Zooplankton response to cyanotoxins

Mass development of cyanobacteria (blue-green algae) is common in many lakes and coastal waters worldwide as a result of an increased nutrient loading. These blooms of cyanobacteria are undesirable for several reasons; thick mats of algae and unpleasant odour reduces the recreational values, bad taste reduces the quality of drinking water and the potent toxins produced by some species of cyanobacteria endanger human health. Aquatic organisms, especially zooplankton, grazers of algae, are also affected by cyanotoxins.

In my thesis I have studied the effects of cyanobacterial toxins on zooplankton. A survey of Swedish lakes revealed that elevated concentrations of the cyanobacterial toxin, microcystin, correlated with reduced abundance of large zooplankton. That is, large zooplankton seems to be negatively affected by microcystin while smaller zooplankton less affected.

In a laboratory experiment I found that the ability of a large zooplankter, Daphnia, to feed on and tolerate toxic algae is improved if the animal has been previously exposed to cyanotoxins. Pre-exposed individuals show higher survival rates and higher reproductive output compared to individuals that have not encountered toxic cyanobacteria before.

Further studies revealed that this tolerance is an induced defence developed during an individual's lifetime. Moreover, this trait could be transferred from mother to offspring, a phenomenon called maternal effects. Daughters from mothers that had experienced toxic cyanobacteria had higher fitness compared to daughters of non-exposed mothers.

Tolerance to toxic cyanobacteria differs among populations from different lakes, among clones inhabiting the same lake, and also among individuals within a clone. Difference in tolerance among populations and clones probably reflect the animals? historical experience of toxic cyanobacterial blooms. The difference in tolerance among individuals, however, is more difficult to explain and needs to be studied further.