Spatial and Temporal Variability of Zooplankton on Georges Bank: Results from the GLOBEC Study

Edward Durbin, Maria CasasGraduate School of Oceanography,

University of Rhode Island

Acknowledgements: NOAA for financial support. The “Planktoneers” who participated in the cruises and counted the samples, other participants in the Broadscale Surveys, and the Captains and crews of the NOAA ship Albatross IV, and the R/V Endeavor and R/V Oceanus.

Talk Outline:

1. Briefly review circulation on Georges Bank and major physical processes affecting the zooplankton.

2. Show 5 year mean distribution patterns of the dominant copepods on Georges Bank during the January-June period from the US GLOBEC Georges Bank Study.

3. Discuss how life history characteristics and physical processes determine the different spatial and temporal patterns of abundance observed for each taxon.

Grand Manan Basin

Roseway Basin

Cape CodBay

Gulf of Maine

Georges Bank

Map of Georges Bank showing GLOBEC Broad-scale Survey station locations. The climatology positions (1985-1996) of the tidal mixing fronts determined fromSST for March and August are also shown (from Mavor and Bisagni. 2001. Deep-Sea Res II. 48 215-243). Isobaths are for 50, 100, 200 and 1000 m.

1 m2 MOCNESS -Pump Comparison

• Data from 30 Broadscale Survey cruises• MOCNESS used at all stations• Plankton pump used at Priority 1 and 2 stations. Samples counted at all Priority 1 and for March, April, May 1995, 1998 and 1999 at Priority 2.• A centrifugal pump used from Feb. 1995 to May, 1998, and a diaphragm pump between June 1998 and June 1999.• For analysis depth ranges were chosen so that the bottom of each MOCNESS and pump profile were within 15 m of each other. Numbers m2 were determined for each profile.• A selection index was used to compare abundances of each taxa caught by the different sampling methods where

I = (MOC-Pump)/(MOC+Pump). Index values range from +1 to -1.

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Calanus Mocness-Centrifugal Pump Comparison

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MOCNESS

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Calanus MOCNESS-Diaphragm Pump Comparison

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Calanus finmarchicus catch curves for the MOCNESS-Centrifugal pump and MOCNESS-Diaphragm pump. No m-3

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Pseudocalanus Mocness-Centrifugal Pump Comparison

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Pseudocalanus MOCNESS-Diaphragm Pump Comparison

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Pseudocalanus spp. catch curves for the

MOCNESS-Centrifugal pump and

MOCNESS-Diaphragm pump. No m-3

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MOC:Pump Index, Centrifugal Pump (EN261-OC322)

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Calanus

Pseudocalanus

Nichols and Thompson (1991) found that a mesh size of 75% of the copepod width captures approx. 95% of the individuals of that size present in the water. With larger mesh sizes extrusion becomes a problem.

75% of 0.15 mm, the mesh size we used, is 0.2 mm.

We find animals up to 0.35 mm in width are under-represented in the MOCNESS samples compared with the pumps.

The pump flow volumes were measured by an in-line flow meter which was calibrated by directly measured volumes of water.

We do not believe extrusion is the cause because of the large size of the animals relative to the mesh size.

Escapement of these small copepods from the 1 m2 mouth opening is not likely either.

We suggest that there was an over-estimation of the volume filtered because of mesh clogging. The MOCNESS flow meter is mounted away from the mouth opening so we can never be certain of the volume filtered.

Smith et al (1968) found a progressive clogging of nets during the course of a plankton net tow. Using his formula for “green water” gives a net filtering area/mouth area of 3.26:1. Our nets have a ratio of 5.5:1leading us to believe that they should have been satisfactory. However, our data suggest otherwise.

If we assume Calanus C1 and C2 captured by the diaphragm pump are correct, their mean abundance(54 m3), is 37% higher than the mean C1 and C2 measured by the MOCNESS (34.2 m3). Based on thiswe suggest that the MOCNESS undersampled by this amount.

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Estim. MOCNESS

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Estim. MOCNESS

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Georges Bank, 0-30 m Mean Temperature, 1995-1999

Stratification Index 0-30 Sigma-t diff.

Calanus finmarchicus

Egg broadcaster.

Overwinters in the Gulf of Maine.

Advected from the Gulf of Maine onto the NE Peak and around the S Flank beginningin winter.

The Scotian Shelf provides an additional more distant source, either directly across the NE Channel, or via the central Gulf of Maine.

January February March

April May June

Calanus finmarchicus total copepodids. Monthly mean abundance (1995-1999), [log10(No m-2+1)].

• Absence from the bank in January, but higher numbers in the Gulf of Maine (mostly Go C5). • Advection of animals onto the NE Peak and along the Southern Flank in February. • A build-up of the population in April and May and a decline on the bank in June.• A persistent region of low abundance in the shallow, tidally mixed crest of the bank.

Calanus finmarchicus adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• Just beginning to extend onto the NE Peak in January, absent elsewhere on the bank, but higher numbers in the Gulf of Maine.• Advection of animals onto the NE Peak and partially along the Southern Flank in February. • A build-up of the population in April and May and a decline on the bank in June.• A persistent region of low abundance in the shallow, tidally mixed crest of the bank.

Surface Salinity Surface Temperature

Calanus copepodids

Pathway of Calanus advection onto Georges Bank, February, 1995.

• Calanus copepodids advected onto the NE Peak of Georges Bank and to the S Flank.• Cold, low salinity Scotian Shelf water blocking advection of animals onto the easternmost region of the bank.

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Calanus finmarchicus

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Calanus finmarchicus copepodids. Interannual variability in abundance, March

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1998 1999

High abundance of Calanus over the Crest of Georges Bank in March,1995 compared with other years. A result of the strong wind event during February replacing water on the crest with water from the Gulf of Maine?

Calanus finmarchicus: total copepodid abundance during June (Log10 [No m-2 +1])

1995 1996 1997

1998 1999

This figure illustrates the consistently low abundance of Calanus copepodids on the Crest of the bank during late spring/early summer.

Metridia lucens. adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

An egg broadcasterStrong diel vertical migratorHigh abundance in the GOM but low on GB

Pseudocalanus spp.•An egg carrier.

•Two species. •P. moultoni: more inshore and carried onto the bank from western Gulf of Maine coastal waters.

•P. newmani: more offshore and the source is probably Scotian Shelf water, either crossing over the NE Channel, or indirectly through the central Gulf of Maine.

•Pseudocalanus does not overwinter in the Gulf of Maine as does Calanus and is not normally present in the central Gulf during winter.

•Because of the less direct pathways of advection onto the bank compared with Calanus,Pseudocalanus shows more interannual variability in abundance.

Pseudocalanus spp. adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• Animals remain on the bank at the beginning of winter (January).• Advection of animals onto the NE Peak and along the Southern Flank in February. • A build-up of the population in March, April and May.• A persistent region of low abundance on the crest of the bank through April.• By June Pseudocalanus has successfully colonized all parts of the bank, including the crest.

Pseudocalanus spp. Adult female abundance during February (Log10 [No m-2 +1])

1995, 940 m-2 1996, 800 m -2 1997, 2610 m -2

1998, 590 m -2 1999, 1300 m -2

This figure illustrates the considerable interannual variability in abundance of Pseudocalanus spp. on Georges Bank. Mean abundances at Georges Bank stations are indicated.

Oithona spp. adult females (primarily O. similis but includes low numbers of O. colcarva): Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• No seasonal pattern in abundance: abundant throughout the sampling period.• Always a greater abundance in the Gulf of Maine.• Distribution pattern suggests an advection onto the bank from the Gulf of Maine.• Unlike Pseudocalanus, another egg carrier, it tends to have lower abundance on the crest.

Temora longicornis adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• Very low abundances during winter (January, February, March).• The population increases in April. • Coming from resting eggs: no obvious off-bank sources.• Continues to increase in May with highest abundances on the crest within the tidal mixing front.• During June there is a suggestion of an off-bank source from across the Great South Channel.

Centropages hamatus adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• Moderate numbers are present during winter and then the population increases during May and June.• During all months the population is concentrated in the shallow western part of the crest.• A resident population coming from resting eggs: no obvious off-bank source at any time.• The distribution pattern suggests that water in the shallowest regions of the crest are isolated from surrounding water, even during winter.

Centropages typicus adult females: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• A Summer-Fall species which lays resting eggs.• Large numbers of animals remain on the bank during early winter.• The population is lost from the crest and the eastern half of the bank during March-April.• There is a suggestion of an increase on the crest during May-June.• There are animals in the Gulf of Maine and Great South Channel during winter, and in the Great South Channel during spring providing a source of animals to the bank.

Planktonic hydroids: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• The population increases from low numbers in January to very high numbers in April and May. A small decrease in June.• It is confined to the crest of the bank, even during winter suggesting a source in this region.• The population is confined within the tidal mixing front during April-June.

Chaetognaths: Monthly mean abundance (1995-1999), (Log10 [No m-2 +1])

January February March

April May June

• The population increases from low numbers in January to very high numbers in May and June.•There is a consistently low abundance on the southern edge of the NE Peak.

Summary

Each taxon shows a distinct temporal and spatial pattern of abundance on Georges Bankwhich appears to be related to life history characteristics and physical factors.

Calanus finmarchicus. Egg broadcaster. Gulf of Maine and Scotian Shelf origin. Advected onto the NE Peak and around the S Flank beginning in winter. Does not normally appear to be able to colonize the crest because of the isolation of this region within the tidal mixing front, and due to high egg mortality.

Pseudocalanus spp.Carries eggs. A resident population which is supplemented from outside sources. Althoughmore variable than Calanus because of less predictability in off-bank sources, it tendsto show the same patterns as Calanus during winter. However, unlike Calanus, it is able to successfully colonize the crest during summer.

Oithona spp.Carries eggs. Also has an off-bank source in the Gulf of Maine and shows a similar spatial pattern to Calanus during winter. No large seasonal pattern in abundance. Appears to be able to achieve relatively high abundances on the crest in summer.

Temora longicornis

• The population does not increase until April and initially appears to be coming from resting eggs with no obvious off-bank sources. • Highest abundances are on the crest within the tidal mixing front. • During June there is a suggestion of an off-bank source from across the Great South Channel.

Centropages hamatus

• Moderate numbers are present during winter and then the population increases during May and June.• During all months the population is concentrated in the shallow western part of the crest.• A resident population coming from resting eggs with no obvious off-bank source at any time.• The population is able to sustain itself on the crest in late spring-early summer even in the face of high predation.• The distribution pattern suggests that water in the shallowest regions of the crest are isolated from surrounding water, even during winter.

Centropages typicus

• A summer-fall species. Only abundant during early winter and then appears to be advected towards the western end of the bank. • Possible re-establishment of the population on the crest in May and June supplies both from resting eggs and the western Gulf of Maine.