This document analyzes data on zooplankton vertical distribution and its relationship to oxycline depth in different regions, size classes, and times of day. The data was collected using optical sensors and processed to determine zooplankton biovolume, median depth, and oxycline depth. Relationships between these metrics were compared between the Gulf of Mexico and Chesapeake Bay, different size classes, and day versus night to understand how hypoxia affects zooplankton behavior.

1. Zooplankton vertical distribution and oxycline depth Comparisons between regions, size classes, and time of day 1 Pierson, J. J. 1 Roman, M. R. 1 Kimmel, D. G. 2 Zhang, X. 1 Boicourt, W. C. 1 Loveland, B 1 2

2. Potential effects of hypoxia on plankton Direct Lethal Sub-lethal, e.g. Slowed development Reduced reproductive success Indirect Habitat reduction Trophic interactions altered Refuge from less tolerant predators Habitat reduction could enhance prey encounter rates Selection for some species

4. Data collection and processing SCANFISH with O ptical P lankton C ounter Processed data resolution: ~1 km × 1 m Oxycline: 2 mg ml -1 Sample rate: up to 2 second -1 Sizes of particles based on light attenuation 8 Size bins

5. Size Classes 250 – 500 μm ESD 500 – 750 μm ESD 750 - 1000 μm ESD 1000 - 1250 μm ESD 1250 - 1500 μm ESD 1500 - 1750 μm ESD 1750 - 2000 μm ESD 2000 - 5000 μm ESD

6. Data sets available Gulf of Mexico 2003 - Survey 2004 – Survey 2006 – Process (Diels) 2007 – Process (Diels) Multiple Longitudinal transects Chesapeake Bay 1996 1997 1999 2000 Single axial transect

7. Chesapeake Bay Gulf of Mexico

8. Raw Data  Processed Data Dissolved O 2

9. Raw Data  Processed Data Zooplankton biovolume

10. Quantitative metrics Median Depth Depth of 50% of abundance Oxycline Depth 2 mg ml -1 dissolved oxygen Normalize the relationship Variable depth ÷ sampling depth

11. Quantitative relationship Slope of oxycline depth and zooplankton median depth Compare between regions, sizes, and times

12. Chesapeake Bay vs. Gulf of Mexico

13. Chesapeake Bay Sizes and Years

14. Diel changes in median depth Gulf of Mexico

15. Day vs. Night distribution Hypoxic vs. Normoxic water

16. Gulf of Mexico vs. Chesapeake Bay

17. The effect of hypoxia on mesozooplankton… … appears to vary between the Gulf of Mexico and Chesapeake Bay But could be due to oxygen conditions … differs across mesozooplankton size classes Steeper response for larger sizes … is seen in the diel vertical migration Weak reverse migration for small sizes Vertical migration extent truncated in hypoxia