Prior to 1990, Laguna Madre, the southernmost bay system along the Texas coast, was known for its clear water. In 1990, a bloom of a single algal species called Aureoumbra lagunensis colored the water brown—a condition known as Texas brown tide. A combination of unusual environmental conditions coupled with unusual characteristics associated with that species triggered the initial bloom and caused the bay’s ecosystem to transition to conditions that favored the resurgence of brown tide in subsequent years. Factors causing brown tide in Laguna Madre include the following:
- Salinity increased in Laguna Madre as a result of a long drought period and lowered water levels. lagunensis tolerates high-salinity water, unlike most other algae.
- High-salinity and low-water conditions combined with a hard freeze effectively crashed the resident ecosystem and wiped out other species of phytoplankton, as well as grazers, invertebrates, fish, and submerged aquatic vegetation (SAV).
- Unlike green algae, lagunensis cannot assimilate nitrate and, consequently, its nitrogen needs must be met by ammonium. In most aquatic ecosystems, the predominant form of nitrogen is nitrate. In Laguna Madre, however, the large quantity of decaying fish, invertebrates, SAV, and other organisms provide ample supplies of ammonium for brown tide growth.
- lagunensis is slow-growing compared to most algal species and, to persist and thrive, it needs a system that has a low flushing rate. Laguna Madre provides that condition.
Laguna Madre is an ecosystem that favors Texas brown tide because of its high salinity, which eliminates potential competitors and grazers; low flushing rate, which makes slow growth possible; and a supply of ammonium from decaying organisms. As the internal ammonium sources dwindle, they might be supplemented by external sources of ammonium from increasing agriculture and urban and suburban development in the watershed (Bricker et al. 2007).
Reference:
Bricker, S., B. Longstaff, W. Dennison, A. Jones, K. Boicourt, C. Wicks, and J. Woerner. 2007. Effects of Nutrient Enrichment In the Nation’s Estuaries: A Decade of Change. NOAA Coastal Ocean Program Decision Analysis Series No. 26. National Oceanic and Atmospheric Administration, National Centers for Coastal Ocean Science, Silver Spring, MD. Accessed October 2016. https://yosemite.epa.gov/oa/EAB_Web_Docket.nsf/(Filings)/3BE82A42C7ED8C3585257B120059CB8A/$File/Opposition%20to%20Petition%20for%20Review%20–%20Ex.%2010%20Part1…23.53.pdf.