Elemental turnover rates and trophic discrimination in juvenile Lebranche mullet Mugil liza under experimental conditions

Trophic niche variation driven by water level fluctuations facilitates the co-occurrence of invasive tilapia species in a subtropical reservoir in China

Investigating how multiple invasive fish species with similar ecological traits respond to different environmental conditions is crucial to understanding their successful invasion and coexistence. Here, we used stomach content analysis and stable isotope analysis to analyse the effects of water level fluctuation on the trophic niche plasticity of three dominant co-occurring invasive tilapia species (Coptodon zillii, Sarotherodon galilaeus, and Oreochromis niloticus) in the Shanmei Reservoir, southern China. We found that the tilapia species exhibited an iliophagous habit with dietary variations between the high-water (HW) and low-water (LW) level periods. During the LW period, tilapia fishes primarily fed on periphytic algae, whereas during the HW period, they reduced their consumption of epiphytic algae and increased their intake of plant remains. Biofilms were the most assimilated resource by the species during the LW period, whereas riparian plants dominated during the HW period. The niche width and niche overlap of the three tilapia species were significantly greater in the HW period than in the LW period. However, their trophic positions were not significantly affected by water level fluctuations. Our findings indicate that temporal variation in diet composition and trophic niche, driven by water level fluctuations, may favor food resource partitioning and facilitate the coexistence of these invasive tilapia species.

Hydroclimatic influence on the trophic ecology of Atlantic goliath grouper juveniles (Epinephelus itajara) in a tropical estuary using non-lethal methodologies

The Atlantic goliath grouper Epinephelus itajara is the largest species of groupers in the Atlantic Ocean, reaching over 2 m total length and 400 kg. It has an ontogenetic migration, with adults using nearshore and offshore marine areas and juveniles inhabiting estuarine/mangrove areas. Despite of its high ecological relevance and classification as a threatened species for several decades in the IUCN red lists, critical phases of its life cycle, like juveniles inhabiting estuaries, are still poorly investigated. In this study, we evaluated if hydroclimatic changes may affect the trophic ecology of juveniles in a tropical estuary. We described their diet composition, isotopic niche area, food assimilation, and trophic position of juveniles across seasons using two non-lethal techniques: stomach content analysis using a stomach flushing procedure and stable isotope analysis using a dermal biopsy. A total of 87 individuals with TL ranging from 38 to 331 mm had their stomachs analyzed, of which 31 had food content. We identified 11 prey items, shrimp being the most abundant prey group (IRI% = 88.4), followed by fish (6.6), blue crab (2.8), and crab (2.2). A total of 93 specimens had their muscle's carbon (δ13C) and nitrogen (δ15N) stable isotope ratios analyzed (38-332 mm TL). Average δ13C values showed statistically significant changes across seasons, but no significant changes were observed for δ15N. As initially predicted, isotopic niche size of juveniles changed consistently along the entire hydroclimatic cycle, reaching its peak at the end of the wet season. We also observed changes in the proportion of prey assimilation by juveniles between seasons. Shrimp (92.1%) was the most assimilated prey group during the late dry season, whereas tidal crabs (36.7%), blue crab (27.8%), and shrimp (25.1%) were the most important in the late wet season. Average trophic position of juveniles remained similar across wet (3.32 ± 0.10) and dry (3.23 ± 0.11) seasons. These findings provide important information to help decision makers to promote more efficient management and conservation legislations to protect early life stages of the Atlantic goliath grouper in tropical estuaries.

Legacy and novel brominated flame retardants in a lab-constructed freshwater ecosystem: Distribution, bioaccumulation, and trophic transfer

The extensive utilization of both legacy and novel brominated flame retardants (BFRs) leads to high environmental concentrations, which would be bioaccumulated by organisms and further transferred through the food webs, causing potential risks to humans. In this study, five BFRs, that showed high detection frequencies and concentrations in sediments from an e-waste dismantling site in Southern China, namely 2,3,4,5,6-pentabromotoluene (PBT), hexabromobenzene (HBB), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE), decabromodiphenyl ethane (DBDPE), and decabromodiphenyl ether (BDE209), were selected as target pollutants in the lab-constructed aquatic food web as part of a micro-ecosystem, to investigate their distribution, bioaccumulation, and trophic transfer patterns. The significant correlations between different samples in the food web indicated that the dietary uptake appeared to influence the levels of BFRs in organisms. Significant negative correlations were observed between the trophic level of organisms and the lipid-normalized concentrations of BTBPE and DBDPE, indicating the occurrence of trophic dilution after 5-month exposure. However, the average values of bioaccumulation factors (BAFs) were from 2.49 to 5.17 L/kg, underscoring the importance of continued concern for environmental risks of BFRs. The organisms occupying higher trophic levels with greater bioaccumulation capacities may play a pivotal role in determining the trophic magnification potentials of BFRs. This research provides a helpful reference for studying the impacts of feeding habits on bioaccumulation and biomagnification, as well as for identifying the fate of BFRs in aquatic environment.

Coastal upwelling influences population structure of dusky grouper Epinephelus marginatus: An integrative approach based on otolith chemistry and muscle stable isotopes

The dusky grouper (Epinephelus marginatus) is an overfished and threatened fish species with coastal distribution. In the Southwestern Atlantic, it occurs across a broad region influenced by two major oceanographic features: the Cabo Frio (23°S) and the Cabo Santa Marta (28°S) upwelling systems. Along the Brazilian coast, the species may present continuous or discrete populations, depending on the methodological approach used. In this study we combined otolith chemistry and muscle stable isotope analyses to examine the population structure of dusky groupers and its association with the two upwelling systems. Fish were collected in shallow coastal waters of the Southwest Atlantic Ocean, covering the southeastern and southern Brazilian coasts, among Macaé (22°S), Santos (24°S), Florianópolis (27°S), and in Rio Grande (32°S). The results show three statistically well-separated population groups along the region. We named these population groups as North (north of Cabo Frio); Center (between upwelling regions); and South (south of the Cabo Santa Marta system). Our findings allow to suggest that the upwelling systems may influence the distribution of E. marginatus stocks along the Brazilian south-western coast, even though a causal effect may not be attributed at this point. Overall, this combined approach, leveraging information from distinct natural tags, and reflecting variability of water chemistry and food webs with latitude, allowed us to enhance our understanding on how major upwelling systems influence the structuring of fish populations along the southwestern Atlantic Ocean.

Contrasting energy pathways suggest differing susceptibility of pelagic fishes to an invasive ecosystem engineer in a large lake system

Species invasions can lead to ecological regime shifts by altering food web structure and changing nutrient cycling. Stable isotopes are a powerful tool to understand the potential and realized impacts of invasive species on food webs, especially when used in tandem with other dietary tracers. An invasion by one of the most notorious freshwater invaders in North America, the quagga mussel (Dreissena bugensis), is imminent in Lake Champlain, United States. An invasion by this filter feeder has the potential to drastically alter energy pathways and destabilize pelagic fisheries via bottom-up impacts. However, the extent and magnitude of these impacts depend on the current food web structure of the mid-trophic pelagic food web, which was previously not well described. We used Bayesian stable isotope mixing models informed by stomach content analysis to identify which energy pathways are currently most important to mid-trophic level fishes. We determined that in the Main Lake basin, the spring phytoplankton bloom and deep chlorophyll layer – the resources most vulnerable to quagga mussels – provide a disproportionate amount of support to the pelagic food web via zooplankton and the migrating macroinvertebrate Mysis. The food web in the Northeast Arm of Lake Champlain is supported by epilimnetic phytoplankton, which is more protected from the filtration effects of quagga mussels than the deep chlorophyll layer. However, the Northeast Arm will likely not provide a high-quality foraging refuge to coldwater pelagic fish due to unfavorable oxythermal conditions. The mid-trophic food web of Lake Champlain—and consequently piscivores who rely on these prey—may be vulnerable to the impending quagga mussel invasion if migratory Mysis are not able to shift their diet to benthic resources.

Effects of seasonal contaminant remobilization on the community trophic dynamics in a Brazilian tropical estuary

In tropical estuaries, wet seasons are responsible for the downstream transport of allochthonous material from the upper basin and flooded plains. Although allochthonous matter is commonly associated to nutrient and detritus input, pollutants are also transported throughout the basin or suspended from the river bottom via strong streamflow remobilization and rainfall dynamics. We assessed community and population trophic niche-based patterns using organisms' stable isotopes signatures in the wet and the dry seasons to test if estuarine trophic diversity is affected by remobilization of metal-contaminated material from a mining dam collapse that occurred in the Doce river basin, Brazil. Trophic depletion was detected community-wide and in a key consumer group (bottom-dwelling fishes) at the end of the wet season in the impacted Doce river estuary (DRE). Conversely, higher trophic diversity values were recorded in a well-preserved estuary used as control site. Stable isotopes mixing models indicated in the DRE that G. genidens, a predator fish species, presented poor-quality diet based on pollutant-tolerant tiny organisms, a finding that strongly contrasts from diet described in other, little-impacted Brazilian estuaries. Although wet seasons are expected to increase trophic, functional and taxonomic diversity in tropical estuaries, in the DRE the rainfall-driven dynamics poses a threat to the community due to the presence of ore tailings.

Early evidences of niche shifts in estuarine fishes following one of the world's largest mining dam disasters

The Fundão dam collapse occurred on November 2015 in Mariana city (Brazil), provoking a series of ecological impacts over the Doce river basin and its nearshore environment. However many impacts over fishery target fauna (fish and shrimp) are still unknown or underestimated due to the lack of baseline data in the region. In the present study we assessed the isotopic niches modeled from δ¹³C and δ¹⁵N signatures of six estuarine fish species before and after the impact to assign potential shifts at the population- and community-level. We showed isotopic niche has altered in all studied species irrespective of its trophic group and habitat use. Niche community metrics indicated a depletion of trophic diversity and basal resources of the whole community after the impact. Food web changes as we reported here can impair the energy transfer through the food chain and put at risk the sustainability of small fisheries that rely upon local resources.