Harmful algal blooms may color the water red or brown, and are often referred to as red tide or brown tide. The discoloration of the water is caused by the millions of microscopic phytoplankton that bloom.

The "brown" tint often seen in Long Island’s ocean water is typically caused by sediment, organic matter, and algae blooms, rather than pollution. In 2026, environmental scientists note that heavy rains often wash soil and "Colored Dissolved Organic Matter" (CDOM) from the island's many bays and inlets into the Atlantic. Additionally, certain types of microscopic phytoplankton—specifically diatoms and dinoflagellates—thrive in the nutrient-rich coastal waters, creating a natural "brown tide." While this can look unappealing compared to the turquoise Caribbean, it is actually a sign of a highly productive ecosystem. The churning surf of the Atlantic also keeps fine sand and silt suspended in the water column near the shore, which reflects light in a way that appears tea-colored or murky, especially after a storm or during the peak summer growing season.