Spatial variations in the levels of nutrients, chlorophyll a, and dissolved oxygen in summer and winter in Doubtful Sound, New Zealand

Abstract

The influence of a strong surface low‐salinity‐layer (LSL) on the distribution of macro‐nutrients, chlorophyll a, and dissolved oxygen in a fiord was investigated in February and August 1994 in Doubtful Sound, New Zealand. Phosphate and nitrate levels were significantly lower in the LSL than in the underlying saline layer (SL) and increased with depth towards the open ocean suggesting exchange of open ocean water across the deep fiord sill and then vertical mixing with the LSL. Nitrate/phosphate ratios in the range 12.3–16.5 (summer) and 14.9–17.4 (winter) for the LSL compared with 9–10 for the SL for both seasons, may reflect significant differences in phytoplankton species and/or nitrogen cycling between the layers. Elevated silicate levels in the LSL decreasing towards the open ocean were attributed to conservative mixing of the freshwater discharge from the Tail Race with the SL. Biomass (chlorophyll a) in the LSL was quite low even in summer and generally increased towards the open ocean but exhibited several local surface maxima perhaps related to variable zooplankton grazing and/ or light levels. Levels of dissolved oxygen were above saturation in the LSL and decreased with depth, but never to anoxic levels.

Keywords:

• salinity
• temperature
• phosphate
• nitrate
• silicate
• chlorophyll a
• dissolved oxygen
• low salinity layer
• mixing
• fiord
• New Zealand