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  Services » Publications » Technical reports - by year » tr200508

Verification of plankton depletion models against the Wilson bay synoptic survey data

Report: TR05/08
Author: N. Broekhuizen, J.W. Oldman, K. Image, M. Gall, J. Zeldis (NIWA)


This report presents the results of a verification exercise applied to the models NIWA utilised to assess the ecological sustainability of the Firth of Thames shellfish aquaculture: the 'original biophysical model', the 'revised biophysical model' the 'logistic plankton model', and the 'hydrodynamic model' which drives transport in the biological models.

The models were verified against data from a synoptic plankton survey carried out by NIWA on behalf of the Wilson Bay Group A Mussel Farming Consortium in May 2003. The analyses were based on quantitative comparisons between simulations and observations. For each of the biological models a pair of simulations was made: 'with-farms simulations' and 'no-farm simulations'.

The verification revealed that the hydrodynamic model reproduces observed vertical temperature, salinity and velocity profiles during the synoptic survey period adequately, although there was a small tendency to underestimate the vertical gradients. This deficiency could be amended by reducing the upper limit to which the vertical diffusivity of momentum is allowed to climb.

In the verification of the biological models, distinctions were made between (a) local patterns (within the farmed zone); near-field patterns (within 5 km of Area A); and far-field patterns (more than 5 km from Area A). The analyses suggested that, within the vicinity of Wilson Bay Area A, the models are neither dramatically over-predicting, nor dramatically under-predicting the degree of phytoplankton change induced by the Wilson Bay Area A farms. The authors caution that the verification was restricted to short periods of time, and that the models may not perform equally well under different conditions. In addition, the predictions for the putative Western Firth of Thames Aquaculture Management Area were less robust than those made for the Wilson Bay area.

View the entire technical report on Auckland Council's website (TP 256).(external link)

Table of contents

1. Executive summary 1
2. Introduction 5
  2.1 Nature of the verification exercise for the biological models 6
  2.2 The biological models 6
  2.3 The Wilson Bay farming operations 7
  2.4 Synoptic survey 8
  2.5 Environmental conditions around the survey period 11
3. Methods (Hydrodynamic model) 15
  3.1 Hydrodynamic model 15
  3.2 Model set-up 15
  3.2.1.Bathymetry 16
  3.2.2.Vertical grid structure 16
  3.2.3 Turbulence closure 16
  3.2.4 Temperature and salinity dispersion 17
  3.2.5 Seabed resistance 17
  3.3 Forcing inputs into the model 17
  3.3.1 Tide 17
  3.3.2 Stratification 18
  3.3.3 Temperature 19
  3.3.4 Salinity 20
  3.3.5 Wind 21
  3.3.6 Heat exchange 22
  3.3.7 Relative humidity 23
  3.4 Model output 23
4. Methods (Biological models) 24
  4.1 Implementing the farming system 24
  4.2 Biological simulations 24
  4.3 Implementing the synoptic survey sampling protocol 25
  4.3.1 Fixed station sampling and analysis of virtual data 25
5. Results (Hydrodynamic model) 26
  5.1 Tides 26
  5.2 Currents 27
  5.3 Stratification (Salinity and temperature) 28
6. Results (Biophysical and logistic models) 30
  6.1 Biophysical model 30
  6.1.1 Temporal changes 30
  6.1.2 Comparison of depletion inferred from the field data and that inferred from the difference between no-farms and with-farms simulations 33
  6.1.3 Comparison of depletion inferred from the field data and simulated data using the surfaces method 37
  6.2 Empirical model 39
  6.2.1 Depletion inferred from a comparison of no-farms and with-farms simulations 41
  6.2.2 Depletion inferred from the surfaces method applied to the simulation results from the logistic model 43
7. Discussion 44
  7.1 A comment upon carbon chlorophyll ratios 44
  7.2 'Sampling Error in the Simulations' 44
  7.2 1 Biophysical model 46
  7.2.2 Empirical model 47
8. Implications for confidence in the earlier, scenario simulations 49
  8.1 Biophysical model 49
  8.2 Logistic model 50
  8.3 General comments 50
  8.4 Implications for interpretations in the earlier, scenario simulations 51
9. Acknowledgements 53
10. References cited 54
11. Appendix 1: Recalibration of the Hydrodynamic Model 56
  11.1 Tides 56
  11.2 Currents 57
  11.3 Stratification (Salinity and temperature) 59
12. Appendix 2: Verification of the revised biophysical model 60
  12.1 Summary of changes to the Biophysical model 60
  12.2 Summary of verification results for revised model 60
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