Guano-derived nutrient subsidies drive food web structure in coastal ponds

A stable isotope study was carried out seasonally in three coastal ponds (Marinello system, Italy) affected by different gull guano input to investigate the effect of nutrient subsidies on food web structure and dynamics. A marked 15N enrichment occurred in the pond receiving the highest guano input, indicating that gull-derived fertilization (guanotrophication) had a strong localised effect and flowed across trophic levels. The main food web response to guanotrophication was an overall erosion of the benthic pathway in favour of the planktonic. Subsidized primary consumers, mostly deposit feeders, switched their diet according to organic matter source availability. Secondary consumers and, in particular, fish from the guanotrophic pond, acted as couplers of planktonic and benthic pathways and showed an omnivorous trophic behaviour. Food web structure showed substantial variability among ponds and a marked seasonality in the subsidized one: an overall simplification was evident only in summer when guano input maximises its trophic effects, while higher trophic diversity and complexity resulted when guano input was low to moderate

Co-existing with the alien: evidence for environmental control on trophic interactions between a native (Atherina boyeri) and a non-indigenous fish species (Gambusia holbrooki) in a Mediterranean coastal ecosystem

Biological invasions are a widespread problem worldwide, as invasive non-indigenous species (NIS) may affect native populations through direct (e. g., predation) or indirect (e.g., competition) trophic interactions, leading to changes in the food web structure. The trophic relationships of the invasive eastern mosquitofish Gambusia holbrooki and the native big-scale sand smelt Atherina boyeri coexisting in three Mediterranean coastal ponds characterized by different trophic statuses (from oligotrophic to hypereutrophic) were assessed in spring through isotopic niche analysis and Bayesian mixing models. The two fish relied on the distinctive trophic pathways in the different ponds, with the evidence of minimal interspecific niche overlap indicating site-specific niche divergence mechanisms. In more detail, under hypereutrophic and mesotrophic conditions, the two species occupied different trophic positions but relying on a single trophic pathway, whereas, under oligotrophic conditions, both occupied a similar trophic position but belonging to distinct trophic pathways. Furthermore, the invaders showed the widest niche breadth while the native species showed a niche compression and displacement in the ponds at a higher trophic status compared to the oligotrophic pond. We argue that this may be the result of an asymmetric competition arising between the two species because of the higher competitive ability of G. holbrooki and may have been further shaped by the trophic status of the ponds, through a conjoint effect of prey availability and habitat complexity. While the high trophic plasticity and adaptability of both species to different environmental features and resource availability may have favored their coexistence through site-specific mechanisms of niche segregation, we provide also empirical evidence of the importance of environmental control in invaded food webs, calling for greater attention to this aspect in future studies

Ontogenetic trophic segregation between two threatened smooth-hound sharks in the Central Mediterranean Sea

Elasmobranchs are among the species most threatened by overfishing and a large body of evidence reports their decline around the world. As they are large predators occupying the highest levels of marine food webs, their removal can alter the trophic web dynamic through predatory release effects and trophic cascade. Suitable management of threatened shark species requires a good understanding of their behaviour and feeding ecology. In this study we provide one of the first assessments of the trophic ecology of the \u201cvulnerable\u201d smooth-hounds Mustelus mustelus and M. punctulatus in the Central Mediterranean Sea, based on stomach contents and stable isotope analyses. Ontogenetic diet changes were addressed by comparing the feeding habits of three groups of individuals: juveniles, maturing and adults. Our results highlighted that the two species share a similar diet based mostly on the consumption of benthic crustaceans (e.g. hermit crabs). Their trophic level increases during ontogeny, with adults increasing their consumption of large-sized crustaceans (e.g. Calappa granulata, Palinurus elephas), cephalopods (e.g. Octopus vulgaris) and fish (e.g. Trachurus trachurus). Our results provide also evidence of ontogenetic shifts in diet for both species showing a progressive reduction of interspecific trophic overlap during growth. The results of this study contribute to improve the current knowledge on the trophic ecology of these two threatened sharks in the Strait of Sicily, thus providing a better understanding of their role in the food web

Trophic connections by means of biotic factors in transitional areas

Le aree di transizione sono sistemi naturali d’interfaccia di elevato valore ecologico, strettamente connessi agli ambienti adiacenti tramite flussi di materia ed energia. Tuttavia, la conoscenza dei meccanismi e degli effetti innescati dalla connettività tra sistemi adiacenti risulta ancora alquanto limitata, specialmente per quanto concerne il ruolo del biota. La grande eterogeneità e la complessità delle aree di transizione rende, inoltre, impossibile una generalizzazione ed implica pertanto la scelta di oculate ipotesi e adeguate tecniche d’indagine in grado di comprendere le dinamiche che si innescano nelle diverse realtà ecologiche. Al fine di valutare il ruolo dei fattori biotici come potenziale veicolo di materia organica, nutrienti e contaminanti tra sistemi adiacenti di diverse aree di transizione, nell’ambito di questa tesi di dottorato, sono state scelte due diverse aree di studio, una mediterranea, i laghetti di Marinello (Sicilia, Italia), ed una tropicale, Gazi Bay (Mombasa, Kenya), che rappresentano modelli di aree indipendenti in cui testare le ipotesi ecologiche. Attraverso l’uso congiunto di diversi approcci, quali lo studio del trofismo di acque e sedimenti, dei marker trofici e dei contaminanti ambientali, è stato possibile indagare gli obiettivi specifici scelti in relazione alle diverse peculiarità e dinamiche ecologiche delle aree di studio. Nell’area mediterranea dei laghetti di Marinello (Sicilia, Italia) la struttura ed il funzionamento dei vari laghetti sono risultati profondamente influenzati dalla presenza di una colonia di gabbiani che rappresenta un’importante fonte di input alloctoni (guano). Nelle stazioni maggiormente interessate dall’influenza dell’input aviario sono stati osservati un incremento della produttività primaria e del carico organico, bassi livelli di diversità delle comunità macrobentoniche con la dominanza di specie opportuniste, una struttura trofica complessivamente orientata verso l’uso del fitoplancton rispetto al fitobenthos ed elevati livelli di contaminazione ambientale. Nell’area tropicale di Gazi Bay (Mombasa, Kenya) è emersa una notevole importanza degli habitat di back-reef come aree di alimentazione e nascondiglio per i giovanili di specie ittiche piuttosto che l’habitat a mangrovie, più importante, invece, come feeding ground per gli adulti. Diverse specie, tra le più importanti economicamente, hanno inoltre dato indicazioni, specialmente nella fase adulta, di effettuare migrazioni alimentari e/o ontogenetiche tra le mangrovie ed il coral reef supportando così alti livelli di connettività inter-habitat. Mentre in letteratura è assodato il ruolo delle forzanti abiotiche nell’influenzare le dinamiche ecosistemiche costiere, dalla presente tesi emerge che nelle aree di transizione i fattori biotici giocano un forte ruolo nel connettere i sistemi adiacenti, secondo modalità strettamente legate alle caratteristiche intrinseche dei relativi sistemi, ma le cui conseguenze si ripercuotono comunque sui processi trofici e più in generale ecosistemici.Transitional areas are interface natural systems, featured by a high ecological value and strictly linked to adjacent habitats through flows of materials and energy. However, the knowledge of both mechanisms and effects of inter-habitat connectivity is still limited, especially about the role of biota. Further, both elevated heterogeneity and complexity of transitional areas make impossible to generalize trends and patterns, need the choice of careful hypotheses to be tested and appropriate techniques to understand the specific dynamics of the different ecological contexts. In order to evaluate the role of biotic factors as possible carriers of organic matter, nutrients and contaminants between adjacent habitats of transitional areas, two study areas were chosen: a Mediterranean area, the Marinello ponds (Sicily, Italy) and a Tropical area, Gazi Bay (Mombasa, Kenya), representing two independent models of areas where to test the specific ecological hypotheses. By the combined use of several approaches, as the study of trophic status of water and sediment, the study of trophic markers and environmental contaminants, it was possible to fulfill the objectives relating to the different ecological features and dynamics of study areas. In the Mediterranean area, the pond structure and functioning were highly influenced by the presence of a seagull colony, important source of allochtonous input (guano). Increased primary productivity and organic load, low levels of biodiversity of macrozoobentonic communities and dominance of opportunistic species, trophic structure shifted towards the use of phytoplankton more than to phytobenthos and high levels of environmental contamination were found in the sites more influenced by the avian input. In the Tropical area of Gazi Bay (Mombasa, Kenya), a high importance of back-reef habitat as feeding ground and refuge area for juveniles of fish was found. Instead, the mangrove habitat was more important as feeding ground for adult fish. Several species, among the more commercially important in the area, gave evidence to carry out feeding and ontogenetic migrations between mangroves and coral reef, supporting a high level of inter-habitat connectivity. While the role of abiotic factors in influencing ecosystem dynamics of coastal areas is well recognized in the literature, in this thesis we show that, in transitional areas, the biotic factors play an important role in connecting adjacent systems in a way strictly linked to the specific features of the areas, but raising consequences that affect anyway the trophic and ecosystem processes

Macroalgae transplant to detect the occurrence of anthropogenic nutrients in seawater of highly tourist beaches in Mediterranean islands

In the Mediterranean region, islands are among the most important tourist destinations, being sites of considerable naturalistic, historical and cultural importance. This is highly beneficial for local economies, but may also represent a threat for the environment, especially during the peak season (i.e. summer). Indeed, the sharp demographic increase concentrated in a relatively short period may negatively affect the quality of coastal marine systems and the provision of ecosystem services, producing in turn negative feedbacks on tourism industry. Nevertheless, the assessment of the environmental impact of tourism on coastal seawater has been seldom addressed. Here, we show the results of the biomonitoring approach adopted in the tourist Island of Rhodes (Greece), in the context of the Interreg Med BLUEISLANDS project. The study involved short-term macroalgae transplantation and incubation, and, through the analysis of nitrogen stable isotopes, provided a time-integrated picture of the occurrence of anthropogenic nutrients, which are indicators of water quality and might be missed by routine water quality monitoring programs. Main findings ruled out a marked input of anthropogenic nutrients potentially threatening the functioning of coastal ecosystems and highlighted overall good environmental conditions. In addition, this approach provided spatial data useful to produce GIS maps, useful tools that may help the decisional process of policy-makers, for adopting management practices to mitigate the environmental impact and foster sustainable touris

Study of the origin and fate of organic matter in a tropical bay using elemental, isotopic and fatty acid biomarkers.

Organic matter (OM) exchanges between adjacent coastal ecosystems affect the role of the different primary producers as energy and nutrient sources in food webs. Elemental and isotope parameters (C/N ratios, δ13C) and fatty acid (FA) biomarkers were used to assess the origin and distribution of OM in both surface sediment and water along a strongly tidally-influenced tropical area, Gazi Bay (Kenya), in two climatic seasons (dry and rainy). Dominant primary producers (i.e. mangroves, macroalgae and seagrasses) were also characterized through the same combined approach. Export of the mangrove-derived OM, highly depleted, to the adjacent bay was evident from a gradual 13C-enrichment of primary producers and sediment along the land-sea transect, with more depleted values close to the mangroves and more enriched ones in the coral reef. Contextually, a step-by-step decrease of both C/N ratio and mangrove FA biomarker concentration was also evident in the surface sediment. This export was higher in the rainy season but reduced from the wide seagrass bed area. This result is consistent with the buffer action of seagrasses, which efficiently act in trapping the suspended OM. Isotopic and FA signatures of the particulate OM gave indication of a mixed contribute of both allochthonous and autochthonous OM sources, but, unlike sediment, were overall homogeneous within the bay because of the mixing effect of tides. FA biomarkers revealed that brown algae, seagrasses and diatoms were also important OM sources in the whole bay. These results suggest an high ecological connectivity in Gazi Bay, which may have important consequences on the functioning of the whole ecosystem and on the local food webs