Ben Thornes

Cadmium release from Spartina Anglica dieback

Tuesday 15th July 2014

Cadmium release caused by the die-back of the saltmarsh cord grass Spartina anglica in Poole Harbour (UK).

Over the past 100 years the perennial Spartina anglica C.E. Hubbard has colonised many estuaries worldwide, for example, the USA, UK, France, Australia, China and the Netherlands have all experienced colonisation. Spartina anglica was introduced into Poole Harbour in around 1900; it was extremely successful for the first quarter century, reaching its maximum extent of approximately 800 ha in 1924, covering approximately 63% of the intertidal area of Holes Bay. Since 1924/25 however, this plant has died back in many areas of the estuary, and in 1964 little active spread was found in the area. Aerial photographs indicate that 38.6% of the total saltmarsh had been lost between 1947 and 1993 with latest figures showing approximately 56.3% of saltmarsh has been lost.

Of the 7 million m3 of sediment accumulated by Spartina growth in Poole Harbour, approximately 4 million m3 of this sediment has already been released through die-back, and the approximately 3 million m3 remaining are in the process of further decline. The mechanisms of the die-back are complex, and still not fully understood. There have been several hypotheses put forward for this die-back, including the erosion of saltmarsh edges, higher temperatures, algal mat deposition during the growing season, anaerobic conditions causing the death of the rhizomes, invasion of other species, reaching the end of its natural life, and infaunal effects, especially caused by the polychaete Nereis diversicolor through bioturbation and herbivory.

The interplay of sea-level rise and population density is expected to shape the saltmarshes of the future. This study was undertaken to evaluate whether a relationship between metal contamination and die-back could be established. However, if sea levels were to rise too much, it must be expected that the die-back will increase, which would release even more potentially contaminated sediment, forming a significant part of any newly generated marshlands.

Spartina anglica have been reported as having higher sediment accretion rates than any other saltmarsh vegetation, tending to strongly accumulate cadmium. Cadmium is of special interest as it has a very high mobility, i.e it can be assumed to be released rapidly from the sediment after a large-scale die-back, and can be deleterious to a wide variety of marine organisms.

To investigate this, a large survey of the Poole Harbour estuary was undertaken. 53 cores were obtained, 11 of these cores were extracted from the Southern Bights region of Poole Harbour during autumn 2008 and were analysed for this study. No obvious impact of metal contamination on the Spartina anglica growth/die-back could be detected in the study zone, although the die-back did seem to have substantially influenced the metal concentrations in the sediments of the estuary. Based on the core samples taken for this study, the remaining patches of Spartina anglica still retain elevated concentrations and the overall cadmium concentrations in the sediments have risen since 1925. The data collated in the study indicates that considerable quantities of cadmium have been released; therefore it can be assumed that the remaining patches are potentially at risk of further cadmium release. The quantity of sediment that could potentially be released by Spartina anglica’s die-back is very high. If die-back is accelerated, for example by rapidly rising sea-level or increased storm frequency, then sudden and high levels of cadmium release may cause harmful effects to estuary biota and be detrimental to nearby shellfish farms.

Read the full research article here

Find out more about Dr. Roger Herbert here

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