The protist that can heavily populate the oceans when nutrients are ample and create red tides is dinoflagellates.

Here's why:

* Abundant nutrients: Dinoflagellates thrive in nutrient-rich waters, often fueled by agricultural runoff or sewage discharge.

* Rapid reproduction: They reproduce quickly, leading to massive blooms.

* Pigments: Many dinoflagellate species contain pigments that give the water a reddish-brown color, hence the term "red tide."

* Harmful effects: Some dinoflagellate species produce toxins that can harm marine life, including fish, shellfish, and mammals. These toxins can also accumulate in shellfish and pose a threat to human health.

While other protists like diatoms can also bloom in nutrient-rich conditions, dinoflagellates are specifically known for their association with red tides due to their pigmentation and toxin production.