Seaweed is a sink for isotopically light molybdenum in temperate coastal environments

The content and stable isotopic (⁹⁸Mo/⁹⁵Mo) composition of bladder wrack (Fucus vesiculosus) were investigated for their potential as a sink for dissolved molybdate in coastal environments. The macrophytes were grown in mesocosms fed with brackish coastal waters from a temperate coastal bay (Kiel Bight) under ambient conditions of simulated environmental stress, e.g., enhanced temperature and/or CO₂ partial pressure. Conditions were set up to simulate possible future climate change scenarios applying a delta-approach. Dissolved molybdate in brackish Baltic seawater was isotopically found to be close to the open North Sea, with a slight trend towards isotopically more negative values with decreasing salinity. This is in-line with a fresh water contribution originating from weathered minerals in the catchment area. It was found that the organic tissue of Fucus vesiculosus was substantially enriched in ⁹⁵Mo compared to dissolved seawater molybdate by up to -1.5 mU. Isotope fractionation was slightly enhanced by increasing temperature but no effect was observed for the other or combined treatments. Seasonal effects in the contents and isotope signatures of the tissue were observed with diminished incorporation of Mo during summer time and an associated lowered isotope signature. No clear trend in the fractionation of the different Mo isotopes can be predicted for different complex climate change scenarios, considering an increase in carbon dioxide partial pressure, in combination with temperature. Mainly temperature seems to impact Mo incorporation and associated isotope signature. A mass balance approach indicates, that the impact of Fucus growth on the total Mo budget in the coastal bight is small due to a continuous water exchange. The results for Mo in seaweed are compared to other trace elements and stable isotope signatures (C, N, S) incorporated into the tissue, too. The results from the present study demonstrate the potential of seaweed to act as an environmental multi-element biomonitor.