Embryos of an Antarctic zooplankton require anoxia for dormancy, are permeable to lipophilic chemicals, and reside in sediments containing PCBs

Abstract

Zooplankton in Antarctic maritime lakes face challenges imposed by anthropogenic chemicals. Studies on temperate species suggest that lipophilic chemicals will accumulate in dormant embryos of Antarctic zooplankton and decrease hatching success, thereby threatening centuries of accumulated genetic diversity that would increase population resilience in the face of climate change. We evaluated the potential for lakes to act as sinks for legacy pollutants in the maritime Antarctic by testing sediments for polychlorinated biphenyls (PCBs) previously identified in soil, flora and fauna of lake catchments. Direct tests of embryo permeability to chemicals are confounded by potential adhesion of chemicals to the embryo surface and limited biomass available. Therefore, in order to assess the potential for lipophilic chemicals to penetrate and passively accumulate in dormant embryos of Antarctic lacustrine zooplankton, we evaluated the effect of anoxia on post-diapause development in the calanoid copepod, Boeckella poppei, and then used chemical anoxia induced by rotenone as a reporter for permeability of these embryos to moderately lipophilic chemicals. The data presented demonstrate that embryos of B. poppei from Antarctic lake sediments will passively accumulate moderately lipophilic chemicals while lying dormant in anoxic sediments. Implications for legacy POPs in sediments of Antarctic maritime lakes are discussed.

Figure 1

Post-diapause development, emergence, and hatching as observed by light microscopy in live B. poppei isolated from sediment of lake on King George Island, Antarctica. (a) Early stage of development (ED Embryo). (b) Intermediate stage of development (ID Embryo). (c) Pre-nauplius. (d) Emergent pre-nauplius. (e) Outer wall (white arrow) of cyst shed. (f) Inner cyst wall (white arrowhead) shed as the hatching membrane (black arrow) expands. (g) Hatching membrane fully expanded; nauplius begins burst swimming (Supplementary Video S1). (h) Free-swimming nauplius larva.

Figure 2

Effect of salinity and light on hatching success of B. poppei. (a) Lack of sensitivity to salinity between 0.35‰ and 4.5‰ ASW. Hatching success was evaluated as the percent of all individuals reaching the free-swimming nauplius stage with T = 0 as day of isolation of red embryos from sediment, and initiation of plate culturing. One way ANOVA demonstrated that there were no significant differences among treatment salinities when 1 d, 5 d, 10 d, 15 d, 20 d, 25 d or 30 d were considered as endpoints (n = 3, α = 0.05). (b) Hatching success was greater under light than in total darkness.

Figure 3

Sensitivity of B. poppei to chemical anoxia induced by rotenone exposure. Hatching success was evaluated as the percent of all individuals reaching the free-swimming nauplius stage with T = 0 as day of isolation of red embryos from sediment, and initiation of plate culturing. One way ANOVA was used to evaluate effect of treatments when 5 d, 10 d, 15 d, 20 d, 25 d or 30 d were considered as endpoints (n = 3; α = 0.05; †rotenone treatments significantly less than - vehicle control; ^‡rotenone treatments significantly less than + vehicle control, *+vehicle control significantly less than − vehicle control).

Figure 4

Development of B. poppei following 14 d, 30 d or 90 d of oxygen limitation produced by incubation in 0.35‰ ASW sparged with N₂ gas in the presence (+Af) or absence (−Af) of A. franciscana embryos as oxygen scavengers. The addition of A. franciscana provided additional biomass to consume residual oxygen quickly. Artemia franciscana also serve to demonstrate that anoxic conditions were achieved, because embryos of this species go dormant under anoxia; 88–93% of A. franciscana in +Af treatments became dormant before initiation of emergence, demonstrating that anoxia was achieved in all treatments. Five categories of red B. poppei were evaluated based on development and hatching events: (a,b) early development (ED), (c,d) ED embryo emerging from cyst wall, (e,f) intermediate development (ID), (g,h) unhatched nauplius, and (i,j) free-swimming nauplius. Sum of stages shown may not equal 100, because individuals that turned white are not plotted. The same aerobic control (0 d of oxygen-limited pretreatment) data are plotted in left and right panels for comparison with each preincubation treatment condition. Data points represent mean (n = 3).

Figure 5

Early emergence from the cyst wall is lethal in B. poppei, and is more likely to occur when embryos are exposed to oxygen limitation early in post-diapause development. Two categories of white embryos are graphed for exposures with (+Af) or without (−Af) A. franciscana embryos in the hypoxic preincubation: (a,b) all white embryos and (c,d) white embryos emerging from cyst wall (mean values plotted, n = 3). (g,h) Regression of relative abundance of red and white emerging embryos indicates that red embryos turn into white embryos (p < 0.0001 for +Af and −Af treatment types). Reasoning for use of Af explained in Fig. 4.

Figure 6

Abundance of red/orange B. poppei embryos in sediment from one lake on Barton Peninsula, King George Island, Antarctica. Day 0 is the first day each Whirl-pak® storage bag was opened since the sediment was subsampled on King George Island. Samples 1 and 2 were collected in February 2015 and stored for 16 months at 4 °C prior to opening. Sample 3 was collected in 2016 and stored for 10 months 4 °C prior to opening. Linear regressions for samples 1, 2 and 3, p = 0.0145, p = 0.9775, and p = 0.0103, respectively.