Which term refers to the presence of a toxic "red tide" in certain algal blooms?

The term that refers to the presence of a toxic "red tide" in certain algal blooms is dinoflagellate. Dinoflagellates are a group of single-celled organisms that can undergo rapid population growth under favorable conditions, leading to algal blooms. Some species of dinoflagellates produce toxins, which can accumulate in the water and cause harmful effects to marine life and humans, hence the term "red tide."

Algal blooms are not synonymous with the specific toxicity associated with dinoflagellates; rather, they are a broader category that can include various types of algae, some of which are harmless. A zygospore is a resting or dormant stage of certain algae and fungi, which does not relate to algal blooms directly. Euglenoids are another type of algal species that are primarily photosynthetic and do not typically contribute to red tides. These distinctions clarify why dinoflagellate is the most accurate term in this context.