Limited data coverage on harmful algal blooms (HABs) in some regions makes assessment of long-term trends difficult, and also impedes understanding of bloom ecology. Here, observations reported in a local newspaper were combined with cell count and environmental data from resource management agencies to assess trends in Karenia brevis “red tide” frequency and duration in the Nueces Estuary (Texas) and adjacent coastal waters, and to determine relationships with environmental factors. Based on these analyses, the Coastal Bend region of the Texas coast has experienced a significant increase in the frequency of red tide blooms since the mid-1990s. Salinity was positively correlated with red tide occurrence in the Nueces Estuary, and a documented long-term increase in salinity of the Nueces Estuary may be a major factor in the long-term increase in bloom frequency. This suggests that freshwater inflow management efforts in Texas should consider impacts on red tide habitat suitability (i.e., salinity regime) in downstream estuaries. Natural climate variability such as the El Niño-Southern Oscillation, which is strongly related to rainfall and salinity in Central and South Texas, was also an influential predictor of red tide presence/absence. Though no significant change in the duration of blooms was detected, there was a negative correlation between duration and temperature. Specifically, summer-like temperatures were not favorable to K. brevis bloom development. The relationships found here between red tide frequency/duration and environmental drivers present a new avenue of research that will aid in refining monitoring and forecasting efforts for red tides on the Texas coast and elsewhere. Findings also highlight the importance of factors (i.e., salinity, temperature) that are likely to be altered in the future due to both population growth in coastal watersheds and anthropogenic climate change.


  • Red tides formed by the marine dinoflagellate Karenia brevis have affected Gulf of Mexico coastlines for centuries [1, 2], typically during late summer-fall

  • Beginning with the time frame covered by NOAA (2005–2013) and/or Texas Health Department (TXHD) (1996–2016) cell counts, there was near perfect corroboration of annual red tide occurrence reported in the newspaper (S2 Table)

  • In the Coastal Zone there were two instances (2000, 2012) of a red tide being reported in the newspaper that did not appear in the TXHD or NOAA records

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Red tides formed by the marine dinoflagellate Karenia brevis have affected Gulf of Mexico coastlines for centuries [1, 2], typically during late summer-fall. Magaña et al [1] reported that the frequency of red tides on the Texas coast increased over the period of 1996–2000 compared to earlier years. In both instances, availability of historical data limited the scope of inferences that could be drawn from study findings [8,9,10]


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