ALBERT

Description: In many small aquatic ecosystems, watershed loading of organic C exceeds autochthonous primary production. Although this allochthonous organic C has long been thought of as refractory, multiple lines of evidence indicate that substantial portions are respired in the receiving aquatic ecosystem. To what extent does this terrestrial C support secondary production of invertebrates and fish? Do current models adequately trace the pathways of allochthonous and autochthonous C through the food web? We evaluated the roles of allochthonous and autochthonous organic C by manipulating 13C content of dissolved inorganic C in a small, softwater, humic lake, thereby labeling autochthonous primary production for about 20 d. To ensure rapid and sufficient uptake of inorganic 13C, we enriched the lake with modest amounts of N and P. We constructed a carbon flow model based on the ambient and manipulated levels of 13C in C compartments in the lake, along with information on key rate processes. Despite the short nature of this experiment, several results emerged. (1) Fractionation of photosynthetically assimilated 13C‐CO2 by phytoplankton (ɛ) is lower (~6‰) than physiologic models would estimate (~20‰). (2) Bacteria respire, but do not assimilate, a large amount of terrestrially derived dissolved organic C (DOC) and pass little of this C to higher trophic levels. (3) The oxidation of terrestrial DOC is the major source of dissolved inorganic C in the lake. (4) Zooplankton production, a major food of young‐of‐year fishes, is predominantly derived from current autochthonous carbon sources under the conditions of this experiment.

Description: We tracked flows of carbon and nitrogen during an experimental phytoplankton bloom in a natural estuarine assemblage in Randers Fjord, Denmark. We used 13C‐labeled dissolved inorganic carbon to trace the transfer of carbon from phytoplankton to bacteria. Ecosystem development was followed over a period of 9 d through changes in the stocks of inorganic nutrients, pigments, particulate organic carbon and nitrogen, dissolved organic carbon (DOC), and algal and bacterial polar‐lipid‐derived fatty acids (PLFA). We quantified the incorporation of 13C in phytoplankton and bacterial biomass by carbon isotope analysis of specific PLFA. A dynamic model based on unbalanced algal growth and balanced growth of bacteria and zooplankton adequately reproduced the observations and provided an integral view of carbon and nitrogen dynamics. There were three phases with distinct carbon and nitrogen dynamics. During the first period, nutrients were replete, an algal bloom was observed, and carbon and nitrogen uptake occurred at a constant ratio. Because there was little algal exudation of DOC, transfer of 13C from phytoplankton to bacteria was delayed by 1 d, compared with the labeling of phytoplankton. In the second phase, the exhaustion of dissolved inorganic nitrogen resulted in decoupling of carbon and nitrogen flows caused by unbalanced algal growth and the exudation of carbon‐rich dissolved organic matter by phytoplankton. During the final, nutrient‐depleted phase, carbon and nitrogen cycling were dominated by the microbial loop and there was accumulation of DOC. The main source (60%) of DOC was exudation by phytoplankton growing under nitrogen limitation. Heterotrophic processes were the main source of dissolved organic nitrogen (94%). Most of the carbon exudated by algae was respired by the bacteria and did not pass to higher trophic levels. The dynamic model successfully reproduced the evolution of trophic pathways during the transition from nutrient‐replete to ‐depleted conditions, which indicates that simple models provide a powerful tool to study the response of pelagic ecosystems to external forcings.

Description: Lake Tanganyika, East Africa, has a simple pelagic food chain, and trophic relationships have been established previously from gut‐content analysis. Instead of expected isotopic enrichment from phytoplankton to upper level consumers, there was a depletion of 15N in August 1999. The isotope signatures of the lower trophic levels were an indicator of a recent upwelling event, identified by wind speed and nitrate concentration data, that occurred over a 4‐d period several days prior to sampling. The isotope structure of the food web suggests that upwelled nitrate is a nutrient source rapidly consumed by phytoplankton, but the distinctive signature of this nitrate is diluted by time averaging in the upper trophic levels. This time averaging is a consequence of the fact that the isotopic signature of an organism is related to variable nitrogen sources used throughout the life of the organism. This study illustrates the importance of recognizing differences in time averaging among trophic levels.

Description: Gross and net O2 production between May 1996 and February 1999 was determined in bottle incubation experiments with H218O spike and from the change in O2 concentration. Carbon fixation rates were obtained from 14C incubations. In general, production rates determined using the H218O‐spike were about twice the primary production determined by the 14C method, where the latter was close to net oxygen evolution. These relationships are similar to results for the open ocean. During the spring bloom, when the dinoflagellate Peridinium was abundant, the ratio of gross O2 production to carbon fixation was about 7.5, and net O2 production was greater than carbon fixation. The difference between O2 gross production and carbon fixation results, at least in part, from uptake by Mehler reaction and from recycling of the 14C tracer by dark respiration and the alternative oxidase (AOX). We used the difference in isotopic discrimination against 18O, occurring during O2 consumption by various biological pathways, to place constraints on the relative engagement of these pathways. We estimated the overall discrimination against 18O in the lake from O2 isotopic mass balance as 20.5—29‰. The only mechanism that can explain the strong overall fractionation in the lake is AOX, which strongly discriminates against 18O (~31‰). Our results show, for the first time, that uptake by AOX is widespread and quantitatively important to oxygen consumption in aquatic systems.

Description: Use of stable isotope techniques to quantify food web relationships requires a priori estimates of the enrichment or depletion in δ15N and δ13C values between prey and predator (known as trophic fractionation; hereafter Δδ15N and Δδ13C). We conducted a broad‐scale analysis of Δδ15N and Δδ13C from aquatic systems, including three new field estimates. Carnivores had significantly higher Δδ15N values than herbivores. Furthermore, carnivores, invertebrates, and lab‐derived estimates were significantly more variable than their counterparts ( f‐test, p 〈 0.00001). Δδ13C was higher for carnivores than for herbivores (p = 0.001), while variances did not differ significantly. Excluding herbivores, the average Δδ15N and Δδ13C were 3.4‰ and 0.8‰, respectively. But even with unbiased fractionation estimates, there is variation in isotopic fractionation that contributes to error in quantitative isotope model outputs. We simulated the error variance in δ15N‐based estimates of trophic position and two‐source δ13C diet mixing models, explicitly considering the observed variation in Δδ15N and Δδ13C, along with the other potential error sources. The resultant error in trophic position and mixing model outputs was generally minor, provided that primary consumers were used as baseline indicators for estimating trophic position and that end member d13C values in dietary mixing models were sufficiently distinct.

Description: Oxygen regulation of nitrification and denitrification in sediments was investigated with 15N isotope techniques. Sediment cores were incubated in a continuous ftowthrough system in which the O2 concentration was varied in the overlying water while the NO3− concentration was kept constant. Nitrification was stimulated with increasing O2 concentrations in the overlying water from 0 to 100% of atmospheric saturation, whereas only a slight stimulation was observed above 100%. At O2 concentrations below 100% of atmospheric saturation, NO3− from the overlying water was the most important source of N for denitrification, whereas above 100% of atmospheric saturation, NO3− produced by nitrification was the main source of N for denitrification. The converse effects of the O2 levels on the source of NO3− can be explained by applying a simple one‐dimensional model: O2 in the overlying water controls the diffusional distance of NO3− to the anoxic zone of denitrification and consequently the location of NO3− vertically in the sediment as well as the magnitude of the nitrification activity. Our results suggest that in aquatic environments containing low NO3− concentrations in the overlying water (such as coastal waters), higher O2 conditions will stimulate denitrification, while the opposite will occur in systems containing high NO3−concentrations (such as eutrophic lakes and streams).

Description: Seasonal variations in the stable isotope composition (d13C and d15C) of crustacean zooplankton and their putative food sources in oligotrophic Loch Ness were recorded during 1998. Bulk particulate organic matter (POM) showed d13C values consistent with a terrestrial plant origin from the catchment and exhibited little seasonal variation, whereas POM d15 was more variable, probably due to associated microbial action. In contrast, phytoplankton d13C was relatively light and showed some seasonal variation, but d15 values were more constant. The isotopic signatures of both POM and phytoplankton remained sufficiently distinct from each other throughout the period of study to allow their relative contributions to zooplankton diet to be assessed. Zooplankton isotopic signatures shifted seasonally, reflecting a dietary switch from a reliance on allochthonous carbon derived from POM during winter and early spring to heavy dependence on algal production during summer. Annually, crustacean zooplankton in Loch Ness derive approximately 40% of their body carbon from allochthonous sources, likely mediated via microbial links. Separate determination of isotope ratios for the main zooplankton species allowed a more detailed trophic investigation. The most abundant zooplankton species in the loch, Eudiaptomus gracilis, incorporated appreciable allochthonous carbon even during the peak of phytoplankton productivity. By contrast, Daphnia hyalina grew mainly in late summer and autumn and derived almost 100% body carbon from algal sources. This study is the first to quantify such a seasonal switch in zooplankton dependence between allochthonous and autochthonous sources of organic matter in a large lake.

Description: We investigate the carbon cycle in Lake Washington for the year 1980 using monthly measurements of the dissolved inorganic carbon (DIC) and its 13C : 12C isotopic composition. Mass balances of DIC and 13C : 12C yield estimates of CO2 gas exchange rates and net organic carbon production rates. Between 24 June and 13 August, the calculated CO2 gas invasion rate of 0.80 × 106 mol C d−1 is nearly equal to the river DIC inflow rate. The calculated epilimnetic net organic carbon production rate is 0.68 × 106 mol C d−1, about 20–30% of primary productivity estimated from 14C‐fixation experiments and ETS‐derived respiration rates. Metalimnetic and hypolimnetic DIC increase rates and porewater DIC gradients in hypolimnetic sediments indicate that remineralization of particulate organic carbon (POC) previously deposited in the sediments is a major (0.5 × 106 mol C d−1) DIC source to the lake during summer. For the whole year, summertime CO2 gas invasion balances wintertime CO2 gas evasion and DIC and POC outflow balance DIC and POC inflow rates, implying no net carbon burial in the sediments during 1980. This contrasts with the measured long term sedimentation‐rate‐derived carbon burial rate of 0.8 × 106 mol C d−1. Year‐to‐year variability in summertime primary production rates largely determines net gains or losses of carbon via CO2 gas exchange and sedimentation.

Description: Emplacement of a tracer mixture containing 13C‐labeled green algae on the sea floor of the continental slope offshore of Cape Hatteras, North Carolina, elicited a rapid response over 1.5 d from the dense benthic community. Certain deposit‐feeding annelids (e.g. Scalibregma inflatum and Aricidea quadrilobata) became heavily labeled with 13C as a result of ingestion of the algae. “C‐labeled organic matter was transported to a depth of at least 4–5 cm into the seabed during the 1.5‐d period, presumably as a consequence of a feeding‐associated activity. Nonlocal transport produced subsurface peaks in organic 13C at 2–3 cm. Dissolved inorganic 13C, produced by the oxidation of the labeled algae, penetrated to 10‐cm depth. The transport of highly reactive organic matter from the sediment surface at initial velocities ≥3 cm d‒1 is expected to be an important control of subsurface benthic processes in slope environments characterized by abundant macrofaunal populations. Anaerobic processes, which are enhanced on the Cape Hatteras slope relative to adjacent areas, may be promoted by the rapid injection of reactive material into subsurface sediments. The transport, in turn, is a consequence of the dense infaunal populations that are supported by the rapid deposition of organic carbon in this region.

Description: It is clear that anthropogenic nitrogen inputs from watersheds to estuaries stimulate eutrophication. It has been difficult, however, to explicitly link anthropogenic N entering estuaries to N found in estuarine producers. To explore this link, we compared stable isotope ratios of N in groundwater and producers from the Waquoit Bay watershedestuary system, Cape Cod, Massachusetts. The δ15N values of groundwater nitrate within the Waquoit Bay watershed increase from −0.9‰ to + 14.9‰ as wastewater contributions increase from 4 to 86% of the total N pool. As a result, the average δ15N of dissolved inorganic nitrogen (DIN, nitrate + ammonium) received by different estuaries around Waquoit Bay increases from +0.5‰ to +9.5‰. This increase is strongly correlated to increases in δ15N of eelgrass, macroalgae, cordgrass, and suspended particulate organic matter. The increase of all producers examined in Waquoit Bay with increasing δ15N of DIN in groundwater demonstrates a tight coupling between N contributed to coastal watersheds and N used by primary producers in estuaries. The ability to identify effects of increasing wastewater N loads on δ15N of estuarine producers may provide a means to reliably identify incipient eutrophication in coastal waters.