Water Quality Trends and Drivers of the Pelorus and Queen Charlotte Sounds

Abstract

Seasonal and spatial patterns in water column data were investigated in the Marlborough Sounds, New Zealand. Water quality data of the last six to seven years were used to give insight into water column structure, nutrient concentrations and phytoplankton biomass distribution. Monitoring sites are located along the main channel of each sound, and in major side-arms. The particular focus in the Pelorus Sound was on riverine vs climatic influences and the horizontal physical and chemical extent of the effects. The water column was thermally stratified in summer, and salinity stratified in winter with much stronger salinity stratification present closer to freshwater inputs from the Pelorus River. Freshwater inputs and associated nutrient and sediment inputs, and increased flushing rates, divided inner sites between outer sites where these effects become progressively diluted. El Niño Southern Oscillation phase did not appear to be correlated with the predicted environmental effects of lower SST during summer or higher rainfall during winter. Nutrients in Pelorus Sound were received from both riverine and oceanic sources. Freshwater inputs of inorganic nutrients such as nitrate and silicate were evident in the surface water and became diluted towards the outer sites, until effects in the outer Sound could only be seen during floods. Mean July chlorophyll-a levels reached a maximum at the mid Sound site, where mean monthly nitrate concentrations reached a minimum. Residence times and light conditions were more favourable in this location, suggesting that nutrients from inner and outer Sound decrease in availability towards the centre, where phytoplankton growth is promoted. For the purpose of this study, the inner Sound is defined as PLS-1 to -3, mid Sound PLS-4 and -5, and outer Sound PLS-6 and -7. In this study the inner Sound consists of QCS-1 and -2, outer Sound as QCS-4 and -5, and QCS- 3 represents Tory Channel. Queen Charlotte Sound experienced density stratification in summer, albeit weak in the outer Sound where tidal mixing prevented calm conditions. For this reason the Tory Channel site was very different from the other sites, characterised by high tidal velocities, higher turbidity and nutrient levels, and a predominantly diatom phytoplankton assemblage. Phytoplankton from this location were neither nitrogen nor silicon limited in summer. Conversely, phytoplankton in the inner Sound were nitrogen limited in summer. The inner Sound sites are located in a sheltered area that experiences long residence times. This, in combination with a persistent phytoplankton community deeper in the water column, supported a gradual decline in dissolved oxygen concentration (to a minimum of 43% saturation) in the bottom water over summer. As this took place as a steady decline over several months it would be beneficial to monitor dissolved oxygen concentrations over this period to ensure that bottom water conditions do not become more severe, leading to ecosystem degradation.