Harvest

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Harvest

Tracadie Bay Models: Influence of

Mussel Aquaculture on Nitrogen and

Particle Dynamics

Peter J. Cranford, Peter Strain,

Barry Hargrave,

Fisheries and Oceans Canada

Bedford Institute of Oceanography

Michael Dowd, Jon Grant &

Marie Archambault

Dalhousie University

Mar 2010 Netherlands


Objectives

1. Provide knowledge on the influences of suspended mussel aquaculture

on ecosystem structure and function.

2. Develop modelling approaches for assessing the capacity of coastal

ecosystems to support the future growth of the mussel culture industry.

3. Assess the cumulative effect of intensive mussel aquaculture and

watershed nutrient inputs on major components of the nitrogen cycle.

4. Compare strengths and limitations of different risk assessment

approaches:



Observation-based energy budget

Dynamic ecosystem model

5. Test previous hypotheses:



Role of mussel aquaculture in coastal nutrient dynamics

Capacity of aquaculture to mitigate impacts from land-based nitrogen

enrichment


Tracadie Bay

N

Tracadie Bay

Area:

Watershed (km 2 )

Low tide (km 2 )

Aquaculture (%)

Mussel Culture:

No. socks

Biomass (tons)

Harvest (tons)

Value

117.4

14.0

50%

~300,000

~4,000

~1,900

$2.66M

TRACADIE BAY

0 1 2 km


Freshwater Nitrogen Input

TRACADIE BAY

0 1 2 km

Aug. 20, 2003

6

5

4

3

2

1

0


Anthropogenic Interactions with N cycle

Excess N addition

Top-down control of phytoplankton

Excess N removal

Run-off

Sunlight

Harvest

N 2

Detritus

Tidal Exchange

Phytoplankton

Nutrients

Bivalves

Sediment

Seston

Grazers

&

Bacteria

Biodeposits

Benthic plants

Aerobic

Sediment

NO 3

-

Nitrification

NO 2

-

NH 4

+

Organic N & P

Burial

Anaerobic

Sediment

NO 3

-

NO 2

-

Denitrification

N 2

Interactions Bottom-up are highly control complex: of phytoplankton

High potential for unexpected results.


N Budget: Reservoirs (t N)

9

Harvest

MUSSELS

9

BENTHIC

TIN

13

PHYTO

1.2

DETRITUS

3

km

0 1.5 3 6

Extensive sampling 2002-03 plus

previous data:

Nitrate, nitrite, ammonia

Chlorophyll, SPM, POM

Conversion to N


N Budget: Freshwater inputs (t N y -1 )

Method 1) GIS analysis of drainage sub-basins and land use (PEI DOE) and

application of published 9 Harvest export coefficients for N (kg N ha -1 y -1 ).

= 76 t N y -1 (50% Agri)

River

Input

100

Drainage Basin

% Land-use

Agriculture

Forest

Wetland/Beach

Other

16.3 km 2 10.6 km 2 28.5 km 2

69.7 km 2

21.3 km 2

Method 2) Winter River flows (1964-2004) scaled to watershed and N levels

extensively monitored in Winter River

= 88 - 124 t N y -1

MUSSELS

9

BENTHIC

TIN

13

PHYTO

1.2

DETRITUS

3

Offshore

Exchange


Offshore Exchanges (t N y - 1 )

9

Harvest

Based on seasonal cycles in

offshore and Bay water and

tidal flushing volumes.

River

Input

100

MUSSELS

9

TIN

13

PHYTO

1.2

DETRITUS

3

-22

-654

-1

Net

Offshore

Exchange

BENTHIC


Mussel interactions (t N y - 1 )

9

Harvest

Ingestion & Excretion → literature

values scaled to population

River

Input

100

MUSSELS

9

154

26

138

92

TIN

13

PHYTO

1.2

DETRITUS

3

-22

-654

-1

Net

Offshore

Exchange

BENTHIC

Biodeposition → predicted from

ingestion rate and diet quality.


Budget Conclusions

Relatively simple approach, although data

intensive, can reveal mussel/ecosystem

interactions.

Dominant role of extensive mussel culture in N

cycling is confirmed:

• High potential for food depletion with turnover

of Phytoplankton by mussels in ~5 days (topdown

control).

• Biodeposition and excretion are major

pathways of N flow to Phyto (bottom-up control).

River

Input

100

9 Harvest

MUSSELS

9

154

BENTHIC

26

138

92

TIN

13

PHYTO

1.2

DETRITUS

3

-22

-654

-1

Net

Offshore

Exchange

Mussel aquaculture is not effective for controlling excess N runoff in Tracadie Bay:

• Mussels clarify water but remove


Lower Trophic Level N Model




Dowd (2005) ecosystem box model includes major interacting ecosystem components,

mixing processes and exchanges, predator-prey dynamics, benthic-pelagic coupling &

biogeochemical processes

3 Boxes with boundary conditions provided by extensive field data.

3 model applications (2 y spinup to steady state):

1. Cumulative effects: present day conditions of N enrichment and aquaculture

2. Enrichment only

3. Baseline: no enrichment or aquaculture

Harvest

9

River

Inputs

TIN

100

TIN

P

M

9

D

Offshore

Exchanges

P

TIN

D

2

1

3

B

0 1 2km

Burial


Scenario Comparisons

1

0.7

0.65

RESERVOIRS

P

100

80

FLUXES

B TIN

2

0 1 2km

3

0.6

15% reduction

60

40

21 X increase

0.55

20

0.5

9

8

7

Baseline

B

Enrichment Cumulative

TIN P

50 20

45 15

Baseline Enrichment Cumulative

M TIN

6

5

4

3

2

21 X increase

40 10

355

63% fueled

by runoff

1

0

Baseline Enrichment Cumulative

300

Baseline Enrichment Cumulative


N Dynamics Conclusions

N Budget

River

Inputs

TIN

(100)

LTLM

River

Inputs

TIN

(100)

TIN

13

TIN

3.40

26

20

122

P

1.2

B

P

1.31

78

B

8.43

92

Burial

63

56

99 39

79

19

Burial

Harvest

M

9

D

3.0

9

138 154

Harvest

M

D

2.14

9

122 156

Offshore

Exchanges

P

TIN

D

(-22)

(-654)

(-1)

Offshore

Exchanges

(106)

P

TIN

D

(-2.4)

(-176)

Results from both approaches are generally

consistent and reach similar conclusions on the

dominant role of mussel culture in Tracadie Bay.

• Mussels influence all aspects of the N cycle.

• Large fraction of P growth depends on N runoff.

• Agriculture helps fuel mussel production.

• Mussels promote N retention in Bay via

biodeposition and reduction in P export.

• Biodeposition represent a very large flux with

potentially serious organic enrichment effects.

N Budget relatively simple but LTLM provides:

• Finer resolution → temporal & spatial differences

• Ability to test different scenarios

Shellfish/ecosystem interactions are complex and

difficult to predict without models.


Modelling Seston Depletion






Fully coupled physical/biological model of Tracadie Bay ecosystem.

Two-dimensional finite element circulation model (Aquadyn) linked to

biological components written in Matlab

Forced at inlet mouth by tides and

boundaries with data (CHL, Nuts, T).

Run for 40 days at which equilibrium

conditions were observed to occur.

Results viewed as maps of Phyto-C.

Mussel density

= 10/m 3


Bay-Scale

Seston Depletion: Model

mg C m -3

63 02’ 63 00’

63 02’ 63 00’

46 25’

46 24’

46 23’

46 22’

A

B

June

June

0 mussels m -3

10 mussels m -3

63 02’ 63 00’

63 02’ 63 00’

(Grant, Bacher & Cranford, submitted)

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