Stable isotope‐based trophic structure of pelagic fish and jellyfish across natural and anthropogenic landscape gradients in a fjord estuary

Abstract

Identifying causes of structural ecosystem shifts often requires understanding trophic structure, an important determinant of energy flow in ecological communities. In coastal pelagic ecosystems worldwide, increasing jellyfish (Cnidaria and Ctenophora) at the expense of small fish has been linked to anthropogenic alteration of basal trophic pathways. However, this hypothesis remains untested in part because baseline description of fish–jellyfish trophic dynamics, and the environmental features that influence them are lacking. Using stable isotopes of carbon (δ13C) and nitrogen (δ15N), we examined spatiotemporal patterns of fish and jellyfish trophic structure in greater Puget Sound, an urbanizing fjord estuary in the NW United States. We quantified niche positions of constituent species, niche widths and trophic overlap between fish and jellyfish assemblages, and several community‐level trophic diversity metrics (resource diversity, trophic length, and niche widths) of fish and jellyfish combined. We then related assemblage‐ and community‐level measures to landscape gradients of terrestrial–marine connectivity and anthropogenic influence in adjacent catchments. Relative niche positions among species varied considerably and displayed no clear pattern except that fish generally had higher δ15N and lower δ13C relative to jellyfish, which resulted in low assemblage‐level trophic overlap. Fish assemblages had larger niche widths than jellyfish in most cases and, along with whole community trophic diversity, exhibited contrasting seasonal patterns across oceanographic basins, which was positively correlated to landscape variation in terrestrial connectivity. In contrast, jellyfish niche widths were unrelated to terrestrial connectivity, but weakly negatively correlated to urban land use in adjacent catchments. Our results indicate that fish–jellyfish trophic structure is highly heterogeneous and that disparate processes may underlie the trophic ecology of these taxa; consequently, they may respond divergently to environmental change. In addition, spatiotemporal variation in ecosystem connectivity, in this case through freshwater influence, may influence trophic structure across heterogeneous landscapes.

A trophic latitudinal gradient revealed in anchovy and sardine from the Western Mediterranean Sea using a multi-proxy approach

“…daytime). Determining jelly organisms ingestion by fish is of special interest, since such expanding organisms could be considered indicators of global warming and changes in the pelagic ecosystem 64 , 65 . However, it is not clear whether hydrozoans have not been ingested in the past, or the lack of such information is related to methodological issues.…”

Mixing models and stable isotopes as tools for research on feeding aquatic organisms

“…Other research has been developed with animals in their natural environment, in order to understand the relations and trophic interactions in their habitat (RAO et al, 2015;ENDJAMBI et al, 2016;NAMAN et al, 2016;NACHEV et al, 2017). These stable isotope surveys also allow more precise understanding the biology and ecology of different groups of aquatic organisms.…”

“…These stable isotope surveys also allow more precise understanding the biology and ecology of different groups of aquatic organisms. Among these researchs, we can highlight the one developed by NAMAN et al (2016) that used the technique of stable isotopes and mixing model to establish and understand the complexity of the trophic structure of pelagic fish and jellyfish in estuarine environments.…”

Floating nurseries? Scyphozoan jellyfish, their food and their rich symbiotic fauna in a tropical estuary

“…As a result, trophic models assessing food web structure and energy flow often ignore jellyfish or include them as a single functional group with the characteristics of an ‘average’ jellyfish whose parameterization frequently varies greatly among models ( Pauly et al, 2009 ). In fact, the hypotheses proposed to explain changes in jellyfish dominated ecosystems remain untested, partially because there is a recognition that more basic research on feeding ecology is still required ( Pauly et al, 2009 ; Richardson et al, 2009 ; Naman et al, 2016 ).…”

“…Much of the prevalent view about fish-jellyfish trophic dynamics is derived from scyphozoans of subtropical and temperate areas that support large pelagic fisheries. In estuarine systems, the scarce evidence suggests a low trophic overlap between fish and jellyfish ( Nagata et al, 2015 ; Naman et al, 2016 ) and a high occurrence of symbiotic associations with juveniles of fish and invertebrates (e.g., Rountree, 1983 ; Costa, Albieri & Araújo, 2005 ; Martinelli et al, 2008 ), perhaps reflecting the abundance of early stages of coastal fauna in nursery areas. Hence, there is a need to study the trophic ecology of scyphozoan jellyfish and their multiple biological interactions to truly understand population dynamics, their position in food webs and their functional role in estuarine ecosystems.…”