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Effects of oil and dispersed oil on temperate seagrasses

A summary of the effects of oil and dispersed oil on temperate sea grasses.

Seagrasses are a vital component of many near-shore areas of coastal Australia, providing habitat and food for a variety of marine, estuarine and beach dwelling animal species, as well as being important in the nutrient cycle of near-shore and foreshore areas.

Oil spills may affect seagrasses by:

  • direct smothering
  • toxic effects of the water-soluble fraction of oil.

Although Australian approved oil spill dispersants rate predominantly as slightly toxic to practically non-toxic by the International Maritime Organization GESAMP classification system, there is limited knowledge on the specific effects on seagrasses exposed to dispersants.

The use of dispersants may also increase the exposure of submerged seagrasses to oil as dispersed oil enters the water column.

To address the knowledge gap on the impact on seagrasses exposed to oil spills, AMSA entered into a funding agreement on behalf of the National Plan with the University of Technology Sydney (UTS). The National Plan also provided support and technical advice to the research team. The UTS contributed additional funding and were also successful in securing an Australian Research Council Industry Linkage grant to expand the project.

The UTS researchers used a combination of laboratory and field experiments to compare the toxicity of several oils and dispersant/oil mixtures on seagrasses, and to provide advice on the best approach to the use of dispersants on oil spills in the vicinity of seagrasses.

The project outcomes were:

In this work, the toxic impact of oil and dispersants was measured on three temperate seagrasses:

  • Zostera capricorni
  • Zostera muelleri
  • Halophila ovalis.

A range of experiments were performed in the field and in the laboratory using whole plants and seagrass leaf sections. Seagrasses were exposed to the water accommodated fraction (WAF) of Tapis Crude oil and IFO-380 (Intermediate Fuel Oil) produced with and without the addition of dispersants, Corexit 9527, Ardrox 6120 (for dispersing the Tapis Crude oil) and Corexit 9500 and Slickgone LTSW (for dispersing the IFO-380).

The findings of the study were that non-dispersed oil, in general, leads to less photosynthetic stress to Zostera capricorni and Halophila ovalis compared with the addition of a chemical dispersant. Zostera muelleri did not appear to be negatively impacted by either dispersed or non-dispersed Tapis crude oil.

In most treatments, where photosynthetic stress was detected in the seagrasses, full recovery was evident prior to the conclusion of the experiment, whilst in other cases recovery occurred within the ten hour exposure period.

In conclusion, when the addition of a chemical dispersant is deemed necessary to protect other resources in the area, the seagrass may still recover depending on the dispersant used.

Research report

Read the research report Effects of oil and dispersed oil on temperate seagrass: scaling of pollution impacts.

Last updated: 9 November 2020