Biomass and Compositional Characteristics of Kaneohe Bay, Oahu, Hawaii, Phytoplankton Inferred from Regression Analysis

Abstract

Concentrations of chlorophyll a (chl a), particulate carbon (PC), and particulate nitrogen (PN) measured on a weekly basis in the picoplankton and nano-plus-microplankton size fractions over a 2-yr period from 1986 to 1988 at a station near a former sewage outfall in Kaneohe Bay, Oahu, Hawaii, were compared to similar data collected in 1970, 1972, 1974, and 1976-1977 while sewage was being discharged into the bay, and in 1978-1979 immediately after diversion of the sewage. Particulate concentrations showed considerable temporal variability both within and between years. High concentrations were associated with periods of above-average rainfall. Heavy rains that occurred during two successive periods of spring tides produced chl a concentrations of over 40 mg m-3 in January 1988, almost four times the highest concentration measured during the period of sewage discharges. Nutrients from land runoff as well as from decomposition of organisms killed by salinity stress were the apparent cause of this spectacular bloom. The bloom consisted almost entirely of nanoplankton and microplankton, but picoplankton accounted for 45 ± 14% of the chl a during the remainder of the 1986-1988 study. Phytoplankton C:N ratios were apparently unaffected by diversion of sewage from the bay and averaged within 10% of the Redfield ratio. This result implies that phytoplankton were growing at close to nutrient-saturated rates both before and after the sewage diversion. Nutrient budget calculations indicated that most of the growth has been supported by recycling within the bay. Phytoplankton C: chl and N: chl ratios estimated by regression analyses increased after the sewage diversion, apparently in response to the increase in average irradiance in the water column caused by the decline in seston concentrations. C: N ratios of picoplankton and nano-plus-microplankton under nutrient-saturated conditions were about 4.6 ± 0.3 and 6.2 ±O.8, respectively; the difference probably reflected the high concentration of nitrogen-containing pigments in some picoplankton.