Planktonic Properties of Honokohau Harbor: A Nutrient-Enriched Subtropical Embayment

Abstract

The phytoplankton properties of this nutrient-enriched, vertically stratified harbor show remarkably oligotrophic characteristics considering the eutrophying potential of groundwater nutrient input. Generally, low phytoplankton biomass levels (0.06-0.70 mg chlorophyll a/m3 ) and productivity rates (0.30-5.1 mg C/m3/hr) prevail within the harbor. Zooplankton standing stocks within Honokohau Harbor were highest in the most inland reaches of the basin and showed marked increases (numerically about 28 times and 6 times by dry weight) over levels in adjacent coastal areas. Enteric bacterial (total and fecal coliforms and fecal streptococci) levels were low, despite evidence of sewage leakage into the harbor, reflecting both low input and rapid flushing rates of the basin. The vertical profiles of phytoplankton parameters reflect the peculiar thermohaline stratification of the water column. Despite high surface nutrient levels, a strongly stratified nutricline apparently limits availability to the subsurface (oceanic) phytoplankton populations. This study discusses the importance of advective removal (via flushing) and grazing in controlling phytoplankton stocks within the basin.