Dietary fatty acid transfer in pelagic food webs across trophic and climatic differences of Chinese lakes.

In eutrophic lake ecosystems, cyanobacteria typically lead to unbalanced phytoplankton community structure and low dietary quality for consumers at higher trophic levels. However, it still remains poorly understood how zooplankton manage to respond to seasonal and spatial differences in lake trophic gradients and temperature factors to retain highly required dietary nutrients from phytoplankton. In this field study, we investigated seston and different size classes of zooplankton of temperate and subtropical large lakes of different trophic conditions in China. We used fatty acids (FA) as dietary nutrients from seston to zooplankton to investigate how eutrophication affects the FA composition of various zooplankton size classes. This study revealed a curvilinear relationship between total phosphorus (TP) and polyunsaturated fatty acids (PUFA) contents of edible phytoplankton ("seston") across 3 seasons and 2 climatic areas. The PUFA content of seston increased until mesotrophic lake conditions (TP: 11-20 µg L-1), after which the dietary provision of PUFA for respective consumers declined. Seston FA, rather than trophic condition or water temperature, primarily predicted changes in zooplankton FA, while this predictive power decreased with zooplankton size. Despite increasing eutrophic lake conditions, LC-PUFA content of the zooplankton consistently increased per unit biomass. The results indicate that the nutritional value of phytoplankton was highest in mesotrophic lakes, and lake zooplankton selectively increased their LC-PUFA retention with body size and/or were able to convert dietary FA endogenously to meet their size-specific FA demands, independent of lake location or time (season) or the measured trophic condition of the lake (from oligo- to eutrophic).

Considering zooplankton as a black box in determining PAH concentrations could result in misjudging their bioaccumulation.

Zooplankton play an important role in energy transfer in the marine food web and form the dietary basis for the size of important fish stocks and the maintenance of their resources. Although zooplankton include numerous taxa with significantly different ecological characteristics and the interspecific differences in optimum body size and taxonomic specificity in fish feeding on zooplankton are remarkable, they are always considered as a whole (like a "black box") in current studies about the transport of persistent organic pollutants through the food chain. This approach might result in misjudgment of their bioaccumulation. In this study, the distribution properties of each taxa of zooplankton community were discerned using data from two cruise surveys conducted in the northern South China Sea. Twelve groups of zooplankton were identified, all of which had distinct ecological and functional characteristics. The carbon-based community structure of zooplankton could explain their variability with respect to polycyclic aromatic hydrocarbons (PAHs). Smaller-sized zooplankton (smaller calanoids and cyclopoids) were more likely to accumulate low molecular weight PAHs (LMW-PAHs), while larger-sized zooplankton (larger calanoids) were more likely to accumulate high molecular weight PAHs (HMW-PAHs). The bioaccumulation capacity of the zooplankton community for LMW-PAHs was negatively correlated with the proportion of omnivores and carnivores, while the opposite was true for HMW-PAHs. These results suggested that the effects of complex community structure within plankton communities should be taken into account when assessing the transfer and bioaccumulation effects of PAHs in the marine food chain.

Transport of trace metals and their bioaccumulation in zooplankton from Changjiang (Yangtze River) to the East China Sea.

The present study conducted a comprehensive field investigation on the transport and bioaccumulation of six trace metals (Cr, Cd, Pb, Mn, Ni, Cu) along a transect from Changjiang (Yangtze River) to the East China Sea continental shelf, which exhibited large variations in physiochemical properties (salinity, turbidity, pH, chlorophyll a, total nitrogen, dissolved oxygen, dissolved and particulate organic matter). From riverine sites to marine sites, dissolved Cr and Cd significantly increased, dissolved Pb and Mn showed less variations, while dissolved Cu and Ni showed complex spatial distribution patterns. Particulate trace metals (for Cr, Mn, Ni and Cu) were significantly negatively correlated with salinity. As a result, partition coefficients of trace metals (except Pb) were all significantly negatively correlated with salinity, indicating high salinity facilitated desorption/dissolution of metals from particulate phase. Additionally, the Changjiang derived particulate Pb, Mn, Ni and Cu sharply decreased (particularly for Mn) at the downstream of turbidity maximum zone, suggesting the efficient trapping of metals within this region. We further investigated the site-specific bioaccumulation of trace metals in size-fractionated zooplankton. Metal contents in macro-zooplankton were lower than micro- and meso-zooplankton owing to size-dependent zooplankton communities, while site-specific metal bioaccumulation mainly driven by site-specific zooplankton communities and salinity. The bioaccumulation factors of metals (Cr, Cd, Ni and Cu) were significantly negatively correlated with salinity, indicating high salinity hampered metal uptake which might attribute to competition of cations and formation of less bioavailable inorganic complexes with anions. Overall, high salinity generated two-sided effects (elevated dissolved metal concentrations Vs. reduced metal bioaccumulation) on metal contents in zooplankton (especially for Cr, Cd, Ni, and Cu), resulting in metal- and site-specific metal contents. We noticed relatively higher metal contents in zooplankton at hypoxia sites which could further transfer to predators in the East China Sea, and the underlying mechanisms still require future investigation.

Role of jellyfish in mesozooplankton community stability in a subtropical bay under the long-term impacts of temperature changes.

To understand zooplankton community changes in the context of climate change and anthropogenic disturbances, we analyzed mesozooplankton samples from four seasons in the subtropical Daya Bay, which is susceptible to perceived disturbances in the South China Sea. The zooplankton community was found to be divided into two clusters, namely the Outer-bay Cluster (OC) comprising Noctiluca scintillans, Temora turbinata, and Paracalanus spp., and the Inner-bay Cluster (IC) which was dominated by Pseudevadne tergestina, Oikopleura rufescens, and Paracalanus spp. The OC was recorded in waters with low Chl a concentrations and high salinity, coinciding with open seawater intrusion. The IC occurred in waters with high Chl a concentrations, low salinity, with terrestrial inputs from the Dan'ao River. The dominant cladoceran species has changed in spring from Penilia avirostris to Pseudevadne tergestina owing to suitable temperature conditions and the low wind speed in this region. Most of the keystone species recorded during all seasons were found to be copepods based on co-occurrence network analysis. Numbers of keystone jellyfish (cnidaria) species, such as Geryonia proboscidalis, Chelophyes contorta, and Aeginura grimaldi were significantly higher in summer than in other seasons due to a low-temperature seawater intrusion, which can result in the highest stability of community structures and affect coastal food webs and fishery resources. Our results highlight that zooplankton community succession may occur with long-term temperature changes in the subtropical Daya Bay under global climate change conditions.

Rapid measurement of total lipids in zooplankton using the sulfo-phospho-vanillin reaction.

Zooplankton provide a vital source of nutrition to a variety of fish and marine predators. Measuring the total lipid content of zooplankton provides important information about diet quality available to predators, revealing details about trophic dynamics and ecosystem status. We analyze the performance of a microplate assay, utilizing the sulfo-phospho-vanillin (SPV) reaction, to quantify the total lipid content of various large crustacean zooplankton in a rapid and high throughput manner. Pilot experiments were performed by measuring the total lipid content of purchased freeze-dried zooplankton (Calanus finmarchicus and Euphausia superba) by both SPV and gravimetric analysis (low throughput and requires large sample size). The results of the SPV assay were not statistically different from gravimetric analysis for either species (p > 0.05). Further, an inter-laboratory comparison study was performed to measure the total lipid content (% of wet mass) of field-collected Arctic and North Pacific zooplankton (copepods (n = 19) and euphausiids (n = 29)) of various species utilizing multiple analysis methods. Results from thin layer chromatography with flame ionization detection (TLC-FID) demonstrated that lipid classes in zooplankton samples varied in composition of steryl/wax esters (3-95%), triacylglycerols (1-52%), free-fatty acids (0.4-25%), sterols (0-4%) and polar lipids (1-42%). Despite this variation in lipid class composition among samples, the results of the SPV assay agreed well with gravimetric analysis. The mean absolute and relative differences between SPV and gravimetric analysis for all zooplankton lipids in this study were 1.0% and 11.6%, respectively. The SPV assay is rapid (<2 hours), high throughput (25 samples processed in parallel), low cost (supplies <$ 0.67 per sample), precise (inter assay CV = 6.9%, intra assay CV = 6.0%), sensitive (limit of detection < 1.7 micrograms of lipid per analysis), and accurate when calibrated with appropriate standards.

Mycosporine-Like Amino Acids from Marine Resource.

In the last 10 years, a great number of publications (both regular papers and reviews) have been published on the interesting molecules-mycosporine-like amino acids (MAAs). Despite significant advances in the research of MAAs, current overviews in the recent publications involving MAA research still need reporting. The aim of this Special Issue is to join, as an interdisciplinary approach, the photochemical and photobiological aspects, with emphasis on new natural resources to obtain both algae and zooplankton MAAs, advances in methodology of extraction and chemical identification of new MAAs. Finally, this Special Issue reviews the bioactivities of MAAs including UVR screen, antioxidant, immunostimulant, growth factor, DNA protection, inhibition of collagenase, elastase and hyaluronidase, and anti-photoaging, among others, and their potential use as nutracosmeceutic molecules (i.e., oral and topic photoprotector).

Lipid content and stable isotopes of zooplankton during five winters around the northern Antarctic Peninsula.

The Southern Ocean zooplankton community is diverse, yet most species are understudied, especially with respect to their overwinter feeding ecologies. Here we present body condition and trophic biomarker data (lipid content and stable isotopes of carbon and nitrogen) from 19 zooplankton species collected over five consecutive winters (August and September 2012-2016) around the northern Antarctic Peninsula. We report environmental data (percent sea-ice cover, sea-ice type, water temperature, salinity, and integrated chl-a) as well as species abundance data at each sampling location to provide additional context for interpreting the lipid and stable isotope data. For most species, these are the first winter measurements or time series of body condition, trophic position, and abundance in relation to environmental variables. These data are critical for evaluating changes in ecosystem structure and predator-prey relationships in a region of Antarctica that is warming faster than most other areas on Earth as a result of climate change.

Comparison of Bayesian and numerical optimization-based diet estimation on herbivorous zooplankton.

Consumer diet estimation with biotracer-based mixing models provides valuable information about trophic interactions and the dynamics of complex ecosystems. Here, we assessed the performance of four Bayesian and three numerical optimization-based diet estimation methods for estimating the diet composition of herbivorous zooplankton using consumer fatty acid (FA) profiles and resource library consisting of the results of homogeneous diet feeding experiments. The method performance was evaluated in terms of absolute errors, central probability interval checks, the success in identifying the primary resource in the diet, and the ability to detect the absence of resources in the diet. Despite occasional large inconsistencies, all the methods were able to identify the primary resource most of the time. The numerical optimization method QFASA using χ2(QFASA-CS) or Kullback--Leibler (QFASA-KL) distance measures had the smallest absolute errors, most frequently found the primary resource, and adequately detected the absence of resources. While the Bayesian methods usually performed well, some of the methods produced ambiguous results and some had much longer computing times than QFASA. Therefore, we recommend using QFASA-CS or QFASA-KL. Our systematic tests showed that FA models can be used to accurately estimate complex dietary mixtures in herbivorous zooplankton. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Short- and long-term diets of the threatened longhorned pygmy devil ray, Mobula eregoodoo determined using stable isotopes.

Most mobulids are listed as near threatened to endangered. Nonetheless, effective conservation measures are hindered by knowledge gaps in their ecology and behaviour. In particular, few studies have assessed diets and trophic ecologies that could inform methods to avoid fishing mortality. Here, a shortfall in data for the longhorned pygmy devil ray, Mobula eregoodoo was addressed by describing temporal variability in dietary preferences using stable isotope analysis. During summer and autumn in 2017, five bather-protection gillnets were deployed off eastern Australia (29° S, 153.5° E). From the catches of these gillnets, 35 adult M. eregoodoo had liver, muscle and stomach contents sampled to determine δ13 C and δ15 N profiles. Analyses revealed that surface zooplankton and zooplanktivorous teleosts were important dietary components across short- and long-term temporal scales. Large quantities of undigested sandy sprat, Hyperlophus vittatus, in the stomachs of some specimens unequivocally confirm feeding on teleosts. A narrow isotopic niche and minimal isotopic overlap with reef manta rays, Mobula alfredi from the same geographic region in eastern Australia implies M. eregoodoo has unique and highly specialised resource use relative to other mobulids in the area. The species is clearly vulnerable to capture during inshore migrations, presumably where they feed on shallow-water shoaling teleosts. Female M. eregoodoo likely have a low annual reproductive output, so population recoveries from fishing-induced declines are likely to be slow. Measures to reduce the by catch of M. eregoodoo in local bather-protection gillnets, and artisanal fisheries more broadly, should be given priority.

Mercury bioaccumulation in zooplankton and its relationship with eutrophication in the waters in the karst region of Guizhou Province, Southwest China.

Zooplankton play an important role in the transfer of mercury (Hg) from the lower to upper trophic positions in the food chain. In this study, total mercury (THg) and methylmercury (MeHg) levels were measured in three size fractions of zooplankton collected from three reservoirs (Hongfeng, Baihua, and Aha Reservoir) and one wetland in karst areas to understand mercury accumulation in zooplankton from alkaline environments. The results showed that the alkaline waters had lower zooplankton MeHg levels (0.1 to 66.8 ng g-1) than most of the acidic waters reported. However, the zooplankton THg levels (6.3 to 494.9 ng g-1) were comparable. The macro-zooplankton (> 500 µm) had significantly higher THg and MeHg levels than meso-zooplankton (116 to 500 µm) in the three reservoirs at all seasons, which showed biomagnification of mercury in the food chain. The correlation between Hg in water and zooplankton and Hg in zooplankton of different sizes indicated that THg bioaccumulation in zooplankton was related to the THg levels in water; however, MeHg bioaccumulation in zooplankton was controlled by many other factors, such as their feeding and living habits. In the three reservoirs, the THg and MeHg concentrations in zooplankton decreased with increasing eutrophication. However, compared with the three reservoirs, Caohai Wetland, with large amounts of aquatic plants, had a much lower trophic level and higher MeHg content in water but much lower zooplankton MeHg levels and bioaccumulation factors (BAFs). The large amounts of plant residue might dilute mercury in the food chain, revealing that high primary production could result in lower Hg bioaccumulation, rather than only being influenced by nutrient levels.

Deficiency syndromes in top predators associated with large-scale changes in the Baltic Sea ecosystem.

Vitamin B1 (thiamin) deficiency is an issue periodically affecting a wide range of taxa worldwide. In aquatic pelagic systems, thiamin is mainly produced by bacteria and phytoplankton and is transferred to fish and birds via zooplankton, but there is no general consensus on when or why this transfer is disrupted. We focus on the occurrence in salmon (Salmo salar) of a thiamin deficiency syndrome (M74), the incidence of which is highly correlated among populations derived from different spawning rivers. Here, we show that M74 in salmon is associated with certain large-scale abiotic changes in the main common feeding area of salmon in the southern Baltic Sea. Years with high M74 incidence were characterized by stagnant periods with relatively low salinity and phosphate and silicate concentrations but high total nitrogen. Consequently, there were major changes in phytoplankton and zooplankton, with, e.g., increased abundances of Cryptophyceae, Dinophyceae, Diatomophyceae and Euglenophyceae and Acartia spp. during high M74 incidence years. The prey fish communities also had increased stocks of both herring and sprat in these years. Overall, this suggests important changes in the entire food web structure and nutritional pathways in the common feeding period during high M74 incidence years. Previous research has emphasized the importance of the abundance of planktivorous fish for the occurrence of M74. By using this 27-year time series, we expand this analysis to the entire ecosystem and discuss potential mechanisms inducing thiamin deficiency in salmon.

Calanus oil in the treatment of obesity-related low-grade inflammation, insulin resistance, and atherosclerosis.

Calanus oil (COil) is a natural product extracted from marine zooplankton Calanus finmarchicus found in the North Atlantic Ocean. This oil is rich in wax esters of polyunsaturated fatty acids (PUFAs) and has been projected as the best alternative to fish oil because its production cannot keep pace with the demands from the growing markets. The COil is the only commercially available marine source of wax esters, whereas classic ω-3 PUFAs comes from triglycerides, ethyl esters, and phospholipids. It has, in recent decades, been seen that there is an unprecedented rise in the use of PUFA-rich oil in the aquaculture industry. A simultaneous rise in the demand of PUFAs is also observed in the health care industry, where PUFAs are suggested preventing various disorders related to lifestyles such as obesity, diabetes mellitus, chronic low-grade inflammation, atherosclerosis, and brain and cardiovascular disorders (CVDs). In this review, we will explore the metabolic aspects related to the use of COil as an antioxidant, anticholesterinemic, and anti-inflammatory dietary source and its impact on the prevention and therapy of obesity-related metabolic disorders.

Mercury and selenium in the filter-feeding whale shark (Rhincodon typus) from two areas of the Gulf of California, Mexico.

Mercury and selenium were measured for first time in the endangered species whale shark (Rhyncodon typus) from two areas of the Gulf of California, Bahía Los Angeles (BLA) and Bahía La Paz (LAP) using dermal biopsies of seventy specimens. Additionally, nineteen zooplankton samples from LAP were analysed. Concentrations (ng/g, wet weight (ww)) in biopsies of BLA ranged from 1 to 40 for Hg and 100 to 680 for Se; while in LAP varied from 1 to 9 for Hg and 11 to 850 for Se. A positive correlation was found for Hg in BLA males biopsies with length. Hg and Se concentrations in the zooplankton from LAP were 1.6 ±â€¯1.8 and 770 ±â€¯930 ng/g, respectively. Hg biomagnification factor ranged from 0.8 to 5.3 in sharks. A molar excess of Se over Hg was found in the biopsies and the zooplankton.

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Zooplankton plays a mediating role in the food web of aquatic ecosystems, the stable carbon and nitrogen isotopes (δ13C and δ15N) of which have been widely used to study the utilization of food resources, material cycling pathways, and trophic relationships. The δ13C and δ15N values of zooplankton have been used to predict primary productivity, sources and sinks of pollutants and environmental changes. To better use δ13C and δ15N of zooplankton as ecological and environmental indicators, it is particularly important to understand their temporal and spatial variations and the influencing factors. Based on related literature, we synthesized spatial and temporal variations in δ13C and δ15N of zooplankton in different aquatic ecosystems and taxa groups, and the use of δ13C and δ15N indicators for ecological processes and environmental changes. The δ13C and δ15N of zooplankton are largely affected by its food sources, and its stable isotope compositions are in turn affected by primary productivity and nitrogen sources. We proposed that the combination of δ13C and δ15N in zooplankton with transportation and transformation of emerging pollutants would form a multi-means, multi-disciplinary and multi-scale research direction in the fields of earth science and biology.

Environmental effects on the bioaccumulation of PAHs in marine zooplankton in Gaoping coastal waters, Taiwan: Concentration, distribution, profile, and sources.

Polycyclic aromatic hydrocarbons (PAHs) are toxic and ubiquitous in the environment and pose great risks. Bioaccumulation by plankton is the outset for PAHs entering marine food web. The long-term driving environmental factors for bioaccumulation of PAHs in zooplankton have not been well investigated. In this study, high variation of PAH concentrations in zooplankton (5 to 5440 ng g-1 dry weight) was found, with the highest PAH levels near the transect Kaohsiung Harbor. Precipitation significantly enhances the PAH concentration in zooplankton and affects the PAH distribution in the water column, indicating PAH input from terrestrial runoff. The sources of PAHs in the coastal waters are a mixture of petroleum and combustion. Biomass dilution dominates bioaccumulation of PAHs in zooplankton, especially for the dominant species, copepods. This study provides the first information on PAH bioaccumulation in zooplankton to understand PAH transport in the plankton food web in subtropical coastal waters.

Trophic Structure and Biomagnification of Total Mercury in Ray Species Within a Benthic Food Web.

Stable isotopes of C (δ13C) and N (δ15N) were used to explore the trophic structure and evaluate mercury (Hg) biomagnification in the food web of muscle of three commercially important ray species from the Pacific coast of Baja California Sur (PCBCS): the shovelnose guitarfish (Pseudobatos productus), banded guitarfish (Zapteryx exasperata), and bat ray (Myliobatis californica). The food web of these ray species predominately consisted of zooplankton, three species of fish, and five species of invertebrates. Mean δ15N values in all species ranged from 10.54 ± 0.18‰ in zooplankton to 17.84 ± 0.81‰ in the shovelnose guitarfish. Mean δ13C values ranged from - 22.05 ± 0.75‰ in the red crab to - 15.93 ± 0.78‰ in the bat ray. Mean total Hg concentration ([THg]) in all species ranged from 0.0009 ± 0.0002 mg kg-1 ww in zooplankton to 0.24 ± 0.19 mg kg-1 ww in the banded guitarfish. The food web magnification factor was 6.38 and significantly greater than 1.0. The present study describes [THg] biomagnification in the benthic food web of three ray species of the PCBCS. This provides an important baseline knowledge of the biomagnification dynamics and pathways of Hg in this environment for these multiple interacting species.

Insensitivities of a subtropical productive coastal plankton community and trophic transfer to ocean acidification: Results from a microcosm study.

Ocean acidification (OA) has potential to affect marine phytoplankton in ways that are partly understood, but there is less knowledge about how it may alter the coupling to secondary producers. We investigated the effects of OA on phytoplankton primary production, and its trophic transfer to zooplankton in a subtropical eutrophic water (Wuyuan Bay, China) under present day (400 µatm) and projected end-of-century (1000 µatm) pCO2 levels. Net primary production was unaffected, although OA did lead to small decreases in growth rates. OA had no measurable effect on micro-/mesozooplankton grazing rates. Elevated pCO2 had no effect on phytoplankton fatty acid (FA) concentrations during exponential phase, but saturated FAs increased relative to the control during declining phase. FA profiles of mesozooplankton were unaffected. Our findings show that short-term exposure of plankton communities in eutrophic subtropical waters to projected end-of-century OA conditions has little effect on primary productivity and trophic linkage to mesozooplankton.

Trophic transfer of cyclic methyl siloxanes in the marine food web in the Bohai Sea, China.

Trophic transfer of cyclic methyl siloxanes (CMS) in aquatic ecosystems is an important criterion for assessing its environmental risks. This study researched the trophic transfer of four CMS (octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and tetradecamethylcycloheptasiloxane (D7)) in marine food web from zooplankton up to seabirds in the Chinese Bohai Sea. In the zooplankton-invertebrate-fish-seabird based food web, the significant trophic magnifications were found for D4 to D6 (D4: R2 = 0.040, p < 0.05, D5: R2 = 0.26, p < 0.0001, D6: R2 = 0.071, p < 0.001), and the significant trophic dilution was found for D7 (R2 = 0.026 and p < 0.05). The trophic magnification factors (TMF) for D4 to D7 were 1.7 (95% confidence interval: 1.1-2.6), 3.5 (2.5-5.0), 1.8 (1.3-2.6), and 0.63 (95% CI: 0.40-0.99) respectively. In the zooplankton-invertebrate-fish based food web, both significant trophic magnification for D5 (R2 = 0.16, p < 0.0001, TMF = 3.0) and significant trophic dilution for D7 (R2 = 0.073, p < 0.01, TMF = 0.4) were found, but for D4 and D6, the trophic magnifications were not significant (D4: R2 = 0.010, p = 0.23, D6: R2 = 0.010, p = 0.23). The trophic transfer of the legacy contaminant BDE-47 and BDE-99 were also conducted as the benchmark chemicals and significant positive correlation was found. As far as we know, this is the first research on the trophic transfer of CMS in the zooplankton-invertebrate-fish-bird food chain which provided new insight of these compounds in the area.

Terrestrial organic matter increases zooplankton methylmercury accumulation in a brown-water boreal lake.

Increases in terrestrial organic matter (tOM) transport from catchments to boreal lakes can affect methylmercury (MeHg) accumulation in aquatic biota both directly by increasing concentrations of aqueous MeHg, and indirectly through effects on MeHg bioavailability and on energy pathways in the lower food web. We carried out a detailed seasonal study of water chemistry, zooplankton diet, and MeHg accumulation in zooplankton in two lakes with contrasting tOM concentrations. Between-lake differences explained 51% of the variability in our water chemistry data, with no observed effect of season or sampling depth, contrary to our expectations. Higher tOM was correlated with higher aqueous Hg concentrations, lower areal pelagic primary productivity, and an increased contribution of terrestrial particles to pelagic particulate organic matter. Based on dietary marker analysis (δ13C, δ15N, and fatty acid [FA] composition), zooplankton diet was strongly linked to feeding mechanism, with dietary reliance on phytoplankton highest in the selective-feeding calanoid copepods, and lowest in filter feeding cladocerans. Zooplankton dietary reliance on phytoplankton and their concentrations of high-quality lipids, including polyunsaturated fatty acids, were higher in the clear-water lake than in the brown-water lake, where bacterial and terrestrial food sources were more prevalent. MeHg was highest in zooplankton from the brown-water lake, with highest concentrations in the 200-500 µm zooplankton size fraction for both lakes. Contrary to our expectations, there was no effect of season on zooplankton dietary markers or MeHg. Our results suggest that, overall, higher tOM results in higher MeHg concentrations in water and zooplankton, and reduces zooplankton dietary reliance on phytoplankton. Increased tOM thus leads to a decrease in the nutritional quality of zooplankton (i.e. higher MeHg concentrations, and lower concentrations of essential fatty acids), which may cascade up the food web with negative implications for higher trophic levels.

Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues.

RATIONALE: It is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound-specific isotope analysis of amino acids (CSIA-AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acid δ15 N values. METHODS: We evaluated the effects of chemical preservatives on bulk tissue δ13 C and δ15 N and amino acid δ15 N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares) and squid (Dosidicus gigas) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species, Calanus pacificus and Eucalanus californicus, which were preserved in formaldehyde for 24-25 years. RESULTS: Tissues in formaldehyde-ethanol had higher bulk δ15 N values (+1.4, D. gigas; +1.6‰, T. albacares), higher δ13 C values for D. gigas (+0.5‰), and lower δ13 C values for T. albacares (-0.8‰) than frozen samples. The bulk δ15 N values from copepods were not different those from frozen samples, although the δ13 C values from both species were lower (-1.0‰ for E. californicus and -2.2‰ for C. pacificus) than those from frozen samples. The mean amino acid δ15 N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanine δ15 N values were altered to a larger extent (range: 0.5-4.5‰). CONCLUSIONS: The effects of preservation on bulk δ13 C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulk δ15 N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation of δ15 N values used in ecological studies. The preservation effects on amino acid δ15 N values were also mostly minimal, mirroring bulk δ15 N trends, which is promising for future CSIA-AA studies of archived specimens. However, there were substantial differences in phenylalanine and valine δ15 N values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation.