Responses of Kaneohe Bay, Hawaii, to Relaxation of Sewage Stress

Abstract

Kaneohe Bay is a subtropical coral reef/estuary complex which was subjected to increasing sewage loading; that sewage was diverted in 1977 and 1978. We have treated the loading and diversion as a controlled, total-ecosystem experiment to evaluate the chemical and biological responses to external subsidy of nutrients. We here consider the Bay's response to sewage loading and to its diversion largely in the context of a nitrogen budget. Even the most heavily impacted portion of the Bay showed only modest increases in dissolved nitrogen levels within the water column. Particulate materials including plankton biomass, as well as dissolved inorganic phosphorus, were elevated substantially. The benthos showed increased biomass and metabolic rates, especially of heterotrophs responding to fallout of organic particulate materials. Nutrient recycling within the Bay was the major immediate source for the nutrients for the observed rapid metabolic activity. Sewage accounted for approximately 80 percent of the inorganic nitrogen and 90 percent of the inorganic phosphorus delivery to Kaneohe Bay. Diversion lowered the land-derived inorganic nitrogen and phosphorus input to the Bay by 70–80 percent. Virtually all components of the system have responded to this diminished nutrient subsidy, but the water column nutrient washout and biological recovery are predictably occurring more rapidly than the benthos responses.