Cryptic Constituents: The Paradox of High Flux–Low Concentration Components of Aquatic Ecosystems

The interface between terrestrial ecosystems and inland waters is an important link in the global carbon cycle. However, the extent to which allochthonous organic matter entering freshwater systems plays a major role in microbial and higher-trophic-level processes is under debate. Human perturbations can alter fluxes of terrestrial carbon to aquatic environments in complex ways. The biomass and production of aquatic microbes are traditionally thought to be resource limited via stoichiometric constraints such as nutrient ratios or the carbon standing stock at a given timepoint. Low concentrations of a particular constituent, however, can be strong evidence of its importance in food webs. High fluxes of a constituent are often associated with low concentrations due to high uptake rates, particularly in aquatic food webs. A focus on biomass rather than turnover can lead investigators to misconstrue dissolved organic carbon use by bacteria. By combining tracer methods with mass balance calculations, we reveal hidden patterns in aquatic ecosystems that emphasize fluxes, turnover rates, and molecular interactions. We suggest that this approach will improve forecasts of aquatic ecosystem responses to warming or altered nitrogen usage.

Download Full-text

Large herbivorous wildlife and livestock differentially influence the relative importance of different sources of carbon for riverine food webs

10.1101/2021.07.29.454315 ◽

2021 ◽

Author(s):

Frank O. Masese ◽

Thomas Fuss ◽

Lukas Bistarelli ◽

Caroline Buchen-Tschiskale ◽

Gabriel Singer

Keyword(s):

Organic Matter ◽

Food Webs ◽

Aquatic Ecosystems ◽

Carbon Sources ◽

Large River ◽

Livestock Grazing ◽

Relative Importance ◽

River Continuum ◽

Aquatic Food ◽

Different Sources

In many regions around the world, large populations of native wildlife have declined or been replaced by livestock grazing areas and farmlands, with consequences on terrestrial-aquatic ecosystems connectivity and trophic resources supporting food webs in aquatic ecosystems. The river continuum concept (RCC) and the riverine productivity model (RPM) predict a shift of carbon supplying aquatic food webs along the river: from terrestrial inputs in low-order streams to autochthonous production in mid-sized rivers. Here, we studied the influence of replacing large wildlife (mainly hippos) with livestock on the relative importance of C3 vegetation, C4 grasses and periphyton on macroinvertebrates in the Mara River, which is an African montane-savanna river known to receive large subsidy fluxes of terrestrial carbon and nutrients mediated by LMH, both wildlife and livestock. Using stable carbon (δ13C) and nitrogen (δ15N) isotopes, we identified spatial patterns of the relative importance of allochthonous carbon from C3 and C4 plants (woody vegetation and grasses, respectively) and autochthonous carbon from periphyton for macroinvertebrates at various sites of the Mara River and its tributaries. Potential organic carbon sources and invertebrates were sampled at 80 sites spanning stream orders 1 to 7, various catchment land uses (forest, agriculture and grasslands) and different loading rates of organic matter and nutrients by LMH (livestock and wildlife, i.e., hippopotamus). The importance of different sources of carbon along the river did not follow predictions of RCC and RPM. First, the importance of C3 and C4 carbon was not related to river order or location along the fluvial continuum but to the loading of organic matter (dung) by both wildlife and livestock. Notably, C4 carbon was important for macroinvertebrates even in large river sections inhabited by hippos. Second, even in small 1st -3rd order forested streams, autochthonous carbon was a major source of energy for macroinvertebrates, and this was fostered by livestock inputs fuelling aquatic primary production throughout the river network. Importantly, our results show that replacing wildlife (hippos) with livestock shifts river systems towards greater reliance on autochthonous carbon through an algae-grazer pathway as opposed to reliance on allochthonous inputs of C4 carbon through a detrital pathway.

Download Full-text

Food web controls on mercury fluxes and fate in the Colorado River, Grand Canyon

Science Advances ◽

10.1126/sciadv.aaz4880 ◽

2020 ◽

Vol 6 (20) ◽

pp. eaaz4880 ◽

Cited By ~ 1

Author(s):

D. M. Walters ◽

W.F. Cross ◽

T.A. Kennedy ◽

C.V. Baxter ◽

R.O. Hall ◽

...

Keyword(s):

Food Web ◽

Food Webs ◽

Species Interactions ◽

Colorado River ◽

Grand Canyon ◽

Terrestrial Ecosystems ◽

Species Traits ◽

Functional Trait ◽

Contaminant Exposure ◽

Aquatic Food

Mercury (Hg) biomagnification in aquatic food webs is a global concern; yet, the ways species traits and interactions mediate these fluxes remain poorly understood. Few pathways dominated Hg flux in the Colorado River despite large spatial differences in food web complexity, and fluxes were mediated by one functional trait, predation resistance. New Zealand mudsnails are predator resistant and a trophic dead end for Hg in food webs we studied. Fishes preferred blackflies, which accounted for 56 to 80% of Hg flux to fishes, even where blackflies were rare. Food web properties, i.e., match/mismatch between insect production and fish consumption, governed amounts of Hg retained in the river versus exported to land. An experimental flood redistributed Hg fluxes in the simplified tailwater food web, but not in complex downstream food webs. Recognizing that species traits, species interactions, and disturbance mediate contaminant exposure can improve risk management of linked aquatic-terrestrial ecosystems.

Download Full-text

Comparing community response indices in aquatic food web models

Advances in Oceanography and Limnology ◽

10.4081/aiol.2019.8621 ◽

2019 ◽

Vol 10 (2) ◽

Author(s):

Ágnes Móréh ◽

Ferenc Jordán

Keyword(s):

Food Web ◽

Food Webs ◽

Aquatic Ecosystems ◽

Single Species ◽

Community Response ◽

Individual Species ◽

Relative Importance ◽

Response Functions ◽

Local Perturbations ◽

Aquatic Food

Aquatic ecosystems face several major challenges from the introduction and invasion of species, to overfishing. In order to better manage these situations, we need predictive models, where diverse scenarios can be simulated and tested. One key challenge to address is how to quantify the relationships between single-species disturbances and their multispecies effects. Mapping the spread of direct and indirect effects in food webs helps to link species to communities. Since food webs are complex networks of interactions, it is typically not easy to make predictions, so modelling and simulation may help to reveal general patterns. In food web simulations, one can quantify the effects of local perturbations on other species, i.e., community response. This may provide information about the relative importance of individual species and it is also useful to assess the vulnerability of the whole community to local changes. However, community response can be measured in several ways and various response functions give different results. In order to better understand their similarities and differences, we present a comparative study on a reasonable set of community response functions in food web simulations. These results contribute to build more predictive, multi-species models for systems-based conservation and management.

Download Full-text

Isotope fractionation by plants and animals: implications for nutrition research

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y90-146 ◽

1990 ◽

Vol 68 (7) ◽

pp. 960-972 ◽

Cited By ~ 29

Author(s):

Brenda V. Kennedy ◽

H. Roy Krouse

Keyword(s):

Food Webs ◽

Aquatic Ecosystems ◽

Isotope Fractionation ◽

Inorganic Compounds ◽

Isotope Effects ◽

Trophic Levels ◽

Future Research ◽

Turnover Rates ◽

Nutrition Research ◽

Biological Interest

The isotopic compositions of animal tissues, minerals, and fluids reflect those of ingested food and water and inhaled gases. This relationship is illustrated by a review of data pertaining to five light elements of biological interest (carbon, nitrogen, hydrogen, oxygen, and sulphur). Processes affecting the isotopic composition of inorganic compounds in reservoirs are summarized, and isotope fractionation during transfer of elements from these inorganic reservoirs through progressive trophic levels of food webs is discussed. Variability of δ values within and among individuals, populations, and species of plants and animals is attributed to at least six factors: locality, dietary selectivity, biochemical composition of food, isotope effects in metabolic processes, turnover rates, and stress. Features of a variety of terrestrial and aquatic ecosystems are used to illustrate basic concepts. Future research should seek to clarify specific mechanisms affecting δ values during the transfer of elements through food webs.Key words: food webs, stable isotopes, isotope fractionation, ecosystems.

Download Full-text

Faculty Opinions recommendation of Neonicotinoids disrupt aquatic food webs and decrease fishery yields.

Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature ◽

10.3410/f.736836591.793566835 ◽

2019 ◽

Author(s):

Michael Symonds

Keyword(s):

Food Webs ◽

Aquatic Food Webs ◽

Aquatic Food

Download Full-text

Elevated temperature and browning increase dietary methylmercury, but decrease essential fatty acids at the base of lake food webs

Scientific Reports ◽

10.1038/s41598-021-95742-9 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Pianpian Wu ◽

Martin J. Kainz ◽

Fernando Valdés ◽

Siwen Zheng ◽

Katharina Winter ◽

...

Keyword(s):

Climate Change ◽

Fatty Acids ◽

Organic Matter ◽

Food Webs ◽

Essential Fatty Acids ◽

Trophic Levels ◽

Climate Change Scenarios ◽

Essential Nutrients ◽

Aquatic Food Webs ◽

Aquatic Food

AbstractClimate change scenarios predict increases in temperature and organic matter supply from land to water, which affect trophic transfer of nutrients and contaminants in aquatic food webs. How essential nutrients, such as polyunsaturated fatty acids (PUFA), and potentially toxic contaminants, such as methylmercury (MeHg), at the base of aquatic food webs will be affected under climate change scenarios, remains unclear. The objective of this outdoor mesocosm study was to examine how increased water temperature and terrestrially-derived dissolved organic matter supply (tDOM; i.e., lake browning), and the interaction of both, will influence MeHg and PUFA in organisms at the base of food webs (i.e. seston; the most edible plankton size for zooplankton) in subalpine lake ecosystems. The interaction of higher temperature and tDOM increased the burden of MeHg in seston (< 40 μm) and larger sized plankton (microplankton; 40–200 μm), while the MeHg content per unit biomass remained stable. However, PUFA decreased in seston, but increased in microplankton, consisting mainly of filamentous algae, which are less readily bioavailable to zooplankton. We revealed elevated dietary exposure to MeHg, yet decreased supply of dietary PUFA to aquatic consumers with increasing temperature and tDOM supply. This experimental study provides evidence that the overall food quality at the base of aquatic food webs deteriorates during ongoing climate change scenarios by increasing the supply of toxic MeHg and lowering the dietary access to essential nutrients of consumers at higher trophic levels.

Download Full-text

Eutrophication induces shifts in the trophic position of invertebrates in aquatic food webs

Ecology ◽

10.1002/ecy.3275 ◽

2020 ◽

Author(s):

Gea H. Lee ◽

J. Arie Vonk ◽

Ralf C.M. Verdonschot ◽

Michiel H.S. Kraak ◽

Piet F.M. Verdonschot ◽

...

Keyword(s):

Food Webs ◽

Trophic Position ◽

Aquatic Food Webs ◽

Aquatic Food

Download Full-text

Assessing the utility of sulfur isotope values for understanding mercury concentrations in water and biota from high Arctic lakes

Arctic Science ◽

10.1139/as-2018-0022 ◽

2019 ◽

Vol 5 (2) ◽

pp. 90-106 ◽

Cited By ~ 1

Author(s):

Gretchen L. Lescord ◽

Meredith G. Clayden ◽

Karen A. Kidd ◽

Jane L. Kirk ◽

Xiaowa Wang ◽

...

Keyword(s):

Food Webs ◽

Total Mercury ◽

Nitrogen Isotopes ◽

Sulfur Isotopes ◽

High Arctic ◽

Arctic Lakes ◽

Carbon And Nitrogen ◽

Resolute Bay ◽

Aquatic Food ◽

Globally Stable

Methylmercury (MeHg) biomagnifies through aquatic food webs resulting in elevated concentrations in fish globally. Stable carbon and nitrogen isotopes are frequently used to determine dietary sources of MeHg and to model its biomagnification. However, given the strong links between MeHg and sulfur cycling, we investigated whether sulfur isotopes (δ34S) would improve our understanding of MeHg concentrations ([MeHg]) in Arctic lacustrine food webs. Delta34S values and total mercury (THg) or MeHg were measured in water, sediments, and biota from six lakes near Resolute Bay, NU, Canada. In two lakes impacted by historical eutrophication, aqueous sulfate δ34S was ∼8‰ more positive than sedimentary δ34S, suggestive of bacterial sulfate reduction in the sediment. In addition, aqueous δ34S showed a significant positive relationship with aqueous [MeHg] across lakes. Within taxa across lakes, [THg] in Arctic char muscle and [MeHg] in their main prey, chironomids, were positively related to their δ34S values across lakes, but inconsistent relationships were found across entire food webs among lakes. Across lakes, nitrogen isotopes were better predictors of biotic [THg] and [MeHg] than δ34S within this dataset. Our results suggest some linkages between Hg and S biogeochemistry in high Arctic lakes, which is an important consideration given anticipated climate-mediated changes in nutrient cycling.

Download Full-text