A global trophic functional analysis of estuary-associated fishes

The global trophic structure and trophic level of estuarine-associated fishes was analysed in relation to biogeographic and selected environmental parameters. Meta-assemblages of estuary-associated fishes were collated at the marine ecoregion level and were analysed at a broad biogeographic (bioregion) level. The fish assemblages were compared based on feeding mode functional groups (guilds) and fish trophic levels. Relationships between these guilds and fish trophic levels and selected environmental characteristics and fish functional attributes were also examined. Significant differences were observed between bioregions in the relative species contribution of functional groups and their trophic levels. Sea surface temperature, which is often related to latitude, appeared to be a major factor associated with the relative contribution and trophic levels of estuary-associated fishes within each bioregion. Zoobenthivores dominated the species composition in all bioregions, highlighting the importance of the benthic food chain in estuarine fish communities. The relative contribution of detritivores, herbivores, and omnivores declined from tropical bioregions to cool-temperate bioregions, with the trophic level of these groups of fishes also increasing from tropical to cool-temperate ecoregions. This trend was linked to the diversity and ability of detritivorous fishes to process low quality (trophic level) food at low latitudes. The relative contribution and trophic level of zooplanktivores increased from tropical to cool-temperate bioregions; this related to higher zooplankton abundance and quality associated with temperate regions. The relative contribution of piscivorous fishes exhibited a decline from tropical to cool-temperate bioregions but the trophic level of this group showed the opposite trend. Through consuming high quality (trophic level) food, piscivores are able to occupy and thrive in cool-temperate estuarine habitats. Generalised estuarine food webs that contrast tropical and cool-temperate bioregions are also presented, with the key role of detritus in conferring stability to estuarine food webs being emphasized.