Effect of Sewage Enrichment on the Phytoplankton Population of a Subtropical Estuary

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1979-04
Authors
Laws, Edward A.
Redalje, Donald G.
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University of Hawaii Press
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Phytoplankton primary production; concentrations of chlorophyll a, particulate carbon and nitrogen, adenosine triphosphate, inorganic nitrogen and phosphorus; and secchi depths were measured at four stations in Kaneohe Bay, Oahu, on a biweekly basis for 20 months prior to diversion of sewage discharges from the bay. Nutrient enrichment experiments designed to determine biomass limitation indicated that phytoplankton biomass, as measured by chlorophyll a, was nitrogen-limited in all parts of the bay, and that phosphorus was simultaneously limiting in the sector of the bay furthest from the sewer outfalls. Mean light-saturated productivity indices in all parts of the bay were about 11-12 mg C mg -1 chl a . hr -1, values close to the maximum reported for phytoplankton in eutrophic marine environments. Based on the results of dawn-to-dusk C-14 incubations and an estimated phytoplankton C: chi a ratio of 50 by weight, phytoplankton growth rates were estimated to fall in the range of 4-6 percent per hour in all parts of the bay. Such growth rates are close to the maximum growth rates reported for marine phytoplankton grown on light-dark cycles in continuous culture, suggesting that phytoplankton growth rates (as opposed to biomass) were limited primarily by suboptimal or supraoptimal light intensities rather than by nutrients. Based on these growth rates and an assumed phytoplankton C : N ratio of 5.68 by weight, nitrogen recycling was estimated to account for 80 percent of phytoplankton nitrogen uptake in the part of the bay receiving direct sewage inputs, and for over 90 percent of phytoplankton nitrogen uptake in the other sectors of the bay. Estimates of living and detrital particulate carbon were made based on an assumed C: ATP ratio in living organisms of 285 by weight. From this partitioning, living carbon was found to vary by a factor of 3-4 between the sewage-enriched and unenriched sectors of the bay. However, estimated detrital carbon concentrations were uniform throughout the bay, as were the measured concentrations of inorganic nitrogen. These results are consistent with the interpretation that the population of microorganisms, both bacteria and phytoplankton, are substrate-limited in all sectors of the bay.
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Laws EA, Redalje DG. 1979. Effect of sewage enrichment on the phytoplankton population of a subtropical estuary. Pac Sci 33(2): 129-144.
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