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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 exposed to hypoxia • Hypoxic mud (27m), oxygen < 2 mg/L • With hypoxia, declines in • Richness • Overall biomass • Biomass of equilibrium (long-lived) species • Increase in: • Opportunist biomass (Modified from Dauer et al. 1992)
Study Organisms • Macomabalthica 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
Results: Mesocosm • 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) 1999 (Seitz et al. MEPS 2003)
Field Hypoxia Experiment • Deep areas experience hypoxia • Caging: clam survival deep vs. shallow (28 days) • Predation higher in deep during hypoxia • Clams reduced burial depth in mud (lab exps) (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 Before hypoxia, predators feed in shallow areas, where prey densities are high. Before hypoxia, predators feed in shallow areas, where prey densities are high.
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 ms in prep with Dan Dauer and Roberto Llanso
B-IBI by depth • 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) • 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 • Positive effects of hypoxia on predators • Habitat degradation leading to loss of species may affect resilience of the system and must be addressed Chesapeake
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)