IMPACT OF LAND RUNOFF ON WATER-QUALITY IN AN HAWAIIAN ESTUARY

Water quality studies were carried out for six-week periods during the summers of 1991 and 1992 in the Ala Wai canal, an artificial estuary heavily impacted by land runoff on the island of Oahu in the Hawaiian Islands. Circulation in the upper reaches of the estuary is greatly re stricted by a sill of accumulated sediment which has washed in from th e surrounding watershed. Both the biomass of phytoplankton and photosy nthetic rates increased from the mouth to the head of the estuary. Gro ss photosynthetic rates near the head of the estuary were comparable t o net production rates reported from intensive algal cultures. Pigment analyses indicated that the phytoplankton community was dominated by diatoms and dinoflagellates, with dinoflagellates most abundant at the head and diatoms at the mouth of the estuary. Productivity indices de clined from 15 to 4 g C g-1 chl a h-1 between the mouth and the head o f the estuary, presumably due to light limitation effects. Respiration rates were relatively uniform throughout the estuary. Although physic al processes significantly reduced the magnitude of diel O2 changes wh ich would otherwise have resulted from photosynthesis and respiration, O2 concentrations at the head of the estuary nevertheless fell as low as 3 mg liter-1 near the bottom at night and as high as 13 mg liter-1 in the surface waters during the day. A complete carbon budget for th e system indicates that the estuary is heterotrophic, with a photosynt hesis/respiration ratio of about 0.6. Of the organic inputs to the sys tem, 70% are respired, 25% are buried in the sediments, and about 5% w ash out of the estuary. In terms of allochthonous nitrogen loading, th e Ala Wai canal appears to be one of the most heavily fertilized estua ries in the world.