Modelling the upper limit to oceanic phytoplankton production as a function of latitude in the New Zealand exclusive economic zone

Abstract

Abstract A simplified model was developed to examine the potential influence of temperature (θ) and irradiance (I) on hourly and daily primary production as a function of latitude. Phytoplankton growth rates (P) were described as functions of θ and I by existing empirical relationships, and the potential total production for the water column was estimated from an exponential integral solution of the P versus I curve. The model was then applied to the New Zealand Exclusive Economic Zone (EEZ) which spans 30° of latitude and thereby embraces a wide range of temperature and irradiance conditions. The proportional extent of light inhibition decreased with latitude, but integral production was more sensitive to variations in the light limitation parameter (a) than in the light inhibition (β) parameter. The absolute seasonal range of production was greater at low than at high latitudes, but on a proportional basis the degree of seasonal fluctuation was greatly reduced at low latitudes. Seasonality of production as measured by the coefficient of variation for daily values throughout the year increased almost linearly with increasing latitude. Annual production rates per unit area decreased linearly with latitude in the EEZ and were strongly correlated with mean annual irradiance and to a lesser extent with mean annual temperature. In the real ocean other processes may reduce production rates below the values calculated here, but the model results provide a foundation for more complex oceanic modelling for specific locations within the EEZ.