Origins of Dissolved Organic Matter in Southern California Coastal Waters: Experiments on the Role of Zooplankton

186

163

Dissolved free amino acids (DFAAs) play a major role in the f l u of organic carbon and nitrogen in marine biotic systems. In t h s study, DFAA release and uptake rates in samples from Long Island Sound and the Atlantic continental shelf were measured simultaneously using a tritium isotope dilution approach in conjunction with h g h pressure liquid chromatography (HPLC). Numerous measurements over a 20 mo period showed that release and uptake rates were usually similar, and net changes in DFAA concentration were much slower than (typically less than 30 '10 of) the gross uptake or release rates; this indicates close coupling between these 2 processes. DFAA turnover was rapid, with summer turnover times typically 0.5 h or less, and concentrations of individual DFAAs were usually a few nanomolar. Comparisons of total uptake rates with estimates of bacterial heterotrophic produchon confirm that DFAAs represent a significant source ( > l 0 %) of carbon and nitrogen for bacterial growth. DFAA release mediated by copepods, either through 'sloppy feeding' or by excretion, can be of comparable magnitude to direct release by microplankton.

Section: Discussionmentioning

confidence: 65%

Close coupling between release and uptake of dissolved free amino acids in seawater studied by an isotope dilution approach

Fuhrman¹

1987

186

163

“…However, the arguments above do not require most DOM to be released directly from phytoplankton, they only suggest it 1s the simplest pathway. Release as a result of grazing processes ('sloppy feeding', egestion, excretion) may be equally or more important (Eppley et al 1981, Jumars et al 1989), yet as discussed above, grazing as the major agent of CYANO turnover implies that grazers are highly selective against BACT. An interesting alternative to phytoplankton as a source of DOM is solar-induced photochemical production of labile low molecular weight organic compounds (e.g.…”

Section: Ecological Implications Of Biomass Distributionsmentioning

confidence: 99%

“…Loss from phytoplankton can be through processes such as grazing or through leakage of DOM. Of the 2, direct leakage is more efficient at providing bacteria with C because, from grazing, the DOC usable by bacteria must come via indirect routes such as 'sloppy feeding' escretion, and egestion (Eppley et al 1981), and much of what is assimilated by grazers is respired.…”

Section: Ecological Implications Of Biomass Distributionsmentioning

confidence: 99%

Dominance of bacterial biomass in the Sargasso Sea and its ecological implications

Fuhrman¹,

Sleeter²,

Ca³

et al. 1989

Self Cite

332

191

Despite the recently discovered importance of heterotrophic microorganisms in the structure and function of marine planktonic systems, little is known about the relative significance of these organisms in open-ocean oligotrophic environments. Here we report that depth profiles of planktonic nonphotosynthetic bacteria (BACT), cyanobacteria (CYANO), and both photosynthetic (PNAN) and heterotrophic (HNAN) nanoplankton (i.e. eukaryotes C 2 0 pm diam.) to 2600 m in the Sargasso Sea in summer and fall (the least productive times of year) indicate that BACT biomass dominates these microorganisms at all depths. From the microscopic counts and our 'best guess' conversion factors, we determined that BACT contain >70 and >80 '5, of the euphotic zone microbial C and N, respectively, and >90 YO of the biological surface area, C Y A N O , P N A N , and HNAN b~ovolume each make up 7 to 17 % of the total. Even calculations based on maximum probable phytoplankton biomass (from chlorophyll) and minimum probable BACT biomass still showed domination by BACT in the euphotic zone. An important practical implication is that conventional measurements of particulate organic C and N collected on glass fiber filters may be missing most of the biomass. Independent bacterial heterotrophic production estimates based on tritiated thymidine incorporation and clearance by nanoplanktonic grazers suggest bacterial division rates of about once per 5 to 15 d. Interpretation of these results suggest that BACT consume significant amounts of C in this system, probably released from phytoplankton dlrectly or via herbivores. These results imply a fundamental difference between oligotrophic and eutrophic waters with respect to biomass distribution and that BACT are major particulate reservoirs of C and N in the sea.

“…Copepods can be 'sloppy feeders' (Dagg 1974, Lampert 1978, Eppley et al 1981. Through sloppy feeding, internal pools of phytoplankton are disrupted.…”

Section: Copepodsmentioning

confidence: 99%

NH4+ regeneration and grazing: interdependent processes n size-fractionated 15NH4+ experiments

Glibert¹,

Miller²,

Garside³

et al. 1992

We measured size-dependent NH,' regeneration by 15N isotope dilution on a seasonal basis in the plume of the Chesapeake Bay estuary (USA) using several different approaches. In our short-term (1 h) size-fractionation experiments, the mean rate of NH,' regeneration by total or by c 202 pm plankton increased from winter to summer a s a linear function of temperature. On the other hand, the mean NH,' regeneration rate by c 10 pm plankton was lowest in February, increased nearly 10-fold from February to April, then remalned virtually unchanged through the end of summer. In all seasons during which experiments were conducted, we observed on occasion that the rate of NH,' regeneration measured in the c l0 pm fraction exceeded the rate measured in the <202 pm fraction by 8 to 1000 %. In other experiments in which size-fractionated plankton were maintained in large carboys for 2 24 h, we also found that NH,' regeneration in the c10 pm fraction exceeded that in the <64 or c202 pm fractions. In the third type of experiment conducted, we artificially manipulated the density of copepods in unfiltered seawater and measured NH,' regeneration. In 5 out of 6 such experiments, we found that a s the numbers of copepods were increased to ca 20 I-', the measured rate of NH,' regeneration increased to a maximum level, then decreased when additional copepods were added. From all of these experiments, we hypothesize that total NH,' regeneration in a natural food web can be significantly different from that measured in component size fractions, and that this effect is a nonlinear function of zooplankton density and/or multiple trophic interactions. These effects may, in addition, b e a function of internal cycling of isotopically labelled substrate. This hypothesis implies that commonly used size-fractionation techniques may be insufficient or inappropriate for describing the true regeneration rates of different size classes in situ.