Underwater light and photosynthesis at three sites in Pelorus Sound, New Zealand

Abstract

Optical and photosynthctic measurements were made at 3 sites in Pelorus Sound, New Zealand, in late summer 1985. The attenuation of underwater light increased towards the landward end of the Sound, with an abrupt rise in scattering parameters (the scattering coefficient b, asymptotic reflectance Ra, and the ratio of scalar to downwelling irradiance Eo/E d) in the inner sound region (Mahau Sound). The effects of scattering were offset by changing water column depth and a high average irradiance was experienced by the phytoplankton community (22–34% of surface irradiance) at the 3 sites. Spectral scans of the water from Pelorus Sound showed an increasing absorbance by dissolved organic material (DOM) as well as paniculate materials towards the inner sound. Maximum photo‐synthetic rates for the water column and per unit chlorophyll a were similarly high at all 3 sites, but chlorophyll a concentrations were consistently low (0.5–2.5 mg nr^‐3). Population losses to zooplankton or other herbivores seem the most likely explanation of the low phytoplankton standing stocks despite the high productivity per unit biomass. A close adaptive coupling of the phytoplankton to nutrient supply and the underwater light field was suggested by large site‐to‐site differences in the photosynthesis versus irradiance parameters.

Key words:

• algal growth
• estuary
• light
• optical properties
• Pelorus Sound
• photosynthesis
• phytoplankton
• primary production
• tidal inlet