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

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Title:Water Quality in a Subtropical Embayment More Than a Decade after Diversion of Sewage Discharges
Authors:Laws, Edward A.
Allen, Colleen B.
Date Issued:Apr 1996
Publisher:University of Hawaii Press
Citation:Laws EA, Allen CB. 1996. Water quality in a subtropical embayment more than a decade after diversion of sewage discharges. Pac Sci 50(2): 194-210.
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.
Pages/Duration:16 pages
Appears in Collections: Pacific Science Volume 50, Number 2, 1996

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