Effects of sinking and zooplankton grazing on the release of elements from planktonic debris

Abstract

Laboratory radiotracer experiments compared the effects of zooplankton grazing and microbial decomposition on the release of organic C and Ag, Cd, CO, Pb and PO from phytoplankton cells. After 40 h incubation of diatoms with copepods + microorganisms, 70% of the diatom cellular C was remineralized to CO, (40 Yo), assimilated in copepod tissue (20 %), excreted in fecal pellets (5 %) or released as DOC (5 %), microorganisms alone removed half this amount of diatom C. Copepod grazlng enhanced the conversion of Ag from diatom cells to other iorms (particulate and dissolved) by 27 %, PO by 25 %, Pb by 20%, Cd by 13 % and CO by 10% over those cells incubated with only microorganisms; zooplankton grazing increased by 5 to 15 % the release of elements from diatom cells into the dissolved phase. Decomposing copepod fecal pellets, held free-falling on a spinning wheel, lost about 20 % more C, Ag, CO and Pb than did undisturbed fecal pellets, while no appreciable difference was observed for release of metals from copepod carcasses treated similarly. The results suggest that retention of an element contained primarily in the cytoplasm of phytoplankton cells (such as C or Cd) is largely governed by microbial activity and leaching. Microbial activity has a smaller effect on particle-reactive elements (Ag and Pb) bound to structural components of cells; zooplankton are effective in removing these unassimilable elements by grazing and packaging them into rapidly sinking fecal pellets.

Grazer-mediated regeneration and assimilation of Fe, Zn and Mn from planktonic prey

“…fecal pellets, DOC). Microbial release of trace metals from phytoplankton cells and debris has been observed to be a relatively slow process, occurring on a time frame of days to weeks (Frey & Small 1979, Lee & Fisher 1992, even when the microbial community is artificially enriched (Lee & Fisher 1994). Extensive microbial degradation of live plankton cells in the time frame of the crustacean grazing experiments presented here (hours) is unlikely.…”

“…Lee & Fisher (1994) present the results of a similar study in which the release of a suite of mostly nonessential trace metals (Ag, Cd, CO, PO and Pb) from phytoplankton debris was examined in the presence and absence of microorganisms and/or copepods. They concluded that microbial degradation processes can account for the bulk of trace metal regeneration in the ocean, with zooplankton grazing accounting for a fractional enhancement (5 to 15% over a 40 h incubation).…”

Microbial degradation of organic carbon and nitrogen on diatom aggregates

“…Carbon turnover times measured on the fresh diatom detritus were much shorter than those calculated on natural diatom aggregates in the sea (Smith et al 1992). The short half-time of POC determined by 14 CO 2 release (Lee and Fisher 1994) was attributed to physicochemical leaching, as well as to activities of free enzymes that can temporarily exceed microbial transformation. A substantial release of DOM from 14 C-labeled seston by solubilization was also found by Berman et al (1999) in Lakes Kinneret and Constance.…”

Heavy metals in zooplankton and decapod crustaceans from the Barents Sea

“…That zooplankton play an important role in the biogeochemical cycling of metals in marine systems in general is well known, especially regarding particle-reactive metals in the water column Lee and Fisher, 1994;Stewart et al, 2005). Thus, in several studies macro-and meso-zooplankton organisms have been specifically used as biomonitors for assessment of the bioavailability of elements in marine systems, covering a variety of spatial and temporal scales (e.g.…”