Lower Trophic Levels Research Articles

Stoichiometric stability of aquatic organisms increases with trophic level under warming and eutrophication

The balance of stoichiometric traits of organisms is crucial for nutrient cycling and energy flow in ecosystems. However, the impacts of different drivers on stoichiometric (carbon, C; nitrogen, N; and phosphorus, P) variations of organisms have not been well addressed. In order to understand how stoichiometric traits vary across trophic levels under different environmental stressors, we performed a mesocosm experiment to explore the impacts of warming (including +3 °C consistent warming above ambient and heat waves ranging from 0 to 6 °C), eutrophication, herbicide and their interactions on stoichiometric traits of organisms at different trophic levels, which was quantified by stable nitrogen isotopes. Results showed that herbicide treatment had no significant impacts on all stochiometric traits, while warming and eutrophication significantly affected the stoichiometric traits of organisms at lower trophic levels. Eutrophication increased nutrient contents and decreased C: nutrient ratios in primary producers, while the response of N:P ratios depended on the taxonomic group. The contribution of temperature treatments to stoichiometric variation was less than that of eutrophication. Heat waves counteracted the impacts of eutrophication, which was different from the effects of continuous warming, indicating that eutrophication impacts on organism stoichiometric traits depended on climate scenarios. Compared to environmental drivers, taxonomic group was the dominant driver that determined the variations of stoichiometric traits. Furthermore, the stoichiometric stability of organisms was strongly positively correlated with their trophic levels. Our results demonstrate that warming and eutrophication might substantially alter the stoichiometric traits of lower trophic levels, thus impairing the nutrient transfer to higher trophic level, which might further change the structure of food webs and functions of the ecosystems.

Carcass appearance does not influence scavenger avoidance of carnivore carrion

The selection or avoidance of certain carrion resources by vertebrate scavengers can alter the flow of nutrients in ecosystems. Evidence suggests higher trophic level carrion is scavenged by fewer vertebrate species and persists longer when compared to lower trophic level carrion, although it is unclear how scavengers distinguish between carcasses of varying species. To investigate carnivore carrion avoidance and explore sensory recognition mechanisms in scavenging species, we investigated scavenger use of intact and altered (i.e., skin, head, and feet removed) coyote—Canis latrans (carnivore) and wild pig—Sus scrofa (omnivore) carcasses experimentally placed at the Savannah River Site, SC, USA. We predicted carnivore carcasses would persist longer due to conspecific and intraguild scavenger avoidance. Further, we hypothesized visually modifying carcasses would not reduce avoidance of carnivore carrion, given scavengers likely depend largely on chemical cues when assessing carrion resources. As expected, mammalian carnivores largely avoided scavenging on coyote carcasses, resulting in carnivore carcasses having longer depletion times than wild pig carcasses at intact and altered trials. Therefore, nutrients derived from carnivore carcasses are not as readily incorporated into higher trophic levels and scavengers largely depend on olfactory cues when assessing benefits and risks associated with varying carrion resources.

Sinking krill carcasses as hotspots of microbial carbon and nitrogen cycling in the Arctic

Krill represent a major link between primary producers and higher trophic levels in polar marine food webs. Potential links to lower trophic levels, such as heterotrophic microorganisms, are less well documented. Here, we studied the kinetics of microbial degradation of sinking carcasses of two dominant krill species Thysanoessa raschii and Meganyctiphanes norvegica from Southwest Greenland. Degradation experiments under oxic conditions showed that 6.0-9.1% of carbon and 6.4-7.1% of nitrogen were lost from the carcasses after one week. Aerobic microbial respiration and the release of dissolved organic carbon were the main pathways of carbon loss from the carcasses. Ammonium release generally contributed the most to carcass nitrogen loss. Oxygen micro profiling revealed anoxic conditions inside krill carcasses/specimens, allowing anaerobic nitrogen cycling through denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates were up to 5.3 and 127.7 nmol N carcass-1 d-1 for T. raschii and M. norvegica, respectively, making krill carcasses hotspots of nitrogen loss in the oxygenated water column of the fjord. Carcass-associated DNRA rates were up to 4-fold higher than denitrification rates, but the combined activity of these two anaerobic respiration processes did not contribute significantly to carbon loss from the carcasses. Living krill specimens did not harbor any significant denitrification and DNRA activity despite having an anoxic gut as revealed by micro profiling. The investigated krill carcasses sink fast (1500-3000 m d-1) and our data show that only a small fraction of the associated carbon is lost during descent. Based on data on krill distribution, our findings are used to discuss the potential importance of sinking krill carcasses for sustaining benthic food webs in the Arctic.

Meltwater Discharge From Marine‐Terminating Glaciers Drives Biogeochemical Conditions in a Greenlandic Fjord

AbstractAn increasing body of work has shown the potential impacts of subglacial discharge from marine‐terminating glaciers on the marine environment around Greenland. Upwelling of nutrients associated with rising buoyant plumes near the front of marine‐terminating glaciers plays a key role in maintaining the high productivity of connected fjords. The response of protist communities to subglacial discharges into fjords nevertheless remains poorly understood. Here we show data of water properties, nutrients, and protist communities during two summers in 2018 and 2019 in a Greenlandic fjord system fed by marine‐terminating glaciers. This study included the period of intense summer melting of the Greenland Ice Sheet in 2019. The data revealed high nutrient concentrations in 2019 that were attributed to intensified upwelling of nutrients and dissolved iron into the subsurface layer. The source of the iron and the nutrients was subglacial discharge and deep fjord water, respectively. Intense glacial discharges in 2019 mitigated nitrate and phosphate limitations of phytoplankton in the fjord and resulted in an increase of chlorophyll a in the subsurface layer and growth of large diatoms. Heterotrophic protists such as dinoflagellates, tintinnids, and nanoflagellates were more abundant in the summer of 2019. We concluded that nutrient upwelling by subglacial discharges was the major driver of the structure of lower trophic levels in fjord ecosystems. We hypothesize that the more intense melting of glaciers and related increase in subglacial discharge will enhance nutrient upwelling, and increased summer productivity in fjords until the glaciers retreat and terminate on land.

An assessment of the aquaculture potential of indigenous freshwater food fish of Fiji, Papua New Guinea, Vanuatu, Solomon Islands, Samoa and Tonga as alternatives to farming of tilapia

AbstractAn important driver behind introductions for aquaculture of alien fish species into Pacific Island Countries and Territories (PICTs) is a lack of knowledge about domestication suitability and specific culture requirements of indigenous taxa. Introductions may be appropriate in some circumstances, but in other circumstances, the associated risks may outweigh the benefits, so greater understanding of indigenous species' aquaculture potential is important. This review summarises literature for indigenous freshwater food fish species from Papua New Guinea, Fiji, Vanuatu, the Solomon Islands, Samoa and Tonga, and evaluates their aquaculture potential for food security and/or small‐scale livelihoods. A species selection criteria incorporating economic, social, biological and environmental spheres was used to score 62 candidate species. Tilapia (Oreochromis mossambicus and O. niloticus) now established in PICTs were evaluated for comparison. Results show that 13 species belonging to the families Mugilidae (Mullets), Terapontidae (Grunters), Kuhliidae (Flagtails) and Scatophagidae (Scats) have the highest culture potential according to selection criteria. These feed at a relatively low trophic level (are herbivores/detritivores), have comparatively fast growth rates and overall possess characteristics most amenable for small‐scale, inland aquaculture. The four top‐ranked candidates are all mountain mullets Cestraeus spp., followed by Nile tilapia (Oreochromis niloticus). Lower ranked candidates include three other mullets (Planiliza melinoptera, P. subviridis and Mugil cephalus) and rock flagtail Kuhlia rupestris. Importantly, many species remain data deficient in aspects of their reproductive biology or culture performance. Species profiles and ranked priority species by country are provided with logistical, technological and environmental assessments of country capacities to culture each species.

Sex and size matter: foraging ecology of offshore harbour porpoises in waters around Greenland

Individuals of different sex or age can vary in their prey and habitat resource use due to differences in behaviour, life history, energetic need, or size. Harbour porpoises are small cetaceans that need to feed constantly to meet their high metabolic demands. In West Greenland, the species has a unique offshore, deep-water ecology. Here, we use bone collagen carbon (δ13C) and nitrogen (δ15N) isotope compositions to elucidate sex and size differences in the foraging ecology of harbour porpoises from this region. Female harbour porpoises are larger than males; we find females have a higher trophic level, and δ15N significantly positively correlates with size for females. This indicates that size may matter in the ability of females to handle larger prey and/or dive deeper to catch higher trophic level prey. The results suggest that females, which also nurse their calves, may be under different ecological constraints than males. We also analysed the harbour porpoise data with available stable isotope data from Greenland populations of belugas and narwhals. We find that harbour porpoises have a lower trophic level than the other species, which is consistent with their smaller body size, and their diet consisting primarily of capelin. Furthermore, harbour porpoises have the largest ecological niche of the three species, in accordance with tagging studies indicating they have a wider range than belugas and narwhals and occur in shelf and deep offshore waters of the sub-arctic and North Atlantic.

Stable isotopes reveal that bottom-up omnivory drives food chain length and trophic position in eutrophic coastal ecosystems

Abstract In coastal marine food webs, food chain length and the distribution of trophic levels mediate the impacts of emerging threats such as overfishing, pollution, and climate change, through their effect on essential properties such as productivity, connectivity, and energy transfer efficiency. These two components of food web structure are therefore an essential element of ecosystem-based management; however, what drives them remains poorly understood. It has been hypothesized that high primary production drives lower trophic levels and shorter food chain length in coastal regions. Here, we evaluate this hypothesis on the British Columbia (BC) coast as this coastal marine ecosystem is comprised of regions with contrasting levels of primary production. We measured nitrogen stable isotopes from zooplankton, micronekton, and nekton collected from four of the main water bodies of southern BC during a survey completed in August of 2019. We used Sentinel-3 satellite data to determine overall production in each region across a 5-year climatology and during the 2019 season. Results showed that primary production varied significantly between the four regions and that increases in phytoplankton biomass were driven by increases in microphytoplankton. Nitrogen isotope data demonstrated a significant inverse relationship between a region’s primary production and both total food chain length and trophic level in species sampled in multiple regions, but no change in the trophic structure of the zooplankton community across regions. Changes in species’ trophic levels were therefore driven by shifts in the level of zooplanktivory. These results support a model of bottom-up omnivory driving coastal food web structure, where levels of primary production, specifically biomass of microphytoplankton, determine food chain length and the distribution of trophic levels. High microphytoplankton biomass supports a large biomass of large grazing zooplankton, which drive increases in omnivorous feeding behaviour among the micronekton and nekton.

Maternal carryover, winter severity, and brown bear abundance relate to elk demographics.

Ungulates are key components of ecosystems due to their effects on lower trophic levels, role as prey, and value for recreational and subsistence harvests. Understanding factors that drive ungulate population dynamics can inform protection of important habitat and successful management of populations. To ascertain correlates of ungulate population dynamics, we evaluated the effects of five non-exclusive hypotheses on ungulate abundance and recruitment: winter severity, spring nutritional limitation (spring bottleneck), summer-autumn maternal condition carryover, predation, and timber harvest. We used weather, reconstructed brown bear (Ursus arctos) abundance, and timber harvest data to estimate support for these hypotheses on early calf recruitment (calves per 100 adult females in July–August) and population counts of Roosevelt elk (Cervus canadensis roosevelti) on Afognak and Raspberry islands, Alaska, USA, 1958–2020. Increasing winter temperatures positively affected elk abundance, supporting the winter severity hypothesis, while a later first fall freeze had a positive effect on elk recruitment, supporting the maternal carry-over hypothesis. Increased brown bear abundance was negatively associated with elk recruitment, supporting the predation hypothesis. Recruitment was unaffected by spring climate conditions or timber harvest. Severe winter weather likely increased elk energy deficits, reducing elk survival and subsequent abundance in the following year. Colder and shorter falls likely reduced late-season forage, resulting in poor maternal condition which limited elk recruitment more than winter severity or late-winter nutritional bottlenecks. Our results additionally demonstrated potential negative effects of brown bears on elk recruitment. The apparent long-term decline in elk recruitment did not result in a decline of abundance, which suggests that less severe winters may increase elk survival and counteract the potential effects of predation on elk abundance.

Beyond the fish-Daphnia paradigm: testing the potential for pygmy backswimmers (Neoplea striola) to cause trophic cascades in subtropical ponds.

Trophic cascades, or indirect effects of predators on non-adjacent lower trophic levels, are a classic phenomenon in ecology, and are thought to be strongest in aquatic ecosystems. Most research on freshwater trophic cascades focused on temperate lakes, where fish are present and where Daphnia frequently dominate the zooplankton community. These studies identified that Daphnia often play a key role in facilitating trophic cascades by linking fish to algae with strong food web interactions. However, Daphnia are rare or absent in most tropical and subtropical lowland freshwaters, and fish are absent from small and temporary water bodies, where invertebrates fill the role of top predator. While invertebrate predators are ubiquitous in freshwater systems, most have received little attention in food web research. Therefore, we aimed to test whether trophic cascades are possible in small warmwater ponds where Daphnia are absent and small invertebrates are the top predators. We collected naturally occurring plankton communities from small fishless water bodies in central Texas and propagated them in replicate pond mesocosms. We removed zooplankton from some mesocosms, left the plankton community intact in others, and added one of two densities of the predaceous insect Neoplea striola to others. Following an incubation period, we then compared biomasses of plankton groups to assess food web effects between the trophic levels, including whether Neoplea caused a trophic cascade by reducing zooplankton. The zooplankton community became dominated by copepods which prefer large phytoplankton and exhibit a fast escape response. Perhaps due to these qualities of the copepods and perhaps due to other reasons such as high turbidity impairing predation, no evidence for food web effects were found other than somewhat weak evidence for zooplankton reducing large phytoplankton. More research is needed to understand the behavior and ecology of Neoplea, but trophic cascades may generally be weak or absent in fishless low latitude lowland water bodies where Daphnia are rare.

Niche expansion by indigenous fish species following the introduction of rainbow trout (Oncorhynchus mykiss) in a subtropical river system, upper Blyde River, South Africa

The introduction of alien predatory fish such as rainbow trout (Oncorhynchus mykiss) can have a significant effect on indigenous fish communities such as altering the structure and dynamics of food webs. Quantifying the trophic niche utilised by the alien fish species is therefore important to aid our understanding of how their feeding strategies might aid establishment, define their functional role and inform on potential impacts. This study assessed food web interactions between fish communities in river reaches that are invaded by O. mykiss and sections that are free of invasions in the upper Blyde River catchment, South Africa. It specifically evaluated the hypothesis that O. mykiss invasion is likely to lead to a decrease in the trophic functional diversity through predation and that indigenous fish will shift their trophic niche to either minimise competition with, or avoid predation, by O. mykiss. Gut content and stable isotope analyses were used to determine trophic interactions. Fish communities in uninvaded areas utilised fewer and similar food sources and occupied lower trophic levels, than fish communities in invaded areas. Oncorhynchus mykiss fed mainly on invertebrates and at trophic levels similar to and or lower than indigenous fishes. This suggests that their current impact on indigenous fish communities is mainly through competition for resources. We posit that O. mykiss invasions reduced the abundance of indigenous fishes and thereby reduced predation pressure on aquatic invertebrates, with a knock-on effect on the trophic interrelationships among fish assemblages. Our findings are consistent with other studies in South Africa that have shown that the impacts of O. mykiss invasion can lead to a decline and fragmentation of indigenous species populations and niche shifts.

The fear diet: Risk, refuge, and biological control by omnivorous weed seed predators

Weed seed biocontrol by omnivorous mice and insects can limit weed seedbanks, but this ecosystem service can be difficult to predict given the broad diet breadth of seed predators and their potential for intraguild predation. Seed foraging behavior is further modified by fluctuating cues of predation risk from higher trophic levels and the availability of refuge habitat. Uncertainty about whether co-occurring insects and mice additively contribute to weed biocontrol or interfere with each other via intraguild predation limits our ability to recommend habitat management strategies that reliably promote seed destruction. Using seed removal assays, fluorescent powder tracking, and stable isotope analyses, we assessed effects of a predation risk cue (moonlight) on mouse foraging patterns in a patchwork of vegetated and exposed plots in a cultivated field. Mouse foraging activity decreased on exposed ground during the full moon, compared to dark nights, yet foraging movements were unaffected by moon cycle within refuge patches. Weed seed consumption was more than three times higher in cover than exposed soil, and 78% of that difference was attributable to invertebrate granivores. Mice and invertebrate granivores both exhibited higher foraging activity in cover, indicating co-occurrence of intraguild predators and prey. However, stable isotope analyses of fecal samples revealed that mice captured in refuge habitats fed at slightly lower trophic levels than those in exposed habitats (suggesting minimal intraguild predation in refuge habitat), and mouse diet was unaffected by moonlight. Despite increased availability of invertebrate prey in cover patches, mice do not appear to preferentially exploit prey when avoiding their own predators or interfere with weed seed predation. Therefore, functional redundancy of mice and invertebrate seed predators in cover crops and other refuge habitats may strengthen and stabilize weed seed biocontrol.

Variations in Fish Community Structure at the Lagoon of Yongshu Reef, South China Sea between 1999 and 2016–2019

Owing to climate change and anthropogenic stressors, the abundance and diversity of reef fishes have globally decreased. However, we know little about the long-term change in reef fishes in the South China Sea (SCS). To reveal the response of reef fishes to these threats in the SCS, based on the fish specimens at Yongshu Reef lagoon collected in 1999 and 2016–2019, this study investigated variations in fish composition and community structure. Additionally, we inferred the changes in the coral cover of Yongshu Reef based on the change of the coral cover of near Meiji Reef in the SCS, sea surface temperature (SST) in Yongshu Reef, and total annual catch of Epinephelinae from China in the SCS. We found that compared with 1999, the number of dominant species in 2016–2019 decreased. The relative dominance of the first dominant species increased from 24.46 in 1999 to 39.44 in 2016–2019. Indices of catch per unit effort, species diversity and richness declined from 1999 to 2016–2019. Community pattern changed with some species with a higher trophic level (3.7–4.5) decreased, while those with lower trophic level (2.2–4.1) increased greatly. Some species with high economic value and important ecological functions decreased or disappeared locally. Correlation analysis indicated that declining coral cover, increasing fishing and rising SST were clearly associated with changes in fish community structure at Yongshu Reef. We speculated that declining coral cover and increasing fishing had a direct impact on the degradation of fish communities at Yongshu Reef lagoon, but increasing SST had an indirect impact on that. To conserve coral reef fish, it is recommended to take rigorous measures for improving habitat and protecting resource.

SIGNS OF STABILITY OF AQUATIC ECOSYSTEMS IN MATHEMATICAL MODELS

To date, various data on water resources have been accumulated, but hydrobiological and hydrochemical indicators remain available to assess the current state of aquatic ecosystems, which can be the basis for assessing the environmental situation within the water body. Systematization of multiyear and diverse data on the lakes and rivers of the country, the use of mathematical tools for assessing and forecasting the state of the aquatic ecosystem is impossible without the use of information and communication technologies. Quality mathematical modeling of aquatic ecosystems and the development of information and analytical system for the study of aquatic ecosystems is an urgent task, including databases of various-quality data on the water body and its ecosystem, data management and processing tools, as well as a set of mathematical models for the functioning of the water body ecosystem. Research is based on information technology, statistical data processing, and mathematical modeling. Mathematical models are based on systems of differential equations, solutions are sought with the help of own computing programs and software suites (Maple, Matlab, Mathematics, etc.). When possible, modeling includes analytical studies of the properties of solutions, primarily this concerns stationary or spatially homogeneous solutions, as well as asymptotic properties of solutions. The lower trophic levels of the water body ecosystem are studied, as this determines the functioning of aquatic ecosystems. The species composition of phytoplankton is an indicator of the ecological state of the water body. Based on the quantitative characteristics of phytoplankton, the bioproductivity of the aquatic ecosystem is calculated. The physical and chemical characteristics of water allow drawing conclusions about the pollution of the water body and the composition of mineral nutrition for phytoplankton.

Stable isotopes reveal dietary shifts associated with social change in Hellenistic, Roman and Late Antique Knossos

Knossos was an important city on Crete and within Mediterranean networks in terms of trade and political status, though its status differed throughout the Hellenistic, Roman and Late Antique periods. This paper uses stable carbon and nitrogen isotope analysis to consider whether people at Knossos had differential diets due to the social, political, cultural, and economic changes across this time frame, factoring in age, sex and social status. Samples of human bone were selected to represent this range of time periods and variables.In this initial study, a small but insignificant increase in δ13C values was observed between the Hellenistic and Roman periods and there was a significant increase in δ15N values for the Late Antique period. No relationship between δ13C or δ15N and age was observed and while the female and male means were similar, the females had wider ranging values. No significant differences were detected by social status as represented by tomb type but there were small sample sizes for several of the tomb types.The results indicated a C3 terrestrial diet with meat or other animal products included for most individuals. The slight increase in δ13C values in the Roman period may represent either the introduction of a small amount of C4 plant or marine food, or very low trophic level marine foods into some Roman diets. The higher δ13C and, in particular, δ15N values observed in the Late Antique samples, suggests an increased consumption of seafood, potentially linked to Christian dietary practices or advances in fishing technologies and preservation techniques. The wider spread values of females compared to males, indicating a more varied diet, could have resulted from differential participation in religious institutions connected to food or may have been caused by greater nutritional stress in females in relation to pregnancy and reproductive issues.This study does not show a pattern of higher animal protein consumption in times of economic and cultural growth and prosperity but differences were detected between the different time periods in connection with the concurrent socio-economic changes.

Top-down response to spatial variation in productivity and bottom-up response to temporal variation in productivity in a long-term study of desert ants.

Under the Ecosystem Exploitation Hypothesis ecosystem productivity predicts trophic complexity, but it is unclear if spatial and temporal drivers of productivity have similar impacts. Long-term studies are necessary to capture temporal impacts on trophic structure in variable ecosystems such as deserts. We sampled ants and measured plant resources in the Simpson Desert, central Australia over a 22-year period, during which rainfall varied 10-fold. We sampled dune swales (higher nutrient) and crests (lower nutrient) to account for spatial variation in productivity. We asked how temporal and spatial variation in productivity affects the abundance of ant trophic guilds. Precipitation increased vegetation cover, with the difference more pronounced on dune crests; seeding and flowering also increased with precipitation. Generalist activity increased over time, irrespective of productivity. Predators were more active in more productive (swale) habitat, i.e. spatial impacts of productivity were greatest at the highest trophic level. By contrast, herbivores (seed harvesters and sugar feeders) increased with long-term rainfall; seed harvesters also increased as seeding increased. Temporal impacts of productivity were therefore greatest for low trophic levels. Whether productivity variation leads to top-down or bottom-up structured ecosystems thus depends on the scale and dimension (spatial or temporal) of productivity.

Effect of fishing effort on the trophic functioning of tropical estuaries in Brazil

A trophic web is a network of complex interactions and energy links between species. These interactions can be simplified into trophodynamic models, such as Ecopath (EP) and EcoTroph (ET), important tools providing the holistic view needed for the ecosystem approach to fisheries. We described food web structure and trophic interactions by developing an EP model for the Santa Cruz Channel (SCC), a large tropical estuarine system in northeastern Brazil, surrounded by mangroves and highly subject to the impacts of domestic pollution, industry, artisanal fisheries, and aquaculture. In addition, considering ecological and fisheries perspectives, we developed ET models in three neighboring Brazilian estuaries (SCC; Sirinhaém – SIR and Mamanguape – MAM) to explore levels of exploitation that affect their trophic functioning. Our EP and ET models consisted of 32 compartments (three primary producers, six invertebrates, 22 fish, and detritus). Keystone Index and Mixed Trophic Impact analysis pointed that several groups of commercial relevance are also ecologically relevant and lack fishing regulations, such as Snooks (Centropomus spp.), Jacks (Caranx spp.) and Barracudas (Sphyraena spp). Fishery impacts across the trophic level spectrum differ between ecosystems, which causes top-down effects depending on the exploitation dynamics of each system. The fishing pressure affects mainly the low and intermediate Trophic Level (TLs) in MAM and SCC and high TLs in the SIR estuary. Consequently, a decrease of biomass for low and high TL was found with the increasing of fishing effort, respectively. These findings are an important contribution to the trophic modelling of tropical estuaries, indicating that both EP and ET approaches can be effective tools to improve the understanding of the trophic functioning and fishery effect on estuarine ecosystems. Additionally, increasing the knowledge of key ecosystem processes in estuarine systems may help to enhance conservation initiatives for sustainable use of the ecosystem, such as protected areas, temporal control of fishing, and the catch size limit.

Sympatric otariids increase trophic segregation in response to warming ocean conditions in Peruvian Humboldt Current System.

Determining trophic habits of predator communities is essential to measure interspecific interactions and response to environmental fluctuations. South American fur seals, Arctocephalus australis (SAFS) and sea lions Otaria byronia (SASL), coexist along the coasts of Peru. Recently, ocean warming events (2014–2017) that can decrease and impoverish prey biomass have occurred in the Peruvian Humboldt Current System. In this context, our aim was to assess the effect of warming events on long-term inter- and intra-specific niche segregation. We collected whisker from SAFS (55 females and 21 males) and SASL (14 females and 22 males) in Punta San Juan, Peru. We used δ13C and δ15N values serially archived in otariid whiskers to construct a monthly time series for 2005–2019. From the same period we used sea level anomaly records to determine shifts in the predominant oceanographic conditions using a change point analysis. Ellipse areas (SIBER) estimated niche width of species-sex groups and their overlap. We detected a shift in the environmental conditions marking two distinct periods (P1: January 2005—October 2013; P2: November 2013—December 2019). Reduction in δ15N in all groups during P2 suggests impoverished baseline values with bottom-up effects, a shift towards consuming lower trophic level prey, or both. Reduced overlap between all groups in P2 lends support of a more redundant assemblage during the colder P1 to a more trophically segregated assemblage during warmer P2. SASL females show the largest variation in response to the warming scenario (P2), reducing both ellipse area and δ15N mean values. Plasticity to adapt to changing environments and feeding on a more available food source without fishing pressure can be more advantageous for female SASL, albeit temporary trophic bottom-up effects. This helps explain larger population size of SASL in Peru, in contrast to the smaller and declining SAFS population.

Effects of forest management on mercury bioaccumulation and biomagnification along the river continuum

Forest management can alter the mobilization of mercury (Hg) into headwater streams and its conversion to methylmercury (MeHg), the form that bioaccumulates in aquatic biota and biomagnifies through food webs. As headwater streams are important sources of organic materials and nutrients to larger systems, this connectivity may also increase MeHg in downstream biota through direct or indirect effects of forestry on water quality or food web structure. In this study, we collected water, seston, food sources (biofilm, leaves, organic matter), five macroinvertebrate taxa and fish (slimy sculpin; Cottus cognata) at 6 sites representing different stream orders (1–5) within three river basins with different total disturbances from forestry (both harvesting and silviculture). Methylmercury levels were highest in water and some food sources from the basin with moderate disturbance (greater clearcutting but less silviculture). Water, leaves, stoneflies and fish increased in MeHg or total Hg along the river continuum in the least disturbed basin, and there were some dissipative effects of forest management on these spatial patterns. Trophic level (δ15N) was a significant predictor of MeHg (and total Hg in fish) within food webs across all 18 sites, and biomagnification slopes were significantly lower in the basin with moderate total disturbance but not different in the other two basins. The elevated MeHg in lower trophic levels but its reduced trophic transfer in the basin with moderate disturbance was likely due to greater inputs of sediments and of dissolved organic carbon that is more humic, as these factors are known to both increase transport of Hg to streams and its uptake in primary producers but to also decrease MeHg bioaccumulation in consumers. Overall, these results suggest that the type of disturbance from forestry affects MeHg bioaccumulation and trophic transfer in stream food webs and some longitudinal patterns along a river continuum.

Copepod community structure in the transition region of the North Pacific Ocean: Water mixing as a key driver of secondary production enhancement in subarctic and subtropical waters

The North Pacific Ocean subtropical–subarctic gyre transition region is recognized as an important small pelagic fish nursery and feeding ground. To clarify the lower trophic level variability of the transition region pelagic ecosystem, we investigated the large-scale variation in copepod communities in relation to the oceanographic environment. We net sampled zooplankton in the upper 150 m from the coast to approximately 4,500 km offshore to the east (140° E − 165° W) during early summer in 2017 and 2018. Cluster analysis based on abundance and species composition at 233 sampling stations showed that the subarctic boundary, defined by a salinity of 34.0 at 150 m depth, separated subarctic and subtropical copepod communities, as a clear biogeographical border. The two communities were each further classified into four subgroups. Among the subgroups, a typical subarctic subgroup dominated by Neocalanus spp. was broadly distributed in the subarctic HNLC (high-nutrient and low-chlorophyll a concentrations) region, whereas a typical subtropical subgroup characterized by a higher overall species diversity occurred in the warm oligotrophic water. In addition, in both these subgroups, the composition of the chlorophyll a size fraction suggested that the food web was mainly based on small-sized phytoplankton. Further analysis of the other subgroups indicated that both the subtropical and subarctic communities showed distinctive succession patterns depending on the degree of water mixing. During succession, a consistent increase in copepod abundance, especially large-sized suspension feeders (e.g., Eucalanus and Calanus) associated with the high productivity of large diatoms was observed in both main communities. The present study revealed that water mixing in the transition region consistently enhances the grazing food chain in both subtropical and subarctic communities, showing the potential underlying mechanisms of control of prey availability for small pelagic fish that exploit the region as a key nursery and feeding ground.

Seasonal assessment of the trophic status in the coastal waters adjoining Tuticorin harbor in relation to water quality and plankton community in the Gulf of Mannar, India

Seasonal variations in hydrography, trophic status, and plankton community structure were studied along the Tuticorin coastal waters (TCWs) in the Gulf of Mannar (GoM). Samples were collected in 2015 and 2016 to analyze physico-chemical (temperature, pH, salinity, suspended particulate matter (SPM), dissolved oxygen (DO), nutrients, etc.) and biological parameters (chlorophyll-a, phytoplankton, and zooplankton). The trophic index (TRIX) and eutrophication index (EI) were calculated to describe the eutrophication status of TCWs. Temperature, pH, salinity, DO, and SPM showed seasonality due to the impact of the northeast monsoon. A massive bloom of Trichodesmium erythraeum was observed, with a visible impact on water quality and the plankton community. The cluster and principal component analysis indicated the bloom event as a distinct phenomenon. ANOVA results showed significant seasonal variations rather than spatial variations. According to the trophic indices, the area had a low trophic level in 2015 and a high trophic level in 2016. The Tuticorin inshore waters had the highest TRIX and EI values during the dominance of cyanobacteria bloom in the post-monsoon 2016. The trophic indices had a positive relationship with phytoplankton abundance but showed an inverse relation with zooplankton abundance. The overall range of the TRIX index (3.18–5.96) indicated that the environment was oligotrophic to eutrophic, and EI values (0.72–21.61) indicated that the state of coastal waters was moderate to poor. This study states that periodic monsoonal flow and frequent algal bloom events have a significant impact on the GoM coastal waters.