FY 1986

Temporal variations in the concentration and settling flux of carbon and phytoplankton pigments in a deep fjordlike estuary

Baker, E.T., R.A. Feely, M.R. Landry, and M.F. Lamb

Estuar. Coast. Shelf Sci., 21, 859–877 (1985)

The weekly mass flux at C and phytoplankton pigments at five depths in the main basin of Puget Sound, a deep (~200 m) fjordlike estuary, was sampled for a year with moored sequentially-sampling sediment traps. Flux measurements were compared with weekly samples of suspended pigments in the euphotic zone and bi-monthly samples of total suspended matter and particulate C throughout the water column at the mooring site. Seasonal changes in the total mass flux at all depths were small; instead, physical (river runoff, bottom resuspension) and biological (phytoplankton blooms) events caused occasional sharp increases on a weekly scale. The dry weight concentration of pigments in the trap samples mirrored the concentration of pigments in the euphotic zone suspended matter, increasing from 0.01% in winter to a maximum of 0.65% in the late summer. Bloom-induced changes in the pigment concentration were observed almost simultaneously in the euphotic zone and in the traps to a depth of 160 m, indicating a rapid vertical transfer of surface-originating particles by organic aggregates. In contrast to the strong seasonal signal in the pigment concentration, C concentration varied by only a factor of three during the year. The seasonal trend of C/pigment ratios in the C flux arises from at least two sources: (1) a balance between terrestrial sources of C during the high-runoff winter season and in-situ primary production in spring and summer, and (2) cycling of C through the zooplankton population. Budget calculations suggest that the loss of primary-produced C and pigment from the euphotic zone by settling is ~5% regardless of season. On an annual basis, this C flux (16 g m) is sufficient to support previously measured values of benthic aerobic respiration at the mooring site. To account for other C sinks such as burial, predation and chemical oxidation, however, terrestrial C sources and alternate transport pathways, such as vertical advection and sediment movement down the steep basin walls, are necessary.