Hypersalinity During Regional Drought Drives Mass Mortality of the Seagrass Syringodium filiforme in a Subtropical Lagoon
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Seagrasses are sensitive to local environmental conditions such as salinity, the underwater light environment, and nutrient availability. To characterize seagrass coverage and condition, as well as to relate changes in community structure to local environmental and hydrologic conditions, we monitored seagrass communities in the Upper Laguna Madre (ULM), Texas annually from 2011 to 2015. In 2011 and 2012, the lagoon was dominated primarily by Halodule wrightii, with mixed meadows of H. wrightii and Syringodium filiforme located in the northwest of our study area. By 2013, the expansive S. filiforme meadows had disappeared and the species was restricted to the northernmost reaches of the lagoon. The S. filiforme mortality occurred following an extended period of extremely high salinity (salinities 50–70) during a regional drought. Continuous measurements of underwater photosynthetically active radiation and stable carbon isotopic signatures of seagrass blade tissues did not suggest light limitation, and H. wrightii N/P molar ratios near 30:1 were not indicative of nutrient limitation. Based on the absence of strong evidence for light or nutrient limitation, along with the known tolerance of H. wrightii for higher salinities, we conclude that hypersalinity driven by regional drought was likely the major driver behind the observed S. filiforme mortality. With a substantial portion of the global seagrass distribution threatened by drought in the next 50 years, the increased frequency of hypersaline conditions is likely to exacerbate stress in seagrass systems already vulnerable to the effects of rising water temperatures, eutrophication, and sea level rise.
KeywordsSeagrass Syringodium Hypersalinity Laguna Madre Monitoring
We thank K. Jackson, K. Darnell, V. Congdon, and many others for assistance with the Texas Seagrass Statewide Monitoring Program. We are appreciative to J. Meiman for field assistance within Padre Island National Seashore (PINS) and for providing salinity data from Bird Island and Baffin Bay. We thank J. Tunnell for field assistance in ULM and continued support of our monitoring efforts. We thank T. Whiteaker and S. Schonberg for assistance with GIS mapping.
Funding for ULM seagrass monitoring was provided by the Coastal Bend Bays & Estuaries Program (#1201 and 1336), and monitoring within PINS was funded by the National Park Service (#P11AT51021). This is contribution #68 of the Marine Education and Research Center in the Institute for Water and Environment at Florida International University.
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