Title : 
 Temporal trends of mercury levels in fish ( dab , Limanda limanda ) and sediment from the German Bight ( North Sea ) in the period 1995–2020 

 Abstract : 
 As a toxic and harmful global pollutant , mercury ( Hg ) enters the marine environment through natural sources , and human activities . It bioaccumulates through the food chain and therefore , Hg is of great importance for environmental monitoring . This study aims to answer the question if Hg contamination in fish and sediment from the German Bight follows temporal trends . Therefore , 496 individual female dab ( Limanda limanda ) were analyzed . The Hg concentrations in the muscle of dab from the German Bight showed significant increase in function of time with an annual percental change of 1.4 % , leading to a 41 % increase in Hg contamination level within 25 years of monitoring . At the same time , Hg concentrations in sediment — analyzed in 86 samples — significantly decreased in the nearby North Sea environment . This surprising contradiction is shown in the present study and possible causes are discussed . It could be clearly shown that contamination in sediment and biota can follow completely different time courses and therefore , different environmental matrices should be considered in future monitoring studies . Age of the fish turned out to be a biological factor of particular importance for temporal trend analysis . Methode Mercury in fish The determination of Hg in fish from 1995 to 2009 followed the protocol described by Harms ( 1981 , 1998 ) . Briefly , wet samples were subjected to a pressure digestion by nitric acid in PTFE tubes heated up to 135 ° C for 2 h. This was partly supported by a microwave system , “ MLS - Ethos - plus ” ( MLS Mikrowellen Labor - Systeme GmbH , Leutkirch , Germany ) . Mercury was determined by cold vapor atomic absorption spectrometry using a flow injection mercury system Model 420 ( Perkin Elmer , Rodgau , Germany ) with background compensation and Hg lamp . A clean bench was used to avoid contamination during sample processing . The limit of detection ( LD ) was calculated according to DIN 32,645 ( DIN , 1994 ) and was always below 1.2 μg Hg / kg wet weight ( ww ) . The method used from 2010 to determine Hg in fish is in detail described in Kammann et al . ( 2021 ) . Briefly , samples were freeze - dried using a lyophilizer ( LD 1–2,Christ , Osterode , Germany ) and subsequently homogenized using an agate mortar or an ultra Turrax tube drive dispenser ( IKA , Staufen , Germany ) to obtain a dry and homogenous sample powder . Total Hg was determined by atomic absorption spectrometry using a Direct Mercury Analyzer ( DMA-80 , MLS , Leutkirch , Germany ) . A total of 20–30 mg of each sample were weighted into the boat containers of the DMA-80 . Direct analysis for total Hg content was performed using a 10 - level calibration with standards in 0.5 M nitric acid . The LD and the limit of quantification ( LQ ) were calculated from a standard curve according to DIN 32,645 ( DIN , 1994 ) with a confidence level of 99 % . Considering the sample preparation , a LD of 0.080 μg / kg ww and a LQ of 0.230 μg / kg ww were determined for Hg . No values below these limits were found in any sample under investigation . Preparation steps were carried out under clean benches all the time . Clean lab conditions of ISO class 7 were available since 2018 . Nitric acid ( 69 % , ultrapure quality ) and certified standard solutions of Hg were purchased from Carl Roth , Karlsruhe , Germany , in 0.5 M nitric acid . Ultrapure water was obtained from a Purelab Flex 3 device ( Elga Veolia ; High Wycombe , UK ) . The accuracy of the Hg measurement in fish was determined by analysis of Certified Reference Materials ( e.g. , DORM-4 ) obtained from the National Research Council ( NRC ) in Canada which was taken through the same analytical procedure as the samples . All samples were measured in triplicates . External quality assurance was done by participation in laboratory proficiency tests conducted by QUASIMEME ( www.wepal.nl ) designed for marine environment analytics from 1997 to 2021 . Results of 55 QUASIMEME intercalibrations are presented in Table S1 with a mean z - score of 0.09 and only 7 % of unsatisfying results ( z - score   >    =   │ 2.5 │ ) .