Kaneohe Bay Background Information
Several important studies that relate to this research have been carried out in Kaneohe Bay.
The earliest and most well known is Smith et al.’s (1981) Kaneohe Sewage Diversion Experiment.
Kaneohe Bay received increasing amounts of sewage from the 1950s to the late 1970s.
Three sewage treatment plants had outfalls in Kaneohe Bay during this time. The Kaneohe
Marine Corps Air Station discharged sewage at an outfall in the southeast corner of the
bay in 7 m of water from 1951 through 1977; the Kaneohe Sewage Treatment Plant discharged
sewage at an outfall in the southern corner of the bay in 8 m of water from 1963 through
1977; and the small Ahuimanu Sewage Treatment plant began discharging into a stream flowing
into the northwestern portion of the bay in 1970.
In December 1977 the Kaneohe sewage was
permanently diverted to a deep ocean outfall and in May 1978 the Marine Corps sewage was
also permanently diverted. Smith et al. documented the bay’s transition from a eutrophic
algae-dominated system to its recovery as an increasingly oligotrophic coral-dominated system.
This comprehensive paper covered all aspects of the bay including discussion of bay dimensions
and subdivisions, freshwater inputs, nutrient concentrations, organic matter concentrations,
plankton distributions and production, sediment fluxes, and benthic organisms before and after
sewage diversion.
Following Smith et al.’s study was the EPA-, NOAA-, and NASA-sponsored long term time-series Coastal Intensive Site Network (CISNet) study (Summers et al. 2000). CISNet monitored four sites, one each in the northern, central, and southern bay and one near the stream mouth of Kaneohe Stream, bimonthly from 1998 to 2001. Basic water quality parameters including chlorophyll and dissolved inorganic nutrients as well as sediment processes were assessed; data are available online. This study provides valuable post-sewage, oligotrophic state baseline data for the bay. For example, during the CISNet study period nitrate and phosphate concentrations were routinely below 0.5 µM in the central and southern bay and below 1.0 µM in the northern bay.
More recent studies include those of Hoover (2002) on nutrient inputs via streams in southern Kaneohe Bay and Ringuet and Mackenzie’s (2005) investigation of the storm response of phytoplankton in the southern bay. Storms in Kaneohe Bay bring in excess nutrients via increased stream and land runoff. Laws and Allen (1996) concluded from nutrient enrichment experiments that phytoplankton production in Kaneohe Bay was nitrogen limited and Ringuet and Mackenzie (2005) determined that excess nutrients from storms stimulated phytoplankton blooms that were largely dominated by diatoms. In fact, due to the high ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) in storm runoff (DIN:DIP, 25-29), phytoplankton production may be driven from nitrogen limitation to phosphorus limitation following some storm events. These blooms can be expected to have an effect on the PCO2 of bay waters.
The first time-series evaluation of air-sea CO2 exchange for a subtropical high island of the Pacific was performed in Kaneohe Bay by Fagan and Mackenzie (2007). From September 2003 through September 2004 surface water samples were collected bimonthly throughout the bay for total alkalinity (TA) and dissolved inorganic carbon (DIC) analysis in order to calculate surface water partial pressure of carbon dioxide (PCO2) and air-sea CO2 exchange flux. PCO2 values were above the atmospheric level (378 µatm) throughout Kaneohe Bay (400 – 500 µatm) and Kaneohe S tream waters (600 – 1300 µatm) during baseline conditions. PCO2 levels above atmospheric in bay water were driven mainly by calcification while PCO2 levels above atmospheric in stream water were driven mainly by remineralization of terrestrial organic matter in soil and groundwater systems that connect to the flow of the stream. PCO2 values above the atmospheric level were observed in surface water that extended almost 4 km beyond the boundary of the bay indicating that processes inside the bay such as calcification affect the inorganic carbon properties of the surrounding near shore open ocean. Precipitation from storm events in the Kaneohe watershed increases stream and land runoff that brings excess nutrients to bay waters. These excess nutrients stimulate phytoplankton blooms which draw down CO2 in Kaneohe Bay water through photosynthesis. Kaneohe Bay water was observed to have PCO2 values below the atmospheric level three times following storm events during the study period and bay water PCO2 remained below baseline values throughout most of the bay for the six months during which storm events occurred. Despite the significant effects of the storms, Kaneohe Bay was a net annual source of CO2 to the atmosphere of -0.34 kton C yr-1 during the study period with an area-specific flux of -0.91 mol C m-2 yr-1. The source of CO2 in Kaneohe Bay could be stronger on a long term basis since dry years may be stronger net annual sources than wet years such as the period investigated during Fagan and Mackenzie’s study. The dynamic nature of Kaneohe Bay makes closely spaced time-series data, such as that collected by our CO2 mooring, imperative for characterizing accurately the inorganic carbon system and the net annual air-sea exchange of CO2.