Variation in nutrient loading from the Motueka River into Tasman Bay, New Zealand, 2005–2009: implications for the river plume ecosystem

Abstract

The monthly mass transport of nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), total nitrogen (TN), dissolved reactive phosphorus (DRP), total phosphorus (TP) and dissolved reactive silica (DRSi) from the Motueka River catchment into Tasman Bay (South Island, New Zealand) was estimated for the period 2005–2009 based on observed historical relationships between recorded river flows and the associated measured concentrations of each nutrient. The resulting average annual loading estimates for TN, TP and DRSi ranged from 397–829, 25–72 and 4053–6576 tonnes, respectively, over the 5-year period. Large seasonal variations in nutrient loadings were also estimated with the highest rates generally, but not always, occurring during winter. The observed patterns suggest that phytoplankton biomass within the river outwelling plume may also vary in response to seasonal and inter-annual variations in catchment nutrient loading rates with potential follow-on food web effects including the growth of cultured GreenshellTM mussels. The estimated average annual TN loading rate (2005–2009) from the Motueka River catchment (613 tonnes) is only c. 20% of the annual loss of nitrogen based on extrapolated literature values for denitrification rates in Tasman Bay. We therefore suggest that, under the present land use scenario in the Motueka catchment, there is little potential for resulting dysfunctional river plume ecosystem enrichment effects to occur.