Conceptualization, S.U. and S.W.F.; Methodology, M.B. and S.U.; Validation, S.U. and A.U.G.; Formal analysis, M.B., S.U. and S.D.; Investigation, M.B., S.U., A.U.G., Y.A.-E., L.A.-M. and V.V.K.; Resources, S.D.; Data curation, M.B. and M.F.; Writing—original draft, M.B. and S.U.; Writing—review & editing, M.B., S.U. and S.W.F.; Visualization, S.D. and M.F.; Supervision, S.U.; Project administration, M.B. All authors have read and agreed to the published version of the manuscript.

Figure 1. (a). Experimental setup of 3 cylindrical enclosures in a sealed compartment and 3 divisions in each aquaria. The color represents the type of copepod in triplicate. (b). The design of the enclosure with plastic wall and 45 µm base and sealed bottom.

Figure 1. (a). Experimental setup of 3 cylindrical enclosures in a sealed compartment and 3 divisions in each aquaria. The color represents the type of copepod in triplicate. (b). The design of the enclosure with plastic wall and 45 µm base and sealed bottom.

Figure 2. Copepod mass (in g) for the 3 copepod species (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis) and the two pHs (8.2 and 7.7). Results are presented as mean ± SEM.

Figure 2. Copepod mass (in g) for the 3 copepod species (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis) and the two pHs (8.2 and 7.7). Results are presented as mean ± SEM.

Figure 3. Fecal pellet mass (in g) for the 3 copepod species (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis). Results are presented as mean ± SEM.

Figure 3. Fecal pellet mass (in g) for the 3 copepod species (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis). Results are presented as mean ± SEM.

Figure 4. Effect of pH on polonium activity in the copepod fecal pellets (in % per 12 h) for each combination of copepod (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis) and microalgae species (Chaetoceros muelleri, Dunaliella salina and Isochrysis galbana). Results are presented as mean ± SEM. Significant difference between pH treatments are indicated with * (ANOVA; p < 0.05) and ** (p < 0.01).

Figure 4. Effect of pH on polonium activity in the copepod fecal pellets (in % per 12 h) for each combination of copepod (Acartia pacifica, Euterpina acutifrons and Parvocalanus crassirostis) and microalgae species (Chaetoceros muelleri, Dunaliella salina and Isochrysis galbana). Results are presented as mean ± SEM. Significant difference between pH treatments are indicated with * (ANOVA; p < 0.05) and ** (p < 0.01).

Figure 5. Effect of pH on polonium activity (in %) in the copepod for each combination of copepod species (Acartia pacifica, Euterpina acutifrons, and Parvocalanus crassirostis) and microalgae species (Chaetoceros muelleri, Dunaliella salina, and Isochrysis galbana). Results are presented as mean ± SEM. Significant difference between pH treatments are indicated with * (ANOVA; p < 0.05) and ** (p < 0.01).

Figure 5. Effect of pH on polonium activity (in %) in the copepod for each combination of copepod species (Acartia pacifica, Euterpina acutifrons, and Parvocalanus crassirostis) and microalgae species (Chaetoceros muelleri, Dunaliella salina, and Isochrysis galbana). Results are presented as mean ± SEM. Significant difference between pH treatments are indicated with * (ANOVA; p < 0.05) and ** (p < 0.01).

Figure 6. Average 209Po concentration in fecal pellets at 7.7 and 8.2 pH in three copepod species for the three algal feeds.

Figure 6. Average 209Po concentration in fecal pellets at 7.7 and 8.2 pH in three copepod species for the three algal feeds.

Figure 7. Average 209Po bioaccumulation in Parvacanus crassitoris, Euterpina acutifrons, and Acartia pacifica ingesting different phytoplankton species at 7.7 and 8.2 pH.

Figure 7. Average 209Po bioaccumulation in Parvacanus crassitoris, Euterpina acutifrons, and Acartia pacifica ingesting different phytoplankton species at 7.7 and 8.2 pH.

Table 1. Average 209Po concentration in water with microalgae.

Table 1. Average 209Po concentration in water with microalgae.

Microalgae209Po Concentration (Bq mL−1)Isochrysis galbana0.088 ± 0.004Chaetoceros muelleri0.075 ± 0.003Dunaliella salina0.050 ± 0.002

Table 2. Summary of the carbonate chemistry: measured pH on the total scale, alkalinity (AT in µmol/kg), salinity (in ‰), and calculated pCO2 (in µatm). Difference between target the two target pHs and between replicates was tested using an ANOVA 2. Results are presented as mean ± SEM.

Table 2. Summary of the carbonate chemistry: measured pH on the total scale, alkalinity (AT in µmol/kg), salinity (in ‰), and calculated pCO2 (in µatm). Difference between target the two target pHs and between replicates was tested using an ANOVA 2. Results are presented as mean ± SEM.

pH 8.2pH 7.7ModelpHReplicateF5,23pF1pF4ppH8.20 ± 0.017.69 ± 0.013828.0<0.000119,137.3<0.00011.10.41AT2890 ± 12610 ± 155,171.0<0.0001275,848.0<0.00011.40.26Salinity42 ± 042 ± 11.00.45....pCO2307.0 ± 1.41131.0 ± 9.91394.7<0.00016969.6<0.00011.170.36

Table 3. 209Po concentration in the experimental samples at 8.2 pH.

Table 3. 209Po concentration in the experimental samples at 8.2 pH.

Isochrysis galbanaChaetoceros muelleriDunaliella salina SampleTimeMass (g)Activity (mBq)Concentration (Bq/g)Mass (g)Activity (mBq)Concentration (Bq/g)Mass (g)Activity (mBq)Concentration (Bq/g)Parvocalanus crassirostisFecal Pellet120.051755010.640.05333997.490.05622815.00Fecal Pellet240.05664988.800.05454097.500.05332675.01Fecal Pellet360.05905198.800.05614217.500.05452735.01Fecal Pellet480.05835138.800.05494127.500.05372695.01Copepod480.003834.08.950.004036.49.100.003919.55.00Water-microalgae48 1213 1176 808 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 193 146.6 82.5 Euterpina acutifronsFecal Pellet120.07216348.790.06725047.500.07273645.02Fecal Pellet240.06886058.790.06955217.500.06983495.00Fecal Pellet360.07306428.790.07335507.500.07093555.01Fecal Pellet480.07106258.800.07185397.510.07313665.01Copepod480.004248.911.640.003943.811.230.004120.55.00Water-microalgae48 807 733 439 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 158.1 109.2 106.5 Acartia pacificaFecal Pellet120.06705908.810.07115337.500.06833425.01Fecal Pellet240.06986148.800.07365527.500.06853435.01Fecal Pellet360.06635838.790.06444837.500.06313165.01Fecal Pellet480.07026188.800.06855147.500.06743375.00Copepod480.00404110.250.003838.510.130.0040225.50Water-microalgae48 910 779 551 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 164 100.5 89.0

Table 4. 209Po concentration in the experimental samples at 7.7 pH.

Table 4. 209Po concentration in the experimental samples at 7.7 pH.

Isochrysis galbanaChaetoceros muelleriDunaliella salina SampleTimeMass (g)Activity (mBq)Concentration (Bq/g)Mass (g)Activity (mBq)Concentration (Bq/g)Mass (g)Activity (mBq)Concentration (Bq/g)Parvocalanus crassirostisFecal Pellet120.05725249.160.05123907.620.05672905.11Fecal Pellet240.05455139.410.05484177.610.05422735.04Fecal Pellet360.05725098.900.05814387.540.05512785.05Fecal Pellet480.05755108.870.05544197.560.05422725.02Copepod480.0043399.070.004640.88.870.004522.95.09Water-microalgae48 1268 1147.2 769.1 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 157 148 95 Euterpina acutifronsFecal Pellet120.06896128.880.06825187.600.07223615.00Fecal Pellet240.06936108.800.06985247.510.07033545.04Fecal Pellet360.07076238.810.06995317.600.07093585.05Fecal Pellet480.06986218.900.07115367.540.07253645.02Copepod480.00455211.560.004349.111.420.004422.55.11Water48 869 729.9 440.8 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 133 112 99.7 Acartia pacificaFecal Pellet120.06886108.870.07035297.520.06913485.04Fecal Pellet240.06936128.830.07215467.570.06953495.02Fecal Pellet360.06866058.820.06955237.530.06883465.03Fecal Pellet480.06996168.810.06925217.530.06943515.06Copepod480.00475110.850.00454610.220.0044255.68Water48 868 737 507 Ʃ algal food added 4840 mL3520 40 mL3000 40 mL2000 Loss48 158 98 74