To kick the jellies off, I found on the NSF site a great primer on jellyfish:

Gulf of Mexico

THE BIGGEST DEAD ZONE IN THE WESTERN HEMISPHERE

The white sands and sparkling emerald waters of the Gulf of Mexico’s beaches belie a dirty little (open) secret: a huge Dead Zone that is devoid of almost all life except jellyfish is expanding in the Gulf of Mexico. During the summer of 2008, the Gulf’s Dead Zone covered about 8,000 square miles, about the size of Massachusetts. It is expected to soon reach about 10,000 square miles.

CREATION OF THE DEAD ZONE
The Gulf’s Dead Zone is produced every summer by tons of fertilizer, sewage and animal wastes that are continuously dumped into coastal waters by the Mississippi and Atchafalaya Rivers. These pollutants do their dirty work by fertilizing huge algae blooms that decay through a process that robs Gulf waters of oxygen. Most sea creatures flee or suffocate to death in the Dead Zone’s oxygen-starved waters, leaving highly adaptable jellyfish to proliferate unrestrained by predators and competitors and to gorge on the Gulf’s bounty of plankton.

GROWING JELLYFISH POPULATIONS
The most abundant species of jellyfish in the Gulf are the sea nettle and moon jellyfish, which typically swarm over hundreds and perhaps even thousands of square miles each summer. Studies show that these species became significantly more abundant and expanded their ranges during the 1980s and 1990s. Moreover, since 2000, the Gulf has hosted invasions of several non-native jellyfish species, including the Australian jellyfish.

Signs that the Australian jellyfish is satisfied with its adopted Gulf home include its tendency to swell from its usual fist-size to the size of dinner plates in the Gulf. In addition, the Gulf’s population of Australian jellyfish is steadily growing and expanding its range; this species recently reached North Carolina.

– https://www.nsf.gov/news/special_reports/jellyfish/textonly/locations_gulfmexico.jsp

Is it hyperbole to say devoid of almost all life? I mean, people still fish, there are still dolphins. Just not very much of anything.