Primary Production in the Columbia River Estuary. II. Grazing Losses, Transport, and a Phytoplankton Carbon Budget

Abstract

Mean loss of phytoplankton carbon as a result of microcrustacean grazing ranged from 0.03 to 8.94 mg C m- 3 day-1", depending upon time of year and location in the estuary. On an annual basis, median grazing rate was 1893 mt C yr-1, which represented 6.3% of the annual primary production in the estuary. Daily transport of phytoplankton carbon decreased from the fluvial regions to the ocean, with the largest decrease occurring at the freshwater-brackish water interface. Annual import to the study area from upriver (excluding the May 1980 data, which were affected by the eruption of Mt. Saint Helens) was 61,440 mt C yr-1", while annual export to the ocean was 40,560 mt C yr–1. Total phytoplankton carbon input to the estuary was 91,316 mt C yr-1, a summation of import from upriver and in situ primary production (29,876 mt C yr-1", from a previous report). Export to the ocean plus median in situ grazing loss equalled 42,453 mt C yr-1", so that there was an unaccounted balance of 48,863 mt C yr-1. Some of this was night-time respiratory loss from the phytoplankton, estimated at 12,209 mt C yr-1. If the standing stock of phytoplankton carbon in the estuary was reasonably in steady state on an annual basis, the remaining balance must have been satisfied by conversion of phytoplankton carbon to non -chlorophyllous detrital particulate carbon (mostly at the freshwater-brackish water interface) and/or by conversion to dissolved organic carbon during transit through the estuary. Some indirect evidence suggests that phytoplankton carbon was mainly converted to detrital particulate carbon during the short transit time through the estuary, but verification requires further data.