Modelling of biogeochemical dynamics in New Zealand seasHelen Macdonald, Niall Broekhuizen, Mark Hadfield, Joanne O'Callaghan, Graham Rickard.

New Zealand oceanography

Motivation

• Hauraki Gulf and ocean acidification– pH and aragonite saturation state are seasonally depressed in the

Firth of Thames, to levels approaching those considered deleterious for some marine biota.

– The Firth of Thames is home to the largest blocks of mussel farms in NZ and much of its oyster culture, and is the most intensive snapper spawning and larval nursery site for our largest snapper fishery management area (SNA 1).

• Large Aquaculture industry (in The Marlborough Sounds Aquaculture ($4.6% GDP)

Looking at different scales of complexity

• New Zealand wide exclusive economic zone (EEZ)

• Harbor/Estuary scale

• Mussel farms

New Zealand scale models

• Need to reproduce main – largescale features of the New Zealand's EEZ.

• Main aims of this scale model are climatological questions such as – Large scale interactions– NZ EEZ climatology– Forcing of smaller grids

• The model needs to cover a wide range of latitudes – the factors that are important in one area are different to the factors that are important in another

Which models works well in this region ?

Good ranking ------------------------- Bad ranking

Models 11, 12 and 13 are PISCES

Introduction to PISCES

Introduction to physical model - ROMS AGRIF

• CROCO• Climatological

forcing for the 90s• 10x10 km

resolution

CROCO and PISCESChlorophyll SLA

0 Mg Chl/m^3

0.5 Mg Chl/m^3

0 Mg Chl/m^3

0.5 Mg Chl/m^3

-1 m

1 m

-1 m

1 m

Shelf scale models

• These models need to capture processes such as

– Coastal currents

– Transient wind-driven events

– Effect of runoff

Introduction to Fennel

• Advantages include –

– Denitrification

– Chlorophyll calculated internally

– 2 classes of nutrients so preferential take up of NO3 can be included

ROMS set up• Resolution: 2km x 2km

• Atmospheric forcing: CCMP and NCEP

• Boundary forcing: From CROCO/PISCES

Dep

th

(m)

Fennel output

• Fennel reproduces the pattern of chlorophyll as seen in the satellite

Longitude (^oE)

NE shelf gliders • A recent glider mission on the Northeast shelf provides

information on the vertical structure.

• ROMS does reproduce some vertical structure of chlorophyll.

Mussel farms

• NZ has a thriving aquaculture industry

• There is this ongoing question as to how many farms are sustainable

• To answer this question we have added the effect of mussel farms into the Fennel model.

• The farms affect the amount of nutrients and detritus in the Fennel code.

Fennel et al. 2006. Global Biogeochemical Cycles 20: GB3007

Mussel farms continued

The Marlborough Sounds

• Marlborough District Council:– “The jewel in our

crown”

• Tourism (5.2% GDP)• Aquaculture ($4.6%

GDP)• How can we best

manage the Sounds to protect their economic and cultural worth?

Simulated effects of mussel farms

• Winter: phytoplankton may have been twice as abundant during pre-farming days • Summer: phytoplankton may have been 20-40% less abundant in inner Sound in pre-

farming days

Next steps

• Inner Hauraki Gulf grid with riverine input

• Biogeochemical grids to other regions of New Zealand

Summary.

• 3 different biogeochemical models were introduced for different model configurations and regions

• These models are an important tool for managing the multiple uses of New Zealand oceans.