The effects of preservation methods, dyes and acidification on the isotopic values (δ15N and δ13C) of two zooplankton species from the KwaZulu-Natal Bight, South Africa

Food-Web Complexity in Guaymas Basin Hydrothermal Vents and Cold Seeps

In the Guaymas Basin, the presence of cold seeps and hydrothermal vents in close proximity, similar sedimentary settings and comparable depths offers a unique opportunity to assess and compare the functioning of these deep-sea chemosynthetic ecosystems. The food webs of five seep and four vent assemblages were studied using stable carbon and nitrogen isotope analyses. Although the two ecosystems shared similar potential basal sources, their food webs differed: seeps relied predominantly on methanotrophy and thiotrophy via the Calvin-Benson-Bassham (CBB) cycle and vents on petroleum-derived organic matter and thiotrophy via the CBB and reductive tricarboxylic acid (rTCA) cycles. In contrast to symbiotic species, the heterotrophic fauna exhibited high trophic flexibility among assemblages, suggesting weak trophic links to the metabolic diversity of chemosynthetic primary producers. At both ecosystems, food webs did not appear to be organised through predator-prey links but rather through weak trophic relationships among co-occurring species. Examples of trophic or spatial niche differentiation highlighted the importance of species-sorting processes within chemosynthetic ecosystems. Variability in food web structure, addressed through Bayesian metrics, revealed consistent trends across ecosystems. Food-web complexity significantly decreased with increasing methane concentrations, a common proxy for the intensity of seep and vent fluid fluxes. Although high fluid-fluxes have the potential to enhance primary productivity, they generate environmental constraints that may limit microbial diversity, colonisation of consumers and the structuring role of competitive interactions, leading to an overall reduction of food-web complexity and an increase in trophic redundancy. Heterogeneity provided by foundation species was identified as an additional structuring factor. According to their biological activities, foundation species may have the potential to partly release the competitive pressure within communities of low fluid-flux habitats. Finally, ecosystem functioning in vents and seeps was highly similar despite environmental differences (e.g. physico-chemistry, dominant basal sources) suggesting that ecological niches are not specifically linked to the nature of fluids. This comparison of seep and vent functioning in the Guaymas basin thus provides further supports to the hypothesis of continuity among deep-sea chemosynthetic ecosystems.

Freezing and fractionation: effects of preservation on carbon and nitrogen stable isotope ratios of some limnetic organisms

RATIONALE

Stable isotopes of carbon and nitrogen have become important natural tracers for studying food-web structure and function. Considerable research has demonstrated that chemical preservatives and fixatives shift the isotopic ratios of aquatic organisms. Much less is known about the effects of freezing as a preservation method although this technique is commonly used.

METHODS

We conducted a controlled experiment to test the effects of freezing (-10 °C) and flash freezing (–79 °C) on the carbon and nitrogen isotope ratios of zooplankton (Cladocera), Mysis diluviana and Rainbow Trout (Oncorhynchus mykiss). Subsamples (~0.5 mg) of dried material were analyzed for percentage carbon, percentage nitrogen, and the relative abundance of stable carbon and nitrogen isotopes (δ13C and δ15N values) using a Carlo Erba NC2500 elemental analyzer interfaced to a ThermoFinnigan MAT Delta Plus isotope ratio mass spectrometer.

RESULTS

The effects of freezing were taxon-dependent. Freezing had no effect on the isotopic or elemental values of Rainbow Trout muscle. Effects on the δ13C and δ15N values of zooplankton and Mysis were statistically significant but small relative to typical values of trophic fractionation. The treatment-control offsets had larger absolute values for Mysis (δ13C: ≤0.76 ± 0.41‰, δ15N: ≤0.37 ± 0.16‰) than for zooplankton (δ13C: ≤0.12 ± 0.06‰, δ15N: ≤0.30 ± 0.27‰). The effects of freezing were more variable for the δ13C values of Mysis, and more variable for the δ15N values of zooplankton. Generally, both freezing methods reduced the carbon content of zooplankton and Mysis, but freezing had a negative effect on the %N of zooplankton and a positive effect on the %N of Mysis.

CONCLUSIONS

The species-dependencies and variability of freezing effects on aquatic organisms suggest that more research is needed to understand the mechanisms responsible for freezing-related fractionation before standardized protocols for freezing as a preservation method can be adopted.

Effects of distilled water rinsing on stable isotope ratios of acid-treated marine invertebrate (Paguridae) samples

RATIONALE

Stable isotope ratios are widely used to infer trophic relationships, although a growing number of studies show that sample pre-treatments (such as acidification to remove carbonates) can cause changes in isotope ratios. Samples are often rinsed in distilled water after acidification, and we examine the effects of this step in particular on the isotope ratios of marine invertebrate samples.

METHODS

Samples of whole hermit crabs (Paguridae) were subjected to one of three treatments: acidification using dilute hydrochloric acid without subsequent distilled water rinsing; acidification with rinsing; and rinsing with no acidification. Continuous-flow isotope ratio mass spectrometry was used to compare the mean carbon and nitrogen stable isotope ratios of treated and untreated material.

RESULTS

Acidification (both with and without subsequent distilled water rinsing) resulted in reductions in mean δ(13)C values (1.939 and 3.146‰, respectively), while rinsing without prior acidification led to a smaller (but still significant) increase. Nitrogen isotope ratios were not affected by acidification, but subsequent rinsing with distilled water caused a decrease of approximately 1‰.

CONCLUSIONS

Acidification of samples is clearly necessary in the presence of carbonates to obtain useful carbon isotope ratio data. However, post-acidification rinsing can result in further (potentially undesirable) changes to both carbon and nitrogen isotope ratios. Ideally, rinsing should be avoided, but the impacts are small enough to be of little concern in many studies. Rinsing (or not) should be considered carefully on the basis of the aims of a study.

The effects of freeze/thaw periods and drying methods on isotopic and elemental carbon and nitrogen in marine organisms, raising questions on sample preparation

Stable isotopes are an increasingly important tool in trophic linkage ecological studies. In studies of large marine animals, isotopic sampling is often given secondary priority to sampling for diversity and biomass aspects. Consequently, isotopic samples are frequently collected subsequent to repeated freezing and thawing of animals, and the results of these studies are often based on the assumption that this pre-treatment does not affect the isotopic values. Our study tested this assumption and examined the difference between oven- and freeze-drying on isotopic values and elemental carbon-to-nitrogen (C:N) ratios. The values for δ(15)N and δ(13)C, percentage nitrogen and carbon, and the C:N ratios were determined from the tissues of six marine species, including invertebrates and fish, as (1) fresh samples, (2) samples thawed once, and (3) samples thawed twice. The drying method, thawing treatment and their interaction did significantly affect the δ(15)N and δ(13)C isotope values for all species. Oven-dried samples had slightly higher δ(13)C and δ(15)N values than freeze-dried samples, although not significant in most instances. For most species, oven-drying produced lower carbon and nitrogen percentage than freeze-drying for samples that had been thawed once, but the C:N ratio was unaffected by the drying method. Repeated freezing and thawing did not affect the isotope values, but it did decrease the percentage carbon and nitrogen for both desiccation methods. We recommend drying samples from fresh wherever possible, and careful choice of desiccation method in light of the fact that most lipid models are based on oven-dried samples and oven-drying could cause enrichment of (15)N or (13)C through evaporation of volatile compounds richer in lighter isotopes such as some lipids. Finally, we recommend that further studies on the specific effects of freezing and desiccation on elasmobranchs is needed. Overall we recommend the use of freeze-drying when possible and to use the samples from freshly caught organisms.