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Peter Tiselius, C. Marc Andersen Borg, Benni Winding Hansen,

High reproduction but low biomass: Mortality estimates of the copepod Acartia tonsa in a hyper-eutrophic estuary. Peter Tiselius, C. Marc Andersen Borg, Benni Winding Hansen, Per Juel Hansen, Torkel Gissel Nielsen, Bent Vismann

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Peter Tiselius, C. Marc Andersen Borg, Benni Winding Hansen,

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  1. High reproduction but low biomass: Mortality estimates of the copepod Acartia tonsa in a hyper-eutrophic estuary. Peter Tiselius, C. Marc Andersen Borg, Benni Winding Hansen, Per Juel Hansen, Torkel Gissel Nielsen, Bent Vismann Presented by Allen N. Nyendi by

  2. Kingdom- Animalia Phylum- Arthropoda Subphylum- Crustacea Class- Maxillopoda Subclass- Copepoda Order- Calanoida Family- Acartiidae Genius- Arcatia Species- A. tonsa

  3. Outline of the article • Production, abundance and mortality of the copepod Acartia tonsa • Effect of phytoplankton bloom consisting mainly of the diatom-Skeletonema costatum • Effect on egg production rate (EPR) • Blue mussel predation • Abundance and decline of the various life stage forms • How the anoxic deeper part of the estuary affects recruitment of the copepod What’s the real cause of the decreasing population?

  4. Introduction Characteristics of the fjord • Mariager Fjord is a highly productive estuary • maximum depth of 30 m • Primary production is high (~1000 g C /m2/ yr) • Phytoplanktons are mainly dinoflagellates and autotrophic ciliates • Permanently anoxic bottom water containing Sulphide. (may prevent hatching) • Exposed to unlimited food for most of the summer • Exposed to predation by suspension feeding blue mussels Mytilus edulis

  5. Experimental methods Abundance • Samples were taken on day 16, 18, 20, 22 and 24 August 2005 at deepest site. • At depths of 0, 2, 4, 6, 8, 10, 12, 15 and 20 m Egg production • Every 2nd day. From upper 10m (using a 70μm net)-lab. • Females separated (5ml poly-CO3) and egg production monitored. Survival of copepods in situ • Incubation performed at 5 different depths: 2,10,12,15,20m • Incubation chambers consisted of 200ml plastic jars with 125 μm mesh lids and bottoms • 3hrs deployment time and 24hrs exposure was given.

  6. Sediment traps • To investigate sedimentation and the fate of copepod eggs • Deployed at 15 m depth in the central part of the fjord from 20th to 24th August. • After 24 h, contents were poured into 2 l bottles and brought to the laboratory • Chlorophyll a and phaeopigments samples were taken, and the remaining sample (~1.7 l) filtered via 45 μm mesh, and copepod eggs counted.

  7. Sediment cores • To determine the abundance of copepod eggs in the sediment • At 4, 11.5 and 18.5 m depth on 22nd August • Cores were frozen (–20°C) until analysis • Sucrose flotation (top 5cm) to reveal eggs • The rest analysed by sieving and inspection. Mortality rates • Calculated from vertical life tables • Mortality estimates are expressed for each stage, not for pairs of stages.

  8. Results

  9. Greatest mortality rate is recorded at depths of 5-10 m. Vulnerable stages are mostly C2 and C3

  10. Some general remarks • EPR had no regular pattern, but hatching success slightly decreased with time. • Female survival in cages was greatest btwn 10-15m. • Sedimentation of eggs greatly reduced in the course of the expt. • Increased egg sedimentation in the Black, sulphide-rich zones, away from Mytilus edulis habitat. • Highest daily mortality rate was recorded with the late nauplii and copepodite stages • Sulphite at deeper levels affect distribution • Anoxic conditions also lead to death (* not always) • Mytilus edulis predation is another source of mortality • Turbulence decreases predation escape

  11. Mytilus edulis occur from 1 -10 m depth in Mariager Fjord, with 1000 to 8000 ind./m2 (C2 & C3). • Fish predation, though not pronounced. • Naupliar mortality averaged 18%/d for all stages combined (uncertain). • Increased EPR in fjord • No hatching and nauplii survival effect from Skeletonema costatum* • Large discrepancy between daily integrated EPR and no. of eggs recovered in the sediment traps at 15m • Dynamics of sinking and hatching (43 m/day) • Resuspension is necessary for hatching even in O2-rich sediments. • Predation rate greatly increases with turbulence.

  12. Conclusion 1. A. tonsa in Mariager Fjord is greatly afected by 2. Predation by mussels and fish from above 3. Anoxic/sulphidic bottom water from below.

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