Examining mummichog growth and movement: Are some individuals making intra-season migrations to optimize growth?

We used a combination of field experiments and stable isotopes to examine mummichog growth and movement within a New England estuary. We documented physical and biological patterns within the estuary by caging individually-marked fish in enclosures at four locations along a coastal river and measuring environmental parameters (e.g., salinity, tidal inundation) and fish characteristics (e.g., gut-contents, growth, and stable isotope values) at each location. The upstream location was fresh (1 ppt) at low tide, and the downstream location was saline at high tide (32 ppt). The upstream and downstream locations had more tidal inundation than the intermediate location. Fish gut contents were dominated by terrestrial insects at the upstream location, by algae and detritus at the intermediate locations, and by aquatic insects at the downstream location. Fish grew fastest at the upstream location and slowest at the downstream location. Stable isotope values (δ¹³C and δ¹⁵N) of fish held in cages were significantly different at upstream, intermediate, and downstream locations. We transferred fish from one location to another in order to document how stable isotope values change when fish switch diets by moving within this estuary. Because differences in rates at which different tissue types approach the isotopic value of new diet sources can be used as a way to estimate the time since diet shift, we used the δ¹³C and δ¹⁵N values of liver and muscle as indicators of short term previous diet (liver) and longer term previous diet (muscle). We collected wild (uncaged) mummichogs from each location, and we compared their liver and muscle isotope values to values of fish that were transferred among locations. When fish were transferred from one location to another, their stable isotope values were intermediate between expected values at the previous and current locations. The liver approached stable isotope values representative of current location faster than muscle. Wild fish showed greater variation in stable isotope values than fish held in cages. Wild fish from the upstream location showed patterns in liver and muscle stable isotope values that were consistent with patterns in fish that were transferred from the downstream location to the upstream location (∼ 10 km away). These patterns in stable isotope values could have multiple causes including intra-season movement between downstream and upstream locations.     

Movement of carbon among estuarine habitats and its assimilation by invertebrates

We measured the extent of movement of carbon and its assimilation by invertebrates among estuarine habitats by analysing carbon stable isotopes of invertebrates collected along transects crossing the boundary of two habitats. The habitats were dominated by autotrophs with distinct isotope values: (1) mudflats containing benthic microalgae (mean −22.6, SE 0.6‰) and (2) seagrass and its associated epiphytic algae (similar values, pooled mean −9.8, 0.5‰). Three species of invertebrates were analysed: a palaemonid shrimp, Macrobrachium intermedium, and two polychaete worms, Nephtys australiensis and Australonereis ehlersi. All species had a similar narrow range of isotope values (−9 to −14‰), and showed no statistically significant relationship between position along transect and isotope values. Animals were relying on carbon from seagrass meadows whether they were in seagrass or on mudflats hundreds of metres away. Particulate organic matter collected from superficial sediments along the transects had similar values to animals (mean −11.1, SE 1.3‰) and also showed no significant relationship with position. The isotope values of these relatively immobile invertebrates and the particulate detritus suggest that carbon moves from subtidal seagrass meadows to mudflats as particulate matter and is assimilated by invertebrates. This assimilation might be direct in the case of the detritivorous worm, A. ehlersi, but must be via invertebrate prey in the case of the carnivorous worm, N. australiensis and the scavenging shrimp, M. intermedium. The extent of movement of carbon among habitats, especially towards shallower habitats, is surprising since in theory, carbon is more likely to move offshore in situations such as the current study where habitats are in relatively open, unprotected waters.     

The primary carbon sources utilised by fishes in the Mngazi and Mngazana estuaries,South Africa: a preliminary assessment

A number of studies have used stable carbon isotopes to analyse aspects of the food web structure in South African estuaries but none has compared mangrove and non-mangrove estuarine food webs. This study compares the primary carbon sources utilised by the ichthyofauna in the Mngazana (with extensive mangroves) and Mngazi (without mangroves) estuaries. In contrast to the Kariega Estuary, where past research identified two basic carbon pathways, there were no clear carbon pathways within the Mngazi and Mngazana fish assemblages. Instead, the carbon isotopic values of fishes in both estuaries displayed a continuum rather than a tight clustering around particular energy sources. Most detritus feeders of the family Mugilidae from both estuaries were relatively more enriched (with carbon isotopic values ranging from ?16.9% to ?12.3%) than other fish taxa. The isotopic values of the mullet species suggest a diet derived from relatively enriched carbon sources such as benthic microalgae, the eelgrass Zostera and associated epiphytes. Based on the isotopic values, piscivorous fishes from both estuaries could not be linked to specific prey fish taxa, but clearly the mullet species were not their main food source. The invertebrate feeders that were common to both estuaries showed greater isotopic variations in the Mngazana Estuary (?24.7% to ?19.3%) than in the Mngazi Estuary (?21.9% to ?18.4%), probably reflecting the higher diversity of habitats and invertebrate prey items in the Mngazana system. Generally, the isotopic signatures of fishes from the Mngazi Estuary were more enriched than those from the Mngazana Estuary, thus indicating the possible effect of δ¹³C-depleted mangrove-derived carbon in the latter system.     

Assessing temporal and spatial trends in estuarine nutrient dynamics using a multi-species stable isotope approach

Coastal urbanisation can alter estuarine nutrient dynamics through the input of point-source and diffuse pollutants, and nutrient concentrations can be highly influenced by seasonal and episodic rainfall and river flow. Understanding of both the spatial and temporal variability of nutrient dynamics is therefore critical to managing these estuaries. This can be achieved by periodically analysing the stable isotopes a range of aquatic taxa with variable nutrient turnover rates, mobility and distribution within the estuary. In two subtropical urban estuaries with different land use patterns, we analysed the carbon and nitrogen stable isotopes of phytoplankton, shrimp, prawns and fish at various proximities to pollution sources in dry and wet seasons. The fast nutrient turnover rates and ubiquity of phytoplankton in the estuary resulted in stable isotopes varying over fine-scale spatial scales, particularly in relation to proximity to point-source pollution. The slower nutrient turnover rates and localised habitat use of prawns, resulted in stable isotopes varying over larger spatial (between pollution sources) and temporal (seasonal) scales. The much slower nutrient turnover rates and high mobility throughout the estuary of fish resulted in stable isotopes varying over very large-scale spatial scales (between estuaries). These results illustrate a wide range of spatial and temporal changes to estuarine nutrient dynamics in subtropical urban estuaries in relation to rainfall conditions and nutrient inputs. This research also highlights the application of stable isotopes in assessing estuarine trophodynamics, and provides direction on the types of organisms that should be used to assess different spatial and temporal trends.     

Analysis of stable isotope ratios to investigate stock structure of red emperor and Rankin cod in northern Western Australia

The ratios of stable isotopes ¹⁸O/¹⁶O and ¹³C/¹²C, in sagittal otolith carbonate from two tropical demersal teleosts, red emperor Lutjanus sebae and Rankin cod Epinephelis multinotatus , from several locations in northern Western Australia, differed between sites. On a broad scale, fish from the four locations, Shark Bay, Ningaloo, Pilbara, and Broome had stable isotope values that were sufficiently different to indicate separate stocks, and it is appropriate to manage these populations of the two species independently in these areas. On a smaller scale, there may be limited mixing of these species between the Pilbara trawl fishery and the trap and line fisheries operating out of Onslow and Broome. Values of stable oxygen isotopes were strongly related to sea surface temperature, although there were some sites in shallow water where low values of stable oxygen isotopes indicated that fish were living in warm water. The use of stable oxygen and carbon isotope values is a valuable, cost effective method of determining the degree of mixing of fish stocks.     

Differences in nitrogen and carbon stable isotopes between planktonic and benthic microalgae

We compiled published data on the nitrogen and carbon stable isotope ratios of phytoplankton and benthic microalgae from lentic systems and explored the primary factors determining the isotope values among systems. Also, we investigated seasonal changes in nitrogen stable isotope ratios of phytoplankton and benthic microalgae in the strongly acidic lake, Lake Katanuma, which has only two dominant species, Pinnularia acidojaponica as a benthic diatom and Chlamydomonas acidophila, a planktonic green alga. From the published dataset, it may be concluded that δ¹³C of benthic diatoms were more enriched than those of phytoplankton at the same sites, although the nitrogen isotope of phytoplankton and benthic microalgae were similar. This differences in δ¹³C between benthic microalgae and phytoplankton could be explained by the boundary layer effect. On the other hand, nitrogen isotope values of both benthic microalgae and phytoplankton were primarily controlled by the same environmental factor, and boundary layer effects are not the primary factor determining the nitrogen isotope values of microalgae. Also, we showed temporal dynamics in nitrogen isotopes of benthic and planktonic microalgae species in Lake Katanuma, and the trends of nitrogen isotopes are similar between benthic and planktonic microalgae, as concluded from the published dataset.     

Variability in the carbon isotope signature of Prochilodus lineatus (Prochilodontidae, Characiformes) a bottom-feeding fish of the Neotropical region

The spatial and seasonal variability of stable carbon isotopes in detritivorous Prochilodus lineatus , primary producers and particulate organic carbon (POC) were examined to promote a better understanding of the trophic dynamics of food webs in Neotropical ecosystems. Spatial variability in carbon isotope composition in the species showed a decreasing gradient in the Upper Paraná River system, from the Paraná River and Itaipu Reservoir subsystems to the Baía and Ivinheima subsystems. Fish δ¹³C followed the isotopic trend of POC along the Paraná River and Baía subsystem. Seasonal differences were not observed in fish. The analysis of percentile contribution of C₃ and C₄ sources in the species diet revealed the predominance of C₃ plants (phytoplanktonic and periphytic algae and riparian vegetation) in most of the analysed subsystems and seasons. Nevertheless, the considerable participation of riparian vegetation was verified, especially in the Ivinheima subsystem in the dry season. C₄ macrophytes presented a maximum contribution along the Upper Paraná River. These results demonstrate the existence of specific variability for the consumer and their food sources in different environments of the same ecosystem.     

Stable isotope analysis indicates microalgae as the predominant food source of fauna in a coastal forest stream,south‐east Brazil

Abstract: Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivore‐detritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well‐preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ¹³C values close to ?30‰, microalgae were ?20‰ and macroalgae were ?25‰. All fauna had δ¹³C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ¹³C signatures. These results support the growing view that tropical stream food chains are primarily algal based.     

Spatial variability of trace elements in fish otoliths: comparison with dietary items and habitat constituents in seagrass meadows

Juvenile trumpeters Pelates sexlineatus , two main prey items (amphipods and polychaetes) and seagrass constituents (detritus, young and old leaves of Zostera capricorni ) were collected from two sites within six estuaries, to compare the spatial variability of elemental composition (Li, Mn, Sr, Cd, Ba and Pb). Average elemental values were lower in the fish otoliths except for Sr. Significant differences in elemental composition of otoliths were detected among estuaries (Mn and Pb). Spatial differences in concentrations of Cd were found in amphipods and polychaetes; the latter also showed differences for Ba and Pb. Detritus and Z. capricorni leaves showed appreciable spatial differences for most of the elements. Spatial variability between sites was generally high. There were differences in the multivariate composition of trace elements found in the otoliths of fish among estuaries. One estuary was consistently separated from the other estuaries in the ordinations. Significant correlations were detected between concentrations of Mn in otoliths and concentrations in prey, Z. capricorni and detritus. A significant relationship was also detected between Pb in otoliths and detritus. There were significant correlations between concentrations of Mn, Sr and Ba in detritus and that in otoliths. The study demonstrated that trophic transfer and habitat constituents may be considered as a potential source for some of the elemental accumulation in fish otoliths.     

Stable-isotope analyses reveal the importance of seagrass beds as feeding areas for juveniles of the speckled worm eel Myrophis punctatus (Teleostei: Ophichthidae) in Florida

The feeding habits and habitats of the speckled worm eel Myrophis punctatus were studied on the mangrove edge of the Indian River Lagoon (IRL, Florida) using gut-content and stable-isotope analyses of carbon (δ(13) C) and nitrogen (δ(15) N). Four taxa were identified through analyses of gut contents, and the index of relative importance suggested that amphipods, microphytobenthos and annelids are the most important food sources in the fish's diet. To assess the feeding habits of the fish after their recruitment to the IRL, these food sources were collected from mangroves and nearby seagrass beds for isotope analyses. Stable isotopes constituted a powerful tool for discriminating fish prey items from mangroves (mean ± s.d.δ(13) C = -20·5 ± 0·6‰) and those from seagrass beds (mean ± s.d.δ(13) C = -16·9 ± 0·6‰), thus providing good evidence of food source origins. The 56 M. punctatus collected [10·0 < total length (L(T) ) < 16·2 cm] had average isotopic signatures of δ(13) C = -16·7 ± 0·2‰ and δ(15) N = 8·2 ± 0·1‰. A significant depletion in (13) C was observed for larger juveniles (15·0 < L(T) < 16·2 cm), suggesting that they found a portion of their food in mangroves. Estimation of the trophic level from stable isotopes (T(Liso)) was similar among different size groups of juvenile fish (T(Liso) = 3·2-3·5); therefore, M. punctatus was considered a secondary consumer, which is consistent with its zoobenthic diet. The concentration-dependent mixing Stable Isotope Analysis in R (SIAR) model revealed the importance of food sources from seagrass beds as carbon sources for all the fish collected, with a significant increase in mangrove prey contributions, such as annelids, in the diet of larger juveniles. This study highlights the importance of seagrass beds as feeding habitats for juveniles of M. punctatus after their recruitment to coastal waters.     

Stable isotope records from otoliths as tracers of fish migration in a mangrove system

The ratios of stable isotopes ¹⁸O:¹⁶O and ¹³C:¹²C were measured in otolith carbon taken from nine species of fishes caught within mangroves and on the reef at Gazi Bay, Kenya. Before analysis, otoliths were divided into 'larval'post-larval' and 'adult' sections using a drill. Fishes were putatively classified as 'mangrove residents'offshore residents' or 'migrants' on the basis of information from the literature, and depending on where they were caught (mangroves only, offshore only or both mangroves and offshore) in the present study. Eight of the species exhibited an increase in otolith ¹³C:¹²C with age, but this was significant only in the two migrant species Lethrinus harak and Lutjanus fulviflammus . There were no consistent patterns in ¹⁸O:¹⁶O with age, or between migrants and non-migrants. These results suggest that comparing absolute values of otolith oxygen and carbon isotope signatures between fish species is not a useful way of determining migration patterns at this site, because of species-specific differences in carbon metabolism and insufficiently steep gradients in temperature and salinity. Changes in carbon isotope signatures between life stages within a species, however, do hold promise as migration tracers.     

Stock structure of Grey Mackerel, Scomberomorus semifasciatus (Pisces: Scombridae) across northern Australia, based on otolith stable isotope chemistry

The stable isotopes of δ¹⁸O and δ¹³C in sagittal otolith carbonates were used to determine the stock structure of Grey Mackerel, Scomberomorus semifasciatus. Otoliths were collected from Grey Mackerel at ten locations representing much of their distributional and fisheries range across northern Australia from 2005 to 2007. Across this broad range (～ 6500 km), fish from four broad locations—Western Australia (S1), Northern Territory and Gulf of Carpentaria (S2, S3, S4, S5, S6, S7), Queensland east coast mid and north sites (S8, S9) and Queensland east coast south site (S10)—had stable isotope values that were significantly different indicating stock separation. Otolith stable isotopes differed more between locations than among years within a location, indicating temporal stability across years. The spatial separation of these populations indicates a complex stock structure across northern Australia. Stocks of S. semifasciatus appear to be associated with large coastal embayments. These results indicate that optimal fisheries management may require a review of the current spatial arrangements, particularly in relation to the evidence of shared stocks in the Gulf of Carpentaria. Furthermore, as the population of S. semifasciatus in Western Australia exhibited high spatial separation from those at all the other locations examined, further research activities should focus on investigating additional locations within Western Australia for an enhanced determination of stock delineation.     

Spatial Patterns of Benthic Macroinvertebrates in Intertidal Areas of a Southern European Estuary: The Tagus, Portugal

This study characterizes the composition and spatial distribution patterns of the benthic macrofauna in the intertidal mudflats of the Tagus estuary, western Portugal. A total of 68 species, more than 226,000 specimens with a total wet weight biomass of approximately 1170 g were identified in 380 sites. The species Streblospio shrubsolii, Cyathura carinata, Tharyx sp., Hydrobia ulvae and Tubificids were the most common and abundant. Scrobicularia plana strongly dominated the biomass. The invertebrate macrofauna of the Tagus estuary shows similarities to what is known from other temperate mudflats. The diversity of species, their overall abundance and the ratio of Molluscs plus Crustaceans to Polychaete species corroborate the distinctiveness between temperate and tropical mudflats and sandflats. The spatial distribution of the fauna reflects the sediment characteristics but the relationship between the environmental and the biological data is not as strong as obtained for sublittoral areas. This relationship diminishes from the sublittoral shelf to sublittoral estuarine areas, showing minimum values in this study, suggesting that such a relationship is less straightforward as natural disturbance increases. Nevertheless, a mixture of grain-size, elevation (inundation time) and particular habitats (relic oyster beds) form the best explanatory factors for the spatial distribution patterns of the intertidal benthic macrofauna of the Tagus estuary.     

Intralocality Variation in Feeding Biomechanics and Prey Use in Archosargus probatocephalus (Teleostei,Sparidae), with Implications for the Ecomorphology of Fishes

Archosargus probatocephalusin a Florida estuary was investigated to explore intraspecific variation in prey utilization and jaw biomechanics. Volumetric contribution of major prey types and seven biomechanical features of the oral jaws that characterize prey-capture and processing performance were contrasted between two locations within the estuary. At Mosquito Lagoon, where A. probatocephalusinhabited mostly oyster beds, mangroves and salt marshes, fish consumed mostly thick-shelled bivalves, gastropods, crabs, and tubiculous polychaetes and amphipods. In contrast, conspecifics at Indian River Lagoon that inhabited mostly seagrass beds and algal turf consumed predominantly algae, seagrass, epiphytic invertebrates and small bivalves and gastropods. Difference in magnitude of durophagy between locations was associated with differences in oral-jaw biomechanics. Analyses of covariance indicated that A. probatocephalusat Mosquito Lagoon had more massive jaw muscles and bones, than conspecifics at Indian River Lagoon. Variations in lever ratios for jaw-opening and jaw-closing between locations were not significant. It is hypothesized that intralocality differences in food habits have induced the development of feeding morphologies that enhance the ability of A. probatocephalusto successfully exploit locally dominant prey resources within the estuary. Plasticity of the feeding mechanism of A. probatocephalusmay buffer the species from the adverse effects of settling on heterogeneous habitats that contain variable prey resources such as those found within estuaries.     

Otolith oxygen and carbon stable isotopes in wild and laboratory-reared plaice (Pleuronectes platessa)

The relationship between water temperature, growth rate, and otolith isotopic ratios was measured for juvenile plaice (Pleuronectes platessa) reared at two temperatures (11 and 17°C) and two feeding regimes (1 and 3 prey items·ml^?1). The otolith isotope ratios in individual fish ranged from ?2 to ?4 for carbon isotope ratios (δ¹³C) and from 0.2 to 1.9 for oxygen isotope ratios (δ¹⁸O). The otolith oxygen isotope ratios were significantly affected by water temperature, but not by feeding level, and there were no significant synergistic effects. The fractionation of oxygen isotopes during otolith growth was independent of individual growth rate. Carbon isotope ratios were not significantly affected by food ration or water temperature, but were related to fish growth rate. The carbon isotope ratios were negatively correlated with fish length in the colder water treatments, and tended to increase with fish length in the warm water treatments. The laboratory-determined relationship between otolith oxygen isotope ratio and water temperature was applied to individuals of five species (plaice, cod, whiting, haddock, gurnard) collected in a single trawl sample. The otolith derived temperatures often overestimated measured water temperatures. The difference between real and estimated water temperatures varied between species, and the closest fit was for field-caught plaice.     

Contribution of water chemistry and fish condition to otolith chemistry: comparisons across salinity environments

This study quantified the per cent contribution of water chemistry to otolith chemistry using enriched stable isotopes of strontium (⁸⁶Sr) and barium (¹³⁷Ba). Euryhaline barramundi Lates calcarifer, were reared in marine (salinity 40), estuarine (salinity 20) and freshwater (salinity 0) under different temperature treatments. To calculate the contribution of water to Sr and Ba in otoliths, enriched isotopes in the tank water and otoliths were quantified and fitted to isotope mixing models. Fulton's K and RNA:DNA were also measured to explore the influence of fish condition on sources of element uptake. Water was the predominant source of otolith Sr (between 65 and 99%) and Ba (between 64 and 89%) in all treatments, but contributions varied with temperature (for Ba), or interactively with temperature and salinity (for Sr). Fish condition indices were affected independently by the experimental rearing conditions, as RNA:DNA differed significantly among salinity treatments and Fulton's K was significantly different between temperature treatments. Regression analyses did not detect relations between fish condition and per cent contribution values. General linear models indicated that contributions from water chemistry to otolith chemistry were primarily influenced by temperature and secondly by fish condition, with a relatively minor influence of salinity. These results further the understanding of factors that affect otolith element uptake, highlighting the necessity to consider the influence of environment and fish condition when interpreting otolith element data to reconstruct the environmental histories of fish.     

Source water partitioning as a means of characterizing hydrologic function in mangroves

Mangrove ecosystems rely on seawater, rain-derived flow, and groundwater for hydrologic sustenance, flushing, and inflow of nutrients and sediments. The relative contribution of these source waters and their variability through time and space can provide key information concerning the hydrologic function of ecosystems. We used hydrologic tracers to partition source waters and trace their movements in the Enipoas stream, a river-dominated mangrove ecosystem on the island of Pohnpei, Federated States of Micronesia (FSM) and in the Yela watershed, an interior mangrove ecosystem on the island of Kosrae, FSM. The Enipoas site was characterized as a salt wedge estuary whose source water contributions alternated between predominantly seawater and rain-derived flow, depending on the tide. The source waters in the interior Yela site were also predominantly seawater and rain-derived flow, however the relative contribution of each was much more stable. The mean groundwatercontribution was 5% (SD 5 5.5) for the Enipoas site and 20% (SD 5 11.0) for the Yela site. Although a small contributor to flow, groundwater was a steady source of freshwater for both systems. Hydrologic linkages between mangroves and adjacent ecosystems were demonstrated by the temporal and spatial distribution of source waters.The 0.8 km Enipoas estuary, with its highly dynamic bi-directional flows, transported source waters along a hydrologic continuum comprised of coral reef, mangroves, and palm forest. In the interior mangroves of the Yela watershed, the presence of rain-derived flow and groundwater demonstrated a hydraulic connection between the mangroves and an upstream freshwater swamp. Interior mangroves with such linkages avoid stresses such as desiccation and heightened salinity, and thus are more productive than those with little or no freshwater flows.     

Seascape-scale trophic links for fish on inshore coral reefs

It is increasingly accepted that coastal habitats such as inshore coral reefs do not function in isolation but rather as part of a larger habitat network. In the Caribbean, trophic subsidies from habitats adjacent to coral reefs support the diet of reef fishes, but it is not known whether similar trophic links occur on reefs in the Indo-Pacific. Here, we test whether reef fishes in inshore coral, mangrove, and seagrass habitats are supported by trophic links. We used carbon stable isotopes and mathematical mixing models to determine the minimum proportion of resources from mangrove or seagrass habitats in the diet of five fish species from coral reefs at varying distances (0–2,200 m) from these habitats in Moreton Bay, Queensland, eastern Australia. Of the fish species that are more abundant on reefs near to mangroves, Lutjanus russelli and Acanthopagrus australis showed no minimum use of diet sources from mangrove habitat. Siganus fuscescens utilized a minimum of 25–44 % mangrove sources and this contribution increased with the proximity of reefs to mangroves (R ² = 0.91). Seagrass or reef flat sources contributed a minimum of 14–78 % to the diet of Diagramma labiosum, a species found in higher abundance on reefs near seagrass beds, but variation in diet among reefs was unrelated to seascape structure. Seagrass or reef flat sources also contributed a minimum of 8–55 % to a fish species found only on reefs (Pseudolabrus guentheri), indicating that detrital subsidies from these habitats may subsidize fish diet on reefs. These results suggest that carbon sources from multiple habitats contribute to the functioning of inshore coral reef ecosystems and that trophic connectivity between reefs and mangroves may enhance production of a functionally important herbivore.     

Use of stable isotope ratios of fish larvae as indicators to assess diets and patterns of anthropogenic nitrogen pollution in estuarine ecosystems

A stable isotope study was carried out to investigate the feeding ecology of the common goby Pomatoschistus microps larvae (Krøyer, 1838), and to assess differences in the response of planktonic food web to nutrient enrichment, in two ecosystems from the Southern European coast with different levels of historical pollution (estuaries of the Minho and Lima Rivers). At each estuary and time (July 2012, November 2012, February 2013, and June 2013), the fish larvae of two size classes (class 0: 0–10 mm; class 1: 10–15 mm), particulate organic matter (POM), and pelagic zooplankton were collected. The stable isotope mixing model SIAR revealed that, despite temporal differences in the relative proportion of prey items ingested, in both estuaries P. microps larvae feed on both planktonic-hyperbenthic food sources, predating mainly on copepods (from 34% to 60%), Mysidacea (from 16% to 28%), and brachyuran zoea (from 14% to 29%). Fish larvae size classes did not differ significantly for δ¹⁵N, and exhibited a very narrow range of the δ¹³C signature. Enriched δ¹⁵N values of all biota in the Lima estuary throughout the study period, with a marked nitrogen enrichment in colder months, are indicative of higher anthropogenic inputs of nitrogen (e.g. sewage and industrial discharges, agriculture) into this system. The δ¹⁵N values of fish larvae and other planktonic groups can be a sensitive bioindicator, because they are highly correlated with the nitrogen content of water (ammonium), indicating that this element has transferred through the planktonic food web. Enriched carbon isotope ratios were observed in warmer months, in both estuaries, and the heavier δ¹³C values in Lima are best explained by differences in the degree of marine influence. This research emphasises the utility of stable isotopes in trophic interactions studies, highlighting the relevance of the stable nitrogen isotope of zooplanktonic communities as a reliable bioindicator to detect patterns of anthropogenic nitrogen contamination in estuarine ecosystems.     

Within-lake variability in carbon and nitrogen stable isotope signatures

1. We assessed spatial and temporal variation in carbon and nitrogen isotopic signatures in different compartments of a single lake ecosystem. Stable isotope analyses were made on samples of particulate organic matter (POM), zooplankton, periphyton, macrophytes, macroinvertebrates and fish collected from several locations throughout the ice‐free period. 2. No spatial variation in δ¹³C or δ¹⁵N values was found for pelagic samples of POM and zooplankton. However, pelagic δ¹⁵N signatures increased steadily through the summer resulting in an almost 6‰ average increase in POM and zooplankton. A concurrent decrease in epilimnetic nitrate concentrations suggested that the increase in δ¹⁵N of POM and zooplankton could have resulted from a progressive ¹⁵N‐enrichment of the available inorganic nitrogen pool as the size of this pool was reduced. 3. Significant spatial variation in isotopic ratios was observed within littoral and profundal communities. Some spatial differences were likely related to lake‐specific characteristics, such as a major inlet and a small harbour area and some were interconnected with temporal events. 4. Marked differences between spring and autumn δ¹⁵N and δ¹³C values of fish at one site probably reflected a spring spawning immigration from a larger downstream lake and also indicated limited dispersal of these immigrants. 5. Our results indicate that restricted sampling of ecosystem components from lakes may provide misleading single values for the isotope end members needed for quantitative uses of stable isotopes in mixing models and for estimating trophic position. Hence we strongly advise that studies of individual lakes, or multiple lake comparisons, that utilise stable isotope analyses should pay more attention to potential within lake spatial and temporal variability of isotope ratios.