Influence of the porewater geochemistry on Fe and Mn assimilation in Laternula elliptica at King George Island (Antarctica)
Introduction
Iron and manganese contents in the tissues of the circum-Antarctic clam Laternula elliptica (King and Broderip, 1832) differ considerably among sites around Antarctica. This led scientists to search for the environmental sources of both metals to explain local differences. At King George Island (KGI; South Shetland archipelago, western Antarctic Peninsula (WAP)) several authors related relatively high contents of both metals in bivalve tissues to a high input of eroded bedrock material transported by melt water streams into the coastal areas (Abele et al., 2008, Curtosi et al., 2010, Husmann et al., 2012). Recently, Monien et al. (2011) reported a tripling of sediment accumulation rates in Maxwell Bay (KGI) during the last century, with the highest increase during the decade 1990–2000. The increased input of lithogenic debris coincides with intensified melt water discharge from retreating land glaciers on the Antarctic Peninsula as a consequence of the strong rise in air temperature in the WAP region during the last decades (Rignot and Thomas, 2002, Vaughan et al., 2003, Braun and Hock, 2004, Cook et al., 2005, Turner et al., 2005, Vaughan, 2006, Domínguez and Eraso, 2007, Steig et al., 2009, Rueckamp et al., 2011). Husmann et al. (2012) proposed the intensified sediment and melt water input at KGI to be responsible for the higher Fe accumulations in L. elliptica from KGI compared to individuals collected at Rothera Point (Adelaide Island). However, eroded bedrock material remains to be verified as source of high tissue Fe levels in L. elliptica.
As with other benthic deposit feeders, Laternula elliptica ingests particles and water from the benthic boundary layer. Trace metals are assimilated from both sources (Rainbow, 2002, Griscom and Fisher, 2004). The proportion of metal assimilation from the particulate and dissolved phase depends on the bioavailability of the metal in each fraction and on the physiological characteristics of the species (e.g., pH-conditions in the gut) (Wang and Fisher, 1999, Rainbow and Wang, 2001, Griscom and Fisher, 2004). Large amounts of lithogenic sediment particles are ingested together with the nutrition. However, the assimilation efficiencies (AE) of metals are generally higher for organic matter compared to inorganic matter, because organic particles are processed more intensely in the gut, due to their nutritional value (Willows, 1992, Decho and Luoma, 1996, Gagnon and Fisher, 1997, Lee and Luoma, 1998, Griscom and Fisher, 2004). Free metal ions are most easily absorbed and readily bioavailable to marine organisms (e.g., Bjerregaard and Depledge, 1994, Fisher et al., 1996).
Concentrations of Fe and Mn usually do not exceed low nanomolar levels in oxic ocean waters (Landing and Bruland, 1987, Bruland and Lohan, 2004, Middag et al., 2012), but bivalves accumulate trace metals even when exposed to low concentrations (Rainbow, 1990). Rainbow (1990) suggested sediment porewater as an alternative source of bioavailable metals for burrowing bivalves. Porewaters in the suboxic sediment zone (preferentially termed as manganous and ferruginous zones; Canfield and Thamdrup, 2009) generally show high concentrations of dissolved Mn(II) and Fe(II) due to the dissimilatory reduction of manganese oxides and iron(hydr)oxides during early diagenesis (e.g., Froelich et al., 1979, Berner, 1981, Lovley and Phillips, 1988, Rutgers Van Der Loeff et al., 1990, Canfield et al., 1993). Dissolved Fe(II) and Mn(II) diffuse into the benthic boundary layer or into the overlying water layers due to concentration gradients between porewater and seawater and depending on the content of organic matter in the sediment, remineralization rate, oxygen penetration depth, and biological or physical reworking of the sediment (Lynn and Bonatti, 1965, Yeats et al., 1979, Sundby and Silverberg, 1985, Elrod et al., 2004, Laës et al., 2007, Pakhornova et al., 2007, Sachs et al., 2009, Severmann et al., 2010, Kowalski et al., 2012).
However, the assimilation of porewater derived Fe and Mn by Laternula elliptica contradicts earlier studies, which assumed a predominating assimilation of both elements from lithogenic particles, in particular eroded bedrock material transported by melt water streams from the glaciers to the cove (Abele et al., 2008, Curtosi et al., 2010, Husmann et al., 2012). We questioned this concept, since the highest metal tissue contents of L. elliptica were found in animals from Deception Island (Deheyn et al., 2005). This island is only marginally covered with glaciers, but highly affected by geothermal influence (e.g., import of dissolved metals; Elderfield, 1972, Rey et al., 1995).
Hence, we wanted to clarify 1) whether the Fe and Mn accumulation by Laternula elliptica differs significantly among animals at sites of high sediment input (located in front of the melt water inlets) and lower sediment impact (outside of the cove) in Potter Cove (PC) and 2) whether high concentrations of dissolved Fe and Mn in sediment porewater are an important source for the assimilation by L. elliptica, similar to the hydrothermal influence at Deception Island.
Section snippets
Sample collection and experimental treatment
Individuals of the Antarctic clam Laternula elliptica were collected by scuba divers at seven stations in PC (Fig. 1) between January and March 2010. Five stations (B; Fig. 1c) located next to the discharge area of melt water streams were chosen pseudo randomly on a Universal Transverse Mercator (UTM)-grid (100 m grid point distance). One station was positioned in a newly ice free area (ID) and one in the outlet of PC to Maxwell Bay (C). Schloss and Ferreyra (2002) reported a decreasing
Results
Animals from stations B and C (N = 28) were older than 13 years, whereas animals collected at ID (N = 9) reached a maximum age of only five years. The lack of older individuals at station ID indicates a colonization of the southern side of the island (close to station ID; Fig. 1) during the Austral summer 2004/05. The island was covered by the Fourcade Glacier until 2002/03, and the results from station ID provide first evidence that benthic colonization around the rocky island commenced only
What controls Fe accumulation in Laternula elliptica?
Bivalves are basically iso-osmotic to the surrounding water, resulting in hemolymph concentrations of major seawater ions (e.g., Na+, Cl−, SO42−) similar to the environmental concentrations, as long as the clam siphons (or shells of a mussel) remain open (Robertson, 1949, Robertson, 1953, Shumway, 1977, Willmer et al., 2000). In line with this, the major ion concentrations (Ca, K, Na, Sr; Table 2) in the hemolymph of Laternula elliptica did not differ from either seawater or porewater
Conclusions and outlook
Fig. 6 summarizes the Fe and Mn uptake pathways for Laternula elliptica in PC. For both elements we assume the flux of Fe- and Mn-rich porewater into the oxic sediment layer or across the benthic boundary as important source for the bivalve, even though the assimilation may differ. Since Mn oxidation is relatively slow compared to the oxidation of Fe and Mn concentrations in hemolymph were as low as in bottom water or in the porewater of the first cmbsf, we expect a predominant assimilation of
Acknowledgments
We would like to thank Stefanie Meyer (AWI) for her support during expedition preparations. Further we would like to thank her and Ilsetraut Stölting (AWI) for their help in the laboratories. The divers and the crew of the Argentinean Antarctic Station Carlini are thanked for their logistic help. Additional analysis of reference materials were done by Norman Loftfield (Institute of Soil Science of Temperate Ecosystems, University of Göttingen) to prove the correctness of our digestion
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