It could be an algal bloom which sometimes are called "red tides" An algal bloom is an abundance of phytoplankton. Sometimes it is harmful to other life but most times it is not. Harmful algal blooms in Florida
often have a neurotoxin called brevetoxin which is caused by a dinoflagellate, Karenia brevis
. It can harm fisheries and can make some shellfish deadly to humans.
The causes can be natural or human caused. Agricultural runoff can be a possible cause which can contain nitrates and phosphates...which is the normal limiting factor for their growth.
We get algal blooms here on the SF Bay
in the spring. I take out scientists who study these and other nutrient related occurrences. Once in a great while the south SF Bay
gets a massive algal bloom which can drop the O2 levels in the water and have caused fish
kills amongst certain species.
Scientists don't really use the term "red tide". The broader and more accurate description is "algal bloom". Sometimes algal blooms are quite visible and sometimes they are not very visible at all. We use a fluorometer mounted to a CTD (Conductivity-Temperature-Depth) to help detect these blooms.