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					 Impacts of hypoxia on key
  benthic infauna and their
predators in Chesapeake Bay

 Rochelle D. Seitz & W. Chris Long
    Virginia Institute of Marine Science,
     The College of William and Mary

     Hypoxia Workshop, March 2007
                        Outline
   Threats to biodiversity

   Overview of hypoxia in Chesapeake Bay

   Recent work on hypoxia: prey and predators
    • Part 1: Mesocosm experiment
    • Part 2: Field predation experiment
    • Part 3: Benthic long-term trends
            Threats to Biodiversity
   Overexploitation and Harvesting
    • Can contribute to habitat degradation

   Introduced Species

   Climate Change

   Habitat Destruction and Loss
    • The single largest threat in terrestrial systems
Importance of habitat destruction
   …The one process ongoing in the 1980s that will
    take millions of years to correct is the loss of
    genetic and species diversity by the destruction of
    natural habitats…
    (E.O. Wilson, 1985)

   …rates of resource collapse increased, and
    recovery potential, stability, and water quality
    decreased exponentially with declining diversity.
    (B. Worm et al., 2006)
 Hypoxia in Chesapeake Bay
                             The most well-studied
                              system in North America
                              for hypoxia

                             Onset of low DO related to
                              European settlement in the
                              17th century

                             How are benthic prey and
                              predators affected?

Human-induced condition (Zimmerman and Canuel 2000)
    Effects of Hypoxia on Benthos
   Mainstem Chesapeake:
    • Polyhaline mud (13m), not                                     100




                                      mg AFDW /0.02 m2 or Percent
      exposed to hypoxia                                                                                            Polyhaline Mud
                                                                                                                    Hypoxic Mud
    • Hypoxic mud (27m),                                            80

      oxygen < 2 mg/L
                                                                    60


   With hypoxia, declines in                                       40

    • Richness
                                                                    20
    • Overall biomass
    • Biomass of equilibrium (long-                                  0
      lived) species                                                               x   5        a   ss       ass        iom
                                                                               ess          iom          iom          nB
                                                                          hn               B         bB            rtu
                                                                      Ric                         uil            po
                                                                                               Eq       Op
   Increase in:
                                                                                           Community Measure
    • Opportunist biomass
                                              (Modified from Dauer et al. 1992)
                  Study Organisms
   Macoma balthica is biomass
    dominant (Baird and Ulanowicz, 1989)

   Important blue crab prey (~50%
    of diet) (Hines et al., 1990)

   Long lived (~3 yr.)

   Can survive for up to 3 weeks in
    hypoxic water (Seitz et al., 2003)

   Crabs not present < 3 mg/l DO
    (VA trawl survey data)
     Mesocosm Hypoxia Experiment
   12 tanks (1m x 2m x 0.5m; 1200 l)
   15 cm muddy sand
   12 M. balthica clams transplanted to 0.25 m2 patch (48/m2)
   Two treatments: Normoxia & low DO (< 2 ,mg/l), 3-5 reps of
    each
   Predator (intermolt blue crab), acclimated to low DO for 24
    hrs, added and allowed to feed for 2 days
                                  (# eaten/# transplanted)
Results: Mesocosm                                            0.5




                                   Proportional mortality
                                                                   Low DO
                                                                   High DO
                                                             0.4

   Proportional mortality sig.                              0.3
    higher under normoxia for
                                                             0.2
    all 3 years (p < 0.01)
                                                             0.1

                                                             0.0
   Siphon protrusion                                              1999      2000   2001
    increased (but no siphon                                                 Year
    nippers)

   Clam burial depth didn’t
    change (in muddy sand)


     (Seitz et al. MEPS 2003)
        Field Hypoxia Experiment
   Deep areas experience
    hypoxia
                                                                                        b




                                 Predation rate (day )
                                                                Shallow




                                 -1
                                                         0.03   Deep

   Caging: clam survival deep
    vs. shallow (28 days)                                0.02
                                                                          a
                                                                                    a


                                                         0.01
   Predation higher in deep                                                  a
                                                         0.00
    during hypoxia
                                                                          Before   During
                                                                              Hypoxia
   Clams reduced burial
    depth in mud (lab exps)               (Long and Seitz, Ecology in review )
        Response to hypoxia




Before hypoxia, predators   During hypoxia, predators
  feed in shallow areas,      move into deep areas to
  where prey densities        take advantage of
  are high                    stressed prey
         Baywide benthic
        sampling methods
   Used benthic data from CBP
    probability-based sampling

   9 years (1996-2004), 2500 points

   Young Grab: samples 0.044 m2
    to 10 cm depth, 0.5 mm mesh

   CTD for water-quality
    parameters

   Means per meter depth
                                      Oxygen, depth, & density
                             8                                                                   8000
Dissolved Oxygen mg/L (SE)




                                                         P < 0.001




                                                                     Number of individuals/m 2
                             7                           R2 = 0.85
                                                                                                 6000               P < 0.001
                             6                                                                                      R2 = 79%

                             5
                                                                                                 4000
                             4
                             3                                                                   2000

                             2
                                                                                                   0
                             1
                             0
                                 0        10        20     30                                           0    2         4        6     8
                                            Depth (m)                                                       Dissolved oxygen (mg/L)


                                     ms in prep with Dan Dauer and Roberto Llanso
                    B-IBI by depth
 Multi-metric index
   Diversity
     Abundance



                               Benthic Index of Biotic Integrity
                                                                   3.5
     Biomass
     Functional groups                                            3.0

                                                                   2.5
   Sig. linear decline with
    depth (P < 0.005)                                              2.0

                                                                   1.5
   Increased variance with                                        1.0
    depth (both low DO &
    normoxic sites)                                                0.5
                                                                         0   10       20   30
                                                                              Depth (m)
   2006 worst B-IBI on
    record
              Conclusions
   Mesocosm: decreased blue crab predation during
    hypoxia (consistent with CSM)
   Field experiment: increased predation during hypoxia
    (consistent with PSM)
   Siphon nipping may be important in field
   Predation occurs immediately after normoxia returns?
   Predators diving into hypoxia?
   Baywide: decreased B-IBI w/ depth, consistent with
    hypoxic trends, lowest values in 2006
   Overall trends: no change in fisheries production with
    bay-wide increases in hypoxia (Kemp et al. 2005)
   Need to better quantify field predation at fine spatial and
    temporal scales & determine food-web effects
                     Summary

   Effects of hypoxia detrimental to
    benthos

                                         Chesapeake

   Positive effects of hypoxia on
    predators

   Habitat degradation leading to loss of
    species may affect resilience of the
    system and must be addressed
                      Acknowledgements




Funding by: National Sea Grant, NOAA – Chesapeake Bay Office, VA
 Commonwealth, EPA, NSF REU program, Chesapeake Bay Program
Assistance from: Community Ecology Group & Marine Conservation
  Biology Group at VIMS, REU students, ODU & Versar (CBP data)

				
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