High reproduction but low High reproduction but low biomass: Mortality estimates biomass: Mortality estimates of the copepod of the copepod Acartia tonsaAcartia tonsa in a hyper-eutrophic estuary.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

Kingdom- Animalia

Phylum- Arthropoda

Subphylum- Crustacea

Class- Maxillopoda

Subclass- Copepoda

Order- Calanoida

Family- Acartiidae

Genius- Arcatia

Species- A. tonsa

Outline of the articleOutline 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?

IntroductionIntroduction 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

Experimental methodsExperimental 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.

Sediment trapsSediment 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.

Sediment coresSediment 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.

ResultsResults

Greatest mortality rate is recorded at depths of 5-10 m.

Vulnerable stages are mostly C2 and C3

Some general remarksSome general remarks• EPR had no regular pattern, but 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 escapeTurbulence decreases predation escape

• 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.

ConclusionConclusion1. A. tonsa in Mariager Fjord is greatly afected

by2. Predation by mussels and fish from above3. Anoxic/sulphidic bottom water from below.