Body size drives allochthony in food webs of tropical rivers

Sources of intraspecific variation in the isotopic niche of a semi-aquatic predator in a human-modified landscape

Intraspecific variation modulates patterns of resource use by species, potentially affecting the structure and stability of food webs. In human-modified landscapes, habitat disturbance modifies trophic interactions and intraspecific niche variation, impacting population persistence. Here, we investigated the relationship of sex, ontogeny, and habitat factors with the trophic niche of Caiman crocodilus in an agricultural landscape. We evaluated temporal variation in the trophic niche parameters using carbon and nitrogen stable isotope analysis from different body tissues. We found that caimans exploit the same carbon and nitrogen pools through time, with low isotopic variability between seasons, partly due to the slow isotope turnover rates of tissues in crocodilians. Conversely, the trophic niche of caimans varied across habitats, but with no evidence of a difference between natural and anthropogenic habitats. It apparently results from the influence of habitat suitability, connectivity, and caiman movements during the foraging. Our findings highlight the broader niches of juvenile caimans relative to adults, possibly in response of territorialism and opportunistic foraging strategy. Although using similar resources, females had a larger niche than males, probably associated with foraging strategies during nesting. Considering the sex and body size categories, caimans occupied distinct isotopic regions in some habitats, indicating apparent niche segregation. Ontogenetic trophic shifts in the isotopes (δ13C and δ15N) depended on sex, leading to resource partitioning that can potentially reduce intraspecific competition. Decision-makers and stakeholders should consider the trophic dynamics of sex and body size groups for the sustainable management and conservation of caiman populations, which implies in the maintenance of wetland habitats and landscape heterogeneity in the Formoso River floodplain.

Influence of life history variation and habitat on mercury bioaccumulation in a high-order predatory fish in tropical Australia

Mercury distribution and bioaccumulation in aquatic ecosystems of tropical Australia is poorly characterised. Barramundi (Lates calcarifer), a widespread high-order predator in both fresh and coastal marine waters of the region, fulfils requirements for a bio-indicator of mercury contamination. In a study of the Mary River system of the Northern Territory, total mercury in the muscle tissue of 300 specimens gathered over four years (2013-2017, across both wet and dry seasons) was determined by direct combustion-atomic absorption spectrometry. Source of nutrition and trophic position of barramundi in the food web was also estimated via carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), respectively, in tissue by stable isotope mass spectrometry, and determination of strontium isotopes (⁸⁷Sr/⁸⁶Sr) in otoliths by laser ablation-ICPMS differentiated between freshwater and saltwater residence. Results showed that fish moving into freshwater floodplain wetlands concentrated mercury in muscle tissue at approximately twice the level of those that remained in saline habitats. Resolving life histories through otolith analyses demonstrated diversity in mercury bioaccumulation for individual fish of the same migratory contingent on the floodplains. Although trophic level (δ¹⁵N), capture location, source of nutrition (δ¹³C), and age or size partly predicted mercury concentrations in barramundi, our results suggest that individual variability in diets, migration patterns and potentially metabolism are also influential. Using a migratory fish as a bio-indicator, and tracking its life history and use of resources, proved valuable as a tool to discern hot spots in a coastal waterway for a contaminant, such as mercury.

Connectivity of fish communities in a tropical floodplain river system and predicted impacts of potential new dams

Longitudinal and lateral connectivity is important for mobile aquatic species in rivers for reproductive migrations, recruitment, gene flow and access to food resources across habitat types. Water resource developments such as dams and levees may disrupt these connections, causing river fragmentation and loss of access to highly productive habitats such as floodplain wetlands. We used sulfur stable isotopes as a tracer to estimate patterns of fish movement in an unregulated river in tropical northern Australia, taking advantage of observed spatial variation in sulfur isotope values of their food resources across the catchment. We also modelled the flow and barrier related impacts of potential dam development scenarios on fish movement. Fish with isotope values significantly different from local prey values were determined to be migrants. In the 'no dams' scenario, movement varied among fish species (0-44% migrant fish within species where n > 5) and sites (0-40% migrant fish within sites where n > 5), and immigration was higher in more connected sites. Impacts of water resource development on fish movement varied between dam scenarios, with predictions that a dam on the main channel of the Mitchell River would have the highest impact of the three individual dam scenarios. This study provides critical information on how flow-mediated connectivity supports patterns of fish community movement in an unregulated river system. The generic quantitative approach of combining tracers of fish movement with connectivity modelling provides a powerful predictive tool. While we used sulfur stable isotopes to estimate fish movement, our approach can be used with other tracers of movement such as otoliths and acoustic telemetry, making it widely applicable to guide sustainable development in other river systems.

Impact of Oreochromis niloticus (Linnaeus, 1758) (Pisces: Cichlidae) invasion on taxonomic and functional diversity of native fish species in the upper Kabompo River, northwest of Zambia

Invasive alien species have been revealed to drastically alter the structure of native communities; however, there is scarce information on whether taxonomic and functional spaces occupied by native species are equally filled by exotic species. We investigated the diversity of native species to understand the impact of exotic Oreochromis niloticus in the upper Kabompo River, northwest of Zambia using taxonomic and functional diversity indices. To achieve this, two tests were performed (Test 1, compared natives in invaded and uninvaded sections; Test 2, compared natives in invaded section). A total of 17 species were collected for functional diversity computation, out of which fourteen (14) functional trait measurements linked to feeding, locomotion, and life history strategy were taken. Findings revealed that taxonomic and functional diversity values changed with invasion in both tests. Taxonomic diversity was 15% more in invaded than uninvaded sections in Test 1 and was not consistent across sampling points of invaded section in Test 2. Invaded areas were taxonomically less diverse, but functionally diverse in both tests. The analysis of similarity and nonmetric multidimensional scaling revealed no difference in Bray-Curtis similarity assemblages in both tests. Our findings revealed that exotic species more often occupy unfilled gaps in the communities often occupied by the native species; this is achieved by occupying functional spaces. Overall, changes in taxonomic and functional diversity of native species documented here partially confirmed impacts of O. niloticus invasion. Therefore, we recommend a multifaceted approach to assess cumulative impacts of invasion on native species.

Unravelling the taxonomy and identification of a problematic group of benthic fishes from tropical rivers (Gobiidae: Glossogobius)

Flathead gobies (genus Glossogobius) include c. 40 small- to medium-sized benthic fishes found primarily in freshwater habitats across the Indo-Pacific, having biodiversity value as well as cultural and economic value as food fishes, especially in developing countries. To help resolve considerable confusion regarding the identification of some of the larger-growing Glossogobius species, a systematic framework was established using nuclear genetic markers, mitochondrial DNA barcoding and phenotypic evidence for a geographically widespread collection of individuals from the waterways of tropical northern Australia. Species boundaries and distribution patterns were discordant with those previously reported, most notably for the tank goby Glossogobius giuris, which included a cryptic species. Genetic divergence was matched with accompanying unique visual characters that aid field identification. Additional taxonomic complexity was also evident, by comparison with DNA barcodes from international locations, suggesting that the specific names applicable for two of the candidate species in Australia remain unresolved due to confusion surrounding type specimens. Although flathead gobies are assumed to be widespread and common, this study demonstrates that unrealised taxonomic and ecological complexity is evident, and this will influence assessments of tropical biodiversity and species conservation. This study supports the need for taxonomic studies of freshwater fishes to underpin management in areas subject to significant environmental change.

New insights into the food web of an Australian tropical river to inform water resource management

Rivers around the world are threatened by altered flow due to water resource development. Altered flow can change food webs and impact riverine energetics. The Fitzroy River, in northern Australia, is targeted for development but uncertainty remains about the sources of carbon supporting the food web, particularly in the lowlands—the region most likely to be impacted by water extraction. This study used stable isotopes to investigate if algal biofilm is the main carbon source sustaining fish in lowland habitats. We also sought evidence that large-bodied migratory fish were transporting remote carbon around the system. Our results revealed that local algal biofilm carbon was the dominant source of energy sustaining fish in wet season floodplain habitats, but that fish in main-channel pools during the dry season were increasingly dependent on other carbon sources, such as leaf litter or phytoplankton. We found no evidence that large-bodied fish were transporting remote carbon from the floodplain or estuary into the lower main-channel of the river. We recommend that water planners take a precautionary approach to policy until sufficient food web evidence is amassed.

Estuarine crocodiles in a tropical coastal floodplain obtain nutrition from terrestrial prey

The estuarine crocodile (Crocodylus porosus) is one of the largest and most widespread crocodilians in the world. Although considered an apex species, the role of the estuarine crocodile in aquatic foodwebs is poorly understood; we know what crocodiles ingest, but not what nourishes them. In this study, we used a combination of stable isotope measurements (δ¹³C, δ¹⁵N, and δ³⁴S) and direct feeding observations to identify the source of nutrition of estuarine crocodiles in Kakadu National Park, Northern Australia. Our results show that most crocodiles sampled (size 850 – 4200mm, with 76% of them being > 2.5 m) consume a large variety of prey, however a large proportion of their nutrition is derived from terrestrial prey. Introduced species such as water buffaloes (Bubalus bubalis) and pigs (Sus scrofa) could contribute between 53 and 84% to the nutrition of the sampled crocodiles. The isotopic composition of large crocodiles (total length > 3 m) suggested possible increase in marine prey consumption with size (R² = 0.30; p = 0.005). Additionally, we found crocodiles sampled in the dry season had on average higher terrestrial contributions compared to crocodiles sampled during the wet season (84.1 ± 2.4% versus 55.4 ± 7.0%). Overall, we found that terrestrial prey are important source of nutrition for many crocodiles in this region where introduced herbivorous mammals are abundant.

Functional diversity measures revealed impacts of non-native species and habitat degradation on species-poor freshwater fish assemblages

Trait-based ecology has been developed for decades to infer ecosystem responses to stressors based on the functional structure of communities, yet its value in species-poor systems is largely unknown. Here, we used an extensive dataset in a Spanish region highly prone to non-native fish invasions (15 catchments, N=389 sites) to assess for the first time how species-poor communities respond to large-scale environmental gradients using a taxonomic and functional trait-based approach in riverine fish. We examined total species richness and three functional trait-based indices available when many sites have ≤3 species (specialization, FSpe; originality, FOri and entropy, FEnt). We assessed the responses of these taxonomic and functional indices along gradients of altitude, water pollution, physical habitat degradation and non-native fish biomass. Whilst species richness was relatively sensitive to spatial effects, functional diversity indices were responsive across natural and anthropogenic gradients. All four diversity measures declined with altitude but this decline was modulated by physical habitat degradation (richness, FSpe and FEnt) and the non-native:total fish biomass ratio (FSpe and FOri) in ways that varied between indices. Furthermore, FSpe and FOri were significantly correlated with Total Nitrogen. Non-native fish were a major component of the taxonomic and functional structure of fish communities, raising concerns about potential misdiagnosis between invaded and environmentally-degraded river reaches. Such misdiagnosis was evident in a regional fish index widely used in official monitoring programs. We recommend the application of FSpe and FOri to extensive datasets from monitoring programs in order to generate valuable cross-system information about the impacts of non-native species and habitat degradation, even in species-poor systems. Scoring non-native species apart from habitat degradation in the indices used to determine ecosystem health is essential to develop better management strategies.

Variation in stable-isotope ratios between fin and muscle tissues can alter assessment of resource use in tropical river fishes

Carbon and nitrogen stable-isotope ratios were compared of fin and muscle tissue from 15 fish species collected from seven headwater rivers in eastern and western Thailand. In addition, two-source stable-isotope mixing models were used to derive estimates of each fish's reliance on allochthonous and autochthonous energy based on fin and muscle tissues. Across the dataset, fish fin was enriched in ¹³ C relative to muscle by c. 1·5‰. Variation in δ¹⁵ N between tissues was below statistically significant levels. Estimates of autochthonous resource use calculated from fin tissue were on average 15% greater than those calculated from muscle. Linear mixed-effects models indicated that inter-tissue variation in estimates of resource use was predominantly related to inter-tissue variation in δ¹³ C. Fish fin is a credible and desirable alternative to tissues such as muscle or liver which require destructive sampling of fishes. Care must be taken, however, when estimating resource use or interpreting previous estimates of resource use derived from different tissues.