Fates of diatom carbon and trace elements by the grazing of a marine copepod

Abstract

Radiotracer experiments were performed to quantify the carbon assimilation and efflux in a subtropical coastal copepod Acartia spinicauda, and the importance of copepod grazing on the release of Cd, Fe, Se and carbon, from diatoms of different sizes (4 to 100 µm) at different food concentrations (0.04 to 9.0 mg C l -1 ). Carbon assimilation was not significantly affected by the diatom food concentration (Thalassiosira pseudonana, T. weissflogii and T. rotula) or the amount of food ingested, but was lower for the large diatom T. rotula than for the small diatom T. pseudonana. No significant relationship between carbon assimilation and gut passage time of food materials was observed. The efflux rate-constant (physiological turnover rate) of carbon ranged between 0.134 and 0.372 d -1 and was not significantly affected by diatom concentration (T. weissflogii). During the physiological turnover period, over 50% of the copepod's carbon metabolic loss was in the form of dissolved organic carbon, whereas only a small fraction of carbon was lost in the form of feces. A significant fraction of the copepod's carbon was also lost through respiration, the relative importance of which decreased with increasing period of depuration. The retention of carbon, Cd, Fe, and Se by the diatoms (T. pseudonana, T. weissflogii, T. rotula, and Cosinodiscus sp.) was further compared in the presence and absence of the copepods. In general, there was no increase in the release of metals and carbon from the diatoms in the presence of grazing copepods regardless of diatom concentrations, suggesting that copepod grazing does not trigger significant releases of diatom metals and carbon directly into the ambient environment. Our results suggested that the excretion by zooplankton as well as leakage from fecal pellets presumably account for the majority of the zooplankton-mediated production and cycling of dissolved organic matter in the ocean.

Egestion of non-pellet-bound fecal material from the copepod Acartia tonsa: implication for vertical flux and degradation

“…This material may have been overlooked in the presence of large discrete fecal pellets, or even if recognized, misinterpreted as a product of fecal pellet break-down. Our results are consistent with recent bioenergetic studies on Acartia, in which production of fecal pellets was compared with ingestion (Besiktepe & Dam 2002, Xu & Wang 2003. Here, the defecation rates appeared low relative to ingestion.…”

“…There is, however, disagreement in the literature as to the relative importance of the different mechanisms. Leakage from fecal pellets has been reported as the major flux of dissolved organic matter from zooplankton (Riemann et al 1986, Hygum et al 1997, while other studies report no such loss (Tande & Slagstad 1985, Strom et al 1997, Xu & Wang 2003. By measuring the DOC production relative to newly voided pellets from calanoid copepods, Møller et al (2003) found that pellets lose 28% of their carbon within the first hour and apparently without alteration of shape.…”

Phosphorus dietary assimilation and efflux in the marine copepod Acartia erythraea

“…The IR of Acartia erythraea generally increased with increasing food concentration (until the saturation concentration was reached), and it was rather variable among different diets, consistent with earlier studies (Liu & Wang 2002, Xu & Wang 2003. Intuitively, one would expect that the P AE would be negatively related to the IR of the copepods, as observed in our diatom food concentration experiment.…”

“…Several recent studies have shown that regeneration into the dissolved phase played a critical role in the elimination of carbon (Xu & Wang 2003) and metals (Hutchins et al 1995, Wang & Fisher 1998, Xu & Wang 2001 in marine copepods. Thus, by efficiently regenerating particulate P (from prey) into the dissolved phase, grazing may increase the P residence time and its recycling in surface waters.…”

Stoichiometric regulation of carbon and phosphorus in P‐deficient Daphnia magna

“…After the pulse feeding, the animals were removed by a mesh and rinsed with 0.22-mmfiltered pond water. After placing them in nonradioactive water, four individuals were immediately picked from each replicate and transferred into vials containing 0.5 mL of 1 mol L 21 NaOH to solubilize the tissues (Xu and Wang 2003). The animals were later counted for their radioactivity (representing the total amount of radioactivity ingested by Daphnia during the 5-min feeding period).…”