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

TR 2005/08
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Report: TR05/08

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

Abstract

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)