Modelling mussel ( Mytilus spp. ) microplastic accumulation.

: Microplastics (MPs) are a contaminant of growing concern due to their widespread 14 distribution and interactions with marine species, such as filter feeders. To investigate the MPs 15 accumulation in wild and cultured mussels, a Dynamic Energy Budget (DEB) model was developed 16 and validated with the available field data of Mytilus edulis ( M. edulis , wild) from the North Sea 17 and Mytilus galloprovincialis ( M. galloprovincialis , cultured) from the Northern Ionian Sea. 18 Towards a generic DEB model, the site-specific model parameter, half saturation coefficient ( X k ) 19 was applied as a power function of food density for the cultured mussel, while for the wild mussel it 20 was calibrated to a constant value. The DEB-accumulation model simulated the uptake and 21 excretion rate of MPs, taking into account of environmental characteristics (temperature and 22 chlorophyll-a). An accumulation of MPs equal to 0.53 particles individual -1 (fresh tissue mass 1.9 g) 23 and 0.91 particles individual -1 (fresh tissue mass 3.3 g) was simulated for the wild and cultured 24 mussel after 4 years and 1 year respectively, in agreement with the field data. The inverse 25 experiments investigating the depuration time of

Considering that these values originate from surface waters and that mussels live in the near surface 289 layer (0-5 m), C env is estimated as a mean value of the upper layer with the methods described by 290 Kooi et al. (2016), who studied the vertical distribution of MPs, considering an exponential 291 decrease with depth. Specifically, in the N. Ionian Sea, mussels were collected from a depth up to 3 292 m (Digka et al., 2018a), while in the North Sea (Van Cauwenberghe et al., 2015), there is no 293 information and thus a maximum depth of 5 m is adopted. 294 In the North Sea simulation, the effect of tides is taken into account by considering that the 295 mussel originated from the intertidal zone, is submerged 12 hours during the day (Van The reproductive buffer (R) is assumed to be completely emptied at spawning (R = 0) 352 (Sprung, 1983, Van Haren et al., 1994. In order to simulate mussel"s spawning, the gonado-somatic 353 index (GSI) defined as gonad dry mass over total dry flesh mass was computed at every model"s 354 time step (Eq. 17 Table 1; the water content of the fresh tissue mass was assumed 80% according to 355 Thomas et al. (2011)). Spawning was induced by a critical value of GSI (GSI th , of April until the end of June (Sprung, 1983, Cardoso et al., 2007.

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In both areas, the model was initialized so that the simulated individual is in the juvenile phase 367 (V < V p ; Table 3) and the reproductive buffer can be considered to be empty (R = 0) (Thomas et al., November until March and the juvenile mussels grow up to 6-6.5cm after approximately one year 374 according to the information obtained from the local farms in the region and Theodorou et al.

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(2011). The initial fresh tissue mass was distributed between the structural volume (V) and reserves 376 energy (E). Energy allocated to those two compartments was firstly constrained by the initial length 377 (L) and then energy allocated to V was in Eq.10 ( Table 1). The initial value of E was set so that the 378 simulated individual has an initial weight that corresponds to the juvenile phase (V < V p ) ( Table 5).

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Finally, for both model implementations, the initial accumulation of MPs in the mussel"s tissue (C) 380 was set to zero.  , 2015). In this simulation, the mussel"s gut clearance is 392 achieved by the excretion of MPs through faeces (3 rd term of Eq. 18), and thus it is necessary to 393 maintain the existence of food in the mussel"s environment in order to ensure that the feeding-394 excretion processes will occur.  Finally, the effect of the environmental forcing data and some model"s parameters on the resulting 399 MPs accumulation by both mussels is explored through sensitivity experiments. These were used to 400 derive a new function that predicts the level of MPs pollution in the environment.
. In order to also examine the effect of tides, in the North Sea implementation, the sensitivity 412 experiments were conducted twice: the first time assuming that the mussel is permanently 413 submerged and the second time assuming that the mussel is periodically exposed to the air.

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Preliminary sensitivity experiments showed that the MPs accumulation is highly depended on 415 the prevailing conditions regarding the CHL-a, temperature and C env and the mussel"s growth that is 416 regulated by the half saturation coefficient (X k ). Therefore an attempt was made using the model"s exposure would be considered, the perturbed temperature will probably affect even more the MPs 569 accumulation on the intertidal than the subtidal mussel. The effect of the C env is slightly higher and 570 lower on the MPs accumulation by the intertidal mussel when perturbed +10% and -10% 571 respectively, however the difference of the sensitivity index (%) between the two mussels (intertidal 572 vs. subtidal) is small, indicating that the environmental MPs concentration affects similarly both 573 mussels, regardless the continuous or intermittent feeding-excretion process.

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The comparison between the mussel sensitivity indexes in the N. Ionian and the North Sea 575 (in conditions of submergence) study areas reveals some important differences. Generally, most of 576 the perturbed (either +10% or -10%) variables and parameters (i.e. CHL-a, temperature, X k ) present 577 higher sensitivity on the MPs accumulation by the mussel from the N. Ionian Sea. This is attributed 578 to the prevailing environmental conditions and specifically the lower food availability (CHL-a) and 579 the higher temperature range in the N. Ionian Sea compared to the North Sea, which greatly 580 determine the feeding processes, the mussel"s growth and hence the MPs accumulation. The

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perturbed C env in both study areas appears to affect similarly the MPs accumulation on both mussels 582 (¬10%), with the small difference probably attributed to the higher abundance of seawater"s MPs 583 present in the North Sea compared to the N. Ionian Sea. Finally, the half saturation coefficient for 584 the inorganic particles (Y k ) has no effect on the MPs accumulation of both North Sea and N. Ionian 585 Sea mussels, indicating that the amount of inedible particles (i.e. MPs) is relatively low in both 586 areas and thus the Y k does not affect the way that the organic particles are being ingested 587 (Kooijman, 2006). According to Ren (2009), when the inorganic matter is low, the K(y) (Eq. 5; 588 Table 1) is approximately equal to X k and then Y k is the least sensitive parameter for the ingestion 589 rate and thus growth.

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The DEB-accumulation model output was used to determine the coefficients in Eq. 19 by the because seawater composition is closely related to the season. As more field data becomes available 622 from various environments, such an approach could result to more generic formulations for the site-623 specific parameter X k , so that the model could be applied in several areas of interest, where field 624 growth data are absent and/or to simulate the potential mussel growth in the 2D space.

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The simulation of MPs accumulation by the mussels, using the DEB-accumulation model, is 626 in good agreement with the available field data. The MPs accumulation by the cultivated mussel 627 (fresh tissue mass 3.42 g) originated from the N. Ionian Sea with mean C env = 0.0012 ± 0.024 628 particles L -1 , is 0.91 particles individual -1 and by the wild mussel (fresh tissue mass 1.87 g) from the 629 North Sea with mean C env = 0.4 ± 0.3 particles L -1 is 0.64 particles individual -1 . If these 630 concentrations are expressed per gram of wet tissue of mussels, the cultivated mussel contamination 631 (0.27 particles g -1 w.w.) is comparable with the wild mussel (0.34 particles g -1 w.w.), despite the 632 much lower environmental MPs concentration (C env ) in the N. Ionian Sea than the North Sea. This 633 comparison aims to highlight the significant impact of the prevailing environmental conditions 634 (CHL-a and temperature) on the MPs accumulation by the mussels, although they originate from 635 https://doi.org/10.5194/os-2020-11 Preprint.  Table 2 for model variables, Table 3 for parameters and Table   1222 4 for initial values 1223 a notation refers to feeding equations handling each type of suspended matter separately (i=1 for algae and i=2 for 1224 microplastics) where units transformation is applied when it is necessary (see Table 3).