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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:

With hypoxia, declines in
• Richness
• Overall biomass
• Biomass of equilibrium (longlived) species
2
• Polyhaline mud (13m), not
exposed to hypoxia
• Hypoxic mud (27m),
oxygen < 2 mg/L
mg AFDW /0.02 m or Percent

100
Polyhaline Mud
Hypoxic Mud
80
60
40
20
0
ess
hn
Ric

Increase in:
• Opportunist biomass
x5
m
ss
ss
Bio
ma
ma
o
o
n
i
i
u
B
B
ort
ilb
p
u
Op
Eq
Community Measure
(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



Proportional mortality sig.
higher under normoxia for
all 3 years (p < 0.01)
Siphon protrusion
increased (but no siphon
nippers)
Clam burial depth didn’t
change (in muddy sand)
(Seitz et al. MEPS 2003)
Proportional mortality
(# eaten/# transplanted)
Results: Mesocosm
0.5
Low DO
High DO
0.4
0.3
0.2
0.1
0.0
1999
2000
Year
2001
Field Hypoxia Experiment

Deep areas experience
hypoxia
-1
Predation rate (day )
b


Caging: clam survival deep
vs. shallow (28 days)
Predation higher in deep
during hypoxia
0.03
0.02
Shallow
Deep
a
a
0.01
a
0.00
Before

Clams reduced burial
depth in mud (lab exps)
During
Hypoxia
(Long and Seitz, Ecology in review )
Response to hypoxia
Before hypoxia, predators
feed in shallow areas,
where prey densities
are high
During hypoxia, predators
move into deep areas to
take advantage of
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
8
8000
P < 0.001
R2 = 0.85
7
6
5
4
3
2
1
Number of individuals/m2
Dissolved Oxygen mg/L (SE)
Oxygen, depth, & density
P < 0.001
R2 = 79%
6000
4000
2000
0
0
0
10
20
Depth (m)
30
0
2
4
6
Dissolved oxygen (mg/L)
ms in prep with Dan Dauer and Roberto Llanso
8
 Multi-metric
index
 Diversity
 Abundance
 Biomass
 Functional groups


Sig. linear decline with
depth (P < 0.005)
Increased variance with
depth (both low DO &
normoxic sites)
Benthic Index of Biotic Integrity
B-IBI by depth
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0

2006 worst B-IBI on
record
10
20
Depth (m)
30
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)