Microzooplankton in the food

web of the coastal Gulf of

Alaska

Suzanne Strom, Russ Hopcroft

Western Washington University

University of Alaska Fairbanks

NOAA Fisheries

The setting: dynamic

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HNLC

Cross-shelf exchange

High runoff

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Sampling regions: 2011 & 2013Spring, Summer, Fall (GOA-IERP)

WEST

wide shelfEAST

narrow shelf

Seward Line

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Strong seasonality: phytoplankton

0

50

100

150

200

250

Spring Summer Fall

IntegratedChl-a

(mg/m2)

2013West

East

0

50

100

150

200

250

Spring Summer Fall

IntegratedChl-a

(mg/m2)

2011West

East

Chlo

rophyll

a (

mg m

3)

Waite & Meuter 2013

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Chlorophyll anomalies

Interannual variability: phytoplankton

May 1-15 May 1-15

May 16-30 May 16-30

Long-term average 2011

0.0

0.2

0.4

0.6

0.8

1.0

0 4 8 12 16

Fra

ctio

n C

hl-

a >

20

µm

Total Chlorophyll-a (µg/liter)

Low-chl water:High-chl water:

2011 spring bloom very low

Large cells (diatoms) sparse

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Questions of our study:

• How does the environment affect

the microzooplankton community?

• How might temporal and spatial

contrasts in microzooplankton

influence the food web?

Image credits: G. Hannach (King County); T. Saxby (ian.umces.edu)

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What we know about CGOA microzoo:

1. Cross-shelf gradient in

biomass, composition2. Moderate – high

grazing rates common

Mean c

ell

vol(µ

m3)

Din

oflagella

tes

(% o

f to

tal bio

mass)

Nearshore Offshore

Strom et al. 2007

including microzoo

grazing on diatoms

Gra

zin

g r

ate

(d

-1)

>20 5-20 <5 µm

Chlorophyll size fraction

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What we know about CGOA microzoo:

3. Prey for dominant mesozooplankton species

Non-bloom Bloom

Ingestion r

ate

(µg C

copepod

-1d

-1)

Neocalanus cristatus

Liu et al. 2005

Also:

Pseudocalanus spp.

Calanus marshallae

(Napp et al. unpub.)

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Taxonomic detail level:

Readily identifiable species/genera

Otherwise, broad size and taxonomic category (functional groups)

All cells sized, converted to C biomass from volume using empirical factors

How are microzoo assessed?DinoflagellatesGymnodinium

Gyrodinium

Ceratium

Polykrikos

Protoperidinium

Amphidinium

Dinophysis

misc (by shape)

CiliatesLaboea

Tontonia

Strombidiniopsis

Strobilidium

Strombidium (var)

Mesodinium

tintinnids (by genus)

misc (by shape)

Increasing distance offshore

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New data: Seasonal cycles in microzoo

0

10

20

30

Spring Summer Fall

Microzoobiomass(µgC/L) 2011

west

east

0

10

20

30

Spring Summer Fall

Microzoobiomass(µgC/L) 2013

west

east

Median values from 10 m samples (n = 6-16)

• Biomass 2-3x higher in spring

• West higher than east

Strom et al. 2016

• Absence of spring phyto bloom seen in 2011 microzoo

Contrasts in microzoo community composition

WEST

EAST

ciliates

dinos

darker = larger

Spring 2013

Spring 2011

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Contrasts in microzoo community composition

2011 2013

E W E W E W

spring summer fall

E W E W E W

spring summer fall

Cili

ate

: d

ino

bio

mass r

atio

1 : 1

W E W E W E

spring summer fall

W E W E W E

spring summer fall

Fra

ction >

40 µ

mMore ciliates in eastMore ciliates in 2011

Large microzoo during

spring bloom

Not so much. . .

Strom et al. 2016

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That is more ciliates than we expected

Year Season Avg % Dinos Reference

1992 spring 48 - 54 Howell-Kübler et al. 1996

1999 summer ~50 Olson & Strom 2002

2004 summer ~70 Strom & Fredrickson 2008

2008-10 spring 65 - 75 Sherr et al. 2013

2008-10 summer 64 Stoecker et al. 2014

Bering Sea:

From Stoecker et al. 2014

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SMALL forms can be abundant

Low production times/places

Feed on <5 µm prey efficiently

Why so many ciliates?

Strombidium dalum

Laboea strobila

Image credits: D. Montagnes (Planktonic Ciliate Project); F. Neidl (Glacier Bay Natl Park); G. McManus

(McManus et al.2010)

Strombidium rassoulzadegani

LARGE forms often most of biomass

Low production times/places

Feed on 5-15 µm prey

Many species RETAIN CHLOROPLASTS

(mixotrophs) – Stoecker et al. 2014

Adaptation to variable environment

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Drivers of microzoo community composition

Biomass

ciliates

dinoflagellates

PC1 related most strongly to largest ciliates and dinoflagellates

PC1 positively correlated with chlorophyll (but not T, S, nutrients)

Strom et al. 2016

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Relationship to mesozooplankton (top down)

SALPS

Unusually

abundant in 2011

Image credits: L. Madin (www.whoi.edu); D. Wrobel (jellieszone.com)

MAX CLEARANCE

(% water column d-

1)

20 1

5 1

Li et al. 2016

Salpa aspera Cyclosalpa bakeri

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Relationship to mesozooplankton (top down)

Neocalanus spp. (spring dominant) similar between regions and years

Calanus marshallae

higher in east

56°

57°

58°

59°

60°

61°

62°

Summer 2011

-155° -150° -145° -140° -135°

56°

57°

58°

59°

60°

61°

62°

Fall 2011

Summer 2013

-155° -150° -145° -140° -135°

Fall 2013

56°

57°

58°

59°

60°

61°

62°

Spring 2011300

30

3

Spring 2013

Summer 2011 Summer 2013

56°

57°

58°

59°

60°

61°

62°

Summer 2011

-155° -150° -145° -140° -135°

56°

57°

58°

59°

60°

61°

62°

Fall 2011

Summer 2013

-155° -150° -145° -140° -135°

Fall 2013

56°

57°

58°

59°

60°

61°

62°

Spring 201150

5

0.5

Spring 2013

Summer 2011

Spring 2011

Summer 2013

Spring 2013

Euphausiids usually

higher in east

Hopcroft et al. 2016

Narrow shelf in east =

MORE OCEANIC =

large-bodied zoop =

larger prey

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Conclusions

• Microzooplankton in the CGOA show seasonal, regional and

interannual variation that reflects the underlying production

regime

• Ciliates made up a large proportion of the community,

especially in lower production seasons and locations

• Large ciliates likely a key functional group in the ecosystem

due to chloroplast retention (mixotrophic life style) and

importance as prey

• Predation by mesozooplankton likely affects microzoo

abundance and community composition

Thank you very much – Tusen takk!