Trophic links in open oceans: combining spatial data, dietary information and biomarkers from top predators to unveil structure and functioning of subtropical pelagic ecosystems
Large marine predators are widely recognized as key elements in the marine environment, delivering invaluable regulation services and many providing substantial economic revenue. Hence, their rapid and widespread decline by excessive harvesting or other human impacts is a matter of high economic and social relevance, gathering serious concerns in the scientific and political arenas alike. Therefore significant resources have been devoted to build ancillary data to feed functional ecosystem models. These models are strongly based on the knowledge of relevant ecological processes and provide vital guidance to management for the sustainable and ecosystem-based exploitation of commercially significant taxa and/or the conservation of threatened species. However, in many areas such process-oriented approach is hampered by limited knowledge on such basic topics as diet, energy fluxes and distribution of predators and of their prey.
This is also true for the deep-ocean regions surrounding the archipelago of Madeira, including the Selvagens. In this project we will undertake the first comprehensive and integrated attempt to unveil the structure and functioning of these offshore pelagic ecosystems. Our broad-scoped team aims at investigating and modelling the trophic links (and ecological derivatives) involving the marine megafauna occurring in this vast oceanic region. The taxonomic coverage of this project includes above and underwater predators: pelagic seabirds (shearwaters and petrels), tunas, billfishes (targeted by commercial or big game fisheries), sea turtles and marine mammals. These charismatic taxa will be used to provide detailed data on the distribution and ecological significance of prey, mostly fish and squid but also crustaceans and jellyfish. The geographical scope of the project includes the huge ranges of well-studied breeding seabirds, notably Cory’s Shearwater (CS) which forages over an area over 1.5million km2. Data from other pelagic-foraging seabird species and from local tuna fisheries will likely extend this area to represent a vast fraction of the 4million km2 comprising the Portuguese EEZ.
The location of major aggregations of seabird taxa in this region is still unknown. We will complement our extensive database on CS trips with new tracking data for most other pelagic seabird (Bulwer’s Petrel, Deserta Petrel, Little Shearwater, and White-Faced Storm Petrel), using bird-born GPS and geolocators. We will employ recent spatially-explicit techniques to produce predictive distributional models, using remote-sensed oceanographic variables.
The diet of seabirds tracked with GPS as well as that of tunas and billfishes captured at known locations will be used to assess the distribution and ecological role of different forage taxa in these subtropical ecosystems. We plan to combine conventional methods with DNA-barcoding to describe diet of predators to high taxonomic detail, using stomach samples of live, stranded and harvested animals. Research on trophic ecology and feeding habits of predator and prey will be furthered by using mixing models and community metrics derived from stable isotopic analysis of predator and prey tissues. Our heterogeneous predator assemblage represents a powerful asset to sample marine prey over wide horizontal (inshore-offshore), and vertical (epi-vs mesopelagic) gradients, in distinct water masses and bathymetric landscapes.
Evidence suggests that even remote oceanic environments are not safeguarded from pollution due to global water and air circulation of several pollutants, most showing bio-accumulation and trophic amplification, like potential toxic trace elements. Levels of biota contamination in this region are poorly documented, a situation in need of remedy given that some species are used for human consumption. Our multi-species study represents an ideal setup to document and interpret geographic patterns in trace elements (Hg and other contaminants) distribution, and to identify factors and processes contributing to enrichment in different taxa.