepod Calanus finmarchicus co-occurs with the said dinoflagellate during the HABs. These invertebrates feed on A. fundyense (Hassett, 2003) but this does not alter mortality rates (Campbell et al., 2005). However, little is known about the possible sublethal effects the toxic dinoflagellates impose on this species, one which supports populations of commercially valuable fish and crustaceans. A paper published this year by Vittoria Roncalli and others however provides added insight on the matter. The team questioned if the fitness of C. finmarchicus was harmed by feeding on C. finmarchicus. Fitness was measured as survival, grazing activity and reproductive success.

The team carried out two field collections of C. finmarchicus which were used for two independent experiments; one in June and another in July. The adult females were fed over a seven day period. The experimental design comprised three treatments: control, low dose and high dose diets of the toxic A. fundyense (50 and 200 cells mL^-1 respectively). The control treatment provided a diet solely consisting of non-toxic Rhodomonas spp. which are routinely used for maintenance of copepods in culture settings. The density of A. fundyense in the low dose treatment were comparable to densities found naturally in the Gulf of Maine. The high dose treatment included just the toxic dinoflagellate.

Over the week fecal pellet production and ingestion rates of the C. finmarchicus were observed to understand grazing activity.  Diet type did not affect either of these processes. They also found that the copepod did not feed discriminately in the low dose treatment (the only diet including both Rhodomonas sp. and A. fundyense). Survival rates too were not affected by the diet. Reproductive success on the other hand was hampered by the presence of A. fundyense. The reproductive success was defined by egg production and naupliar production (egg viability). In both experiments the cumulative egg count was lower in the high dose treatments. Fecundity was also reduced in the low dose treatment, but this was found in just the July experiment. The adverse effect of the dinoflagellate diets was more noticeable in copepod egg viability. After the experiment, total Nauplius production by females on the low and high dose diets was reduced by 35% to 75% compared to the control copepods.

The significance of this paper comes from the finding that the female copepod’s fecundity is affected negatively. Upon ingestion of A. fundyense the copepod experiences sub-optimal conditions requiring physiological adjustments of its energy allocation causing a reduction in reproductive success. This in turn could reduce recruitment to the next generation decreasing the abundance of C. finmarchicus. Over the last several decades, the prevalence and densities of HABs have increased due to human-induced climate change. In the Gulf of Maine this could, at the population level, affect the number of copepods, thus affecting the food source which sustains important fisheries in the Atlantic which has large economic consequences for the fishing industry in the afflicted areas.

Roncalli et al. has provided another example of a harmful algal blooms negatively effecting the fitness of a copepod fitness. However, further research is needed to establish whether the toxins are responsible for the observed effects or if other metabolites produced by A. fundyense are the cause. More importantly, in my opinion is the need for further research on how we can reduce the prevalence of HABs in the Gulf of Maine followed by the administration of these potential solutions.

Referenced papers

Campbell, R.G., Teegarden, G.J., Cembella, A.D. and Durbin, E.G. (2005) ‘Zooplankton grazing impacts on Alexandrium spp. In the nearshore environment of the gulf of Maine’, Deep Sea Research Part II: Topical Studies in Oceanography, 52(19-21), pp. 2817–2833.

Hassett, R.P. (2003) ‘Effect of toxins of the “red-tide” dinoflagellate Alexandrium spp. On the oxygen consumption of marine copepods’, Journal of Plankton Research, 25(2), pp. 185–192.

Roncalli, V., Turner, J.T., Kulis, D., Anderson, D.M. and Lenz, P.H. (2016) ‘The effect of the toxic dinoflagellate Alexandrium fundyense on the fitness of the calanoid copepod Calanus finmarchicus’, Harmful Algae, 51, pp. 56–66.