Monitoring restoration success: Assessing Marine-Freshwater

Food Web Linkages Using Stable Isotopes

Dr. Karen Wilson, University of Southern MaineDr. Graham Sherwood, Gulf of Maine Research

InstituteDr. Jonathan Grabowski, Gulf of Maine Research

InstituteDr. Theodore Willis, University of Southern Maine –

Aquatic Systems Group

Juven

ile lamp

rey m

igrate to

sea

sea lamprey larvae

Sp

awn

ing

sea lam

prey

April May June July Aug Sept Oct Nove

stu

ary

tid

al

bra

ck

ish

tid

al

fre

sh

Ma

ins

tem

o

r tr

ibu

tary

po

nd

s/l

ak

es

Elvers m

ove u

pstream

Matu

re adu

lt eels leave freshw

ater

Ad

ult alew

ife

Spawning ALE

AL

E Y

OY

exit lakes

AL

E Y

OY

exit lakes

ALE YOY

Sh

ad

Spawning shadS

had

YO

Y exit river

shad YOY

Salmon parr

Ad

ult salm

on

Spawningsalmon

Salm

on

smo

lt exit river

Ad

ult b

lueb

acks

Blu

eback

YO

Y

Sp

awn

ing

b

lueb

acks

Blu

eback Y

OY

exit river

Sm

elt Y

OY

exit river

Sm

elt ad

smelt YOY

Sp

awn

ing

Sm

elt

Also in the lower river:Shortnose sturgeonAtlantic sturgeonTomcodStriped Bass

Penobscot River Restoration Project

Food Web Restoration Indicators

Indicator Rationale & desired outcomes

Trophic levels

More trophic levels = •more diverse predator-prey interactions•greater prey availability•greater ecosystem complexity (i.e., more pathways for food web interactions)

Connectivity Greater reliance on prey from non-focal habitats (i.e., marine vs. freshwater) indicating increased marine-freshwater linkages

See http://www.umaine.edu/searunfish/penobscotexchange/PRFramework_final.pdf

Carbon and Nitrogen stable isotopes: “you are what you eat”• Light (common) & heavy (rare) isotope

forms differ by # of neutrons• Heavy nitrogen (15N) is more difficult to

excrete, so is accumulated from prey to predator as it moves up the food chain

• Approximately 3 – 4 δ15N(‰) units between trophic levels (primary producer, primary consumer, secondary consumer, etc)

• Primary producers [plants] assimilate heavy carbon (13C) under carbon-limited conditions, which vary depending upon habitat

• Animals do not distinguish between heavy & light carbon, so carbon reflects the original source of carbon in their prey

You are what you eat:

• 15N indicates trophic position• 13C indicates habitat- in this

case:– freshwater (lighter) – marine (heavier)

• Other critical advantage: stable isotope signatures of slow turn-over tissues integrate feeding relationships over many months

With increases in anadromous fish, particularly alewife, it is

expected that: • Spawning alewife add marine-derived

nutrients through spawning, excretion & mortality, shifting freshwater 15N signatures higher and enriching freshwater 13C towards heavier (marine) values.

• YOY or juvenile alewife may become important forage for larger predators, increasing trophic position of some species (SMB, CP, cod, mackerel).

• Nearshore marine predators may consume out-migrating YOY alewife in greater numbers, shifting marine 13C signatures towards lighter (freshwater) values.

Expectations: carbon

river mouth

13C

(‰)

Marine carbon signature

Freshwater carbon signature

Before Restoration

After Restoration

freshwater marine

Distance from river mouth

- 26

-16

Field collections

• Muscle tissue samples from common fish & top predators (bass, chain pickerel, cod, mackerel)– freshwater fish collected in collaboration

with U. Maine Fish Index Surveys– marine fish collected in part in

collaboration with MeDMR

• Tissue samples from food web base– primary consumers including snails

(benthic algae) & mussels (phytoplankton)

– secondary consumers (crayfish, crabs, insects)

Freshwater sites: Penobscot River and Tributaries

Penobscot River

Marine sites: Penobscot Bay

A

B

C

D

E

RV ‘Gadid’ - GMRI

September 2009

FV Robert Michael – ME/NH inshore trawl survey

May 2010

2

4

6

8

10

12

14

-35 -30 -25 -20 -15 -10 -5

del 13C

de

l 15 N

Penobscot BayPushaw StreamPen Riv Below VZDPen GW ImpoundmentPen VZ Impoundment

SMB

snail

CP

CP

mussels

snail

Odonata

SMB

snail

snail

urchin

brittle star

mussel

predatory snail

sea star

lobster

cod, sculpin, crab, pollock

WS

WS

WS

eel

bivalveOdonata

Odonata

bivavesnail

eel

SMB

RBSF

WSYP

RBSF

YP

GS

PS

PS

eel

FW

Marinelamprey

alewife

blueback

mackerel

-0.2

0

0.2

0.4

0.6

0.8

1

Chain pickerel SMB Mackerel Alewife Blueback

CodPer

cent

FW

influ

ence

Influenced by adult alewife?

Influenced by juvenile alewife?

Connectivity

-35

-30

-25

-20

-15

-10

Pusha

w

GW

IVZDI

Below

VZD A B C D E

RiverMouth

Del

13

C

Inverts_RiverFish_River

Fish_BayInverts_Bay

What’s next?

• Analyze new samples (more fish and inverts from Pen bay and additional tributaries)

• Comparison to Kennebec River system (2010)

• Re-sampling post dam removal

Mechanisms: high abundances of river herring

Spring

Fall

Freshwater

Marine

Summer