Fluxes of organic carbon in Manukau Harbour, New Zealand

Abstract

We collated information on the sources and sinks of organic carbon in Manukau Harbour, a shallow temperate estuary. Two contrasting inner harbor regions were considered; the northern region, which is urbanized and receives a major load of sewage wastewater, and the southern region, where allochthonous inputs are dominated by the runoff from small rural streams. Although high levels of dissolved nitrogen in the wastewater supported phytoplankton blooms in the northern region, total primary production there was similar to that in the southern region (ca. 300 g C m⁻²yr⁻¹). By contrast, high concentrations of organic carbon in the wastewater resulted in an additional input to the northern region of 120 g C m⁻² yr⁻¹. Loads from runoff and streams to both regions were low. At 350 g C m⁻² yr⁻¹, total respiration in the northern region exceeded total production, so the region was slightly heterotrophic. Respiration was lower in the southern region (270 g C m⁻² yr⁻¹), which was net autotrophic. Some carbon was exported from each region to the outer harbour (50–80 g C m⁻² yr⁻¹). Dissolved oxygen levels in the northern region were somewhat depleted at times; and the high numbers of microzooplankton indicated consumption was enhanced there. Apart from a relatively small area of organic enrichment close to the wastewater discharge, benthic consumers in the harbor appeared to be limited by physical disturbance (by wind-waves) rather than by food availability. Improved wastewater treatment is expected to substantially reduce the allochthonous input to the northern region, with the total input of carbon in the future being only slightly higher than that to the southern region.