Department of Oceanography, National University of Ireland, Galway, Ireland

Copepods and other netplankton of the Shannon estuary, Ireland,and two tributary estuaries: Composition and grazing pressure

Ian R. Jenkinson* and Tom Ryan†

Department of Oceanography, National University of Ireland, Galway, Ireland

*Present address:Agence de Conseil et de Recherche Océanographiques,19320 La Roche Canillac, [email protected]

†Deceased

Co-author Tom Ryan was killed in a car accident, and this presentation is dedicatedto his memory.

Co-author Tom Ryan was killed in a car accident, and this presentation is dedicatedto his memory.

Financed as a baseline study by Aughinish Alumina (Ireland) Ltd (Alcan Group)

Thanks to John Bracken and Geoffrey O’Sullivan (National University of Ireland Dublin and Department of the Marine, Ireland)

Thanks to Brian McK. Bary, Mary Hensey and colleagues at this conference for discussions

Work done at National University of Ireland Galway. Thanks to John Coyne and Tom Furey for technical help.

Ireland

Ireland Shannon Estuary

Flowmeter not shown

Calibration

• 37 m3 for adult copepods and larger copepodids, including Oikopleura (as their houses are destroyed)

• 210 m3 for larger plankton

Temperature (°C)

Salinity (ppt)

Secchi depth (m)

1)

3)

4)

Copepods

1)

3)

4)

Eurytemora affinis

Thanks tohttp://www.cnas.smsu.edu/zooplankton/eurytemora_affinis.htm

37 m3.haul-1

1000.m-3

Eurytemora affinis

Thanks tohttp://www.cnas.smsu.edu/zooplankton/eurytemora_affinis.htm

Salinity of abundancedistribution over year and over all stations

• 5 to 26 psu

• Maximum at 16 psu

Acartia bifilosa 1)

3)

4)

Yoon et al. (1998)J. Plankt. Res.

37 m3.haul-1

2000 m-3

Acartia clausi

1)

3)

4)

37 m3.haul-1

35 m-3

Acartia discaudata

Only on cruises in July,August and October (least in August) and at Stations 3 and 4.

Centropages hamatus

1)

3)

4)

37 m3.haul-1

20 m-3

Freshwater cyclopoids

1)

3)

4)

37 m3.haul-1

Other zooplankton

Mesopodopsisslabberi 1)

3)

4)

Supposed to feed mostly on phytoplankton. Good food for fishes.

210 m3.haul-1

~50.m-3

Neomysis integer

1)

3)

4)

Photo: Marco Faasehttp://www.marlin.ac.uk/species/Neomysisinteger.htmSupposed to feed mostly on phytoplankton. Good food for fishes.

210 m3.haul-1

Oikopleura dioica

1)

3)

4)

Length up to ~2 mm.

From Hardy, A. (1962),The Open Sea. After Lohmann(1899).

~100 m3.haul-1 ?

~100.m-3

1)

3)

4)

Pleurobrachia pileus

Length up to 1-2 cmFrom Hardy, A. (1962),The Open Sea. After Marie Lebour and Mrs M.A. Connell

1) 210 m3.haul-1

0.5.m-3

Copepod abundance (nos/haul, corrected for flowmeter reading)

Copepod diversity(Shannon-Wiener index, H’)

Filled triangles: H’Open triangles: abundance

37 m3.haul-1




37 m3.haul-1

Open triangles: H’Filled triangles: abundance




37 m3.haul-1

Clearance rate by Eurytemora affinis,

Poulet (1978) found that ingestion rate [µg . copepod-1.h-1] (wet wt.)

y = 0.218 + 0.135.x

where x is the concentration of particulate matter (g.m-3 wet wt.).

[POC] in the Shannon estuary averages around 600 mg.m-3 (say, 800 mg.m-3 near the bottom, where E. affinis is most abundant). In the Shannon estuary, microplankton (20-200 µm) contributes about 100 mg C.m-3. Using a factor of 20 to convert to wet weight this gives x around 16 g.m-3, and y of 2.4 µg wet wt. copepod-1.h-1 ~2.9 µg C.copepod-1.d-1.

So, clearance rate by Eurytemora

Ge = (y/x) = (0.218/x) + 0.135

Table 4. Estimated values of Ge, the clearance

rate by Eurytemora affinisStation

Cruise 1 3 4

12-May 0 0.029 0.8410-Jul ? 0.029 0.0713-Aug 0 0 03-Oct 0.011 0.006 0

20-Nov 0 0.29 2.921-Dec 0 0.11 0.03213-Feb 0 0.002 0.004

1-Apr 0.048 0.089 0.183-May 0.31 0.41 0.066

Mean 0.046 0.107 0.455

Units: dm3 m-3 day-1

Carbon demand by Eurytemora affinis,

D = [(A.a) + (C.c) . P . (1/E)]

where A and C are the concentrations of adults and copepodids, and a and c are there respective masses. P is the dailyproductivity (a function of temperature) taken from Heinle & Flemer (1975) and E is conversion efficiency, taken to be 50%.

5

Clearance rate by Acartia spp, (Ga)

Poulet (1978) found that ingestion rate [µg . copepod-1.h-1] (wet wt.)

y = 1.43 + 1.09.x

where x is the concentration of particulate matter (g.m-3 wet wt.).

The Acartia spp adults in the Shannon are about the same size. As Acartia need more a higher proportion of phytoplankton than Eurytemora, it has been assumed that only half the POC is available to them, 460 mg.m-3, ~8.8 g.m-3 (wet weight).

This gives Ga for adult Acartia of 3.0 cm3.copepod-1.d-1.

By comparison experimentally determined Ga by Acartia tonsa of 1.5 to 3.7 cm3.copepod-1.d-1 were shown by Roman (1977).

Assuming Ga = (Aa . 3) + (Ca . 1.5)

where Aa and Ca are the respective concentrations of adults and copepodids.

Clearance rate by Oikopleura dioica (Go)

King (1982) found that a concentration of 1,000 m-3 of Oikopleura dioica gave a clearance rate of 0.2 d-1 at 14°C. This gives Go of 200 cm3.larvacean.d-1.

O. dioica feeds on bacterioplankton from less than 1 µm to phytoplankton of 30 µm, and it needs a minimum concentration of 40 to 60 mg C.m-3 of such plankton as POC.

Without this concentration it dies in a few days, but with it, it doubles, on average every day (King, 1982).

76Go

Estimated values of Gco, the clearance

rate due to copepods and larvaceansStation

Cruise 1 3 4

12-May 0 0.029 1.210-Jul ? 9.6 493-Aug 0.12 1.1 2.23-Oct 0.21 0.18 0.0003

20-Nov 0 0.91 3.321-Dec 0 0.11 0.03213-Feb 0 0.002 0.004

1-Apr 0.048 0.092 0.183-May 0.085 1 1.7

Mean 0.058 1.447 6.402

Units: dm3 m-3 day-1

E. affinis

1)

3)

4)

1)

3)

4)

M. slabberi

210 m3.haul-137 m3.haul-1

Table 11 Comparison of the netplankton of five west European estuaries

Parameter ShannonSevern/ Bristol Ch. Thames Gironde Morlaix

Stratification Slight Mixed ? Slight Slight

Temperature 6 to 17 ? ? 5 to 27 ?

Chlorophyll

a (mg.m-3) 0.1 to 60.2 to 1.8 (to 13) ? Up to 60 ?



Aurelia auritaNursery for larvae

In early summer

Pleurobrachia pileus

May to Nov.; sals down to 25; prob. alloch.

Prob. Alloch.; "true estuarine" Occasional

"Marine euryhaline"

Hydrobia ulvae

Abundant, esp. in summer

Acartia granii

"Marine euryhaline"; alloch.

"Endemic", esp. in temp.-strat. water



Acartia bifilosa

Co-dominant, esp. in sals over 8.5

"Estuarine and marine"; authoch.

Co-dominant; authoch.

Acartia clausi

Most in lower est.; sals > 16.4

"Euryhaline marine" Dominant

"Marine-euryhaline"; alloch.?

Lower est.; max in May

Acartia discaudataIn sals only > 22.5

Common; summer only

Lower est. Max in May



Acartia longiremisIn sals only >22.5

Common, summer only

Lower est. Max in May

Acartia tonsaCommon; authoch.

Calanus sppMost in lower est.

"Stenohaline marine"

Occas. Mostly in summer



Centropages hamatus

Abun. In lower est., in sals > 16.4

"Euryhaline marine"

Occas., mostly in summer

"Marine-euryhaline", alloch.

Lower est. in May

Centropages typicus

Common; low similarities with other taxa

"Marine-euryhaline; alloch.

Mostly sals >34

Freshwater cyclopoids

Fairly common; sals of up to 25.6

"Euryhaline fresh water"



Eurytemora affinis

Dominant, with A. bifilosa; Optimum sal ~16

"True estuarine"; co-dom. With A. bif.

Occasionally abundant

Co-dom. Optimum sal. 0.5 to 5

Abun., esp. in winter-spring; mostly sals <15

Eurytemora velox Sporadic(In Crouch est.)

Isias clavipes

"Marine euryhaline"; alloch.?

Dom. In lower est. in summer.



Oithona helgolandica Rare


Oithona nana Rare Occas."Marine euryhaline"

Euterpina acutifrons

V. common in May to Nov.



Paracalanus parvus Rare

"Stenohaline-marine"


In Oct., throughout salinity range (> 30)

Pseudocalanus elongatus

Occas. But may be common mostly lower est.

"Stenohaline-marine"

Fairly common, except in summer


Lower est. in May

Temora longicornis

Occas. In lower and upper est.

"Euryhaline-marine"

Constantly common

"Marine-euryhaline"; alloch.

Lower est. in May



Gasterosaccus normanni

Occurred in winter

Gasterosaccus spinifera Rare

"True estuarine"; sals < 30 Rare

Near mouth in sals > 18

Mesopodopsis slabberi

Often dom. above Tarbert

"Estuarine and marine"

Most abundant

Endemic in sals >5

High biomass; mainly sals <15; autoch.



Schistomysis spiritus

"Estuarine and marine" Frequent

Near mouth, in sals >18

Schistomysis ornata Occasional

Siriella clausi Rare



Sagitta elegens

Most common outside est., but found rarely in est. in winter in low sals.

Juveniles "estiuarine and marine"; authoch.

Found close to shore

Sagitta sp. "euryhaline marine", authoch.

Lower est. in summer

Sagitta setosa As above

As above, but less abun.; adults "stenhaline marine"

Always present See above

Lower est in summer

Oikopleura dioica

Often abun. In summer; mostly below Aughinish

Common May to Oct.

"Euryhaline-marine"; alloch.

Lower est. in spring, summer and Oct.



Evadne

Low similarity with Podon and other taxa

"Euryhaline marine; alloch.

Upper est. in temperatures >17, in temp.-stratified water; alloch.

Podon

Low similarity with Evadne and other organisms Rare

"Euryhaline-marine"; alloch. As above

Tomopteris Lower est."Euryhaline marine".



References: Shannon - Hensey (1980); present study

Severn/Bristol Ch. - Rees (1939), Collins and Williams (1981, 1982) Williams (1984)

Thames - Wells (1938), E. affinis/hirundoides - Hardy (1924)

Gironde - Castel (1981), Sorbe (1981)

Morlaix - Lefèvre-Lehoërhoff (1972)

Eurytemora affinis

Sex

Females with egg sacs, and with spermatophores, present all year, except July (and August).

Male:female ratio 1:0.65 (this study, overall)1:5 (Dutch Wadden Sea - Vaupel-Klein & Weber, 1975)1:8 to 1:0.45 with positive relationship between

males and total abundance (Gironde - Castel, 1981)~1:0.5 to 1:0.7 (Seine - Devreker & Souissi, this

conference)Ladiges (1935) noted that ovigerous females and

copepodids seem not to coexist in the same water mass.

Concluding remarks

Concluding remarks

1. Eurytemora affinis occurred at salinities of 5 to 26 psu,with maximum abundance, ~1000.m-3, at 16 psu.

Concluding remarks


2. The overall ratio ratio of males:females was 1:0.65

Concluding remarks



3. E. affinis occurred maximally from November to May, and was absent at all three stations in August.

Concluding remarks




4. E. affinis was co-dominant with Acartia bifilosa, which reached a maximum abundance of 2000.m-3 in July.

Concluding remarks





5. A. bifilosa was uncommon in August and absent from February to April.

Concluding remarks





5. A. bifilosa was uncommon in August and absent from February to April.

6. Maximum clearance rate by copepods was only 0.6%.day-1, but mean values of [POM], ~600-800 mg C.m-3, with ~100 mg C.m-3 of micro- and nano- plankton (3-200 µm) would have been just sufficient to support E. affinis.

Concluding remarks

7. Copepods became markedly uncommon in August.

Concluding remarks


8. Oikopleura dioica occurred from May to August (max. July), with its maximum clearance rate, 4%.day-1, in July.

Concluding remarks



9. Grazing by mysids, especially Mesopodopsis slabberi, has not been calculated, but may have exceeded that of all the other grazers.

Concluding remarks




10. The predator Pleurobrachia pileus showed a maximum from July to August. It may have decimated copepods and mysids, but Oikopleura’s mucous house may have protected it from Pleurobrachia’s stinging tentacles.

Concluding remarks




10. The predator Pleurobrachia pileus showed a maximum from July to August. It may have decimated copepods and mysids, but Oikopleura’s mucous house may have protected it from Pleurobrachia’s stinging tentacles.

11. Different estuaries have curiously different plankton fauna, whose distributions with salinity are also markedly different. Neighbouring estuaries lend themselves to comparative studies.

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Department of Oceanography, National University of Ireland, Galway, Ireland

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