Stable isotope evaluation of population‐ and individual‐level diet variability in a large,oligotrophic lake with non‐native lake trout

Non‐native piscivores can alter food web dynamics; therefore, evaluating interspecific relationships is vital for conservation and management of ecosystems with introduced fishes. Priest Lake, Idaho, supports a number of introduced species, including lake trout Salvelinus namaycush, brook trout S. fontinalis and opossum shrimp Mysis diluviana. In this study, we used stable isotopes (δ¹³C and δ¹⁵N) to describe the food web structure of Priest Lake and to test hypotheses about apparent patterns in lake trout growth. We found that isotopic niches of species using pelagic‐origin carbon did not overlap with those using more littoral‐origin carbon. Species using more littoral‐origin carbon, such as brook trout and westslope cutthroat trout Oncorhynchus clarki lewisi, exhibited a high degree of isotopic niche overlap and high intrapopulation variability in resource use. Although we hypothesised that lake trout would experience an ontogenetic diet shift, no such patterns were apparent in isotopic signatures. Lake trout growth rates were not associated with patterns in δ¹⁵N, indicating that variation in adult body composition may not be related to adult diet. Understanding trophic relationships at both the individual and species levels provides a more complete understanding of food webs altered by non‐native species.     

Implications of nutrient enrichment for the conservation and management of seagrass Zostera muelleri meadows

1. Nutrient overenrichment in aquatic environments, or eutrophication, is increasingly affecting seagrass habitats around the world, leading to the degradation of seagrass ecosystems and the loss of associated ecosystem services.
2. The resistance of seagrass to nutrient enrichment is believed to be site specific, yet the direct correlations between environmental factors and the resistance of seagrass meadows has not yet been adequately resolved. The response of seagrass beds to long‐term multiple‐level sediment nutrient enrichment represents a current research gap.
3. This study therefore assessed the in situ long‐term influence of four levels of sediment urea fertilizer addition on Zostera muelleri plants using a manipulative experimental design at three sites with diverse sediment characteristics. Specifically, this study investigated the response of porewater nutrient concentration, seagrass morphology indices, and seagrass plant tissue carbon and nitrogen contents under experimental nutrient stress levels.
4. Over a 13‐month experiment, sediment nutrient addition substantially increased the ammonium concentrations in the sediment porewater, significantly decreased seagrass morphology indices, and caused a carbon and nitrogen imbalance in plant tissues. Ammonium toxicity was more pronounced in seagrass meadows growing on sandy substrates with lower relative above‐ground biomass, which showed low resistance to eutrophication at sediment ammonium concentrations of up to 1 × 10⁴ μM. Plants growing in muddy sand substrates with higher relative above‐ground biomass were more resistant to nutrient impacts, however, with a delayed negative response observed in response to higher nutrient concentrations (up to 1 × 10⁵ μM).
5. Our findings suggest that seagrasses in low‐nutrient sandy environments show more rapid responses to eutrophication, although seagrasses in both sandy and muddy substrates exhibited declines in response to a long‐term elevation of sediment nutrient concentrations. While continuing to assess the relative susceptibility of seagrass habitats across differing environments to inform coastal management, conservation efforts should allocate resources accordingly and prioritize reducing nutrient overenrichment in sensitive seagrass meadows.

     

Declines in the tuber‐feeding waterbird guild at Shengjin Lake National Nature Reserve,China – a barometer of submerged macrophyte collapse

• 1. The Yangtze floodplain is globally unique for its extensive ephemeral wetlands, recharged by summer monsoon precipitation. The annual cycle of inundation and water table recession favours submerged macrophytes, including Vallisneria that overwinters in desiccated substrates as tubers, which provide high‐energy winter food for tuber‐feeding waterbirds that were formerly abundant in the region.
• 2. Large declines among the tuber‐feeding waterbirds swan goose Anser cygnoides (L.), tundra swan Cygnus columbianus (Ord) and hooded crane Grus monacha Temminck between 2004/2005 and 2009/2010 at Shengjin Lake, Anhui Province, suggest that major changes in food availability have occurred there.
• 3. Based on observations of feeding behaviour and energy budgets of these species, it was calculated that at least 5.0 and 8.9 km² of Vallisneria beds in 2004 and 2005, respectively, would be needed to support observed numbers of these species, compared with less than 1.5 km² found in 2009 and 2010.
• 4. An incomplete macrophyte survey in summer 2000 located at least 7.7 km² of Vallisneria beds in the Upper Lake, where none was present during resurveys in 2008 and 2009. Declines in tuber‐feeding waterbirds at Shengjin Lake coincide with the disappearance of their submerged macrophyte food plants, possibly as a result of eutrophication since the mid‐2000s.
• 5. Widespread declines and concentration of tuber‐eating wintering waterbirds at other sites elsewhere in the Yangtze floodplain may also reflect the local collapse of submerged macrophytes and of ecosystem services that these wetlands provide to the human communities. Copyright © 2010 John Wiley & Sons, Ltd.

     

Spatial variation in abundance and diversity of the smaller surface and near‐surface eelgrass‐associated intertidal macrobenthos within a warm‐temperate estuarine bay in the Garden Route National Park,RSA

• 1. Smaller members (<10 mm) of the sediment‐surface macrobenthos of Nanozostera capensis meadows across 9 km² of the marine Outer Basin of the Knysna estuarine bay (Garden Route National Park, Western Cape, South Africa) were investigated at a series of 24 stations.
• 2. Ordination (nMDS) disclosed the existence of five clusters of stations related to degree of exposure. Relatively sheltered stations were dominated by two endemic species of deposit‐feeding microgastropod, and they supported significantly higher macrobenthic densities but lower species diversity and less evenness than relatively exposed stations. The latter were dominated by polychaetes and also possessed more suspension feeders, equivalent to more open seagrass beds in other latitudes. Species richness per station, however, was relatively constant across the whole basin.
• 3. The smaller benthic macrofauna, totalling 82 species, was found to include several animals (<5 mm) hitherto unknown from the region, including one gastropod genus (Cornirostra) not previously known from Africa.
• 4. Species were patchily distributed across all scales from 1 m to >1 km, but variance partitioning showed components of total variance to decrease with increasing spatial scale: sample (46.5%), station (30.0%), site (23.5%).
• 5. These findings are discussed in relation to conservation site selection in rich but faunistically heterogeneous seagrass beds that are impacted by bait collection or other human disturbance. Copyright © 2010 John Wiley & Sons, Ltd.

     

Seasonal effects of clams (Panopea generosa) on eelgrass (Zostera marina) density but not recovery dynamics at an intertidal site

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Distribution,structure and function of Nordic eelgrass (Zostera marina) ecosystems: implications for coastal management and conservation

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Fish assemblages of Red Sea backreef biotopes

• 1. Fish assemblages of shallow‐lagoonal biotopes (seagrass beds, coral patches, reef flat and sand) were examined within the Nabq Managed Resource Protected Area (MRPA), South Sinai, Egypt. This protected area supports a small‐scale artisanal Bedouin fishery, managed by gear restrictions and a network of no‐take zones (NTZs).
• 2. Coral patches showed highest species richness and diversity of fish, followed in order by the reef flat, shallow seagrass beds and sandy bottoms.
• 3. There were clear differences in fish assemblages between the biotopes surveyed, little differences between sampling areas and no significant differences in fish assemblages between no‐take and take zones, suggesting species characteristic of these shallow‐water biotopes are dispersed along the coast irrespective of fishing pressure.
• 4. Nine species (Acanthurus nigrofuscus, Asterropteryx semipunctatus, Cryptocentrus caeruleopunctatus, Cheilio inermis, Thalassoma rueppellii, Lethrinus mahsena, Lethrinus nebulosus, Parupeneus forsskali and Pomacentrus trilineatus) had a 95% correlation to the pattern of assemblage distribution, indicating these species are the most important determinants of the fish community.
• 5. Approximately one‐third of fishes recorded appeared to be juveniles, with seagrass beds having a particularly high proportion of juvenile fish, including several commercial species.
• 6. The structure of the fish assemblage and fish size suggests that shallow‐water biotopes in Nabq MRPA may be acting as nursery areas of commercial fish for the Bedouin fishery.

Copyright © 2006 John Wiley & Sons, Ltd.     

Stable isotope ratios of egg albumen of three waterbird species nesting in the Colorado River Delta indicate differences in foraging ground and isotopic niche breadth

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A novel method combining species distribution models,remote sensing,and field surveys for detecting and mapping subtidal seagrass meadows

1. Seagrasses such as Zostera marina L. play a key role in coastal ecosystems because of the ecological goods and services that they provide, enhancing biodiversity, productivity and carbon sequestration. Despite their ecological relevance, their distribution is, to date, insufficiently documented and it is estimated that only one‐quarter of their global extent is mapped.
2. This study aims to develop a new method to accurately detect and map subtidal seagrass meadows, using Irish seagrass populations as a case study. This method consists of four steps: (i) the development of a species distribution model (SDM); (ii) the use of satellite‐derived images to visually appraise the potential presence and extent of seagrass beds; (iii) field surveys to validate the presence or absence of the seagrass; and finally (iv) the construction of an up‐to‐date detailed map of the seagrass distribution for the region under investigation.
3. Results indicate that along the Irish coast, and in western regions in particular, the actual distribution of seagrass is considerably greater than is currently reported. Using the proposed method, 16 new regions occupied by seagrass in areas of interest in County Galway (Kilkieran Bay, Bertraghboy Bay, and Chasla Bay) were identified, accounting for a total of 267.92 ha, which increased the previously documented distribution in this area by 44.74%.
4. In this study, we demonstrate the potential of this novel method to efficiently identify and map undocumented subtidal seagrass meadows. As seagrass habitats are under threat globally, the development of new mapping strategies is a critical contribution to current international efforts in seagrass monitoring and management.

     

Stable carbon (δ13C) and nitrogen (δ15N) isotopes as natural indicators of live and dry food in Piaractus mesopotamicus (Holmberg, 1887) larval tissue

This study proposed the use of the stable isotope technique to track the type of food utilized by pacu Piaractus mesopotamicus larvae during their development, and to identify the moment when the larvae start using nutrients from the dry diet by retaining its carbon and nitrogen atoms in their body tissues. Five‐day‐old pacu larvae at the onset of exogenous feeding were fed Artemia nauplii or formulated diet exclusively; nauplii+formulated diet during the entire period; or were weaned from nauplii to a dry diet after 3, 6 or 12 days after the first feeding. δ¹³C and δ¹⁵N values for Artemia nauplii were ?15.1‰ and 4.7‰, respectively, and ?25.0‰ and 7.4‰ for the dry diet. The initial isotopic composition of the larval tissue was ?20.2‰ and 9.5‰ for δ¹³C and δ¹⁵N respectively. Later, at the end of a 42‐day feeding period, larvae fed Artemia nauplii alone reached values of ?12.7‰ and 7.0‰ for δ¹³C and δ¹⁵N respectively. Larvae that received the formulated diet alone showed values of ?22.7‰ for δ¹³C and 9.6‰ for δ¹⁵N. The stable isotope technique was precise, and the time at which the larvae utilized Artemia nauplii, and later dry diet as a food source could be clearly defined.     

Application of the coastal ecosystem complex concept toward integrated management for sustainable coastal fisheries under oligotrophication

Harmonizing coastal fisheries with water-quality improvement has become an essential factor for the sustainable use of coastal ecosystem services. Here, we present the scope of our study based on an interdisciplinary approach including ecological actions, socio-economic actions and socio-psychological actions. We chose to focus on the interaction between oyster aquaculture and seagrass vegetation as a typical ecological action using the coastal ecosystem complex (CEC) concept. Coastal organisms have adapted their traits to the environment over a long period of time, so that restoration of the CEC represents reconstruction of the original process of coastal production. Subtidal seagrass vegetation with intertidal oyster reefs is the original CEC in Japan, which would be expected to enhance coastal production by improving the production efficiency without adding nutrients. A simple field experiment examining carbon and nitrogen contents and stable isotope ratios revealed that oyster spats cultivated on a tidal flat adjacent to seagrass beds had higher nitrogen contents and higher δ¹³C ratios than spats cultivated in an offshore area using only pelagic production. This result suggests that utilization of the CEC, which enables oysters to use both pelagic and benthic production, has potential to sustain a food provisioning service for humans, even in oligotrophic conditions.     

Importance of various marine coastal habitats during the life cycle of Spratelloides delicatulus in Con Dao,the oldest MPA in Vietnam

• 1. Marine protected areas (MPAs) are set up to conserve biodiversity, but their design is not always based on strictly scientific considerations. Ideally, an MPA should protect all key habitats necessary for a marine species to complete its life cycle. The identification of these key habitats is complex, especially during the early life of marine fishes.
• 2. A widely distributed tropical and important low trophic‐level fish species, Spratelloides delicatulus (Clupeidae), was used to evaluate the significance of various coastal habitats for its larvae and juveniles in the Con Dao Archipelago MPA in Vietnam. Early stages (larvae and juveniles) were sampled monthly over one year (June 2016 to May 2017) using light traps in three main habitats (seagrass beds, coral reefs and harbour). The species was identified using morphometry and DNA barcoding. Age and growth variables were estimated using otolith daily growth increments.
• 3. A total of 3,581 fish were caught. The species was not found in captures between January and February, directly linked to the decrease in seawater temperature and was most abundant from April to June. For a subsample of 248 fish (7–38 mm standard length), ages ranged from 7 to 108 days.
• 4. Captures and back‐calculated birthdates using otolith daily increments showed that S. delicatulus spawns during the period of high seawater temperature, from March to October. The species colonizes all three habitats during the early stages (0–26 days old), with growth rate lowest on the seagrass beds. Nevertheless, the species occupies seagrass beds exclusively during the older stages.
• 5. The conservation of seagrass beds in the Con Dao archipelago is essential for protection of juvenile stages of this species but this habitat is presently not included in the MPA patches. Establishment of a continuum of protected areas linking habitats, rather than the existing patches is needed to conserve the complete life cycle of this species in the Con Dao MPA.

     

Nitrogen loading alters seagrass ecosystem structure and support of higher trophic levels

• 1. Anthropogenic‐derived nutrient inputs to coastal environments have increased dramatically worldwide in the latter half of the 20th century and are altering coastal ecosystems. We evaluated the effects of nitrogen loading on changes in macrophyte community structure and the associated fauna of a north temperate estuary. We found that a shift in primary producers from eelgrass to macroalgae in response to increased nutrient loading alters habitat physical and chemical structure and food webs. As nitrogen load increased we found increased macroalgal biomass, decreased eelgrass shoot density and biomass, decreased fish and decapod abundance and biomass, and decreased fish diversity.
• 2. The central importance of macroalgae in altering eelgrass ecosystem support of higher trophic levels is evident in the response of the ecosystem when this component was manipulated. Removal of macroalgae increased eelgrass abundance and water column and benthic boundary layer O₂ concentrations. These changes in the physical and chemical structure of the ecosystem with lower macroalgal biomass resulted in higher fish and decapod abundance and biomass.
• 3. Both a ¹⁵N tracer experiment and the growth of fishes indicated that little of the macroalgal production was immediately transferred to secondary consumers. δ¹⁵N values indicated that the most abundant fishes were not using a grazing food web based on macroalgae. Fish tended to grow better and have a greater survivorship in eelgrass compared to macroalgal habitats.
• 4. Watershed‐derived nutrient loading has caused increased macroalgal biomass and degradation and loss of eelgrass habitat, thus reducing the capacity of estuaries to support nekton.

Copyright © 2002 John Wiley & Sons, Ltd.     

Linking pipefishes and seahorses to seagrass meadows in the Venice lagoon: Implications for conservation

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The effectiveness of seahorses and pipefish (Pisces: Syngnathidae) as a flagship group to evaluate the conservation value of estuarine seagrass beds

• 1. Syngnathids (Pisces, Syngnathidae: seahorses and pipefish) were investigated for their use as a flagship group to evaluate the conservation value of estuarine seagrass beds in estuaries in south‐east Australia. Some species of syngnathids are listed internationally as vulnerable or endangered, and they are a charismatic group of fish that attracts a high level of public support and sympathy. Syngnathids are also protected in several states of Australia. Conservation of syngnathids might provide coincidental benefits to other species that share their habitats.
• 2. The effectiveness of syngnathids as a flagship group was assessed by (1) testing for correlations with other fish in species richness, density, assemblage variation, and summed irreplaceability value, and (2) determining the number of species of all other fish coincidentally captured in marine protected areas (MPAs) selected for syngnathids. The study was undertaken in a single estuary (scale: tens of square kilometres) and across multiple estuaries (scale: hundreds of square kilometres).
• 3. Densities of syngnathids and other fish were correlated only at the scale of multiple estuaries. Species richness and summed irreplaceability of syngnathids and other fish were not spatially correlated. Spatial variations in assemblages of syngnathids and other fish were correlated. MPAs selected for syngnathids included more non‐syngnathid species than a random selection of locations.
• 4. This study provides evidence that ranking the conservation value of seagrass beds on the basis of the density and assemblage variation of syngnathids, and selecting MPAs to represent syngnathid species, will simultaneously benefit other fish. Synganthids are therefore regarded as a useful flagship group for conservation planning.

Copyright © 2008 John Wiley & Sons, Ltd.     

Trophic relationships between a native and a nonnative predator in a system of natural lakes

Abstract – Bull trout, a species of char listed as threatened under the US Endangered Species Act, have been displaced from portions of their historic range following the introduction of nonnative lake trout. It has been suggested that competitive exclusion as a result of trophic overlap between bull trout and lake trout may be the causal mechanism associated with displacement of bull trout. This study used stable isotope data to evaluate trophic relationships among native bull trout, nonnative lake trout and other fishes in seven lakes in Glacier National Park (GNP), Montana. Bull trout and lake trout had greater δ¹⁵N values relative to other fishes among lakes (δ¹⁵N ≥ 3.0‰). Lake trout had greater δ¹⁵N values relative to bull trout (δ¹⁵N = +1.0‰). Bull trout had greater δ¹³C values relative to lake trout in six of the seven lakes examined. Although both bull trout and lake trout had greater δ¹⁵N values relative to other fishes within lakes in GNP, differences in δ¹⁵N and δ¹³C between bull trout and lake trout suggest that they are consuming different prey species or similar prey species in different proportions. Therefore, displacement of bull trout as a direct result of complete overlap in food resource use is not anticipated unless diet shifts occur or food resources become limiting. Additionally, future studies should evaluate food habits to identify important prey species and sources of partial dietary overlap between bull trout and lake trout.     

Experimental test of oyster restoration within eelgrass

1. Both seagrasses and oysters are foundation species valued for their wide range of ecosystem services, but their space competition sets a constraint on joint benefits. A reserve for native Olympia oysters (Ostrea lurida) was established in lower Hood Canal (Washington State, USA) more than a century ago but is now devoid of that species and dominated by native eelgrass (Zostera marina). This situation sets up a conservation conflict because management activities for one species are at odds with the protection of another.
2. In experimental enhancement plots, Olympia oysters were outplanted at low density, which successfully maintained eelgrass density and production. One method was used in 2013 (seeded cultch, 8% cover) and two additional methods in 2015 (anchored cultch and single oysters, the latter at 4% cover).
3. For all outplant methods, oysters experienced 99% annual mortality, associated with the attraction of non‐native and native predators. Shell cover remained steady for a year and then declined rapidly, as shell accumulation did not exceed sedimentation rates.
4. Eelgrass per se does not preclude Olympia oysters, given that the two species were observed to co‐occur at a coastal estuarine site (Willapa Bay, Washington). However, even when socio‐political constraints on restoration activities were overcome, ecological constraints remained from predation. Competition between these two protected species was avoided, but it may be the case that top‐down control on oysters was particularly acute owing to low oyster density and/or the environmental conditions of eelgrass beds.

     

Distribution of three algicidal <Emphasis Type="Italic">Alteromonas</Emphasis> sp. strains in seagrass beds and surrounding areas in the Seto Inland Sea,Japan

Harmful algal blooms (HABs) often cause great damage to the fish aquaculture industry in the western part of Japan. Bacteria that effectively kill such phytoplankton are found in coastal seawater and are considered to influence the occurrence of HABs. Algicidal bacteria are found in abundance in seagrass beds; however, the distribution of particular bacterial strains has not yet been clarified. In this study, we determined the abundance of three algicidal Alteromonas sp. strains that were isolated as Chattonella antiqua-killing bacteria in seagrass beds and their surrounding areas in summers of 2013–2015. The strains were detected using a quantitative polymerase chain reaction (qPCR) amplification method. Two of the three algicidal bacterial strains were remarkably abundant in the seawater of an enclosed seagrass bed compared to the surrounding areas. In addition, the abundance of the three algicidal bacteria decreased when the eelgrass withered in late summer. These results suggest that growth of these algicidal bacteria was stimulated by eelgrass. The bacterial abundance estimated by qPCR was much greater than that determined by the culture-dependent method in June 2015 when the eelgrass grew thickly. This implies that the qPCR assay could be a sensitive tool to evaluate algicidal bacteria in natural environments.     

Differences in stable isotope ratios of Dall’s porpoises (Phocoenoides dalli) between coastal and oceanic areas of the North Pacific

Carbon and nitrogen stable isotope ratios in the muscle of Dall’s porpoises were measured. Samples were collected from the catches of the hand harpoon fishery, incidental catches of drift net, and scientific research on the use of drift nets. Samples were from the North Pacific, Sea of Japan, Sea of Okhotsk, and Bering Sea. Although no variation in δ¹⁵N was observed, δ¹³C was significantly different between population groups near Japan and the oceanic North Pacific and Bering Sea. The difference may be due not only to local variation in prey species, but also to an overall difference in carbon stable isotope ratios that originate from coastal benthic or oceanic pelagic‐based food webs. We differentiated Dall’s porpoise population groups from both areas using carbon stable isotope ratios with an error rate of <5%. Although further study is needed, our results suggest that carbon stable isotope ratios could possibly be an indicator of whether a Dall’s porpoise belongs to a coastal benthic or oceanic pelagic food web.     

Trophic dynamics in two South American estuaries encompassing industrial development and a biodiversity hotspot

1. Estuaries are critical to the life histories and development of many marine species, and support some of the most productive aquatic ecosystems. However, owing to their geographic accessibility, many estuaries face considerable anthropogenic threats, which can be exacerbated in developing nations.
2. Understanding the ecological implications of anthropogenic activities as a precursor to dedicated conservation activities requires comprehensive assessments of baseline trophic relationships. This study addresses a shortfall in such information for two tropical, coastal ecosystems globally identified as priority areas for biodiversity conservation: the Paranaguá estuarine complex; and the Cananéia–Iguape estuarine lagoon system in southern Brazil.
3. Ecological relationships were investigated through carbon and nitrogen stable isotope (δ¹³C and δ¹⁵N) determination in sediments, plant fragments and the muscle tissues of marine species occupying different trophic levels, including benthic invertebrates, ichthyofauna (Stellifer rastrifer, Paralonchurus brasiliensis, and Isopisthus parvipinnis), and delphinids (Sotalia guianensis and Pontoporia blainvillei).
4. Isotope ratios facilitated distinguishing plant fragments, sediments (which were terrestrially influenced) and biota (predominantly marine influenced). However, despite variation among anthropogenic stressors, isotope ratios did not vary between the studied estuaries, implying similar impacts on trophic relationships which might reflect some inter‐estuary mixing.
5. All ichthyofauna and the delphinids had similar generalist diets and shared the same trophic niche, which may reflect their consistent spatio‐temporal distributions and habitat patterns in the estuaries. The data illustrate the utility of isotopically characterizing biodiversity‐hotspot estuaries as a mechanism for rapidly assessing ecological relationships between key species and regional habitats. Such an approach can be used to evaluate ecological connectivity as a baseline for management and might be applied more broadly, embracing multiple regions as part of estuarine and coastal biome conservation efforts.