Water Quality in a Subtropical Embayment More Than a Decade after Diversion of Sewage Discharges

Abstract

Concentrations of chlorophyll a (chl a), particulate carbon and nitrogen (PC and PN, respectively), inorganic nutrients, and Secchi depths were measured from October 1989 to June 1992 in Kane'ohe Bay, an embayment on the windward coast of O'ahu, Hawaiian Islands. Results were compared with values reported in 1978-1979, the year immediately following diversion of two sewer outfalls from the southeast sector of the bay. Nutrient enrichment experiments indicated that the bay is now distinctly nitrogen limited. In many respects the water column appears more oligotrophic now than in 1978-1979. Inorganic nitrogen and phosphate concentrations now border on the limit of detection by colorimetric methods. Chl a concentrations have declined by 3540% (0.3-0.5 mg m-3 ) and Secchi depths have increased by 15-35% (1.01.5 m) in the southeast sector of the bay since 1978-1979. This has happened despite a population increase of 7,762 persons in the watershed from 1980 to 1990. Characteristics of the water column are now remarkably similar in all sectors of the bay. About 40% of the phytoplankton chl a is accounted for by picoplankton. Pigment analyses indicate that diatoms and cyanobacteria make up ca. 45 and 25%, respectively, of the phytoplankton biomass. It is postulated that the drawdown of inorganic nutrient concentrations and increase in PN/chl a and PC/chl a ratios reflect a shift of the phytoplankton community toward smaller species characteristic of oligotrophic environments. An increase of PN in the central and northwest sectors of the bay is postulated to have been caused by an increase in nitrogen fixation and export from the barrier reef. There is no evidence that human population growth has altered nutrient loading from stream runoff.