Primary Productivity in a Nutrient-Limited Tropical EstuaryI

The results of a survey of phytoplankton productivity, chlorophyll a and nutrient concentrations in Pala Lagoon, American Samoa, are shown to be interpretable in terms of two regimes: an ocean-dominated region where productivity is controlled by fix~d nitrogen concentrations, and a land-runoff-dominated regime where productivity is unrelated to major nutrient concentration. A quantitative relationship between nutrient concentrations in the lagoon and phytoplankton growth rates is presented. This relationship is used as the basis for predicting the effects of a proposed dredging operation within the lagoon as well as for predicting the effects of increased waste discharge into the local marine environment. PALA LAGOON, on the southern coast of Tutuila Island, American Samoa, is a tropical estuary roughly circular in shape of about 1 mile diameter with an entrance channel at the southern end aligned eastward (Fig. 1). The mean depth is about 1.5 meters although much deeper dredged holes exist in and near the entrance channel. The lagoon is moderately turbid, and receives a large amount of seasonal stream runoff, mostly during the period December to March. The annual rainfall is about 130 inches and a considerable portion of the bottom is covered with a layer of silty mud. Sewage treatment is nonexistent for the communities bordering the lagoon, wastes being discharged into the lagoon via the many streams originating in the volcanic mountain range running the length of the island. To improve the economic and recreational value of the lagoon, it has been proposed that rather extensive dredging operations be conducted for the channel and adjacent areas; and construction of a small boat harbor is anticipated. The present study is part of an overall reconnaissance being made by scientists from the University of Hawaii in cooperation with the government of American Samoa to determine the advisability of further altering circulaI This survey was supported by NOAA Sea Grant Year 04, grant no. 253-243, modification 2. Hawaii Institute of Marine Biology Contribution no. 404. Manuscript received 15 November 1972. 2 University of Hawaii, Hawaii Institute of Marine Biology, P.O. Box 1067, Kaneohe, Oahu, Hawaii 96744. tion patterns inside the lagoon. Reported here are the results of a week-long survey in March 1972 of phytoplankton biomass and productivity and associated macronutrient concentrations.