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The effect of dietary nitrogen content on trophic level 15N enrichment (2000)

Thomas S. Adams, Robert W. Sterner

Wiley

In: Limnology and Oceanography

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Publikationsdatum: 2000

Beschreibung: Given the great potential value of stable isotopes in a variety of scientific investigations, surprisingly little attention has been paid to the underlying physiological and biochemical mechanisms that account for trophic increases in ;415N values. This has lead to a general call for controlled studies investigating the relationship between organismal diet and corresponding isotopic composition. We conducted a series of laboratory studies varying dietary nitrogen content and measuring corresponding variations in organismal ;415N values. Specifically, we investigated the relationship between the ;415N values of the anomopod crustacean, Daphnia magna, and the C:N ratio of its food, the green algae, Scenedesmus acutus. Daphnids were raised to a standard life stage on three types of S. acutus as food, which ranged in C:N (atomic) from 7.3 to 24.8. The average C:N of the daphnids was 6.0. 15N enrichment was found to be strongly linearly related to the C:N of the algae, ranging from nearly zero to approximately 6‰, which would normally be considered a span of almost two trophic levels. The d15N values of the daphnids and the diet‐tissue isotope fractionation factor, (Δdt = D. magna ;415N – S. acutus ;415N), were inversely related to the nitrogen content of the algae (R2 = 0.82 and 0.99 in two separate runs). To our knowledge, this is the first controlled study of nitrogen balance and d15N values in animals.

Print ISSN: 0024-3590

Digitale ISSN: 1939-5590

Thema: Biologie , Geologie und Paläontologie , Physik

Publiziert von Wiley im Namen von The Association for the Sciences of Limnology and Oceanography (ASLO).

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The fate of intertidal microphytobenthos carbon: An in situ 13C‐labeling study (2000)

Jack J. Middelburg, Christiane Barranguet, Henricus T. S. Boschker, Peter M. J. Herman, Tom Moens, Carlo H. R. Heip

Wiley

In: Limnology and Oceanography

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Publikationsdatum: 2000

Beschreibung: At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ 13C puls‐chase experiment. Label was added at the beginning of low tide, and uptake of 13C by algae was linear during the whole period of tidal exposure (about 27 mg m−2 h−1 in the top millimeter at both sites). The 13C fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silty stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of 13C in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of 13C into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipid‐derived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:1ω7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and 13C assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria 〉 macrofauna 〉 meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments.

Print ISSN: 0024-3590

Digitale ISSN: 1939-5590

Thema: Biologie , Geologie und Paläontologie , Physik