The impact of tidal flow restrictions on arid marshes

What happens to a wetland when you construct a dyke and cut off tidal flow for over two decades? Silvia Ibarra-Obando and fellow researchers looked at the effects of 22 years worth of tidal exclusion on Estero de Punta Banda, an arid estuary 120 km south of the U.S.-Mexico border.

Their study found that long-term tidal exclusion has led to hypersaline conditions with a decrease in species diversity. These results contrast with the increase in biodiversity typically found in diked marses in humid areas.

Although natural disturbance is important for shaping marsh dynamics, anthropogenic impacts such as berms, dikes and tide gates tend to have lasting detrimental effects on hydrological and hydrographic processes. Limited research has looked at the long-term consequences of these impacts, and this is one of the first studies to focus on the coastal zone of Mexico.

The team compared an undisturbd section of Estero de Punta Banda with an area that has been cut off from the tides via dyke since 1983. The dike was built to accomodate an industrial patio where oil-drilling platforms were assembled, enclosing a 0.45 km2 area.

The researchers measured soil salinity, percent soil humidity, percent vegetation cover, and percent frequency of occurrence of specific species. They also measured topographic profiles for each marsh. 

Soil Results

The interstitial soil salinity at the disturbed region was 3.5 times higher than the reference area. The team even had to underestimate soil salinity because the concentrations were consistently higher than the refractometer’s capabilities (160 ppt). Soil moisture was higher in the tidal area at 36.2% humidity compared to 26% humidity at the non-tidal site.

Vegetation Results

Tidal exclusion reduced species richness and diversity by close to half. Two years after dike construction, three marsh species had disappeared: Salicornia bigelovii, Suaeda esteroa, and Jamea carnosa. After 17 years, four more salt marsh plain species had vanished: Cuscuta salina, Distichlis spicata, Triglochin maritime, and Limonium californicum.

There was also a major dominance shift in the disturbed area leaving Bolbaschoenus maritima prevailing due to its high salt tolerance and deep roots. B. maritime has continued to persist in the disturbed region, while the reference area has experienced several dominance shifts over the years.

Topographic Results

The average elevation loss was -44.6 cm in the non-tidal area while the change at the tidal area fluctuated between gains and losses over time with an average change of -3.4 cm.

Implications

The study findings show that anthropogenic changes in marsh hydrology can lead to hypersaline conditions that in turn reduce species diversity and composition. Entire species disappear and rapid decomposition leads to dramatic elevation declines that will not keep pace with sea-level rise.

These findings also demonstrate a major difference between tidal exclusion in arid regions versus humid ones. Humid-climate estuaries tend to have an increase in vegetation biodiversity after tidal restriction due to high freshwater increases, while this arid marsh demonstrated the exact opposite trend.

--Reviewed by Evyan Borgnis

Ibarra-Obando, S., Poumian-Tapia, M., & Morzaria-Luna, H. (2010). Long-Term Effects of Tidal Exclusion on Salt Marsh Plain Species at Estero de Punta Banda, Baja California Estuaries and Coasts DOI: 10.1007/s12237-009-9253-x