Science Centres: Oceans

Predicting ocean nutrient levels

Repeated measurements of temperature and nitrate concentrations were made on transects sailed by Tangaroa and Munida in subtropical (S2 ) and subantarctic (S4) waters and the Subtropical Front (S3) southeast of New Zealand.

Nitrate availability is one of the main factors controlling primary productivity in the world’s oceans. We are using NIWA datasets to better understand variability in concentrations of this important nutrient. Nitrate concentrations in the surface layers of the ocean vary greatly in time and space, particularly around New Zealand, where subantarctic and subtropical water masses meet.

Measuring this variability directly would require intensive sampling from ships, which is expensive and impractical. As a proxy, scientists estimate nitrate concentrations using models based on satellite measurements of sea surface temperature (SST) and chlorophyll.

NIWA scientists have recently tested how well these models fit the real world. "This dataset is unique by world standards because it includes measurements which repeatedly sample the same locations, in both subantarctic and subtropical waters," says NIWA physicist Dr Vanessa Sherlock.

The team found that while SST alone captured a fair amount of the actual variation in nitrate levels, including measurements of chlorophyll will improve our ability to predict seasonal variations. Ultimately, this information will feed into our understanding of the link between nitrate concentrations and primary productivity in our EEZ. "But that’s a long way off," says Dr Sherlock.