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Magnitudes of natural and mussel farm-derived fluxes of carbon and nitrogen in the Firth of Thames

Report: TR05/30
Author: John Zeldis (NIWA)


The eastern Firth of Thames supports the largest single block of mussel farms in New Zealand, within the Wilson Bay Marine Farming Zone. In addition to this, another Aquaculture Management Area (AMA), is under consideration by Auckland Regional Council in the western Firth. The scale of these developments has made it necessary that EW and ARC assess and predict environmental performance of Firth aquaculture at Firth-wide, as well as local AMA scales.

This study evaluates fundamental ecosystem processes at the scale of the Firth: incorporation of carbon and nitrogen into organic material through system import and primary production, and losses of nitrogen and carbon through system denitrification, respiration and export. These values are compared with carbon and nitrogen assimilation and respiration by mussel farms, at the various AMA development intensities. The intention of the work is to provide perspectives on the relative magnitudes of ecosystem and farm processes, under the various intensities of AMA development.

Information on Firth system primary production, respiration and denitrification were compared with information on mussel biomass, C and N composition, and weight-specific respiration, to draw conclusions about the importance of mussel aquaculture within the Firth ecosystem. At the present level of AMA development, mussel biomass harvest removes 0.2% of Firth C primary production y-1. At projected biomasses of maximum AMA development (= WBMFZ fully developed + Western Firth AMA) the harvest would remove 1.6% of primary production y-1. For these respective scenarios, mussel C respiration would account for 0.3 and 1.8% of present Firth system respiration. Similar to denitrification, the mussel harvest represents a net sink for nitrogen, removing nitrogen from the internal cycle supporting Firth primary production. At maximum AMA development, about 1.4% of Firth N primary production (i.e., DIN fixed) would be removed by the mussel harvest. This is about 2.8% of the size of the denitrification sink.

Magnitudes of natural and mussel farm-derived fluxes of carbon and nitrogen in the Firth of Thames
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Table of Contents

Executive summary i
1 Introduction 1
2 Method 2
2.1 Overview of budgetary approach 2
2.1.1 Water budget 3
2.1.2 Salt budget 3
2.1.3 Budgets of non-conservative nutrients 4
2.1.4 Stoichiometric relationships among non-conservative budgets 4
2.2 Primary production 5
2.3 Mussel elemental composition and respiration 6
2.4 Scenario designation 6
3 Results and Discussion 7
3.1 Carbon and nitrogen fluxes 7
3.1.1 Budget outcomes 7
3.1.2 Primary biomass and production 9
3.1.3 Mussel elemental composition and respiration 9
3.2 Assessing influence of aquaculture 10
3.3 Accuracy and precision 14
3.4 Comparisons with LAC criteria and biological modelling 15
4 Acknowledgements 16
5 References 17