Elevated trace elements in sediments and seagrasses at CO2 seeps

Abstract

Seagrasses often occur around shallow marine CO 2 seeps, allowing assessment of trace metal accumulation. Here, we measured Cd, Cu, Hg, Ni, Pb and Zn levels at six CO 2 seeps and six reference sites in the Mediterranean. Some seep sediments had elevated metal concentrations; an extreme example was Cd which was 43x more concentrated at a seep site than its corresponding reference site. Three seeps had metal levels that were predicted to adversely affect marine biota, namely Vulcano (for Hg), Ischia (for Cu) and Paleochori (for Cd and Ni). There were higher-than-sediment levels of Zn and Ni in Posidonia oceanica and of Zn in Cymodocea nodosa, particularly in roots. High levels of Cu were found in Ischia seep sediments, yet seagrass was abundant there, and the plants contained low levels of Cu. Differences in bioavailability and toxicity of trace elements helps explain why seagrasses can be abundant at some CO 2 seeps but not at others.

Seagrass Ecosystems of India as Bioindicators of Trace Elements

“…The presence of various seagrass species at the land and sea-interface makes them suitable bioindicators of coastal metal contamination (Bonanno and Borg, 2018;Mishra et al, 2019). This suitability of seagrass as bioindicators have been extensively used by the European Water Framework Directive using the endemic seagrass Posidonia oceanica and Cymodocea nodosa of the Mediterranean Sea (Bonanno and di Martino, 2016;Bonanno and Orlando-Bonaca, 2018;Bonanno and Raccuia, 2018).…”

“…However, it has been observed, that is not the case always (Bonanno and di Martino, 2017). The accumulation of trace elements from the water column is species-specific among seagrasses and depends on the plant physiology and the nature of the trace element, i.e., toxic or essential (Millero et al, 2009;Mishra et al, 2019). In India, the trace element studies of the water column above seagrass ecosystems are very less compared to that of the sediment and seagrasses.…”

“…In seagrasses the concentration of elements varies within the tissues; leaves, rhizomes and roots. Where roots accumulate the maximum concentrations and the leaves accumulates less (Bonanno and Orlando-Bonaca, 2018;Mishra et al, 2019) as higher metal concentration in leaves can lead to trace metal toxicity and damage photosynthetic apparatus of seagrass (Govers, 2014;Prange and Dennison, 2000). However, in India, trace metals and their toxicity on seagrass physiology or growth have not been reported.…”

“…Trace elements as the name suggests, occurs in very low concentrations in the marine environment. At these low levels, trace elements are not toxic and play a critical role in marine ecosystem functioning (Avelar et al, 2013;Mishra et al, 2019). Among these trace elements some are non-essential and toxic to organisms (As, Cd, Cr, Hg and Pb), whereas others act as essential micronutrients (Cu, Mn and Zn), provided that their concentrations do not exceed the threshold levels (Millero et al, 2009;Stockdale et al, 2016).…”

“…These trace elements pose serious risk to seagrass eco-physiology, because of their persistent nature in the marine sediment. Once accumulated in the seagrass roots their bioavailability increases (Bonanno and Borg, 2018;Govers, 2014) and under future ocean acidification and anthropogenic pollution scenarios their concentration and toxicity is predicted to increase (Mishra et al, 2019;. Once bioavailability increases, trace elements get absorbed into the root plasmalemma at the root: soil interface and are translocated to the leaves via rhizomes.…”